PALEOCLIMATIC
RECONSTRUCTION FROM
THE NORTHEAST
SUBTROPICAL ATLANTIC:
ATMOSPHERIC AND
OCEANOGRAPHIC SCENARIOS
Graziella Bozzano
Doctoral thesis fulfilled at the Department of
Marine Geology and Physical Oceanography
Instituto de Ciencias del Mar
CSIC
Barcelona, 2004
PROGRAMA DE DOCTORADO DE
CIENCIAS DEL MAR
Bienio 1996-1998
Memoria presentada por
Graziella Bozzano
para optar al título de DOCTOR EN CIENCIAS DEL MAR
por la Universidad de Barcelona
Director de Tesis:
Dra. Belén Alonso
Dept. de Geología Marina y Oceanografía Física
Instituto de Ciencias del Mar
CSIC - Barcelona
Alla mia amata Genova ed al mare della Liguria
Lenta e rosata sale su dal mare
la sera di Liguria, perdizione
di cuori amanti e di cose lontane.
[...]
Sepolto nella bruma il mare odora.
Le chiese sulla riva paion navi
che stanno per salpare.
Vincenzo Cardarelli, “Sera di Liguria”
A little learning is a dang'rous thing;
Drink deep, or taste not the Pierian spring:
There shallow draughts intoxicate the brain,
And drinking largely sobers us again.
Fir'd at first sight with what the Muse imparts,
In fearless youth we tempt the heights of arts,
While from the bounded level of our mind,
Short views we take, nor see the lengths behind,
But more advanc'd, behold with strange surprise
New, distant scenes of endless science rise!
So pleas'd at first, the tow'ring Alps we try,
Mount o'er the vales, and seem to tread the sky;
Th' eternal snows appear already past,
And the first clouds and mountains seem the last;
But those attain'd, we tremble to survey
The growing labours of the lengthen'd way,
Th' increasing prospect tires our wand'ring eyes,
Hills peep o'er hills, and Alps on Alps arise!
Alexander Pope, 1711. "An Essay on Criticism", lines:215-232
AGRADECIMIENTOS
La realización de esta tesis se ha beneficiado de la financiación de los
proyectos CANIGO (MASTIII-CT96-0060), MAYC (CICYT-AMB95-0196),
MARSIBAL (REN2001-3868-CO3-03), y de mi generosa beca predoctoral
Marie Curie, gracias a la cual he disfrutado de una inmensa libertad a la hora
de tomar decisiones sobre mi trabajo.
Escribir estas páginas de agradecimientos me es muy grato porque por
fin puedo dejar plasmado mi ¡gracias! a las muchas personas que me han ido
apoyando, acompañando, ayudando y… aguantando, a lo largo de la
realización de esta tesis. Expreso aquí mi agradecimiento:
a la Dra. Belén Alonso, mi directora de tesis, por la confianza
depositada en mí durante todo el tiempo que esta tesis ha necesitado para salir
a la luz, por su dedicación y por su constante apoyo, y sobre todo por respetar
mi forma de ser y de trabajar.
to Dr. Rainer Zahn, who has supported me and my work with a great
deal of enthusiasm since the beginning of this thesis. I especially acknowledge
his clever advices and recommendations that always enlighten and guide my
work.
al Dr. Miquel Canals de la Universidad de Barcelona por haberme
brindado la oportunidad de participar en CANIGO, la campaña Meteor y en
las reuniones de proyecto, confiando un trabajo de envergadura a una joven
estudiante italiana a la que poco o nada conocía; y al Dr. Toni Calafat, por
ayudarme todas las veces que lo he necesitado.
a los miembros del tribunal por haber aceptado ser parte del mismo,
con todo lo que esto conlleva.
a los miembros del departamento de Ecología de la facultad de
Biología de la Universidad de Barcelona, por aceptarme como alumna del
programa de doctorado y especialmente a los Drs. Mikel Zabala y Felipe
Fernández, tutores del programa, y a la Dra. Isabel Muñoz por agilizar los
trámites necesarios para la presentación de la tesis. Gracias también a Montse
Fresco, Ana Cuchí y Maria Ros, secretarias del programa de doctorado de la
UB, por su disponibilidad y paciencia.
Con la campaña Meteor 37/1 empezó mi aventura… Gracias a mi
compañero de campaña Jordi Targarona por cuidarme y respaldarme. Va aquí
un recuerdo cariñoso a los demás componentes del grupo de los
“mediterráneos” de esa campaña, y sobretodo a Fátima Abrantes. A los
doctores Holger Kuhlmann, Helge Meggers, Tim Freudenthal, Susane Neuer
y Gerolf Wefer, investigadores involucrados en el proyecto CANIGO,
agradezco el impulso dado sobre todo en la primera etapa de mi trabajo. En
particular, gracias a Holger por pasarme los datos del escáner, por su simpatía
y por el intercambio de ideas en su estancia en Barcelona. Estoy muy
agradecida a Ana Moreno por el continuo intercambio de informaciones,
datos y vivencias, y por ser buena compañera de viaje.
A la campaña siguieron muchas horas en el laboratorio. Compartí el
espacio algo reducido del laboratorio de sedimentología del viejo y querido
instituto en la Barceloneta con Ana, Enrique, Mohamed, Susana, Bene y
Neus. Fue muy grato el día a día con todos ellos, tengo muchos y muy buenos
recuerdos relacionados a esa época, caracterizada por el continuo intercambio
de ideas, enseñanzas, experiencias, vivencias, preocupaciones, alegrías… A
Ana le debería decir gracias por las miles de veces que me ha ayudado sin que
se lo pidiera, y por las miles de veces que sí se lo pedí, por sus ánimos, sus
alegrías y por ser una buena amiga. A Susana, que además fue mi compañera
de despacho, le debo las gracias por haberme encarrilado durante las primeras
fases de la tesis y por expandir en el aire su alegría, energía y vitalidad
envidiables que, después de su partida, eché mucho en falta. A Enrique
agradezco los buenos ratos que regala a su entorno de forma tan espontánea,
los consejos y los ánimos que siempre me ha prodigado tanto de cerca como
desde la lejanía de Alemania.
Durante los años de la realización de esta tesis el grupo de Geología
Marina en el que estuve “afincada” fue cambiando y modificando sus
componentes. A Ludovic le tengo especial cariño porque más que compañero
de trabajo fue y sigue siendo muy buen amigo, a pesar de las distancias.
Gracias por haberme hecho descubrir a los Massive Attack! Estoy
especialmente agradecida a mis muchos compañeros de despacho, Ferran,
María José, Susana, Jorge, Fernando, por aguantarme. A Marta y a Sandra,
gracias por cuidarme con mucho cariño, por los buenos momentos pasados en
el despacho a tres, y por las excursiones de los fines de semana. A Marga, mi
actual compañera de despacho, le agradezco enormemente haber tolerado con
humor la crítica fase de la redacción final de la tesis, haberme ayudado
eficazmente y animado muchísimo. En la recta final de tu tesis espero que
tengas una compañera de despacho tan maravillosa como la que he tenido yo.
A Marcel.li un enorme “gracias” por resolverme gran cantidad de problemas,
no sólo informáticos, y por adoptarme en su despacho en las épocas de la
lupa. A las que se fueron a buscar mejor suerte, María José y Annick, gracias
por los buenos ratos compartidos y por los ánimos. Un recuerdo cariñoso a la
memoria de Jesús Baraza, quedan inolvidables las comidas lideradas por él, su
lectura del periódico, y las lágrimas en los ojos por las risas. Gracias también
a todos los demás componentes del grupo, Pere, Albert, Sergi, David, Silvia,
Elena, y especialmente a Marc, Jacobo y Mohamed por las excursiones al
monte que me sentaron de maravilla.
Con mi segunda campaña a bordo del buque Meteor empezó mi
colaboración con el grupo de investigadores del centro GEOMAR de Kiel,
especialmente con los doctores Rainer Zahn, Joachim Schönfeld, y Matthias
Hüls. I again say thank to Rainer for the several genuine conversations about
science and life, and for affording me the opportunity to perform the stable
oxygen isotopes in the Kiel Laboratory, expecting nothing in return. Thanks
also to Matthias for executing the isotope analyses one week before his PhD
defence. I also thank him for sharing the office with me for 3 long months in
Kiel, introducing me to the world of the planktonic foraminifera. Thank to
Joachim for attending my several questions and for reading the chapter 5.
Gracias a la Dra. Menchu Comas, por facilitarme los trámites para realizar mi
estancia en GEOMAR.
Los tres meses pasados en la fría y aburrida Kiel fueron aliviados por
el encuentro afortunadísimo con mis amigos rusos, Jenny y Stas. Thanks
Jenny for being always ready to offer your help and take care of me, for
teaching me a lot about SST reconstructions, for cheering me up, for listening
to me, and for being firm in our friendship despite the distance. Thanks to
Stas for creating good atmosphere in that horrible student residence, for the
weekends together and for teaching me to be not afraid of the rain.
En ocasión de una breve estancia en Inglaterra, estuve colaborando
con investigadores del Centro Oceanográfico de Southampton. Allí aprendí el
manejo del Sensor Core Logger gracias a la paciencia y amabilidad infinita de
Dave Gunn y al beneplácito del Dr. Phil Weaver. Thanks to Dave for the long
hours spent with me in the logger’s room, for the chocolate cookies, for the
excursions to the Forest and for being so patient with my bad English. Pero lo
mejorcito de esas tres semanas en el SOC fue el ambiente internacional y
entusiasta liderado por Roberta, Emilio y Eulàlia. Grazie Roberta per avermi
offerto a prima vista la tua amicizia e disponibilitá. Non dimentico che sei
sempre stata la prima a soccorrermi nei momenti difficili. Gràcies a l'Eulàlia
pels ànims que m'ha donat contínuament, i per tenir sempre una estona per
xerrar amb mi. També gràcies al Raimon per saber-me escoltar.
A lo largo de los años tuve diferentes ocasiones de colaborar con los
investigadores del grupo de análisis químicos del Centro de Investigación y
Desarrollo del CSIC. Gracias al Dr. Joan Grimalt, por animarme a una nueva
realización de los análisis de carbono orgánico en su laboratorio, y a
Constancia López y a Montse Ferrer por facilitarme el trabajo. Un ¡gracias!
muy especial al Dr. Joan Villanueva, su ayuda y sus enseñanzas fueron muy
importantes y valiosas. A Isabel Cacho, Eva Calvo, Carles Pelejero, y demás
componentes del viejo y olvidado “grupo de paleo” de Barna que tanta
energía consiguió crear durante un tiempo. Gracias por el apoyo, los ánimos y
el intercambio enriquecedor de ideas.
A todos los miembros del Instituto de Ciencias del Mar con que
comparto el día a día desde hace muchos años, gracias por la disponibilidad y
la profesionalidad siempre dirigidas a facilitar mi trabajo. Gracias a las Dras.
Gemma Ercilla y Marta Estrada por haber agilizado los trámites burocráticos
para mi permanencia en el Instituto, al Dr. Francesc Peters por permitirme el
acceso al equipo MAC del Departamento de Biología, a Alex Mir y a Evilio
por atender al momento mis peticiones de usuario del SI, a Marta Ezpeleta por
facilitarme los recursos bibliográficos, a Rosa Cabanillas por descifrar mis
confusas peticiones de material, a José María Anguita por quedarse fuera de
horario hasta imprimir mi póster, a Conchita Borruel, Nuria Angosto, Justo
Martínez y a Jordi Estaña por su ayuda a lo largo de estos años.
Mi bienestar dentro y fuera los muros del edificio del ICM lo debo a
la compañía y el apoyo de los amigos, Hugo, Laura, Lella, Roger, Andrea,
Marta “la portuguesa”, Jordi Solé, Alicia, Claudio, Juan Cruz, Alexis,
Agustín, Nora, Pilar, Geert. Especialmente gracias a Laura, mi compañera de
piso hasta que el viento argentino vino a alborotar mi vida, por cuidarme
como una de la familia; a Carles y a Montse, mis cupidos, por haber
conducido la Felicidad hacia mí; a las niñas Lella y Marga, por corromperme
con el café de las 5 y por la entrañable compañía; a Nora por hacerme
descubrir partes olvidadas de mi misma; a Gian y Manu, amigos sin límites de
tiempo y de espacio, gracias por estar, por representar siempre un lugar seguro
de acogida, cariño y de renovación.
Y por fin llego a las personas que forman parte de mi vida, porque
cumplir el objetivo de realizar la tesis doctoral no tendría valor si no pudiera
compartir con ellos mi felicidad. A mi familia, a Anna, y a mis padres, gracias
por poder siempre contar con vosotros, por vuestro incondicional cariño y
apoyo. Y a Demián, por caminar a mi lado en la vereda del sol.
Page
CONTENTS
ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RESUMEN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
11
Part I
INTRODUCTION. . . . . . . . . . . . .
31
Chapter 1.
PRESENTATION AND OBJECTIVES. . . . . . . . . . .
33
1.1.
1.2.
1.3.
Climate change: the social impact. . . . . . . . . . . . .
Framework. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Objectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
43
45
Chapter 2.
CLIMATIC FACTORS. . . . . . . . . . . . . . . . . . . . . . . . . . .
47
2.1.
2.2.
2.3.
50
54
57
Dust impact on climate. . . . . . . . . . . . . . . . . . . . . . .
Continental margins role in global change . . . . .
Sea surface water implications in climate . . . . . .
Part II
SETTING, DATA AND
METHODOLOGY . . . . . . . . . . . .
63
Chapter 3.
THE STUDY AREA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
3.1.
3.2.
3.3.
Geography and climate. . . . . . . . . . . . . . . . . . . . . . .
Physiography and sedimentary processes . . . . .
Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
69
70
Chapter 4.
METHODOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
4.1.
Non-destructive measurements. . . . . . . . . . . . . . . .
4.1.1.
4.1.2.
Sediment colour. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Magnetic susceptibility . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
78
80
4.1.3.
Element intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.
Geochemical analyses . . . . . . . . . . . . . . . . . . . . . . .
4.2.1.
4.2.2.
Calcium carbonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Total organic carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.
Foraminiferal census counts . . . . . . . . . . . . . . . . . .
4.3.1.
4.3.2.
Sea surface temperature . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sea surface salinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.
Stable isotope measurements and age model . .
Part III
RESULTS . . . . . . . . . . . . . . . . . . . .
Chapter 5.
STORMINESS CONTROL OVER AFRICAN
DUST INPUT TO THE MOROCCAN
ATLANTIC MARGIN (NW AFRICA) AT THE
TIME OF MAXIMA BOREAL SUMMER
INSOLATION: A RECORD OF THE LAST 220
KYR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.
5.2.
5.3.
Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summer dust plume: generation and transport
Material and methods . . . . . . . . . . . . . . . . . . . . . .
5.3.1.
5.3.2.
5.3.3.
5.3.4.
5.3.5.
Calcium carbonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sediment colour and magnetic susceptibility . . . . . . . . . .
Geochemical analyses . . . . . . . . . . . . . . . . . . . . . . . . . .
Stable isotope measurements . . . . . . . . . . . . . . . . . . . . .
Spectral analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.1
5.4.2
5.4.3
Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sediment geochemical and physical properties . . . . . . . . .
Spectral analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.
Discussion and conclusions . . . . . . . . . . . . . . . . .
5.5.1.
5.5.2.
Significance of the sediment composition . . . . . . . . . . . .
Precessional forcing of stormy weather conditions . . . . . .
82
84
84
85
85
87
89
92
95
97
99
100
102
104
104
104
105
105
105
106
106
106
109
111
111
114
Chapter 6.
SIGNIFICANCE OF ORGANIC CARBON IN
THE NORTHEAST SUBTROPICAL ATLANTIC: PRODUCTIVITY AND LATERAL ADVECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.
6.2.
6.3.
Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Material and methods . . . . . . . . . . . . . . . . . . . . . . . .
6.3.1.
6.3.2.
6.3.3.
Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Total organic carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Foraminiferal counts and G.bulloides identification . . . . . . . .
6.4.
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.1.
6.4.2
6.4.3
Total organic carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Foraminiferal preservation and G.bulloides abundance . . . . .
NW Africa and Iberian Peninsula: integrated results . . . . . . .
6.5.
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5.1.
6.5.2.
6.5.3.
Marine plankton productivity . . . . . . . . . . . . . . . . . . . . . . . .
Organic carbon preservation . . . . . . . . . . . . . . . . . . . . . . . .
Allochthonous advection . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.6.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 7.
A 220 KYR HISTORY OF SEA SURFACE
HYDROLOGICAL
CHANGES
IN
THE
NORTHEAST SUBTROPICAL ATLANTIC: A
PLANKTONIC FORAMINIFERA APPROACH
7.1.
7.2.
7.3.
Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The oceanographic setting . . . . . . . . . . . . . . . . . . . . .
Material and methods . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.1.
7.3.2.
7.3.3.
Planktonic foraminifera . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sea surface temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Salinity and density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.1.
Planktonic foraminifera . . . . . . . . . . . . . . . . . . . . . . . . . . . .
117
119
120
121
123
124
124
125
126
126
127
128
132
134
135
137
139
141
143
144
146
147
148
148
149
153
153
7.4.2.
7.4.3.
Sea surface temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Salinity and density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.5.
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.5.1.
7.5.2.
7.5.3.
Heinrich fingerprints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seawater δ18O and salinity . . . . . . . . . . . . . . . . . . . . . . . . . .
Structure of the upper water column: LGM snapshot . . . . . . .
7.6.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .
Part IV
DISCUSSION AND
CONCLUSIONS . . . . . . . . . . . . . .
Chapter 8.
DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
155
160
163
163
165
166
170
171
173
8.1.
8.2.
8.3.
Dominant sedimentary processes . . . . . . . . . . . . . .
Controlling factors . . . . . . . . . . . . . . . . . . . . . . . . .. . .
Dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.1.
8.3.2.
8.3.3.
Dust generation and transport . . . . . . . . . . . . . . . . . . . . . . . .
Glacial dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dust contribution to climate change . . . . . . . . . . . . . . . . . . .
8.4.
8.5.
Sea surface temperatures . . . . . . . . . . . . . . . . . . . . . .
Time slices scenarios . . . . . . . . . . . . . . . . . . . . . . . . .
176
176
178
178
179
180
180
181
Chapter 9.
CONCLUSIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
185
9.1.
9.2.
9.3.
Major achievements . . . . . . . . . . . . . . . . . . . . . . . . . .
Major results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Future lines of investigation . . . . . . . . . . . . . . . . . . .
187
188
190
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
193
ABSTRACT
An integrated and comprehensive picture of the climatic and
oceanographic changes experienced by the northeast subtropical Atlantic (NW
African margin) during the last 220 thousand years is here presented. The
study region is characterized by features of local and global relevance. From
one hand, it is part of the Atlantic Subtropical Gyre, one of the most effective
climatic components of heat and salt transfer from low to high latitudes. From
the other hand, local factors (e.g. coastal upwelling and filament proximity,
monsoon, trades, coastline shape, bottom topography, etc.) interplay to create
a certain degree of complexity in the area, concerning both the oceanographic
and the atmospheric realms.
The study has a multi-proxy approach, since a wide range of
parameters were analyzed. In one sediment core retrieved off the Moroccan
margin, north the Canary Islands, geochemical and sedimentological data
were combined with physical properties and micropaleontological analyses. In
addition, stratigraphic, geochemical and hydrological data from other cores
located in nearby regions of the north east Atlantic were used for comparison
and integration.
Results suggest that seasonal variability in the solar radiation exerts
major influence on the tropical and subtropical climates. Atmospheric
dynamics, linked to the precessional cycle of the Earth’s orbital parameter,
control thermal gradients between land and ocean, determining atmospheric
instability and storminess, as well as enhancing monsoon activity. In addition,
atmospheric dynamics influence the strength of coastal upwelling and the
extension of filament formation. However, expected increases of primary
productivity during glacial periods were not found. Apparently, significant
amount of organic carbon was transported to the deep basin by nepheloid
layers during interglacial periods. Sediment mobilization is tentatively
imputed to sea-level fluctuations or storm surges.
Changes of the oceanographic scenario show the effects of the
variability of the Subtropical Atlantic Gyre, in that sea surface temperatures
reflect the dynamics of the Canary Current and of the subpolar front
fluctuations. Local hydrological budget (precipitation/evaporation), controlled
mostly by the continental climatic conditions, determines the changes in local
sea surface salinity. During the Last Glacial Maximum, the study area was
characterized by deeper thermocline and ticker mixed layer than analogue
regions on the other side of the Atlantic. The contrast between eastern and
western sides of the subtropical Atlantic is mainly imputed to different surface
salinity patterns that characterized the two regions.
9
10
Resumen
RESUMEN
11
Reconstrucción paleoclimática en el Atlántico noreste subtropical
RECONSTRUCCIÓN PALEOCLIMÁTICA EN EL
ATLÁNTICO
NORESTE
SUBTROPICAL:
CIRCULACIÓN ATMOSFÉRICA Y OCEÁNICA
El resumen de la presente memoria, que se presenta a continuación,
responde a la voluntad de cumplir con la normativa vigente en la
Universidad de Barcelona, aprobada el pasado 14 de enero de 1997 por
la Comisión de Doctorado de la División de Ciencias Experimentales y
Matemáticas, que así establece “Si en la redacció de la tesi no s'ha
utilitzat cap de les dues llengües d'ús de la UB s'haurà d'adjuntar un
resum amb una introducció i unes conclusions en una d'aquestes
llengües. Aquest resum haurà de donar una idea prou precisa del
contingut de la tesi, la qual cosa tan sols es podrà aconseguir donant-li
una certa extensió, que es recomana que no sigui inferior a una desena
part de l'obra original“.
La presente tesis propone unas reconstrucciones paleoclimáticas en
el Atlántico noreste subtropical, tratando de dar a conocer aspectos de
la circulación atmosférica y oceánica, a partir de una aproximación
multidisciplinar basada en diferentes parámetros y metodologías,
aplicados al sedimento marino del margen distal de la costa occidental
del norte de África.
Parte I.- Introducción
La primera parte de la tesis comprende los primeros dos capítulos:
Capítulo 1.- Presentación y objetivos
Capítulo 2.- Principales factores del cambio climático
El capítulo 1 se divide en dos apartados, el primero dedicado al
cambio climático y a su impacto en la sociedad (apartado 1.1) y el
segundo enfocado a los objetivos científicos de la presente tesis
(apartado 1.2).
12
Resumen
El primer apartado es una introducción muy general al impacto que
el cambio climático tiene sobre las sociedades modernas. El texto se
complementa de una pequeña reseña de imágenes, viñetas irónicas, y
titulares sensacionalistas que casi diariamente aparecen en los
periódicos de todo el mundo. De la lectura de los diarios, en general, se
desprende que no es tanto el cambio climático en si mismo que interesa
realmente, cuanto el efecto y las consecuencias desastrosas que el
cambio climático podría tener en un futuro relativamente cercano. Las
sociedades están preocupadas por las previsiones de incremento de las
temperaturas medias causado por el exceso de CO2 en la atmósfera y al
efecto invernadero, factores éstos que podrían tener repercusiones sobre
la aceleración del deshielo de los casquetes polares y finalmente sobre
la subida del nivel del mar. Por ejemplo, Mallorca se quedaría sin
playas, dañando gravemente el negocio del turismo veraniego!!!! Los
investigadores están obligados a dar un enfoque alarmista al cambio
climático para poder llegar a la sensibilidad de los ciudadanos y a los
proyectos de los políticos. Sea como sea, el cambio climático es un
hecho, ya que tenemos evidencias de que caracterizó la historia
geológica de la tierra durante muchos milenios y millones de años. El
efecto que la actividad humana pueda tener sobre el natural cambio
climático todavía es tema de debate entre la comunidad científica.
Seguramente estamos interaccionando con el clima y sus mecanismos
de regulación, aunque no sepamos la magnitud de esta interacción y de
sus consecuencias a grandes escalas de tiempo. Uno de los motivos de
esta incertidumbre es que poco conocemos de la variabilidad climática
natural de la tierra y menos aún comprendemos las respuestas internas
del sistema a los cambios externos. Esta tesis intenta elucidar algunos
aspectos sobre estas respuestas internas.
El segundo apartado presenta el objetivo general de la tesis, que
consiste en la reconstrucción de la dinámica de la circulación
atmosférica y oceanográfica del Atlántico noreste subtropical a lo largo
de los últimos dos ciclos glaciares-interglaciares. El cumplimiento de
este objetivo general pasa obligatoriamente por etapas intermedias de
conocimiento de la dinámica del área de estudio que inducen a
plantearse objetivos más detallados:
•
identificación de las tasas y modalidades de la transferencia de
polvo desde el continente africano hacia la región de estudio.
Con este objetivo se pretende investigar la dinámica de los
vientos dominantes en la zona de estudio (vientos alisios y
13
Reconstrucción paleoclimática en el Atlántico noreste subtropical
•
•
monzones) y descifrar los principales factores de control sobre
la producción y el transporte de polvo.
evaluación de los factores que controlan el aporte de materia
orgánica en la zona de estudio, con especial énfasis en la
distinción entre producción primaria in situ y aportes alóctonos
procedentes del continente.
caracterización de las condiciones hidrológicas de la zona de
estudio, a través del estudio de la evolución de las temperaturas,
salinidades y densidades de la capa superficial de mezcla.
El capítulo 2 se centra en los factores reguladores del clima. Entre
los muchos y complejos factores que gran impacto tienen sobre el clima
global terrestre, se han elegido tres entre los más relevantes en la zona
de estudio y más destacados en los foros de discusión científica:
1. la producción y el transporte de polvo atmosférico,
2. los procesos sedimentarios que tienen lugar en los márgenes
continentales, con especial enfoque a los que regulan la
producción, transporte y preservación de carbono orgánico en el
sedimento,
3. las condiciones hidrológicas (temperatura, salinidad, densidad)
de las aguas superficiales oceánicas.
El primer apartado trata sobre el polvo atmosférico que, en calidad
de elemento del aerosol, influencia el balance en la radiación de la
Tierra, pudiendo actuar tanto como regulador positivo y negativo sobre
el calentamiento global. También, el polvo con alto contenido en
minerales de hierro actúa como fertilizante de algunas regiones
oceánicas, favoreciendo así la producción primaria, con significativas
consecuencias sobre la regulación del ciclo del carbono.
El polvo de origen africano, procedente de las regiones áridas y
semiáridas del Sahara y Sahel, es transportado largas distancias y en
diferentes direcciones por los vientos, hacia el este en dirección del
Mediterráneo oriental, hacia el norte, en dirección de las Islas Canarias
y Europa, y hacia occidente, hacia las costas atlánticas de América
central. Por lo tanto, el polvo de origen africano tiene un gran impacto
sobre los climas y las vidas de los países mediterráneos, europeos y
parte de los países caribeños.
El contenido mineral del polvo, generalmente constituido por
cuarzo y óxidos de hierro, confiere una característica coloración rojiza
14
Resumen
al sedimento. El color y la composición mineral del polvo son algunas
de las características utilizadas por los investigadores para identificar,
describir y trazar los sedimentos de origen africano, tanto en los
depósitos actuales como en los depósitos fósiles.
En el sedimento marino de los fondos del océano Atlántico es
posible encontrar capas de polvo de origen desértico. Su estudio
permite hacer reconstrucciones del clima del pasado, como por ejemplo
identificar la paleointensidad de los vientos así como las variaciones de
dirección, o comprobar el grado de aridez de las regiones del continente
africano donde se originó el polvo. Se han venido haciendo desde hace
varios años estudios paleoclimáticos basados en el registro de polvo, no
sólo en sedimentos marinos sino también en testigos de hielo. Todos
ellos han contribuido al conocimiento de los climas pasados, aunque
algunos aspectos permanecen sin resolver. Muchos indicadores
climáticos apuntan a que los períodos glaciares eran épocas con mayor
tasa de partículas en la atmósfera, debido probablemente a una extrema
aridez o a una intensificación de los vientos. Aunque esta teoría ha sido
aceptada durante muchos años, recientemente se están planteando
hipótesis alternativas para explicar la gran cantidad de partículas en la
atmósfera. Éstas se relacionan con cambios en el ciclo hidrológico de
las zonas de acumulación o con desplazamientos de los centros de
control de la circulación atmosférica. También queda por discernir si la
mayor o menor presencia de polvo en la atmósfera es una simple
consecuencia del cambio climático o si más bien es un factor regulador
del cambio climático.
El segundo apartado discute el papel de los márgenes continentales
en los cambios climáticos así como los procesos a ellos asociados
(afloramiento costero, entrada de nutrientes de procedencia continental,
transporte de material hacia los fondos oceánicos) cuyo papel en la
regulación del ciclo del carbono es de suma importancia.
A pesar de que las zonas de producción primaria se encuentren en
las plataformas continentales, éstas no son los lugares de acumulación
principales de la materia orgánica, ya que la mayor parte del sedimento
es transportado desde la plataforma hasta las cuencas profundas. Las
principales modalidades de transporte incluyen (1) transporte directo
por superficie (por ejemplo, en presencia de filamentos costeros); (2)
advección lateral a través del descenso continental por medio de capas
nefeloides; y (3) transporte a través de cañones submarinos.
15
Reconstrucción paleoclimática en el Atlántico noreste subtropical
La acumulación y preservación del material orgánico en el
sedimento (carbono orgánico total) es el resultado de diferentes factores
y procesos, entre los cuales los más importantes son el nivel de
producción primaria en las aguas superficiales, la velocidad de
sedimentación, el contenido en oxígeno de las masas de agua profunda,
y el grado de porosidad del sedimento.
El contenido en carbono orgánico (Corg) en los sedimentos marinos
fósiles ha sido y sigue siendo utilizado por los investigadores como un
indicador importantísimo en la determinación de la evolución de la
producción primaria en épocas remotas. Puesto que los factores que
influyen en la preservación del carbono orgánico en el sedimento
marino son muchos y controvertidos, generalmente Corg es estudiado
junto con otros indicadores de paleoproductividad (microorganismos
planctónicos e indicadores geoquímicos).
Estudios clásicos del Cuaternario superior indican que la
producción primaria pudo aumentar durante las épocas glaciares debido
a un incremento en los afloramientos costeros. Éstos últimos se veían
intensificados por la elevada fuerza de los vientos alisios. Aunque la
mayoría de estos estudios estaba enfocada al último máximo glacial, la
teoría sobre una elevada producción primaria durante los períodos
glaciares se extendió a todas las épocas glaciares del Cuaternario
superior. Algunas investigaciones recientes, en cambio, han sugerido
que los afloramientos costeros no siempre y no ubicuamente fueron
reforzados durante las épocas glaciares, sino que también hay
evidencias de aumentos de afloramiento durante las épocas
interglaciares. El efecto combinado entre la anchura de la plataforma
continental, las variaciones del nivel del mar, y los cambios en la
dirección de los vientos, pudieron provocar el desplazamiento de los
centros de afloramiento hacia el mar abierto o aumentar la extensión
del área influenciada por el afloramiento de modo que el resultado final
podría ser un aumento de la producción primaria durante las épocas
interglaciares.
El tercer apartado trata sobre las aguas oceánicas, de cómo la
distribución de temperatura, salinidad y densidad en la superficie puede
afectar a las características de la capa superficial de mezcla, y de cómo
ésta influye en los cambios globales.
La capa superficial de mezcla recubre un papel estratégico en las
variaciones climáticas globales en cuanto es el principal lugar de
16
Resumen
intercambio de calor, vapor acuoso y otros gases, tan importantes como
el oxígeno (O2) y el dióxido de carbono (CO2), entre océano y
atmósfera y entre superficie y profundidades oceánicas.
La capa superficial de mezcla está conectada con las aguas
oceánicas profundas a través de la zona ocupada por la termoclina
permanente u oceánica. Esta conexión ocurre en cuanto que las aguas
que forman parte de la termoclina permanente se originan por lenta
subducción de las aguas superficiales, que se hunden siguiendo las
superficies isoclinas. Puesto que la subducción de la termoclina tiene
lugar mayoritariamente en los mares subtropicales, hay una cierta
tendencia entre los investigadores a considerar las zonas subtropicales,
y en general las zonas de bajas latitudes, de gran relevancia como
motores e impulsores de los cambios climáticos globales. Esta
tendencia, relativamente reciente, es avalada además por otras
evidencias y observaciones. En efecto, las regiones de bajas latitudes
reciben la mayor insolación, exportan grandes volúmenes de vapor
acuoso hacia la atmósfera, intercambian gran cantidad de CO2 y, en
definitiva, transportan calor hacia las altas latitudes.
Actualmente, hay un elevado interés por la aplicación de
simulaciones climáticas al pasado, cuya finalidad es, al fin y al cabo,
predecir los cambios climáticos del futuro con el mayor grado de
precisión posible. No obstante, los modelos matemáticos utilizados en
estas simulaciones, necesitan datos precisos. Es por eso que se está
poniendo mucho interés en deducir los valores “exactos” de
temperaturas y salinidades de las épocas remotas.
Obviamente la tarea no es fácil, ya que no hay indicadores directos
de éstos parámetros. Por ejemplo, en el caso de las paleotemperaturas,
durante muchos años las estimaciones se han basado en el estudio de
los foraminíferos planctónicos, utilizando funciones de transferencias y
métodos analógicos. Con esta aproximación, esfuerzos conjuntos
reunidos en el programa CLIMAP, se calculó que las temperaturas
medias tropicales del último período glaciar eran de apenas 1.5ºC más
frías que hoy en día. Sucesivamente, evidencias procedentes de
indicadores terrestres, desmintieron esta teoría y sugirieron que el
enfriamiento de los trópicos pudo llegar a ser de 5º-6ºC.
Posteriormente, indicadores aplicados a corales y a organismos del
fitoplancton, junto con las mejorías sobre las técnicas aplicadas a los
foraminíferos planctónicos, apuntaron a un enfriamiento de 3º-4ºC.
Actualmente se está todavía trabajando en los indicadores de
17
Reconstrucción paleoclimática en el Atlántico noreste subtropical
paleotemperatura y paleosalinidad, y parece ser que la mejor
aproximación es utilizar una combinación de indicadores diferentes.
Parte II.- Localización y métodos
La segunda parte comprende el tercer y cuarto capítulos:
Capítulo 3.- Presentación del área y del material de estudio
Capítulo 4.- Metodología
El capítulo 3 introduce brevemente el área de estudio y el material
utilizado
para
las
reconstrucciones
paleoclimáticas
y
paleoceanográficas. La presentación más detallada del área de estudio
(circulación atmosférica, circulación oceánica, principales masas de
aguas) está incluida en los capítulos de la tercera parte de la tesis.
El área de estudio es el margen continental africano en el Atlántico
noreste subtropical. Está delimitada al norte por el margen portugués y
hacia el noreste por el estrecho de Gibraltar que conecta el
Mediterráneo con el océano Atlántico. Al este, el área de estudio está
limitada por la costa marroquí, cuyos elementos geográficos relevantes
son las montañas de los Atlas centrales, y el valle del río Souss que
desemboca en la ciudad de Agadir, cerca del cabo Ghir. Hacia el sur, el
área de estudio está delimitada por el archipiélago de las Islas Canarias
y a occidente por el océano Atlántico.
Desde el punto de vista fisiográfico, el margen continental de
Agadir está caracterizado por una línea de costa accidentada, una
estrecha plataforma continental (aprox. 40km de ancho) y un marcado
ascenso continental (aprox. 1º de pendiente). Éste último está surcado
por valles submarinos que confluyen en un gran cañón submarino que
toma el nombre de la ciudad de Agadir. El sistema turbidítico de
Agadir y en concreto el eje principal del cañón de Agadir están
influenciados por la morfología submarina, como por ejemplo la
presencia de altos morfológicos y montañas submarinas. Hacia mar
abierto, el ascenso continental termina en las profundidades de la
cuenca de Agadir y cuenca de Canarias, así como en las llanuras
abisales más occidentales de Madeira y del Seine.
18
Resumen
El continente africano está caracterizado por diferentes regímenes
climáticos que se distinguen los unos de los otros en relación a la tasa
media de precipitación anual. Las principales franjas climáticas son la
mediterránea, la zona de transición hacia el desierto, el desierto
sahariano y el Sahel. Cada una de estás franjas climáticas está
acompañada de una característica cobertura vegetal. A lo largo de los
tiempos geológicos, los cambios climáticos que afectaron al continente
africano, cambiaron la actual disposición de las franjas climáticas. Por
ejemplo, en épocas húmedas el desierto del Sahara pasó a tener una
extensión bastante limitada, mientras que en épocas áridas la superficie
desértica se expandió a expensas de las franjas contiguas.
El material de estudio es un testigo de sedimento marino
identificado con el nombre GeoB4205, recogido durante la campaña
CANIGO a bordo del buque oceanográfico alemán Meteor. Se trata de
5 metros de sedimento cuyo aspecto refleja una sedimentación continua
hemipelágica, sin presencia de niveles turbidíticos, ni superficies de
erosión. El color del sedimento denota una alternancia de bandas
marrón claro con bandas de tonos más grisáceos. El análisis de la
fracción arenosa muestra que ésta se compone mayoritariamente de
organismos de caparazón calcáreo (foraminíferos planctónicos y
bentónicos), y en menor mesura de minerales entre los cuales el cuarzo
transparente es el más abundante. Además del testigo GeoB4205, otros
cuatro testigos de sedimento (GeoB4202, GeoB4204, GeoB4216,
MD952042) localizados en la misma zona de estudio o en regiones
cercanas se han utilizado para comparación de resultados.
El capítulo 4 introduce el conjunto de metodologías aplicadas al
estudio. Aquí se pretende presentar las diferentes metodologías
utilizadas justificándose y explicándose su uso en el ámbito de estudios
paleoclimáticos y paleoceanográficos. En los capítulos de la tercera
parte de esta tesis (Capítulos 5, 6, y 7) las metodologías son explicdas
con más detalle.
Las metodologías empleadas se han dividido en dos grandes
grupos: medidas realizadas con el uso de equipos analizadores y
estudios realizados a partir de inspección visual.
Dentro del primer grupo, se distingue entre:
•
medidas automáticas no destructivas. Comprenden el análisis
del color del sedimento usando un espectrofotómetro Minolta
19
Reconstrucción paleoclimática en el Atlántico noreste subtropical
•
CM-2002; el análisis de la susceptibilidad magnética usando el
Multi Sensor Core Logger de Geoteck; y el análisis de la
intensidad de elementos, entre los cuales el hierro y el calcio,
usando un scanner XRF. Las ventajas de estos métodos son
múltiples: no se destruye el testigo, permitiendo así posteriores
análisis; es posible programar el equipo para que realice las
mediciones según intervalos determinados por el operador; las
medidas se adquieren de forma rápida, permitiendo así obtener
registros de muy alta resolución.
medidas aplicadas a muestras separadas. Comprenden los
análisis del contenido en carbonato cálcico y en carbono
orgánico del sedimento, ambos realizados con un analizador
elemental de carbono, hidrógeno y nitrógeno (CHN); y las
medidas de isótopos del oxígeno a partir de los caparazones del
foraminífero planctónico Globigerinoides ruber (variante
blanca) realizados con un espectrómetro de masa.
El segundo grupo comprende el análisis de la asociación de
foraminíferos planctónicos y su censo estadístico (realizado con una
lupa binocular Leica), base para la sucesiva derivación de las
paleotemperaturas y paleosalinidades de las aguas superficiales.
La reconstrucción de las temperaturas superficiales del pasado
están basadas en la utilización de tres aproximaciones diferentes del
método analógico: MAT, técnica moderna analógica (Prell et al.,
1985); SIMMAX, método analógico basado en el índice de semejanza
y otras modificaciones introducidas por Pflaumann et al. (1996), y
RAM, método analógico revisado por Waelbroek et al. (1998). El uso
en conjunto de las tres diferentes aproximaciones permite valorar la
calidad de los resultados obtenidos reduciendo al máximo posible el
rango del error asociado al método. La comparación entre las
estimaciones de las temperaturas de muestras de sedimento moderno y
los valores actuales observados por medio de satélite y otras medidas
directas (base de datos de NOAA) apuntan a que el error de la
estimación oscila en un rango de ± 2ºC.
El cálculo de las paleosalinidades se basa en los resultados de las
paleotemperaturas y de las mediciones de isótopos del oxígeno de
G.ruber. El conocimiento previo de estos dos parámetros es necesario
para poder resolver la ecuación de paleotemperatura (en esta tesis se
utilizó la de Erez y Luz, 1983):
20
Resumen
T=17.0-4.52 (δ18Oc-δ18Ow)+0.03 (δ18Oc-δ18Ow)2
(1)
donde T es la temperatura del agua de mar, δ18Oc es la relación
isotópica del oxígeno en el caparazón cálcico de G.ruber y δ18Ow es la
relación isotópica del oxígeno en el agua de mar. Puesto que T y δ18Oc
son conocidos, de esta ecuación se puede extraer el término δ18Ow.
Una vez calculado el término δ18Ow con la ecuación (1), se puede
proceder a calcular la salinidad utilizando la relación existente entre
salinidad y relación isotópica del oxígeno en el agua de mar:
Salinidad= a+bδ18Ow
(2)
donde a y b son coeficientes característicos de cada región
oceánica.
Las estimaciones de paleosalinidades están además sujetas a una
serie de correcciones debidas a que la relación (2) actualmente vigente
en los océanos no se puede transferir tal cual a épocas pasadas. Hace
falta introducir términos de correcciones que tengan en cuenta los
cambios climáticos globales, como el volumen de hielo acumulado en
los casquetes polares, las consecuentes variaciones del nivel del mar y
las alteraciones en la salinidad media del agua del mar.
Parte III.- Resultados
La tercera parte de la tesis comprende los capítulos 5, 6 y 7. Cada
uno de estos capítulos tiene forma de artículo, estando constituido por
los apartados de introducción, presentación del área de estudio, material
y métodos, resultados, discusión y conclusiones. El capítulo 5 ha sido
publicado en la revista Palaeogeography, Palaeoclimatology,
Palaeoecology en el año 2002 (vol. 183, pp.:155-168); el capítulo 6 se
ha sometido para su publicación en la revista Quaternary Science
Reviews, y el capítulo 7 se está elaborando todavía para su futura
publicación.
El capítulo 5 pretende determinar el control de la turbulencia sobre
los aportes de polvo al margen Atlántico de Marruecos (África
noroeste) durante las épocas de máximos valores de insolación de
verano basándose en el registro de los últimos 220 mil años. El estudio
21
Reconstrucción paleoclimática en el Atlántico noreste subtropical
se centra en los factores que controlaron la producción de polvo en las
regiones de África noroeste y su transporte hacia el Atlántico este, las
Islas Canarias y Europa occidental durante el Cuaternario superior.
Las condiciones meteorológicas que actualmente favorecen la
generación de tormentas de polvo durante el verano son detalladas en el
apartado dedicado al encuadre geográfico. Varios estudios realizados
con observaciones directas de aportes de polvo y con la utilización de
modelos matemáticos de simulación, apuntan a elementos atmosféricos
denominados líneas de tormenta (squall lines), asociados a ondas
atmosféricas del este, como los factores determinantes en elevar el
polvo hasta la troposfera (a 3-5 Km de altitud). Posteriormente, si bien
el polvo es transportado hacia la costa occidental de América, una parte
puede llegar a las Islas Canarias y a la Península Ibérica gracias al
impulso dado por la célula de alta presión localizada sobre el norte de
África.
Los aportes de polvo a la zona de estudio durante los últimos 220
mil años son investigados a través de las propiedades físicas y químicas
del sedimento. El color del sedimento, la susceptibilidad magnética, la
intensidad de los elementos calcio y hierro, son utilizados como
indicadores de los aportes de partículas eólicas procedentes de las
regiones del Sahara y/o del Sahel.
Los resultados demuestran que la acumulación de polvo fue
regulada por la variación estacional de la insolación, asociada al
movimiento de precesión del eje de la Tierra. Los análisis espectrales
empleados sobre los indicadores de polvo demuestran una periodicidad
dominante en la frecuencia 1/23 mil años. El resultado más destacado e
insólito es que los picos de aportes terrígenos coinciden con momentos
de mínimos valores de la precesión, es decir en momentos de máxima
insolación de verano, cuando supuestamente más intensos son los
monzones y más abundantes las lluvias a ellos asociados. En cambio,
los resultados aquí permiten establecer una conexión directa entre
radiación solar, régimen monzónico y producción de polvo.
Los resultados obtenidos sugieren que, en épocas de máxima
insolación, los contrastes térmicos estacionales se vieron aumentados,
amplificando la turbulencia atmosférica y estimulando la frecuencia e
intensidad de las tormentas generadoras de polvo. Si bien la aridez es
un factor necesario en la generación de polvo, la inestabilidad
atmosférica y el estado de tormentas, también pudieron jugar un papel
22
Resumen
muy importante. El transporte de polvo hacia la zona de estudio
probablemente efectuó el mismo recorrido que hoy en día siguen las
tormentas de polvo al llegar a las Canarias y al sur de Europa.
El capítulo 6 discute el significado del carbono orgánico en el
Atlántico noreste subtropical diferenciando entre productividad
primaria y advección lateral. Se trata de un estudio de los factores que
controlan el aporte y acumulación de carbono orgánico (Corg) en el área
de estudio a lo largo de los últimos 220 mil años. El principal objetivo
es distinguir entre la influencia de la productividad primaria costera y la
influencia de los aportes aloctónos procedentes del continente.
La región es actualmente oligotrófica a pesar de que en las
cercanías, frente al Cabo Ghir, el agua de afloramiento costero sea
transportada hacia el mar abierto y cerca de la zona de estudio por un
filamento cuya formación y estacionalidad está relacionada con la
intensidad de los vientos alisios. La hipótesis planteada en el estudio es
que en épocas pasadas los alisios pudieran ser mucho más vigorosos
que en la actualidad y estimular una mayor extensión del filamento
propagando así la región de afloramiento costero hasta la zona de
estudio.
Los valores de carbono orgánico encontrados en el sedimento del
testigo estudiado oscilan entre 0.3% y 1.7%, estando los picos máximos
centrados en las épocas de mínimos valores de insolación de verano. Se
discuten tres posibles mecanismos de acumulación de dichos
contenidos de carbono orgánico: (1) la producción primaria in situ
asociada a la excepcional extensión del filamento del Cabo Ghir; (2) la
presencia de aguas profundas que podrían favorecer la preservación de
la materia orgánica del sedimento; y (3) la advección lateral de material
orgánico.
Para alcanzar los objetivos planteados, los resultados obtenidos en
el testigo en estudio (GeoB4205) son comparados e integrados con
datos derivados de otros dos testigos, uno procedente de la cercana
cuenca de Canarias (GeoB4216) y otro situado algo más al norte, frente
a las costas de Portugal (MD952042). De ambos testigos se dispone de
una serie de datos de gran calidad: dataciones, isótopos del oxígeno,
contenido de carbonato cálcico y de carbono orgánico.
La hipótesis de una mayor influencia de la producción primaria es
confrontada con el estudio de foraminíferos planctónicos, y en concreto
23
Reconstrucción paleoclimática en el Atlántico noreste subtropical
la especie Globigerina bulloides, conocida por ser típica de zonas
eutróficas asociadas a afloramientos costeros. Los porcentajes de
G.bulloides aumentan en correspondencia con los períodos de
transición entre el estadio 2 y el estadio 1 (Terminación I) y entre el
estadio 6 y el estadio 5 (Terminación II). Por lo demás, la abundancia
de este foraminífero no está relacionada con los picos de carbono
orgánico. Las variaciones en la dinámica de las aguas profundas, es
decir la alternancia entre una mayor influencia del agua del Atlántico
norte (NADW o Agua Profunda del Atlántico Norte) y una mayor
influencia del agua del hemisferio sur (AABW o Agua Profunda del
Antártida) no parece haber jugado un papel importante en la
preservación o destrucción de la materia orgánica acumulada en el
fondo marino.
El aporte lateral de material orgánico parece ser el mecanismo más
plausible para explicar los altos valores de Corg encontrados en el
sedimento de la zona de estudio. En la actualidad, advección lateral y
transporte de nutrientes y materia orgánica desde la zona costera hacia
el mar abierto ha sido observado en diversos puntos de la zona de
estudio. En cambio, todavía no hay información ni datos que
establezcan si estos mecanismos de transporte fueron activos en el
pasado geológico.
Las conclusiones finales de este estudio apuntan a que el sedimento
enriquecido en carbono orgánico procedente de las cercanas zonas de
afloramiento costero de las costas marroquíes sea movilizado por
pequeñas variaciones del nivel del mar o por el efectos de ondas de
tormentas y transportado hacia el mar adentro a través de capas
nefeloides que viajan desde la plataforma continental hacia las cuencas
profundas.
El capítulo 7 pretende evaluar los cambios en las condiciones
hidrológicas de la zona de estudio a lo largo de los últimos 220 mil
años a partir del estudio del registro de los foraminíferos planctónicos.
En concreto, se investiga la evolución temporal de las temperaturas y
salinidades de las aguas superficiales, y como éstos parámetros
influyeron en los cambios de la estabilidad de la columna de agua,
especialmente de la capa superficial de mezcla.
Los datos de temperatura y salinidad de las épocas remotas se han
obtenido indirectamente usando a los foraminíferos planctónicos como
indicadores. La abundancia relativa de las especies dentro de la
24
Resumen
asociación de foraminíferos de una muestra depende de las propiedades
del agua (temperatura, disponibilidad de nutrientes, salinidad y
densidad) en las que vivieron los microorganismos. Además, las
propiedades del agua quedan registradas en la composición química de
los caparazones cálcicos de los organismos, y más concretamente en la
relación isotópica del oxígeno δ18O/δ16O. Integrando los valores de
temperaturas superficiales obtenidos a través de los métodos analógicos
MAT, RAM y SIMMAX y los valores de isótopos del oxígeno medidos
sobre los caparazones de la especie G.ruber, se han calculado los
valores de salinidad superficial de las aguas. Por último, introduciendo
en la ecuación del estado del agua de mar los valores obtenidos de
temperatura y salinidad, se han calculado los correspondientes valores
de densidad de las aguas superficiales.
Los resultados demuestran que la zona de estudio ha
experimentado cambios significativos en sus características
hidrológicas durante los dos últimos ciclos glaciar - interglaciares. Las
temperaturas bajaron unos 4-5ºC durante las épocas glaciares, mientras
que se reestablecieron hacia valores parecidos a los actuales durante el
último interglaciar (inter-estadio 5.5). Las salinidades, en cambio,
durante las fases glaciares aumentaron de alrededor de 1‰ y bajaron
bruscamente a valores ligeramente inferiores a los actuales durante las
épocas de deglaciación. Éstas últimas, en efecto, fueron temporadas de
clima más húmedo y más lluvioso que el presente, debido a una mayor
actividad de los monzones. Bruscos descensos de temperatura (de ~12ºC) y de salinidad (de ~0.5‰) ocurrieron en las épocas denominadas
eventos Heinrich, cuando el Atlántico norte estaba poblado de icebergs
viajando hacia el sur transportados por las corrientes superficiales. El
progresivo derretimiento de los bloques flotantes de hielo modificó la
temperatura y la salinidad de las aguas disminuyendo ambos
parámetros. Sus efectos, aunque muy atenuados, llegaron a ser
transportados por la corriente de Canarias hasta las latitudes
subtropicales.
Los cambios de temperatura y salinidad no sólo afectaron a las
aguas superficiales sino también a toda la columna de agua. Por
ejemplo, el gradiente vertical de densidades cambió de intensidad
cíclicamente a lo largo del Cuaternario tardío. Esta afirmación se basa
en las fluctuaciones observadas en los porcentajes del foraminífero
planctónico G.truncatulinoides (enrollamiento derecho) considerado
indicador de mayor o menor gradiente de densidad en la columna de
agua. Se observa un registro cíclico en las abundancias de
25
Reconstrucción paleoclimática en el Atlántico noreste subtropical
G.truncatulinoides con picos en las épocas de mínima insolación.
Según estudios recientes apoyados en modelizaciones, parece ser que la
temperatura de las aguas de la termohalina (entre 100 y 600 metros de
profundidad) durante el último máximo glacial (hace 18-20 mil años)
disminuyó en el Atlántico este alrededor de 1ºC. Las salinidades, en
cambio, aumentaron globalmente en 1‰ debido a las variaciones del
volumen de hielo en los casquetes polares.
Integrando estas informaciones con los resultados del presente
estudio, se desprende que durante el último máximo glacial la
estabilidad de la capa superficial de mezcla era diferente a la que existe
hoy en día. Los cambios hidrológicos de temperatura y salinidad
devolvieron unas aguas superficiales más densas, mientras que el
aumento de densidad en la sub-superficie fue menor. De aquí se deduce
que el gradiente vertical de densidad entre superficie y subsuperficie
tenía que ser moderado, permitiendo así un gran intercambio entre las
capas de aguas. Estas deducciones se confirman en la mayor
abundancia de G.truncatulinoides durante las épocas de mínima
oblicuidad, épocas de mayor almacenamiento de hielo en los casquetes
polares.
Parte IV. Discusión y conclusiones
Esta parte de la tesis comprende los últimos dos capítulos:
Capítulo 8.- Discusión
Capítulo 9.- Conclusiones
El capítulo 8 pretende dar una visión general de todo lo que se ha
discutido en los capítulos previos 5, 6, 7. El intento consiste en reunir
toda la información y sugerir un escenario integrado de los cambios
climáticos que afectan el área de estudio a lo largo de los últimos 220
mil años. Está compuesto por cinco apartados, el primero sobre los
procesos sedimentarios que actuaron en el área de estudio (apartado
8.1), el segundo sobre los factores que controlaron los procesos antes
mencionados (apartado 8.2), el tercero sobre los aspectos asociados al
polvo atmosférico (apartado 8.3), el cuarto sobre las problemáticas
asociadas a las paleotemperaturas (apartado 8.4), y por último, el quinto
apartado ofrece una panorámica integrada de las épocas clave de los
últimos 220 mil años (apartado 8.5).
26
Resumen
En el primer apartado se da una visión general de los posibles
procesos sedimentarios que afectaron el área de estudio. La
sedimentación hemipelágica prevaleció y continuó durante todo el
lapso de tiempo en estudio. También tuvieron lugar advecciones
laterales de sedimento procedentes de la zona costera especialmente
enriquecido en materia orgánica. Los procesos turbidíticos, asociados a
la presencia del cañón de Agadir, no parecen haber influenciado el
registro sedimentario en estudio, puesto que el testigo estudiado se
encuentra en un interfluvio de dicho cañón.
En el segundo apartado son detallados los principales factores que
controlaron la variabilidad de los procesos sedimentarios en el área de
estudio. Se distinguen entre factores regionales y factores globales. A
los primeros pertenecen factores como la forma de la línea de costa, la
anchura de la plataforma continental, la topografía submarina y el
desarrollo del filamento del Cabo Ghir. La interacción de todos y cada
uno de ellos con la circulación atmosférica y oceánica determina
cambios importantes en el registro sedimentario, especialmente los que
están relacionados con la variabilidad de los aportes de materia
orgánica. Al segundo grupo de factores, los globales, pertenece la
radiación solar, las fluctuaciones del nivel del mar, las aguas profundas,
y la dinámica de los vientos, entre otros.
El tercer apartado está dedicado a las problemáticas asociadas al
tema del polvo de derivación desértica. En primer lugar, se analizan los
factores que influenciaron la generación de polvo y su posterior
transporte por medio de los vientos. La disponibilidad de polvo en las
áreas fuentes parece estar relacionada con situaciones de inestabilidad
atmosférica más que a condiciones de extrema aridez del terreno. En
segundo lugar, se discute sobre las aportaciones de polvo durante las
épocas glaciares. Muchas evidencias apuntan a que las épocas glaciares
fueron caracterizadas por un mayor aporte y acumulación de polvo
atmosférico. En cambio, los resultados de la presente tesis indican que
los aportes eólicos al área de estudio no están relacionados con los
ciclos glaciar-interglaciares, sino a una ciclicidad asociada a los
movimientos de precesión del eje terrestre. En definitiva, el polvo
africano se depositó sobre el fondo marino tanto en épocas glaciares
como interglaciares, y los máximos aportes se registraron en épocas de
máxima insolación de verano. Por último, se aborda la relación entre
polvo atmosférico y clima global. Los resultados de los análisis
espectrales efectuados sobre los indicadores del polvo desértico y los
parámetros orbitales indican que existe una cierta simultaneidad entre
27
Reconstrucción paleoclimática en el Atlántico noreste subtropical
los eventos eólicos y la insolación. Por lo tanto, no se ha podido
concluir si la concentración de polvo en la atmósfera es un factor
determinante o una simple consecuencia del cambio climático.
El cuarto apartado hace hincapié en la fiabilidad de los valores
calculados de paleotemperatura, derivados del estudio de los
foraminíferos planctónicos. Se discute la posibilidad de que los
cambios en las propiedades de las aguas profundas, así como las
fluctuaciones en la profundidad de la lisoclina, puedan haber afectado a
la integridad de los caparazones de los foraminíferos, y en definitiva
haber alterado los resultados. Esta posibilidad queda descartada puesto
que en ninguna sección del testigo estudiado el grado de disolución
cálcica alcanza valores significativos.
En el cuarto apartado también se discute el significado de los
cambios de temperaturas. Se quiere averiguar en especial si las
fluctuaciones de temperatura son debidas a la influencia del cercano
afloramiento costero o más bien forman parte de un escenario más
global, el del giro subtropical del Atlántico norte. Esta distinción es
importante porque permite especificar si la señal de temperatura
estudiada es local o global. A partir de los resultados del capítulo 6, se
concluye que la evolución de las temperaturas es una señal global,
puesto que la influencia del afloramiento costero, a través del filamento
del cabo Ghir, puede haber alcanzado el área de estudio sólo en las
épocas de deglaciación.
El último apartado concluye el capítulo de discusión con una
panorámica sobre los escenarios de las épocas clave de los últimos 220
mil años. Las épocas más representativas del clima pasado como por
ejemplo el Holoceno, el último interglacial (estadio 5.5), el ultimo
máximo glacial (estadio 2.2), y las fases de transiciones entre glaciar e
interglaciar (terminaciones I y II) quedan descritas por el conjunto de
todos los datos elaborados a lo largo de la presente tesis. Por cada uno
de estos intervalos de tiempo se delinean los procesos sedimentarios
dominantes, las características de las aguas profundas y superficiales, el
tipo de asociación de foraminíferos planctónicos y la estructura de la
capa superficial.
El capítulo 9 enumera las conclusiones generales de la presente
tesis.
28
Resumen
El primer grupo de conclusiones trata sobre las metodologías
aplicadas en el presente trabajo. En primer lugar, el material utilizado
para este estudio paleoceanográfico es apropiado, en cuanto se
caracteriza por una acumulación continua de sedimento hemipelágico.
Los datos obtenidos a través de la utilización de dispositivos
automáticos, como el sensor de susceptibilidad magnética, el escáner de
fluorescencia, y el espectrofotómetro, son de excelente calidad.
Susceptibilidad magnética, color del sedimento y contenido en hierro
son indicadores fiables de las aportaciones eólicas procedentes de las
zonas áridas del Sahara y del Sahel. También los porcentajes de
G.bulloides y G.truncatulinoides son indicadores micropaleontológicos
fiables para la evaluación de los cambios en las características de las
aguas (productividad y densidad vertical, respectivamente).
El segundo grupo de conclusiones versa sobre los escenarios
reconstruidos a partir de los datos obtenidos. Los datos elaborados en
esta tesis sugieren que existieron épocas pasadas en las que los aportes
eólicos eran más abundantes que en la actualidad. La estacionalidad de
estos cambios coincide con la variabilidad en la insolación de verano:
épocas de mayor radiación solar en verano coinciden con épocas de
mayor inestabilidad atmosférica, mayor gradiente térmico entre océano
y continente, mayor frecuencia e intensidad de tormentas asociadas a
eventos monzónicos, y así mayor presencia de polvo de origen africano
en la atmósfera. Las condiciones de aridez en las áreas fuentes no
parecen ser los únicos factores que controlan la generación de polvo.
Aunque la región es y ha permanecido oligotrófica casi durante
todo el Cuaternario superior, no se descarta que en las épocas de
tránsito de glaciar a interglaciar (es decir en las terminaciones), la
excepcional extensión alcanzada por el cercano filamento de Cabo Ghir
pueda haber transportado aguas más eutróficas a la zona de estudio. De
todas maneras, el carbono orgánico que se ha encontrado preservado en
el fondo marino no parece ser indicador directo de condiciones de
productividad primaria en la superficie. Por el contrario, se cree que
este material orgánico proceda de la zona costera y haya sido
transportado por capas nefeloides asociadas a inestabilidad del
sedimento acumulado al borde de la plataforma continental. La
inestabilidad sedimentaria podría haber sido inducida por cambios de
nivel del mar o por tormentas.
Las características de las aguas superficiales han cambiado a lo
largo de los últimos 220 mil años. A grandes rasgos, los períodos
29
Reconstrucción paleoclimática en el Atlántico noreste subtropical
glaciares estaban caracterizados por aguas más frías y más saladas que
las actuales, mientras que las condiciones en períodos interglaciares
eran parecidas a las modernas. Los cambios en las temperaturas no son
debidos a factores locales, como el cercano afloramiento costero, sino a
factores globales relacionados a la dinámica del Giro Subtropical del
Atlántico norte. Los cambios de salinidad están ligados a la dinámica
de los vientos que regula el balance entre precipitación y evaporación
en la zona. Por último, las variaciones en la profundidad de la
termoclina y en el grosor de la capa superficial de mezcla parecen ser
influenciadas por el patrón de la salinidad más que por el de la
temperatura.
30
PART I
INTRODUCTION
31
32
Chapter 1.
PRESENTATION AND OBJECTIVES
33
34
Chapter 1. Presentation and Objectives
PRESENTATION AND OBJECTIVES
The climatic variability that punctuated the history of the Earth
along the geological periods has been registered in different archives. A
trained eye can observe the fingerprints of the climate change quite
easily. For example, U-shape valleys or moraine deposits are the most
obvious evidence that thick ice masses shaped the terrestrial landscapes
thousands of years ago. Marine and lake sediments, speleothems and
peat deposits, coral reefs and ice caps also nicely document the past
climatic oscillations. However, the interpretation of the peculiar
information stored in these archives is not immediate.
In this thesis, the marine sediment retrieved from the northeast
subtropical Atlantic was analyzed and examined under several aspects.
For the purpose, different methodologies were applied in what is
generally known as “the multi-proxy approach”. The final climatic
interpretations are thus based on the global conjunction of resulting
data.
Why a thesis on paleoclimatic reconstructions? What for? Because
our society is worried by the perspective of dramatic climate changes
that may alter, or even destroy, our order of the things. The first part of
this chapter is dedicated to the climate change seen with the eyes of the
general public. It is just a brief excursion out of the ivory tower to test
what the societies are currently demanding. Climate changes, and more
specifically future climate predictions, are issues that increasingly drive
the attention of the western societies. Climate predictions depend on the
reliability of climate general models, that in turn, need climatic
constrains as much detailed as possible to better perform the computing
exercises. Climatic reconstructions based on field data, e.g. from the
marine sediments, are a first step towards a deeper knowledge of the
functioning of our system, in the past and in the future. In the second
part of this chapter, an overview about previous scientific contributions
dealing with the study region is presented; finally, in the third part the
scientific objectives of the thesis are formulated.
35
Part I. Introduction
1.1. Climate changes: the social impact
Global warming and the hole in the ozone layer are terms that
appear in the newspapers of the entire world. All western societies are
concerned about the close future effects of the climate change. The
European sixth framework programme for research and technological
development has included sustainable development, global change and
ecosystems among the seven thematic priorities of its supporting
policies.
A small sample of what can be easily found in the newspapers is
reproduced in this section. At this point, the interest is not how precise
or truthful is the information reported in the newspapers. The objective
here is just to show that the climate change is a problem that involves
and absorbs the interest of large portions of the modern society.
The major concern is the
global warming due to the
excess of CO2 in the
atmosphere
and
the
consequences on sea level
change, but also floods,
droughts, human diseases and
fauna
massive
extinctions
provoke a big alarm in the
society.
from http://www.envirocitizen.org/
The Mediterranean and the Atlantic coasts of Morocco and Spain
are subjected to the recurrent dust storms proceeding form the Sahara
desert. The impact on the society is at times undesired. For example,
prolonged atmospheric dust loads over the Canary Islands provoke
sanitary problems and difficulties in the local transports system. The
spectacular images of large quantity of dust transported by the winds
are captured by satellites and reported by the newspapers.
36
Chapter 1. Presentation and Objectives
Schematic and simplified representation of an integrated assessment framework for
considering anthropogenic climate change (The Intergovernmental Panel on Climate
Change, 2001. See: http://www.grida.no/climate/ipcc_tar/vol4/english/208.htm)
Cartoon from http://www.envirocitizen.org/ Loophole: a means of evading a rule
without infringing the letter of it (Oxford Dictionary)
37
Part I. Introduction
This section documents a small collection of extracts from Spanish
and Italian newspapers about the consequences of a general climate
change due to human activity. An extract about the impact of dust
storms over the Canary Islands is from a Catalan newspaper.
El Mundo
20 february 2001
Mil expertos vaticinan que las temperaturas pueden subir
este siglo casi seis grados - El deshielo de los polos y los
glaciares elevaría el nivel de los océanos hasta 88
centímetros en este plazo
La ONU prevé sequías, hambre, epidemias e inundaciones a
causa del cambio climático. Los expertos estiman que la
temperatura de la Tierra aumentará durante este siglo entre
1.4ºC y 5.8ºC grados y que el nivel de los océanos subirá entre 8
y 88 centímetros.
La Rioja
24 march 2003
ALERTA ANTE EL CAMBIO CLIMÁTICO.- Ayer se celebró el
Día Meteorológico Mundial presidido bajo el lema Nuestro
clima futuro
Las consecuencias del calentamiento climático, debido en buena parte
a la actividad del hombre, son inundaciones, tempestades, olas de
calor, inviernos sin apenas nieve y otros fenómenos similares. El
principal gas que contribuye al efecto invernadero, el dióxido de
carbono, ha aumentado el 30 por ciento aproximadamente desde el
comienzo de la industrialización en 1750 y aumentará más si no se
toman medidas urgentes a nivel internacional para evitarlo.
El Grupo Intergubernamental de Expertos sobre Cambio Climático
(IPCC) ha evaluado que, con el constante aumento de los gases de
efecto invernadero, la temperatura media anual, en el escenario más
optimista de emisión, aumentará 1,4ºC hasta el año 2100 y el nivel del
mar subirá en torno a nueve centímetros. En las estimaciones más
pesimistas de emisión, los valores anteriores se sitúan en 5,8ºC y 88
centímetros para 2100, aunque estos valores se verán influidos por la
demografía, el consumo de energía y el desarrollo económico.
38
Chapter 1. Presentation and Objectives
EL MUNDO-BALEARES
26 November 2002
39
Part I. Introduction
EL MUNDO
8 april 2004
C I E N C I A
40
Chapter 1. Presentation and Objectives
Il Corriere della Sera
11 August 2003
Il clima impazzito provoca danni e lutti anche nel Nord Africa
Alluvioni nel deserto del Sahara: 13 vittime
Algeria, allarme per le piogge torrenziali a Sud della capitale: hanno
ingrossato i corsi d'acqua stagionali e travolto le case
ALGERI - L'Europa soffre la
siccità, soffoca per il caldo
anomalo e teme di dover
convivere a lungo con un clima
«africano». Ma le bizzarrie del
clima, con risvolti anche gravi e
tragici, non si fermano ai nostri
confini. E il paradosso della
natura è che proprio mentre
l'Europa somiglia a un deserto
secco, nel deserto del Sahara
Una foto dal satellite del Sahara
invece sono in corso delle vere e
algerino (Afp)
proprie alluvioni che hanno fatto
anche delle vittime. Almeno 13 persone sono morte nelle ultime 48
ore nel Sahara algerino, a causa delle inondazioni provocate da
piogge torrenziali nella zona. Lo riferiscono i quotidiani El Khabar e
Er Ra.
el Periódico
5 March 2004
41
Part I. Introduction
42
Chapter 1. Presentation and Objectives
1.2. Framework
This study is part of CANIGO1 (Canary Islands Azores Gibraltar
Observation), a project funded by the European Commission in 1996.
The main objective of CANIGO is “to understand the functioning of
the marine system in the Canary-Azores-Gibraltar region of the
Northeast Atlantic Ocean and its links with the Alboran Sea through
comprehensive interdisciplinary basin scale studies” (Wefer and
Müller, 1998).
Before CANIGO, the Canary Basin and the nearby Moroccan
margin were poorly studied. General contributions to the history of the
northwest African continental margin have been compiled in 1982
(Diester-Haas and Chamley, 1982; Sarnthein et al., 1982; Seibold,
1982). Some palynological studies, related to the Canary and Moroccan
regions (Agwu and Beug, 1982; Hooghiemstra, 1989; Hooghiemstra et
al., 1992), concluded that upwelling was enhanced by stronger trade
winds during glacial times. Other investigations of the area dealt with
the sedimentary processes associated to the distal turbidite
emplacements in the Madeira and Seine Abyssal Plains (e.g. Weaver
and Kuijpers, 1983; Weaver et al., 1986; Weaver and Rothwell, 1987)
and to the Agadir Turbidite System (Ercilla et al., 1998; Wynn, 2000).
By contrast, plenty of paleoceanographic and paleoclimatic studies
can be found concerning regions to the south of the Canary Basin, such
as the tropical and equatorial Atlantic, and the Cape Blanc Upwelling
System (e.g. Sarnthein et al., 1981; Pokras and Mix, 1985, 1987;
Abrantes, 1991; deMenocal et al., 1993; Tiedemann et al., 1994;
Matthewson et al., 1995, among others).
Most of the CANIGO results are included in two special volumes
of the Journal of Deep-Sea Research (Parrilla et al., 2002a,b), which
also make reference to the contributions published in other journals.
Moreover, two doctoral theses with paleoceanographic objectives were
focused on the CANIGO region (Moreno, 2002; Kuhlmann, 2004).
Abrantes et al. (2002) and Meggers et al. (2002) have studied the
present distribution of microfauna in the surface sediment. Both studies
1
See more details on CANIGO at:
http://www.allgeo.uni-bremen.de/forschung/projects/canigo/
43
Part I. Introduction
reported about a good preservation of the fauna within the sediment, a
close relationship between fauna in the sediment and the hydrographic
characteristics of the overlying waters, and a direct influence of the
coastal upwelling. Proxy taxa for upwelling episodes are G.bulloides
(planktonic foraminifera) and Chaetoceros (diatoms).
Several papers (Neuer et al., 1997; Ratmeyer et al., 1999;
Freudenthal et al., 2001; Neuer et al., 2002) have focussed on the
assessment of the modern processes acting at the transition between the
productive shelf and the oligotrophic subtropical gyre provinces. In
these studies, the authors suggest that the area is characterized by
export production and lateral advection of organic matter from the
primary coastal upwelling region to the deep sea by means of
intermediate nepheloid layers.
Henderiks et al. (2002) and Kuhlmann et al. (2004) have studied
the changes of particle and sediment accumulation during the glacialinterglacial cycle. According to these studies, glacial periods in general
and the Last Glacial Maximum (LGM) in particular were characterized
by enhanced total mass accumulation rates (Henderiks et al., 2002) and
by enhanced productivity (Kuhlmann et al., 2004).
Several papers (e.g. Barton et al., 1998; Nave et al., 2001; Abrantes
et al., 2002; Moreno et al., 2002; Freudenthal et al., 2002; Kuhlmann et
al., 2004) have contributed to the understanding of the temporal
productivity changes, the links with coastal upwelling and of the
filament evolution. In general, all these studies agree in that
productivity was higher during glacial than during interglacial periods.
In addition, productivity seems to have been even higher during
deglaciations (Freudenthal et al., 2002; Moreno et al., 2002; Kuhlmann
et al., 2004).
Torre-Padrón et al. (2002) have focussed on the variability of the
dust inputs to the CANIGO area. They show that today Saharan dust
reach the Canary region both during winter and summer in stormy
episodes that last 3-8 days on average. Moreno et al. (2001) have
analysed the temporal variability of the dust input for the last two
glacial cycles. The conclusions of this study point to insolation
changes, related to eccentricity and precession, as the main control over
dust deposition. Enhanced dust outbreaks were found to be linked to
stronger monsoon activity.
44
Chapter 1. Presentation and Objectives
Taking in mind this state-of-the-art, the present thesis concerns the
reconstruction of past atmospheric and oceanographic circulation of the
northeast subtropical Atlantic, with the specific objectives detailed in
the following section.
1.3. Objectives
The general objective of the thesis is to reconstruct the dynamics of
the atmospheric and oceanographic components controlling the climatic
changes of the northeast subtropical Atlantic (NW African margin)
along the last two glacial-interglacial cycles.
Because the study region is complex due to the interaction of
different climatic components operating at the same time and with
reciprocal influences, the achievement of the global objective goes
trough three intermediate objectives:
1. to identify the rates and mode of the dust supply from the
African continent to the study region. The fulfilment of this
objective allows fixing the influence of the wind systems, trade
and monsoon on the study region and deciphering the principal
factors that trigger the dust production and deposition.
2. to distinguish the factors controlling the organic carbon
deposition in the oligotrophic study region. This includes the
further purpose to differentiate between the influence of the
local productivity and the impact of the allochthonous supplies
derived from the continent.
3. to determine the climatic implications of surface hydrological
conditions changes in the study region. This is achieved through
the identification of the sea surface temperature and salinity
evolution as well as surface density changes.
45
Chapter 2.
CLIMATIC FACTORS
47
48
Chapter 2. Climatic factors
CLIMATIC FACTORS
The past two million years of the Earth’s history have been
distinguished by large cyclic variations in climate. Cold ice age
periods, characterized by a thick cover of continental ice sheets over
much of the polar Northern Hemisphere, alternated with interglacial
ice-free periods.
These glacial-interglacial cycles are statistically linked to cyclic
changes in the orbital parameters of the Earth, with characteristic
frequencies of about 100, 41 and 23 thousand years (Hays et al., 1976;
Imbrie et al., 1984, 1992). The orbitally driven variations in the
seasonal and spatial distribution of solar radiation, known as the
Milankovitch cycles (Milankovitch, 1941), have been considered the
fundamental drivers of glacial-interglacial cycles. However, the orbital
variations alone are insufficient to drive the large amplitude of the
observed glacial-interglacial oscillations and do not explain the rapid
climate transitions shown by the paleoceanographic records.
The central issue of current investigations in Earth science is the
understanding of the factors that affect the Earth’s climate system,
amplifying or reducing orbital forcing. Some examples of these factors
are CO2, water vapour, albedo, cloud radiation, hydrologic cycle and
precipitation, thermohaline circulation, surface mixed layer and oceanic
convection, dust, volcanic emissions and aerosols (see Stocker et al.,
2001 for a review).
The objective of this chapter is to introduce the general context of
the themes addressed in the section dedicated to the results, which
makes the third part of this thesis. It comprises three main parts.
Section 2.1 deals with the climatic implications of the blown-wind dust,
section 2.2 focuses on the organic matter transfer from the continental
margins to the deep oceans, and section 2.3 addresses the relevance of
sea surface mixed layer conditions (temperatures, salinities and vertical
densities) and of low-latitude climates on global climate changes.
49
Part I. Introduction
2.1. Dust impact on climate
Aeolian dust1 dominates the mineral component of pelagic
sediments in regions away from the influence of continental
depositional processes (Windom, 1975). Dust is mobilized by storms
from the arid and semi-arid regions (e.g. North Africa, Middle East,
NW India, central Asia, North America), raised to the upper part of the
troposphere, transported great distances by the zonal winds and washed
out of the atmosphere by the rain (Rea et al., 1994).
Dust has been widely used as a proxy of:
•
aridity/humidity of the nearby continents. It is largely assumed
that the amount of dust transported to the oceans is a function of
source area aridity (Prospero et al., 1981; Prospero and Nees,
1986).
•
strength and directions of the prevailing winds. The intensity of
the transporting winds is recorded by the grain size variation of
the dust (Rea et al., 1985).
Dust is an effective component of the global climate system (Shine
and Forster, 1999; Arimoto, 2001) operating in two principal ways:
1. as part of aerosols, dust influences the Earth’s radiative budget
through absorption and scattering of the incoming solar
radiation (insolation) and outgoing terrestrial radiation (Mitchell
et al., 1995). Dust may cause either a warming or a cooling at
the surface depending on the size distribution of dust particles
and their chemical composition, among other factors (Alpert et
al., 1998; Harrison et al., 2001; Perlwitz et al., 2001).
2. iron-rich dust fertilizes the nutrient-rich, low-chlorophyll
oceanic regions, especially in North Pacific Ocean and
Antarctica, with significant impacts on CO2 cycling. It has been
proposed that addition of iron to oceanic waters may stimulate
nitrogen fixation by plankton and enhance its productivity,
1
Dust: dry solid matter of clay and silt size, which is readily blown about by the wind
and may be carried considerable distances (Dictionary of Geological Terms, 1984).
50
Chapter 2. Climatic factors
which, in turn, may lower atmospheric CO2 concentration and
produce a “reverse greenhouse effect” (Martin, 1990; 1991).
The Saharan desert produces more aeolian dust than any other
world desert. Dust plumes are transported far distances by the
prevailing winds following three main principal trajectories: westward,
over the North Atlantic Ocean as far as Barbados, Cayenne or Miami;
northward, toward the Mediterranean and southern Europe stretching as
north as British Islands and even Norway; and eastward, toward the
Middle East (Fig. 2.1). Important source regions are the Bodèlè
depression (between Tibesti and Lake Chad), Mauritania, Mali,
southern Algeria, the Horn of Africa, southern Egypt and northern
Sudan (Goudie and Middleton, 2001; Prospero et al., 2002) and the
Sahel (Prospero, 1996).
Fig. 2.1. Schematic illustration of the principal trajectories of the Saharan dust
outbreaks.
The 30-50% of the Saharan dust takes the westward trajectory over
the North Atlantic, mainly after dust outbreaks occurring in summer.
The trigger mechanism for dust production seems to be associated with
strong convective disturbances and average air flow instabilities that
develop deep in the Sahara and Sahel regions between 15-20ºN in
summer (Prospero, 1996). More specifically, small-scale structures,
51
Part I. Introduction
called “African Squall Lines”2 are thought to be the main mechanism
for dust uplift (Tetzlaff and Peters, 1986) (Fig. 2.2).
Fig. 2.2. Convective disturbances “squall lines” and associated fast moving cold front
producing thunderstorm and general instability that raises dust upward (modified after
http://www.tpub.com/content/aerographer/14312/css/14312_122.htm
2
Squall line is a type of multicell storm. It consists of a line of severe thunderstorms
that can form ahead of a cold front, with a continuous, well developed gust front at
the leading edge of the line. These storms can produce small to moderate size hail,
occasional flash floods and weak tornadoes. Squall lines, stretching several kilometres
in length and lasting for many hours, fall into the category of mesoscale convective
systems.
52
Chapter 2. Climatic factors
Specific sources for transatlantic dust plumes, although not wellknown, likely are located in Mauritania-Mali area, and further north in
Western Sahara and southern Morocco (Goudie and Middleton, 2001).
Dust plumes, derived from Sahelian Saharan and Moroccan soils, can
be driven to a more northerly direction and reach the Canary Islands
(Coudé-Gaussen et al., 1987; Bergametti et al., 1989) and the
Mediterranean regions (Dulac et al., 1992; Avila et al., 1997).
Dust is mainly made of a mixture of minerals, among which the
most common are quartz, clay minerals, calcite, gypsum and iron
oxides. The mineralogy of dust lends a characteristic reddish colour to
the aeolian deposits. Together, mineralogy and dust colour are good
indicators of the soil composition of the source region and are widely
applied in those investigations aimed to reconstruct dust backward
trajectories (Chiapello et al., 1997).
The study of the dust records, contained in ice cores, marine
sediments and continental deposits (loess3), has contributed
significantly to the understanding of the past global climate changes
(Rea, 1994). Yet, there are basically three issues that make the “dust
problem” still puzzling:
1. is dust unconditionally characteristic of glacial periods? Using
different approaches and proxies, several investigations based on
marine sediment cores have supported a scenario of higher dust fluxes
during glacial periods (especially at the Last Glacial Maximum -LGM)
(Parkin and Shackleton, 1973; Kolla et al., 1979; Sarnthein et al., 1981;
Gasse et al., 1990; Hooghiemistra et al., 1992; Dupont, 1993;
Ruddiman, 1997; Grousset et al., 1998; Zhao et al., 2000). Also ice
cores from Greenland and Antarctica have pointed to greater
concentrations of mineral dust during cold phases (Petit et al., 1981;
Lorius et al., 1985; Grousset et al., 1992a; Jouzel et al., 1996;
Mayewski et al., 1997; Taylor et al., 1997). However, in some regional
contexts, LGM dust fluxes lower than today have been registered (e.g.
Gulf of Guinea and tropical Pacific) (Harrison et al., 2001).
3
Loess: a blanket deposit of buff-coloured calcareous silt, homogeneous,
nonstratified, weakly coherent, porous and friable. Loess covers wide areas in
Northern Europe, eastern China, and the Mississippi Valley. It is considered to be
windblown dust of Pleistocene age (Dictionary of Geological Terms, 1984)
53
Part I. Introduction
2. what are the factors that drive dust production and transport? In
general, two principal processes are usually invoked to explain
enhanced dust deposition: strong atmospheric circulation and extended
dust source areas. However, other factors can also impact on the rate of
dust production and transport (Prospero et al., 2002): North Atlantic
Oscillation (Moulin et al., 1997), small-scale meteorological features
(Sarnthein et al., 1982; Tetzlaff and Peters, 1986; Ruddiman, 1997),
land cover (Tegen and Fung, 1995), and attenuation of the hydrological
cycle (Yung et al., 1996).
3. is dust a driving factor or a consequence of the climatic change?
It is still uncertain whether the atmospheric dust loading is a response
to or a contributory cause of climate changes on glacial-interglacial
time scales (Li et al., 1996; Overpeck et al., 1996; Tegen et al., 1996,
Harrison et al., 2001). For example, there are evidence for dust to be an
important climate-forcing agent over the tropical and subtropical North
Atlantic Ocean (Li et al., 1996), to be a potential source of episodic
warming during the last glacial period (Overpeck et al., 1996), and to
be an important factor that decreases the net surface radiation forcing
enhancing the atmospheric heating (Tegen et al., 1996).
2.2. Continental margins role in global change
Continental margins comprehend three physiographic provinces
(continental shelf, continental slope and continental rise) that extend
between the shoreline and the abyssal ocean floor. The continental
margins play a significant role in the oceanic biogeochemical cycles of
carbon and nitrogen (Walsh, 1991). The understanding of the
distinctive and often complex processes occurring at the ocean margins
provides a base for predictions of the consequence of climatic change
and of other anthropogenic perturbations.
The increase in the human-activity CO2 emissions of the postindustrial epoch and its consequences on the Earth climate system is a
familiar question with great social impact. The rate of changes in
atmospheric CO2 depends not only on human activities but also on
biogeochemical and climatological processes and their interactions with
the carbon cycle. The atmospheric CO2 continuously exchanges with
oceanic CO2 at the surface, thus removal of CO2 from atmosphere
depends basically on ocean circulation (Falkowski et al., 2000 and
references therein). Different natural mechanisms tend to equilibrate
54
Chapter 2. Climatic factors
the rate of atmospheric CO2 emission and uptake (Fig. 2.3). Within the
oceans, two processes, referred to as the solubility4 and the biological
pumps5, improve the CO2 buffering capacity of the ocean and maintain
the CO2 concentration in the surface waters lower than in the deep
waters (Sarmiento, 1993).
Fig. 2.3. Natural processes controlling the carbon cycle at the ocean margins (after
Wollast, 1981).
Atmospheric CO2 is fixed by the photosynthesis6 performed by
phytoplankton. Organic matter and oxygen are the products of this
4
Solubility pump: As CO2 is more soluble in cold waters, the cold and dense waters
that form at high latitudes and sink to the deep oceans are enriched with carbon more
than the average surface warm waters
5
Biological pump: The near-surface living phytoplankton uses CO2 to form organic
matter. The organic matter is partly metabolized by the organisms (respiration) and
partly sinks to the deep ocean where it is partly converted back to CO2 by bacteria and
partly is stored within the sediments. The net results is to keep the deeper ocean
enriched with carbon
6
Photosynthesis: 106CO2 + 16 NO3- + HPO42- + 122H2O + 18H+ + (solar energy +
trace elements) = C106H263O110N16P1 + 138O2
55
Part I. Introduction
process often also referred to as primary production. Photosynthesis is
limited by the availability of nutrients (nitrate, phosphate, silicate, and
trace elements such as iron) that can be introduced to the oceans by
deep waters (upwelling and vertical mixing), by river discharge or wind
inputs.
Although the major productive areas are located onto the
continental shelves, these are not the sink where organic matter is
permanently buried. Due to many high energetic processes taking place
along the shelves, most carbon-rich material is exported from the shelf
to the continental slope and ocean basins (Walsh et al., 1981; Walsh,
1991).
The three principal processes involved in particulate matter
transport to the deep ocean are the followings:
1. direct flux from the surface, when high-productivity zones are
found offshore, for example in presence of filaments extended
onto the sea surface.
2. shelf-slope lateral advection, which operates on the upper slope
through nepheloid layers7 at intermediate and bottom depths.
3. shelf-slope transport through submarine canyons (Puig et al.,
2003, 2004, and references therein). Sediment transport in
canyons can occur either by gravitational mass flow (initiation
mechanisms include sediment suspension, storm surges, sealevel changes and earthquake-triggered mass failures) or by a
variety of channelled currents, at times strong enough to move
fine-grained sediment down-canyon without invoking massfailure.
Total Organic Carbon (TOC) refers to the amount of organic
matter preserved within sediment. Sediment TOC records are widely
used as proxy for paleoproductivity since organic-rich sediments are
usually formed close to high-productive oceanic regions. Actually, this
is only roughly true because many other factors account for TOC
storage. The amount of organic matter found in sediments is a function
of the rate of organic matter production, the quantity of particulate
7
Nepheloid layer: a layer of considerable thickness characterized by a large increase
in light scattering conferred by the presence of increased amounts of suspended
sediment (McCave, 1986).
56
Chapter 2. Climatic factors
organic matter supplied to bottom sediments, the various processes
operating at the sediment surface and the rates at which the organic
matter is degraded8 before burial (e.g. Müller and Suess, 1979; Hedges
and Keil, 1995; Rühlemann et al., 1999). Robust paleoproductivity
studies mostly combine TOC analyses with measurements on other
paleoproductivity proxies, making use of biogenic components (e.g.
planktonic foraminifera, diatoms, radiolarians), geochemical markers or
organic compounds.
Since several years, there has been a great interest in studies
devoted to elucidate past levels of marine productivity. The interest
stems basically from the tight relation existing between marine
productivity and atmospheric CO2 storage. Most evidence have pointed
out that marine productivity was increased during glacial periods (e.g.
at LGM) favoured by enhanced upwelling caused by stronger winds.
However, recent studies focussed on coastal upwelling along the
African margin (Bertrand et al., 1996; Martinez et al., 1999) have
suggested that upwelling enhancing occur not only during glacial but
also during interglacial periods. The combined effect of other factors,
such as shelf width, sea level fluctuations and wind stress, may have
favoured primary production also during interglacial periods and during
deglaciations phases (Terminations).
2.3. Sea surface water implications in climate
Sea surface waters properties are crucial in climatic studies
because several relevant processes take place at the ocean surface. In
particular, surface waters are the important link between atmosphere
and deep ocean. The ocean-atmosphere exchange of heat, gases and
momentum (e.g. CO2, oxygen, and water vapour transfer, wind-stress
action, phytoplankton production, etc.) is transmitted to the deep ocean
through complex mixing processes.
The ocean surface is usually referred to as the mixed surface layer
(MSL), a portion of the water column that extends from the ocean skin
to a depth that usually ranges between 25 and 200m (Fig. 2.4). The
principal characteristic of the MSL is the relative homogeneity of the
8
The decomposition of organic matter is variously referred to as oxidation,
metabolism, degradation and mineralization.
57
Part I. Introduction
vertical density distribution (i.e. density does not change with depth).
The depth reached by the MSL depends on the degree of the vertical
mixing, which in turn is a function of the stability of the water column9,
the incoming energy of the wind stress, and the degree of heat10 and
fresh water exchange. The more stable the surface water, the less
mixing, and the thinner the mixed-layer.
The base of the mixed layer is connected to the ocean’s interior
through the permanent or oceanic thermocline, whose depth ranges
from below the seasonal thermocline to about 1000 m (Fig. 2.4). The
temperature at the upper limit of the permanent thermocline depends on
latitude, reaching from well above 20°C in the tropics to just above
15°C in temperate regions; at the lower limit, temperatures are rather
uniform around 4-6°C depending on the particular ocean.
Thus, thermocline is the transition zone between the warm wellmixed waters of the surface layer and the cold waters of the main body
of the ocean, representing thus the zone within which temperature
decreases markedly with depth. Thermocline waters are formed by
subduction mainly in the subtropics. Water from the bottom of the
mixed layer is pumped downward through a convergence in the Ekman
transport and sinks slowly along surfaces of constant density (Pedlosky,
1990).
Until recently, climate change investigations focussed on temperate
and polar regions, with particular emphasis on past temperatures and on
the North Atlantic region commonly thought to be the prime mover of
climate. The low-latitude regions were neglected despite their central
role in global climate (Gasse, 2001; Kerr, 2001). In fact, low-latitude
oceans are key components of the Earth’s climate system. As part of
the thermohaline circulation (THC), waters of tropical origin are the
vehicle of heat and salt transport to the northern regions, with dramatic
consequences on global climates (e.g. Broecker, 1991).
9
For a water column to be stale, its density must increase downward with increasing
depths. Thus, changes in the surface water properties (temperature and salinity),
which modify surface water density, greatly affect the stability of the water column.
10
The heat budget of the mixed layer is determined by horizontal advection, surface
heating, entrainment and vertical heat flux at the mixed-layer base.
58
Chapter 2. Climatic factors
Fig. 2.4. A: The water column distinctions in: surface mixed layer (green),
thermocline zone (pink) and deep ocean (blue). Seasonal thermocline is a seasonal
feature that takes place in the upper layer. B: Permanent thermocline profiles
according to latitude. Notice that, in polar regions, thermocline is absent (picture from
http://biology.ecsu.ctstateu.edu/courses/Bio440images/Bio440Thermocline.JPG).
Low-latitude climate components have an effective impact on
global climate through the following mechanisms:
•
insolation. Latitudinal variation in the low-latitude
insolation affect the trade winds regime and tropical SST
conditions (e.g. Kutzbach and Guetter, 1986; McIntyre
et al., 1989; Schneider et al., 1999) with consequences
on the ocean-atmosphere feedbacks (Gu and Philander,
1997).
•
hydrological cycle. Low-latitude surface water
conditions control atmospheric water vapour content,
cloud cover and albedo with significant global climate
consequences. The impact of the transfer of heat and
moisture from the low-latitude regions to the high-
59
Part I. Introduction
latitudes is effective through the ocean-atmosphere
exchange processes (Ruddiman and McIntyre, 1981).
•
thermocline. Tropical thermocline waters, subducted at
mid-latitudes, provide a direct link with extratropical
climate conditions (Bush and Philander, 1998).
Reliable paleosea surface and salinity values are instrumental for
modellers and future climate predictions. Efforts for SST
paleoreconstructions date back to the seventies and still are a question
of debate among the scientific community (e.g. Bard, 2001).
The contributions made by the CLIMAP project (Climate Longrange Investigation, Mapping, And Prediction) (1976; 1981) are
considered the first quantitative reconstruction of paleotemperatures.
Using transfer functions on planktonic foraminifera and computer
models, CLIMAP developed a detailed climatological map of the world
at 18,000 years ago. As a major result, average global sea surface
temperatures were computed 1.4ºC-1.7ºC colder than today.
CLIMAP’s findings have not been refuted until the nineties, when
several lines of evidence derived from continental data (measurements
of noble gas concentrations in groundwater, pollen records and glaciers
snowline data) and from geochemical measurements on corals, showed
severe cooling of 5º-6ºC at tropical sites (e.g. Bonnefille et al., 1992;
Stute et al., 1992; 1995; Guilderson et al., 1994; Broecker, 1997).
Recently, Crowley (2000) reviewed the tropical ocean temperature
“problem” and suggested an intermediate solution with an estimated
glacial cooling in the tropics of about 3ºC. The discrepancy between
marine and continental temperature estimations has recently been
reconciled by some modelling studies, according to which cooling rate
is usually several degrees smaller in the oceans than on the continents
(Ganopolski et al., 1998). Thus, it seems that a 5ºC continental cooling
is compatible with a change of 2º-3ºC in the tropical oceans (Bard,
1999).
CLIMAP’s findings have been tested with approaches applied to
different proxies than planktonic foraminifera. Other factors than
temperature (e.g. nutricline and water depth) can influence
foraminifera’s ecology, so that temperature estimations derived from
planktonic foraminifera census counts may give contrasting results. The
most effective contributions to SST calculations came from
60
Chapter 2. Climatic factors
geochemical proxies, such as Mg/Ca and UK37 index. At mid eighties,
the UK37 index11 started to be used as a new paleothermometer (Brassell
et al., 1986; Prahl et a., 1988). Alkenone estimates on a west tropical
Pacific core corroborated that glacial SSTs were 2-3ºC lower than
present (Ohkouchi et al., 1994; Bard et al., 1997). This result fixed the
discrepancy with CLIMAP estimations to 0.5-1ºC. The other
paleothermometer, Mg/Ca ratio12 applied to foraminiferal shells,
confirmed a 2.5ºC drop at glacial times (Hastings et al., 1998).
Bard (2001) completed a review of the existing proxies for SST
reconstruction with a special emphasis on alkenone estimates. From the
comparison of the different methods, it seems that there is a general
agreement on the amplitude of changes at low and middle latitudes.
Major discrepancies are observed at high latitudes. Alkenone estimates
are more in line with Mg/Ca data, rather than with those derived from
statistical analysis of the foraminiferal distribution (Bard, 2001).
11
Uk37 is based on the relationship between lipid compounds produced by marine
coccolithophorid algae (e.g. Emiliania huxleyi) and the temperature of the water in
which the algae lived.
12
Mg/Ca ratio in foraminifera shells shows to be temperature dependant due to the
partition of Mg during calcification.
61
PART II
SETTING, DATA AND METHODOLOGY
63
64
Chapter 3.
THE STUDY AREA
65
66
Chapter 3. The study area
THE STUDY AREA
The objective of this chapter is to present the study region and the
analyzed material. The study region is the northeast subtropical
Atlantic, which is characterized by many components that actively play
and build a complex scenario made of winds systems, oceanic currents
and topographic effects. The area presents interesting aspects both with
local and global significance. Its complexity provides an excellent
palaestra for regional studies, which show to be important for
modelling simulations and to better understand mesoscale-climatic
features. From a global perspective, it receives the influence of large
scale climatic components, both of the atmospheric realm (Trades
Winds, Monsoon, ITCZ, Azores high pressure cell) and of the
oceanographic realm (thermohaline circulation and Subtropical Gyre,
Canary Current, African coastal upwelling). Further, the proximity to
the African continent makes the region extremely useful for monitoring
climatic continental changes.
The detailed atmospheric and oceanographic features are described
in the specific chapters 5, 6 and 7. Here, only the geographical and
morphological regional frames are briefly described. The location and
the sedimentological characteristics of the core on which this study is
based are finally disclosed.
3.1. Geography and climate
The study area is located in the northeast subtropical Atlantic on
the Moroccan passive margin. The nearby continent is at 200km
eastward, being Agadir the principal adjacent town. Here river Souss,
originated from the NW-SW stretching Atlas Mountains and High
Plains, flows into the ocean. The coastline forms bays and capes, being
Cape Ghir (at 31ºN) and Cape Yubi (28ºN) the most prominent of the
zone. The northern limit of the study region is the Portuguese margin,
with the Gulf of Cadiz and the Strait of Gibraltar as the northeastern
67
Part II. Setting, Data and Methodology
boundaries. The southern limit is constituted by the volcanic Canary
Islands, while the open Atlantic Ocean is the western frontier (Fig. 3.1).
38º N
0
550
0
500
0
450
0
400
0
350
0
300
0
250
0
200
0
150
0
100
500
Portugal
36º N
Gulf of Cadiz
Gibraltar strait
34º N
Atlantic Ocean
STUDY AREA
32º N
ins
river Souss unta
o
M
C.Ghir
las
At
Agadir
30º N
Canary
28º N
16º W
Islands
0
C.Yubi
14º W
12º W
10º W
8º W
6º W
4º W
2º W
Fig. 3.1. Geographical location of the study area. Scale bar on the right side of the
figure refers to bathymetric depths, which are given in meters.
The nearby African continent is characterized by a wide range of
climatic regimes with distinct vegetation typologies, which strongly
depend on the amount and distribution of precipitations over the year
(Dupont, 1998). The northern part is characterized by a Mediterranean
climatic regime (250-1000mm precipitation per year) with July and
August as the driest months. The Sahara (<100-150mm annual
precipitation) stretches from 28ºN in Morocco and 34ºN in Tunisia to
18-20ºN in Mauritania and 16ºN in Chad. The Sahel (150-500mm
annual precipitation) borders the southern Sahara, stretching between
15ºN and 18ºN in the western part and between 12ºN and 14ºN in Chad
(Fig. 3.2).
Studies based on palynological record of marine sediment cores
and on the continental lake level fluctuations have shown that NW
Africa climates and vegetation patterns have been changing and
shifting back and forth along the geological times (e.g. Gasse et al.,
1990; Dupont and Hooghiemstra, 1989). More humid as well as more
arid conditions than today characterized NW Africa. For example,
humid conditions prevailed in NW Africa during stage 5 (Eemian) and
68
Chapter 3. The study area
the early Holocene (e.g. Holocene Climatic Optimum), as indicated by
high lake levels and the Mediterranean vegetation reaching the southern
limit of the Atlas Mountains. Large proportion of the present-day desert
was vegetated during the early Holocene (Street-Perrott and Perrott,
1993). By contrast, the Last Glacial Maximum (LGM) was one of the
driest periods of the recent history in NW Africa (Dupont, 1998).
3.2. Physiography and sedimentary processes
The Northwest African margin is characterized by a narrow
continental shelf (40-60km approx.) that slightly widens off the Cape
Ghir forming the Cape Ghir Plateau. This is the submarine extension of
the Atlas Chain on land (Hagen et al., 1996). The shelf break is at a
water depth of about 100-200m. The continental slope extends up to
3000m water depth, where it stretches away into the deep Agadir Basin
(to the SW) and to the Seine Abyssal Plain (to the NE) (Fig. 3.3).
Fig. 3.2. A satellite picture of NW Africa showing the climatic provinces and the
political divisions.
69
Part II. Setting, Data and Methodology
Large sections of the continental slope are punctuated by several
seamounts, canyons and channels (Wynn et al., 2000). The Agadir
canyon, located not far from the mouth of the Souss River, is part of the
Agadir turbidite system, one of the most expanded in the area (Ercilla
et al., 1998).
The sediments of the area fall into two groups: turbidite and
pelagic sediments (Weaver and Rothwell, 1987). Turbidite currents
routing the Agadir canyon have two principal destinations:
•
SW, into the Agadir Basin and toward the southern Madeira
Abyssal Plain. High-CaCO3 (>70% in CaCO3) turbidite
sediments derive from the collapse of nearby seamounts. Other
turbidite sediments, characterized by low CaCO3 content (40%60%), can be rich in organic matter (up to 2% in organic
carbon) and probably originate from the productive NW African
margin (Weaver at al., 1986; Weaver and Rothwell, 1987;
Cowie et al., 1998).
•
NE, into the Seine Abyssal Plain. These turbidite sediments,
characterized by fine volcanic material, reach the plain buoyed
up by turbulent mixing processes (Davies et al., 1997).
3.3. Material
In general, paleoceanographic studies make use of undisturbed
sediment cores, located far from the influence of turbidite and bottom
currents or any other processes that may interrupt or remove part of the
sediment package.
With the objective of studying the Sahara dust load, the coastal
upwelling and the continent-ocean sediment transfer variability due to
climatic changes, sediment core GeoB4205 was retrieved at 200km
from the coast, to the north of the Agadir canyon, on a plane interfluve1
located not far from the Agadir Canyon axis (Fig. 3.3).
1
Interfluve: the relative undissected upland between adjacent streams flowing in the
same direction (Dictionary of Geological Terms, 1984).
70
Chapter 3. The study area
34ºN
Seine Abyssal Plain
Seine sm
Agadir sm
33ºN
Canary Basin
32ºN
GeoB4205
Agadir Basin
CGP
AGADIR
31ºN
Agadir Canyon axis
30ºN
16ºW
14ºW
12ºW
10ºW
8ºW
Fig. 3.3. Location of the sediment gravity core GeoB4205 (32º 10.8’N latitude,
11º38.9’W longitude, and 3296m water depth). Legend: sm: seamount; CGP: Cape
Ghir Plateau. Isobaths at 275m.
The five meters sediment core consisted mainly of foram-bearing
nannofossil ooze2 with intercalation of sandy calcareous ooze. The
sediment colour alternatively changes from brown to grey shades (Fig.
3.4) basically due to the changes in calcium carbonate content and
terrigenous inputs (see Chapter 5).
2
Ooze: pelagic sediment consisting of at least 30% skeletal remains of calcareous or
siliceous pelagic organisms, the rest being clay minerals. Oozes are defined by their
characteristic organisms (Dictionary of Geological Terms, 1984).
71
Part II. Setting, Data and Methodology
Fig. 3.4. Photograph of core GeoB4205 showing the alternation between brownish
and greyish tones of the sediment colour.
72
Chapter 3. The study area
The results obtained from the study of the sediment gravity core
GeoB4205 were combined and integrated with data retrieved from
other four sediment cores: multicores GeoB4202 and GeoB4204,
gravity core GeoB4216, and piston core MD952042. Cores labelled
GeoB (from the Earth Science Institute of Bremen, Fachbereich
Geowissenschaften) are located within the study area, whereas core
MD952042 (a sediment core retrieved on board of the r/v Marion
Dufresne) is sited off the Iberian Margin (Fig. 3.5).
Iberian
Margin
40ºN
Portugal
45ºN
MD952042
35ºN
Agadir
Basin
GeoB4202
Spain
African
Margin
Morocco
GeoB4205
GeoB4204
GeoB4216
30ºN
25ºN
20ºW
10ºW
0ºW
Fig. 3.5. Location of the complete set of the five sediment cores used in the present
thesis. GeoB4202: 32º28.6’N, 13º39.8’W; 4289 m; GeoB4204: 32º01.0’N,
11º56.7’W, 3213m; GeoB4205: 32º10.8’N, 11º38.9’W, 3296m; GeoB4216:
30º37.8’N, 12º23.8’W, 2324m; MD952042: 37.5ºN, 10ºW, 3146m). Note that the
locations of cores GeoB4205 and GeoB4204 are very proximate so that they are
represented by one spot only.
73
Part II. Setting, Data and Methodology
Detailed information of the location and use of these five cores is
as following:
1. GeoB4205, sediment gravity core, located at 3296m water
depth on the continental slope of the NW African margin,
500cm long. The majority of analyses and data used in this
thesis were collected in this core.
2. GeoB4202, sediment multicore, located at 4289m water depth
on the Agadir Basin, 39cm long. From this core, only the
surface sample was used for SST calibration (see Chapter 7).
3. GeoB4204, sediment multicore, located at 3213m water depth
on the continental slope of the NW African margin, 14cm long.
From this core, only the surface sample was used for SST
calibration (see Chapter 7).
4. GeoB4216, sediment gravity core, located at 2324m water
depth, on the continental slope of the NW African margin,
1117cm long. Foraminiferal oxygen isotope, organic carbon,
and calcium carbonate data from this core (Freudenthal et al.,
2002) were used to infer changes in the upwelling variability
(see Chapter 6).
5. MD952042, sediment piston core, located at 3146m water depth
from the Iberian Margin off Portugal, 3250cm long.
Foraminiferal oxygen isotope, organic carbon, calcium
carbonate, and SST data from this core (Cayre et al., 1999;
Eynaud et al., 2000; Shackleton et al., 2000; Pailler and Bard,
2002) were used to characterize the sediment processes and the
upwelling history in the region, and to correlate SST values
with Heinrich events (see Chapters 6 and 7).
74
Chapter 4.
METHODOLOGY
75
76
Chapter 4. Methodology
METHODOLOGY
In this chapter, a compendium of the different methodologies
applied in the thesis is resumed. Several and different proxies were
used to formulate the paleoceanographic and paleoclimatic
reconstructions of this work. This multiproxy approach ensures a robust
constrain for the proposed paleoclimatic scenarios. Moreover, the use
of different proxies applied upon the same set of samples is extremely
valuable because the temporal mismatches between the data series are
avoided.
Each methodology first is introduced in general terms and then is
detailed in line with its use in paleoclimatic and paleoceanographic
studies. Paragraph 4.1 is dedicated to the non-destructive measurements
(sediment color, magnetic susceptibility and element intensities) that
were obtained with the use of automatic devices. Paragraph 4.2 deals
with the geochemical analyses (calcium carbonate and total organic
carbon content) performed on discrete samples. Paragraph 4.3 explains
how the foraminiferal census counts were elaborated and transformed
into sea surface water temperature and salinity values. Finally,
paragraph 4.4 is devoted to the oxygen stable isotope measurements
that were performed on specimens of Globigerinoides ruber (white)
and that were used to generate the stratigraphic framework of the study.
4.1. Non-destructive measurements
The understanding of the lithological variation and the sediment
composition is essential for a correct interpretation of the sedimentary
sequences under the paleoenvironmental, paleoceanographic and
paleoclimatic perspective.
For decades, sediment geochemical and physical properties were
determined through the analysis on discrete samples. These time77
Part II. Setting, Data and Methodology
consuming and destructive procedures prevented to perform high
resolution analyses. Recently, this limit has been overcome by the use
of the automatic devices that enable to quantify millennial-scale climate
fluctuations with high temporal resolution in different oceanic contexts
and time windows. These devices can measure a wide range of
parameters at a user-defined sample interval down the length of the
core.
The advantages:
The disadvantages:
•
•
•
•
•
•
very-high resolution data
continuous records
rapid measurements
non-destructive procedure
semi quantitative analyses
calibration
on
discrete
samples still recommended
In this thesis non-destructive measurements were obtained with the
use of three different devices:
1. Colour spectrophotometer, to determine the sediment colour.
2. Multi Sensor Core Logger, to obtain the sediment magnetic
susceptibility.
3. X-ray fluorescence core scanner, to recover the calcium and
iron element intensities in the sediment.
4.1.1. Sediment colour
Sediment colour can be measured or analysed in different manners
and different methods (Balsam et al., 1999; Ortiz and Rack, 1999).
Geologists are familiar with the Munsell colour system used for
sediment and rock colour analyses. This is the visual examination and
comparison with a colour chart based on HVC (hue, value and chroma)
notation1. Another technique for sediment colour measurement is the
Greyscale analysis. This uses photographic slides, digital photographs
1
Hue is the colour of the rainbow or spectrum, Red, Yellow, Green, Blue, Purple and
intermediates; Value indicates the lightness of a colour. The scale of value ranges
from 0 for pure black to 10 for pure white; Chroma is the degree of departure of a
colour from the neutral colour of the same value. Colours of low Chroma are
sometimes called "weak," while those of high Chroma are said to be "highly
saturated," "strong" or "vivid" (from the Colour Academy web site at
http://www.coloracademy.co.uk).
78
Chapter 4. Methodology
or digital images from a RGB camera2 of rocks and core samples. The
density of the pixels or the intensity values of a group of pixels are
correlated to the sediment colour-grey intensity. Recently, the use of
spectrophotometry in science has increased considerably. A
spectrophotometer measures the quantity of light emitted by a light
source, or reflected or transmitted by a surface (e.g. the core’s surface
or a sediment sample). The resulting parameter, lightness, is the
quantity of light conveyed to the eye from unit area of a source or
surface per second flowing through a cone of 1 unit of solid angle
(Osborne, 1999).
In this thesis sediment colour was measured by diffuse-reflected
spectophotometry using the L*a*b* system, also referred to as
CIE/LAB system after the Commission Internationale d’Eclairage
(CIE, 1986). In this system, L* is lightness and represents the blackwhite component of the sediment colour; a* is the red-green
chromaticity; and b* is the blue-yellow chromaticity (Blum, 1997).
The two most common uses of colour reflectance data are:
1. to infer changes in the composition of the bulk material (e.g.
Barranco et al., 1989; Deaton and Balsam, 1991; Mix et al.,
1995; Balsam et al., 1999). Usually, the correlation between L*
and carbonate content is the best and most obvious one.
2. to operate lithostratigraphic correlation between sites (Balsam
and Deaton, 1991; Balsam et al., 1997; Ortiz and Rack, 1999).
The use of the automatic measurements of the sediment colour has
been largely applied in paleoceanography with the following
objectives:
•
•
•
2
to perform a preliminary stratigraphy thanks to the good
correlation with carbonate content and oxygen isotopes (e.g.
Helmke et al., 2002).
to assess the changes in CaCO3 concentration, related to winddriven productivity variability (Hughen et al., 1996) or other
factors (Balsam et al., 1999).
to estimate the varying input of terrigenous materials (Cortijo et
al., 1995; Helmke et al., 2002), especially the wind-blown dust
RGB stands for the scientific hues of the primary colours of red, green, and blue.
79
Part II. Setting, Data and Methodology
derived from the desertic areas (Balsam and Deaton, 1991;
Balsam et al., 1995).
In this thesis, sediment colour was obtained at a 5-cm resolution by
spectrophotometry
using
a
Minolta
CM-2002
hand-held
spectrophotometer according to the procedure described in Wefer and
Müller (1998) (Fig. 4.3). Sediment colour reflectance lightness
parameter L* was used as a proxy for calcium carbonate content in the
sediment (see Chapter 5).
Fig. 4.3. Example of a Minolta CM-2002 hand-held spectrophotometer used for
sediment colour measurements.
4.1.2. Magnetic susceptibility
The magnetic susceptibility is a physical property of the sediment
that strictly depends on two features:
1. the sediment mineralogical composition. The magnetic
susceptibility provides a measure of the concentration and
mineralogy of magnetic minerals and of Fe2+, Fe3+ and Mn2+
bearing clay minerals in the sediment, which are typically a
trace component of the terrigenous fraction.
2. the grain size of the sediment components and consequently the
void ratio of the sediment.
Magnetic susceptibility is a sediment characteristic widely used to
monitor the presence, the path and the source area of terrestrially
derived sediments and to provide high resolution stratigraphic intercore
80
Chapter 4. Methodology
correlations. Several studies have highlighted the link between
magnetic susceptibility and concentration/accumulation of the
terrigenous fraction in the sediment (e.g. Kent, 1982; Robinson, 1986).
This link applies to sediments both of the marine and the terrestrial
realms and is consistent with a wide range of regional provenances,
transport agents and depositionary environments (Thompson and
Oldfield, 1986; Bloemendal et al., 1988; Frederichs et al., 1999).
The most frequent applications of the sediment magnetic
susceptibility as a proxy in paleoceanographic and paleoclimatic
reconstructions are the followings:
•
•
•
•
proxy for wind transport of mineral dust and for
aridity/humidity conditions that indirectly reflect the
monsoons activity both in Asia and in Africa. It is thus applied
both to continental loess deposits and to the desert-born dust
found in the marine sediments (e.g. Doh et al., 1988;
Bloemendal and deMenocal, 1989; deMenocal et al., 1991;
Moreno et al., 2002).
proxy for Heinrich events (e.g. Grousset et al., 1993; Robinson
et al., 1995; Chi and Mienert, 1996; Lebreiro et al., 1996).
tool for inter core correlation (Robinson et al., 1995).
proxy for sea bottom current activity (deMenocal et al., 1988).
In this thesis, magnetic susceptibility measurements were done at a
1-cm resolution using a GEOTEK Multi Sensor Core Logger (Weaver
and Schultheiss, 1990; Gunn and Best, 1998). A BARTINGTON pointsensor MS2F was used to obtain the volume magnetic susceptibility,
expressed in SIX10-5 U (Fig. 4.4). Downcore magnetic susceptibility
was used as a proxy for the airborne dust contribution to the deepsediment and as a tracer of Saharan-Sahelian provenance of this dust
load (see Chapter 5).
81
Part II. Setting, Data and Methodology
Fig. 4.4. GEOTECK’s multisensor core logger (MSCL) at Instituto de Ciencias del
Mar, CSIC, Barcelona.
4.1.3. Element intensity
Sediment element intensities are proportional to the concentration
of the chemical element components within the sediment. Some of
these elements (e.g. Fe, Al, Ca, Ba, Mn, Ti) are valuable proxies for
environmental changes. A broad range of techniques and methods are
currently employed for chemical analyses. Most of them apply timeconsuming and destructive procedures on discrete samples: X-ray
fluorescence (XRF), atomic absorption spectrometry (AAS)3,
inductively coupled plasma optical emission spectroscopy (ICP-OES)
or inductively-coupled plasma mass spectrometry (ICP-MS)4 are just
some examples.
3
AAS. Technique that involves the determination and measurement of atomic energy
levels (spectrometry) and chemical identification based on how atoms absorb
electromagnetic radiation.
4
ICP. Chemical analysis with an inductively-coupled plasma (a state of matter
containing electrons and ionized atoms) that is based on the principles of
vaporization, dissociation, and ionization of chemical elements when introduced into
the hot plasma. These ions can then be separated according to mass.
82
Chapter 4. Methodology
X-ray fluorescence (XRF) is the technique used for chemical
analyses in this thesis. This method employs X-ray emissions to sense
elements of different atomic number. Every element emits its own
characteristic energy and wavelength spectra when hit by the incident
X-rays (Jansen et al., 1998). The analyses can be performed on discrete
samples or on the surface of split sediment cores. No need to say that
the automatic logging systems for X-ray fluorescence spectrometry
represent a tremendous advantage above the traditional XRF method
because the quantity of data acquired by the scanner from the sediment
archives is extremely high. The two most common types of split-core
XRF scanners are:
1. CORTEX (Jansen et al., 1998), built and developed at the
Netherlands Institute for Sea Research (NIOZ, Texel).
2. XRF core scanner (Röhl and Abrams, 2000; Kuhlmann, 2004),
modified at the Bremen University (Germany) in order to allow
a wider spectrum of elements.
This device is new and still under improvement and some limits
exist. For example, the non-homogeneity of the sediment core surface
or the coarse grain size may alter the results (Jansen et al., 1998). Other
problems concern the detection of Al, Si and Ba, important chemical
elements for paleoenvironmental reconstructions. Hopefully the range
of measurable elements will become wider in the next future.
Nevertheless, because of the high-resolution and semi-quantitative
records, XRF-scanners are useful tools for detailed time series analysis,
inter-core correlation and shipboard analysis (Jansen et al., 1998).
The applications in paleoceanography are the followings:
•
•
•
Ca element intensity is a proxy for calcium carbonate and may
provide a prediction of the downcore δ18O record with a wide
range of stratigraphic applications.
Mn element intensity is a proxy for redox transitions at the base
of, for example, turbidite deposits.
Fe (and Ti) element intensity is a proxy for terrigenous input
and often it correlates with magnetic susceptibility.
In this thesis, element intensities were measured at a 1-cm
resolution using an XRF core scanner (Jansen et al., 1998; Röhl and
Abrams, 2000). Iron (Fe) and calcium (Ca) intensities were employed
as tracers for the lithogenic and marine fraction sedimentary
83
Part II. Setting, Data and Methodology
components (see Chapter 5). Ca element intensity was also used for
stratigraphic correlations (see Chapters 6 and 7).
4.2. Geochemical analyses
There are many types of possible geochemical analyses applicable
to the marine sediment. Calcium carbonate and total organic carbon
contents are two of the most employed proxies for paleoclimatic
reconstructions. These are useful to infer:
•
•
changes in sea surface productivity and upwelling intensity.
depth of the lysocline and degree of deep water dissolution.
Also, calcium carbonate is a valuable tool for direct stratigraphic
correlation due to its link with the δ18O planktonic and benthic
foraminifera.
4.2.1. Calcium carbonate
Traditionally, the calcium carbonate content was determined on
discrete samples of cored material by pressure calcimetry or
coulometric techniques. Recently, high-resolution carbonate data have
been extracted from automatic core logging, applying the good
correlation between bulk density and sediment composition (e.g.,
Mayer, 1991; Weber, 1998).
In this thesis, the Ca element intensity was used as a proxy for
calcium carbonate (see section 4.1.3). In addition, carbonate analyses
on discrete samples were performed at 10cm sampling intervals using
a Carlo Erba CHN analyser. Calcium carbonate percentages were
calculated as:
CaCO3= (Ctot-Corg) × 8.33
Ctot, total carbon, was measured using 2-3mg of homogenised and
powdered sediment that was transferred to a stainless steel cup and
mixed with an aliquot of V2O5, which acts as a catalyser for
combustion. Corg, organic carbon, was measured as explained in the
following section 4.2.2. (method I)
84
Chapter 4. Methodology
4.2.2. Total organic carbon
In this thesis, organic carbon concentrations were determined
using an organic elemental Carlo Erba analyser, which determines the
N, C, H, S content of a different range of organic matters. Sediment
samples are combusted in an oven at about 1000ºC in temperature.
The gases that resulted from the combustion (N2, CO2, H2O or SO2)
are detected and measured by a chromatograph. The effective
sediment sample content in N, C, H, or S is calculated after the
calibration against a reference standard (atropine) with known
elemental characteristics.
In order to ensure the complete removal of calcium carbonate
from the sediment, two different procedures were applied to the same
set of samples:
1. method I. Sediment samples (6-7mg) were homogenised and
powdered. The samples were leached repeatedly with 1M
solution of HCl until the carbonate carbon was completely
removed. The samples were then transferred to a stainless steel
cup with V2O5 catalyser and combusted in an EA 1108 CHNSO Carlo Erba analyser. Replicate analyses showed that the
measurement reproducibility, calculated as the deviation from
the mean, fell within ±0.07.
2. method II. Sediment samples (0.3-0.4g) were homogenized and
powdered. The carbonate fraction was removed by adding HCl
solution of 7% concentration (4-5ml in volume) and heated at
50ºC in a bain-marie overnight. The resulting suspension was
centrifuged for 10 minutes at 3,000 rpm to avoid loss of fine
particles. The decarbonated sediment was neutralized by
washing with ultra-pure water before being freeze-dried. TOC
was determined with a Carlo-Erba (NA 1500) Elemental
Analyzer, and percentages were calculated after correction for
the carbonated fraction. Measurement reproducibility of
replicate analyses fell within ±0.1.
4.3. Foraminiferal census counts
Qualitative and quantitative changes in planktonic foraminiferal
associations are widely used in paleoceanographic reconstructions. The
geographical distribution and the seasonal ecological preferences of
85
Part II. Setting, Data and Methodology
planktonic foraminiferal species have a strong link with biooceanographic parameters and hydrographic conditions, among which
pycnocline position, chlorophyll maximum extension, mixed layer
thickness, food availability, and mixing processes are the most relevant.
The most widespread routine in micropaleontologic works is the
visual identification of foraminiferal species under a stereomacroscope
(Fig. 4.5).
Fig.4.5. Example of GeoB4205 sand fraction, mainly constituted by planktonic
foraminifera.
Planktonic foraminifers’ taxonomy is the base of several studies
with the following paleoceanographic applications:
•
•
•
•
to characterize water masses and oceanic front displacements.
to infer productivity changes related to wind-driven upwelling.
to derive sea water temperature.
to infer ocean thermocline depth and mixed layer thickness.
In this thesis, planktonic foraminiferal census counts have been
employed to reconstruct sea surface temperatures, which are further
needed to derive sea surface water salinities.
86
Chapter 4. Methodology
4.3.1. Sea surface temperature
Sea surface temperatures (SSTs) can be evaluated by means of
different proxies. The most common procedure is to apply transfer
functions and statistical methods to a range of microfossils such as
foraminifera, radiolarians, coccoliths, diatoms, pollen, etc. Other
proxies are: alkenons Uk37 on phytoplankton, Sr/Ca on corals, Mg/Ca
on foraminiferal shell, etc.
In this thesis, statistical methods (analog techniques) on planktonic
foraminiferal census counts were employed. Transfer functions TFT
(Imbrie and Kipp, 1971; Mix et al., 1999) were excluded from
calculations because these procedures are more susceptible to
differential dissolution than the other methods based on faunal census
counts and because they ensure lower accuracy in the calibration to
modern SST data set (Ortiz and Mix, 1997; Malgrem et al., 2001).
Analog techniques are based on a comparison between modern and
paleo faunal assemblages. The modern faunal data set is constituted by
a compilation of core-top samples for which every sample is associated
with modern ocean temperature. Paleofaunal assemblages are the
downcore faunal percentages. Hereafter, the main characteristics of the
analog methods are resumed.
Modern Analog Technique (MAT) (Prell, 1985). This strategy
searches in the modern data base for core tops that have the closest
faunal composition to that of the sample to be analysed. The selected
cores are called “best analogs”. The choice of the best analogs is made
using dissimilarity coefficients that measure the difference between two
given samples. The calculated temperature is acquired by averaging the
SST data associated with the 10 most similar analogs.
Firstly proposed by Hutson (1979), the modern analog approach
was used in paleopalynology by Prentice (1980) and Overpeck et al.
(1985). Prell (1985) used the squared chord distance as a dissimilarity
coefficient. Advantages and disadvantages are discussed by the author
(Prell, 1985).
87
Part II. Setting, Data and Methodology
Some of the advantages:
•
•
•
•
SST estimates are weighted averages of the temperatures of
most similar subset of samples (analogs) and not a complex
linear regression equation like in the case of TFT.
analogs are easily identified in their geographic context.
preservation state does not affect the SST estimates.
the threshold values used to retain or reject the “best analogs”
can be fixed by the user.
Modern Analog Technique with a Similarity Index (SIMMAX)
(Pflaumann et al., 1996) is an analog technique in which the similarity
index is the scalar product of the normalized faunal percentages. The
peculiarity of this approach is the use of a geographical distance
weighting which gives preference to the analogs geographically closer
to the area for which SSTs are estimated.
SIMMAX uses the North Atlantic database computed by
Pflaumann et al. (1996), initially of 738 core tops, but recently enlarged
to 916 core tops. Modern oceanographic database was retrieved by the
worldwide dataset of Levitus (1982). The problem of no-analog
samples is not totally solved by both MAT and SIMMAX, but at least
when it occurs spurious results are evidenced by low values of
similarity and high standard deviations of the residuals (Pflaumann et
al., 1996).
Revised Analogue Method (RAM) (Waelbroeck et al., 1998) is
another analog technique with two important changes:
1. the search for best analogs is based on an objective selection
criterion. This means that when the dissimilarity coefficient
experiences a sharp increase (“jump”), only the analogs before
the jump are selected to average the SST, retaining at least two
analogs. This approach ensures that the temperature estimation
is based on real good analogs, even though less than 10.
2. a remapping of the data-base before looking for the analogs is
performed. This means that core-top data base is interpolated
and new data are artificially created in order to homogenize the
coverage of the reference database. Interpolation is permitted
only between core-top assemblages associated with close SST
conditions (Waelbroeck et al., 1998).
88
Chapter 4. Methodology
RAM seems to do a better job than MAT, more at middle and high
latitudes than at low latitudes (Waelbroeck et al., 1998).
4.3.2. Sea surface salinity
Sea surface salinity (SSS) is an important component of the climate
system. Nevertheless, often paleosalinity reconstructions have been
relegated by the dominant attention on paleotemperature
reconstructions. Today, there is an increasing interest in paleosalinity
reconstructions as they proved to be very useful:
•
•
•
•
•
to better constrain the oceanic salt budget.
to infer hydrological changes (evaporation, precipitation, fresh
water input, etc).
to deduce sea water density and water column structure.
to speculate about deepwater formation and global thermohaline
circulation.
to better define the boundary conditions of the model
simulations.
Unfortunately, still there is no method or technique that allows to
measure paleo seawater salinity contents directly. In fact, there is no
geological parameter that can be related directly to paleosalinity in the
open ocean (Wolff et al., 1999a).
First attempts to derive salinity from transfer functions based on
planktonic foraminifera association have resulted not successful
because the growth, abundance and distribution of the foraminifera is
more related to SST (and to nutrient, thermocline, etc) and less to sea
water salinity content (Maslin et al., 1995, Wolff et al., 1999a).
The most common procedure is to extract salinity from the oxygen
isotopic composition of planktonic and benthic foraminifera (Duplessy
et al., 1991; 1992; Rosteck et al., 1993; Maslin et al., 1995; Wang et al.,
1995). However, foraminiferal δ18O reflects the temperature and the
isotopic composition of the water in which the organisms calcified. In
turn, changes in the mean isotopic composition of the seawater are
influenced by the effect of the ice volume and by the local changes in
precipitation and evaporation (Fig. 4.6).
89
Part II. Setting, Data and Methodology
Calcification temperature
δ18Oc
δ18Oice
δ18Ow
Local hydrological effects (plank. forams)
Deep-sea temperature (benthic forams)
Fig 4.6. Scheme of the different components that are encoded in the foraminiferal
δ18O record. Legend: δ18Oc, foraminiferal δ18O; δ18Ow, seawater δ18O; δ18Oice,
contribution of the ice volume to the change in δ18Oc.
In other words, both salinity and temperature influence the
foraminiferal δ18O composition. For this reason, in order to retrieve
salinity information, temperature effect needs to be isolated.
The relationship existing among water temperature (T),
foraminiferal δ18O (δ18Oc) and seawater δ18O (δ18Ow) is referred to as
paleotemperature equations. Several of these equations have been
elaborated after laboratory experiments using different water
temperature conditions and different foraminiferal species. A good
review can been found in Bemis et al. (1998). A paleotemperature
equation has the following quadratic aspect (e.g. Shackleton et al.,
1974; Erez and Luz, 1983)
T (ºC) = a-b(δ18Oc-δ18Ow)+c(δ18Oc-δ18Ow)2
Or the linear aspect (Bemis et al., 1998):
T(ºC) = a-b (δ18Oc-δ18Ow)
Where T is the seawater temperature, δ18Oc is the foraminiferal
oxygen isotopic composition, δ18Ow is the seawater oxygen isotopic
composition and a, b, and c are parameters that change with the
90
Chapter 4. Methodology
changing equation. The salinity information is encapsulated within the
term δ18Ow.
If temperature is independently known (for example using
planktonic foraminiferal abundances), and if δ18Oc is measured (see
next section 4.4), then the paleotemperature equations can be resolved
according to δ18Ow.
As asserted before, δ18Ow is the result of two contributions (Fig.
4.6.):
1. ice volume effect (i.e. sea level changes). Studies based on
measurements of δ18O in fossil corals in Barbados (Fairbanks,
1989; Bard et al., 1990) constrained the glacial-interglacial sealevel change in about 120m. From this result, the contribution
of the ice volume to the change in δ18Oc (δ18Oice) was calculated
to be 1.3‰ (i.e. 0.011‰ per meter of sea-level change). In the
following decades, the contribution of the ice volume was
obtained from measurements on deep sea pore fluids and was
estimated δ18Oice =1.0±0.1‰ (Schrag et al., 1996), although the
scientific community still has not reached a real consensus
about the exact value (Lea et al., 2002).
2. hydrological effect (i.e. changes in salinity). Salinity and δ18Ow
are linearly linked as follow:
δ18Ow = m Salinity + N
where N is the δ18O of a freshwater end member and m is the
slope of the salinity-δ18Ow relationship (SSS-δ18Ow). The
parameters N and m vary from region to region (Craig and
Gordon, 1965; Berger and Gardner, 1975). Modern regional
SSS-δ18Ow can be derived from GEOSECS (1987) or from the
more recent Global Seawater Oxygen- δ18O Database (Schmidt,
1999a). As for past scenarios, both δ18Ow and SSS underwent
global changes. Thus, the modern relationship cannot be used
and corrections for global changes need to be applied (SchäferNeth, 1998; Schmidt, 1999b; Wolff et al., 1999a).
In this thesis, sea surface water salinity records were derived from
planktonic foraminiferal data, combining stable oxygen isotopes and
SST results. Modern SSS- δ18Ow relationship was calculated from the
Global Seawater Oxygen- δ18O Database (Schmidt, 1999a).
91
Part II. Setting, Data and Methodology
Among the different paleotemperature equations, that from Erez
and Luz5 (1983) was chosen because it has been generated from
laboratory-grown specimens of G. sacculifer in a temperature range
(14º-30ºC) that is applicable to the study region. SSS values were
calculated applying the SSS-δ18Ow equation proposed by Schäfer-Neth
(1998) that allows for corrections attributable to the global salinity
increase due to sea-level change and to the ice-volume effect. The
curve obtained by Vogelsang (1990), partly derived after that of
Labeyrie et al. (1987), and the curve calculated by Waelbroeck et al.
(2002) were used to this purpose (see Chapter 7).
4.4. Stable isotope measurements and age model
Stable isotope measurements were carried out at a 5 cm resolution
on 18-25 specimens of Globigerinoides ruber-white which were
picked from the >149µm size fraction. This species was chosen
because it is present in sufficient numbers throughout the length of the
core.
The isotope analyses were performed at GEOMAR in Kiel using a
Finnigan MAT 252 mass spectrometer with a single-sample "CARBOKIEL" carbonate preparation device. Prior to isotope analysis, all
samples were ultrasonically rinsed in methanol to remove sediment
dust that may adhere to the foraminiferal shells. An internal carbonate
standard (Solenhofen Limestone) was run to monitor reproducibility,
which was ±0.05‰ for δ18O. The isotope values were converted to the
Pee Dee Belemnite (PDB) scale.
The stratigraphy for core GeoB GeoB4205 was based on the
correlation of the G.ruber δ18O record with the orbitally tuned, stacked
record of Martinson et al. (1987) (see Chapter 5). In Chapters 6 and 7,
the results obtained on core GeoB4205 are compared and discussed
with data from other cores located in the study area and along the
Portuguese margin. In order to have a robust stratigraphic frame in
which such comparisons can be operated, the age model of core
GeoB4205 was adjusted to the age model of the core MD952042,
located off the Iberian margin (see figure 6.5). More specifically,
calcium element intensity record was tuned to the calcium carbonate
5
T= 17.0 - 4.52 (δ18Oc-δ18Ow) + 0.03 (δ18Oc-δ18Ow)2
92
Chapter 4. Methodology
curve available for core MD952042 (Pailler and Bard, 2002) whose
age model has been well constrained by Shackleton et al. (2000) (see
Chapter 6).
93
PART III
RESULTS
95
96
Chapter 5.
STORMINESS CONTROL OVER AFRICAN
DUST
INPUT
TO
THE
MOROCCAN
ATLANTIC MARGIN (NW AFRICA) AT THE
TIME OF MAXIMA BOREAL SUMMER
INSOLATION: A RECORD OF THE LAST 220
KYR
97
98
Chapter 5. Storminess control over African dust
STORMINESS CONTROL OVER AFRICAN
DUST
INPUT
TO
THE
MOROCCAN
ATLANTIC MARGIN (NW AFRICA) AT THE
TIME OF MAXIMA BOREAL SUMMER
INSOLATION: A RECORD OF THE LAST 220
KYR
Abstract
Precessional forcing of dust flux from Northwest Africa to the
Atlantic during the last 220kyr is recognised through changes in
physical and chemical sediment properties (sediment colour
reflectance, magnetic susceptibility, X-ray fluorescence element
intensities) in a core from the Moroccan margin. Sediment colour
reflectance, magnetic susceptibility, Ca and Fe element intensities
changes are thought to be driven by dust input from North Africa.
Spectral analysis of the sediment properties records displays a
dominant periodicity in the 1/23kyr-1 frequency band that is associated
with the Earth’s orbital precession. Peaks in terrigenous supply match
precessional minima. This suggests a close link between maxima of
boreal summer solar radiation, monsoon activity, and dust generation.
Enhanced precession-driven solar radiation would have caused
increased seasonal temperature contrasts, which amplified atmospheric
turbulence and stimulated storminess. Such a scenario is similar to
today’s summer conditions, when frequent storms cause distinct dust
plumes to form in the area and transport fine terrigenous material to the
adjacent North Atlantic Ocean.
99
Part III. Results
5.1. Introduction
North Africa is an important source area for dust supply, both
regionally to the North Atlantic Ocean and the Mediterranean Sea, and
also globally (e.g. Prospero, 1996). Prolonged periods of aridity in the
source areas and the subsequent decrease of vegetation cover lead to
enhanced wind-driven erosion and promote conditions favourable to
dust generation (e.g. Janecek and Rea, 1985; Middleton, 1985; Rea,
1994; Tiedemann et al., 1994). According to Sarnthein et al. (1982),
dust generation is not related exclusively to aridity, but rather is driven
by a complex interplay of meteorological conditions that favour the
formation and atmospheric uptake of dust. In particular the intensity of
the Hadley circulation and the vigour of the mid-tropospheric easterly
jet are important factors that determine the rate of dust production and
transport (Prospero, 1996).
The objective of the present study is to identify the rates and
mode of the dust supply during the last 220kyr from the African
continent to the subtropical north-eastern Atlantic, the region north of
the Canary Islands (Fig. 5.1).
We employ a set of independent dust-proxies: sediment colour
reflectance, magnetic susceptibility, and element intensities (e.g. Fe
and Ca).
Sediment colour reflectance is closely correlated with trace
concentrations of antiferromagnetic minerals (primary hematite and
goethite), which are typically contained in dust and give the sediments
a red appearance (Deaton and Balsam, 1991; Balsam et al., 1995).
Sediment colour is also influenced by varying carbonate contents in
that increased CaCO3 content results in higher colour reflectance
values (e.g. Cortijo et al., 1995; Mix et al., 1995; Adkins et al., 1997;
Weber, 1998; Balsam et al., 1999).
Magnetic susceptibility is driven by the concentration of
ferrimagnetic (magnetite) and antiferromagnetic (goethite and
hematite) minerals in deep-sea sediments (Kent, 1982; Doh et al.,
1988; deMenocal et al., 1991; Frederichs et al., 1999). Magnetic
susceptibility records slight changes in the concentration of
ferroginous material that typically coats clay minerals and quartz
grains (Sarnthein et al., 1982). This makes down-core magnetic
susceptibility records valuable documents of airborne dust
100
Chapter 5. Storminess control over African dust
contribution to deep-sea sediments, an approach that has been widely
tested and validated by many studies related to marine deposits
(Bloemendal et al., 1988; Bloemendal and deMenocal, 1989;
deMenocal et al., 1991; deMenocal, 1995) and terrestrial deposits
(Bloemendal et al., 1995).
Iron and calcium are some of the main elements that enter in the
sediment composition. Here, the intensities of Fe and Ca are used as
tracers for the lithogenic and marine fraction respectively.
Geochemical approaches have been extensively employed to infer
African dust source regions and trajectories (e.g. Bergametti et al.,
1989; Chiapello et al., 1997) and to monitor wind strength and
atmospheric circulation changes (Matthewson et al., 1995; Martinez et
al., 1999; Moreno et al., 2001).
38º N
SPAIN
37º N
36º N
35º N
Gibraltar Strait
ATLANTIC OCEAN
34º N
MOROCCO
33º N
GeoB 4205
C. Ghir
32º N
.
s Mt
Atla
31º N
Agadir
30º N
CANARY
ISLANDS
North Sahara
29º N
28º N
14º W
12º W
10º W
8º W
6º W
4º W
Fig. 5.1. The study area and the location of core GeoB 4205 along the Moroccan
Atlantic Margin (NW Africa): 32º10.8’N latitude, 11º 38.9’W longitude; and 3296m
water depth.
101
Part III. Results
5.2. Summer dust plume: generation and transport
Today, the primary sources for aeolian dust supply to the northeastern Atlantic are the arid regions of Mauritania, Mali, southern
Algeria and Morocco (Grousset et al., 1998). During summer, major
dust outbreaks from this region are concentrated between 10 and 25ºN
(Fig. 5.2). Dust hazes are associated with strong convective
disturbances and average air flow instabilities that develop deep in the
Sahara and Sahel regions between 15 and 20ºN in summer (Prospero,
1996). According to Tetzlaff and Peters (1986), small-scale structures,
so-called African squall lines, are the main mechanism for dust uplift.
Squall lines consist of a group of storm clouds located between 10 and
15ºN, which are summer-time features and are active for days before
they disappear. They are normally associated with atmospheric
easterly waves and produce strong surface winds which cause dust to
be lifted up to mid-tropospheric levels (3-5km elevation) (Tetzlaff and
Peters, 1986). From there, the dust is transported west by the Saharan
Air Layer. The dust crosses the Intertropical Convergence Zone
(ITCZ; 16-22ºN in summer) and splits into two branches. The western
branch follows a route that eventually reaches the Caribbean and the
SE United States (Carlson and Prospero 1972, Prospero and Carlson,
1972). The northern branch spreads over the Northeast Atlantic and
the Canary Islands. This region receives additional dust from northern
Morocco and the Moroccan Atlas, which is transported by the shallow
trade winds (Bergametti et al., 1989).
Meteorological processes associated with dust transport and
deposition in the northeast Atlantic are complex and not well
understood (Prospero, 1996). Northward dust transport from African
regions to the Canary Islands seems possible only when the Azores
high pressure system is split into two cells, one located over the
Atlantic Ocean and the other over Africa. The African cell would
allow the dust to move northward (Bergametti et al., 1989).
Satellite images and studies carried out on dust time series
covering the last few decades demonstrate that there is a high intraannual and interannual variability in the dust source areas. The
seasonal shift of the subtropical high-pressure centre seems to govern
the variability in the dust source areas and transport paths (Chiapello
et al., 1997). For instance, the Canary Islands during the spring season
may receive dust from both north Morocco and the south MoroccoAlgerian Saharan regions within less than one week (Coudé-Gaussen
102
Chapter 5. Storminess control over African dust
et al., 1987); in summer, they receive dust from both Sahelian and
Moroccan areas (Bergametti et al., 1989). Variability also depends on
the seasonal and interannual cycle in dust concentration and
precipitation rates. Currently, the primary dust deposition mechanism
along the North African coast and in the Mediterranean, which are
reasonably close to the dust source areas, is by dry removal
(sedimentation and impact). Wet deposition (rain washout) is less
important because periods with persistently high atmospheric dust
concentrations (i.e. summer) coincide with low rates of precipitation in
these regions (Prospero, 1996).
Fig. 5.2. Schematic representation of the conditions for dust generation and transport
toward the study area in summer. H: high-pressure cell; ITCZ: Intertropical
Convergence Zone. Turbulence zones of the squall lines passage are where the dust
is elevated to the mid-tropospheric level. Dust haze is intercepted by the easterly
Saharan Air Layer (SAL) and transported west/north-west to the North Atlantic
(modified after Sarnthein et al., 1981; Tetzlaff and Peters, 1986; Bergametti et al.,
1989).
103
Part III. Results
5.3. Material and methods
Core GeoB4205 was retrieved at 32º10.8N, 11º38.9W at a water
depth of 3296 m. The core is located about 230km off the Moroccan
margin (Fig. 5.1). Sediments at this location mainly consist of forambearing nanno-ooze.
5.3.1. Calcium carbonate
Carbonate and organic carbon analyses were performed at 10cm
sampling intervals using a Carlo Erba CHN analyser. Two repeat
samples were run to ensure reproducibility of the measurements. Total
carbon was measured using 2-3mg of homogenised and powdered
sediment that was transferred to a stainless steel cup and mixed with
an aliquot of V2O5, which acts as a catalyser for combustion. Organic
carbon concentrations were determined using 6-7mg of homogenised
and powdered sediment. The samples were leached repeatedly with
1M solution of HCl until the carbonate carbon was completely
removed. The samples were then transferred to a stainless steel cup
with V2O5 catalyser and combusted. Calcium carbonate percentages
were calculated as CaCO3= (Ctot-Corg) × 8.33.
5.3.2. Sediment colour and magnetic susceptibility
Colour spectrophotometry was performed shipboard using a
Minolta CM-2002TM hand-held spectrophotometer. Colour scans were
done immediately after the core was split to ensure the original colour
was measured. Data were acquired at a 5cm resolution. The instrument
provides the reflectance of the sediment colour over 31 wavelength
channels that cover the visible band (400-700nm). Here, we use the
sediment colour reflectance lightness parameter, labelled L*, which
represents the black-white component of the sediment colour. Its
values are dimensionless and can vary between 0 and 100 (Weber,
1998).
Magnetic susceptibility was achieved post-cruise at the University
of Bremen. Non-destructive measurements were done at a 1cm
resolution, on the split-core archive half using a GEOTEK™ Multi
Sensor Core Logger (Weaver and Schultheiss, 1990; Gunn and Best,
104
Chapter 5. Storminess control over African dust
1998). A BARTINGTONTM point-sensor MS2F was used to obtain the
volume magnetic susceptibility, expressed in SI × 10-5 units.
5.3.3. Geochemical analyses
Geochemical analyses were performed post-cruise at the
University of Bremen. Non-destructive measurements were done at a
1cm resolution on split-core surfaces using an X-ray fluorescence core
scanner (Jansen et al., 1998; Röhl and Abrams, 2000). The central
sensor unit consisted of a molybdenum X-ray source (3-50kV), a
Peltier-cooled PSI detector (KEVEXTM) with a 125µm beryllium
window, and a multichannel analyser with a 20eV spectral resolution.
The system configuration used at the University of Bremen allows the
analyses of a wide range of elements, from K (atomic number, 19) to
Sr (atomic number, 38). The acquired XRF spectrum for each
measurement was processed by the KEVEXTM software Toolbox© and
element intensities were finally expressed in counts per second.
5.3.4. Stable isotope measurements
Stable isotope measurements were carried out at a 5cm resolution
on 18-25 specimens of Globigerinoides ruber-white which were
picked from the >149µm size fraction. We chose this species because
it is present in sufficient numbers throughout the length of the core.
The isotope analyses were performed at GEOMAR in Kiel using a
Finnigan MAT 252 mass spectrometer with a single-sample "CARBOKIEL" carbonate preparation device. Prior to isotope analysis, all
samples were ultrasonically rinsed in methanol to remove sediment
dust that may adhere to the foraminiferal shells. An internal carbonate
standard (Solenhofen Limestone) was run to monitor reproducibility,
which was ±0.05‰ for δ18O. The isotope values were converted to the
Pee Dee Belemnite (PDB) scale.
5.3.5. Spectral analyses
Spectral analysis was applied using AnalySeries software 1.1
(Paillard et al., 1996). We use the Blackman-Tukey method to
determine spectral density in sediment colour, magnetic susceptibility
and element intensity records. Cross-spectral analysis was performed
105
Part III. Results
using the orbital ETP (eccentricity, tilt, precession) as a reference
record (Imbrie et al., 1984). Cross-spectral analysis aims to define the
coherency between two signals when they present a similar
periodicity. The degree of correlation gives the value of coherency (k)
and is considered statistically significant when k exceeds an 80%
confidence level. The analysis also returns the phase angle (φ) of the
proxy series with respect to the ETP record.
5.4. Results
5.4.1. Stratigraphy
The stratigraphy for core GeoB 4205-2 is based on the correlation
of the δ18O record, from the planktonic foraminifera Globigerinoides
ruber, with the orbitally tuned, stacked record of Martinson et al.
(1987) (Fig. 5.3). The core spans the last 220kyr back to the end of
interglacial Stage 7. The events identified with the best confidence
occur at interglacials 7.3 and 7.1 as well as 5.5, 5.3 and 5.1.
Terminations II (6/5 boundary) and I (2/1 boundary) are also easily
recognised by the abrupt decrease in δ18O at 275-280cm and 15-20cm
respectively. The Last Glacial Maximum (event 2.2) was correlated to
the highest oxygen isotope value (δ18O = 2.02) at 33cm in core-depth.
The average sedimentation rate is 2.3cm/kyr.
5.4.2. Sediment geochemical and physical properties
Measurements give CaCO3 proportions that fluctuate between 25
and 65%. The profile shows the typical CaCO3 pattern for the Atlantic
Ocean (Fig. 5.4): higher values during the interglacial and lower
values during the glacial stages. Stage 3 shows comparable carbonate
content values to interglacial stages. The lowest CaCO3 values are
displayed in stage 4 (at 60-65kyr B.P.), in part of Stage 6
(approximately at 150-155kyr B.P.) and throughout Stage 2.
The sediment colour reflectance lightness values range from 50 to
70 L* units, and the record shows two superimposed patterns (Fig.
5.4). One is related to glacial/interglacial variability with high values
during interglacials and low values during glacials, as seen for the
fluctuations in carbonate content. The second pattern is mostly linked
to the Earth's orbital precession forcing where the sediment colour
106
Chapter 5. Storminess control over African dust
reflectance clearly records high values during maxima and low values
during minima in the precession index.
MIS
GeoB4205
Stacked isotope record
0
0
1
2
100
2.2
3
50
4
5.1
100
5
5.3
5.5
300
150
Age (kyr)
Depth (cm)
200
6
400
7.1
7
200
7.3
500
2
18
1
0
δ O (‰) (G.ruber)
-1
1
0.5
0
-0.5
-1
18
δ O (‰)
Fig. 5.3. Oxygen isotope record of core GeoB4205 and its correlation with the
stacked isotope curve of Martinson et al. (1987). MIS stands for Marine Isotope
Stage. Solid lines indicate limits of each stage. Dotted lines indicate some of the
principal substages.
Calcium values span from 2500 and 8500 cps, and the profile
much resembles both CaCO3 and sediment colour reflectance records
(Fig. 5.4). We performed parallel analyses of colour reflectance, Ca
intensity and CaCO3 content in the same levels and found that the
three parameters are significantly correlated. The regression
correlation coefficient was equal to 0.53 for CaCO3 and colour
reflectance, and equal to 0.80 for CaCO3 and Ca correlation.
107
Part III. Results
1
C aC O 3 (% w t.)
70
2
3
2.2
4
5.1
5
5.3
6
5.5
7.1
7
7.3
60
50
40
70
30
65
10000
60
L ightness
20
C alcium (cps)
MIS
55
7500
50
5000
2500
40
35
25
20
15
5000
Iro n (cp s)
10
M ag. S usc. (S I)
30
5
0
3000
0.05
1000
0.00
-0.025
-0.05
0
20
40
60
80
100
120
Age (kyr)
108
140
160
180
200
220
P recession Index
0.025
Chapter 5. Storminess control over African dust
Fig. 5.4. Calcium carbonate content (%wt.), colour reflectance lightness, calcium
intensity (cps), magnetic susceptibility (10-5 SI) and iron intensity (cps) records
plotted against an age scale. The precession index curve (Laskar, 1990) is displayed
at the bottom of the figure and the minima in the precession index are outlined by
shaded areas. Peak-to-peak correlation between magnetic susceptibility and Fe highs
(lightness and Ca lows) and minima in the precession index are recognisable
throughout the length of the core.
Magnetic susceptibility curve does not point to any well-defined
glacial-interglacial pattern. Rather, it shows low background
variability (between 5 and 20×10-5 SI) over which higher, spikeshaped values (between 20 and 40×10-5 SI) occur (Fig. 5.4). The
spikes characterise interglacial Stages 5 and 7 at odd substages, part of
glacial Stage 6 and Stage 4. One of the spikes at Stage 6 (centred at a
core-depth of 378cm = 168kyrs) corresponds to an ash layer, which
has been recognised as a common feature of the entire area (Wefer et
al., 1997).
Iron values span from 1000 and 5000 cps, and the profile matches
magnetic susceptibility record very well (Fig. 5.4). Especially, nearly
every spike in the magnetic susceptibility is confirmed by high values
in the iron record. The exception comes with the volcanic ash layer
that gives a peak in the magnetic susceptibility record only.
As for the sediment colour reflectance and Ca curves, magnetic
susceptibility and Fe records also appeared to be linked to orbital
precession forcing as shown by the peak-to-peak correlation between
spikes in magnetic susceptibility and Fe and minima in the precession
index.
5.4.3. Spectral analyses
We applied spectral analysis to both physical and geochemical
sediment properties time-series. Spectral density in the sediment
colour reflectance and Ca intensity records is significant and crosscoherency is high (k=0.84 and k=0.83 respectively) in the 1/23kyr-1
frequency band (Fig. 5.5A, 5.5B and Table 5.1).
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Part III. Results
A
B
C
D
Fig. 5.5. Spectral analysis performed over physical and geochemical sediment
properties time-series. A: sediment colour reflectance, B: calcium intensity, C:
magnetic susceptibility, D: iron intensity. Spectral density of core profiles is
compared to the spectrum of the ETP (eccentricity, tilt, precession) curve (on the
top) and cross-coherency between the two curves is shown at the bottom of each
box. Blackman-Tuchey method was used; the number of lags was 216. Bandwidth
(BW) and 80% confidence interval (C.I.) are represented as horizontal and vertical
bars respectively. Eccentricity (100kyr), obliquity (41kyr), and precession (23kyr)
periods are indicated as vertical dashed lines.
110
Chapter 5. Storminess control over African dust
VARIABLE
δ18O
Reflectance
Calcium
Iron
Mag. Susc.
K
FORCING FREQUENCY
K
∆T (kyr)
∆T (kyr)
φ
φ
1/41 kyr -1
1/23 kyr -1
0.88
-125±14
14±1.6
0.90
-115±11
7±0.7
0.68
0.76
61±27
39±22
7±3
4±2.5
0.84
0.83
-173±17
178±17
11±1.1
11±1.1
0.68
-136±27
----
-15.5±3
----
0.88
3±14
0.2±0.9
0.89
23±13
1.5±0.8
0.09
Table 5.1. Cross-spectral analyses comparison of core GeoB 4205 proxies with
Eccentricity-Tilt-Precession (ETP) curve. Coherency (K) and phase angles (φ) are
given only for relationships with K>80% level. A positive phase angle indicates
that the proxy series lags ETP.
The phase angle between colour reflectance and the ETP's
precessional component is -173º, the phase angle between Ca and
ETP's precessional component is 178º; both values correspond to a
time lag of ~11kyr. As the precession component, in the ETP
composite, is actually reversed (ETP=E+T-P), the phase angles we
found mean that colour reflectance and Ca minima are in phase with
the maxima in boreal summer insolation. Spectral density in the
magnetic susceptibility and Fe records is also significant in the
1/23kyr-1 frequency range. Cross-coherency values are high for both
magnetic susceptibility and Fe (k=0.89 and k=0.88 respectively) and
the phase angles of 23º (MS) and 3º (Fe) mean that magnetic
susceptibility and Fe maxima lag the maxima in boreal summer
insolation by ~1.5kyr and ~0.2kyr respectively (Fig. 5.5C, 5.5D and
Table 5.1).
5.5. Discussion and conclusions
5.5.1. Significance of the sediment composition
Changes in the sediment composition in our core are attributed
mainly to the variability of the calcium carbonate content of biogenic
origin and to lithic particles.
Calcium carbonate abundance is controlled by the variations in its
preservation and by dilution by lithic components. Changes in
carbonate preservation/dissolution are mainly attributed to a major
entrance/influence of Antarctic Bottom Water (AABW) in the east
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Part III. Results
Atlantic basin during glacial periods. Here, changes in deepwater
circulation and maximum incursion of 13C-depleted glacial AABW are
postulated for early Stage 2 and at the 3/2 stage boundary (Sarnthein et
al., 1994). Moreover, during Stages 2, 4 and 6, important dissolution
events have been recognised throughout the Atlantic (Crowley, 1983;
Curry and Lohman, 1990; Verardo and McIntyre, 1994). This
mechanism explains the "global glacial-interglacial pattern" that we
have recognised in the carbonate and calcium fluctuations as well as in
the sediment colour reflectance curve: low/high carbonate, calcium
and colour values during glacial/interglacial stages. Results of a
parallel study on the foraminifera association in the same core reveal
high test fragmentation as well as benthic/planktonic ratio values at
the change from Stages 2 to 3 and in Stage 4 (see Chapter 6). These
are interpreted as dissolution events because they parallel the lowering
of CaCO3 values (Berger, 1970; Thunell, 1976).
The other factor that may control sediment carbonate content is
the effect of dilution. The delivery of terrigenous material from the
nearby continent plays an important role in the sediment composition
changes. The modes of transfer include aeolian and gravity-induced
transport. The largest fraction of terrigenous material reaches the
Atlantic Ocean as the result of the removal from the dust hazes blown
from the African lands (Windom, 1975). However, it has been
reported that during glacials a great amount of terrigenous material,
previously stored along the shelf, is delivered to the deep ocean
triggered by the lowering of the sea level (Sarnthein and DiesterHaass, 1977).
In our core, the input of the terrigenous material is monitored
through the changes in physical and chemical sediment properties
records. The Sahara and Sahel regions supply dust, the components of
which are mostly quartz, mica and clay minerals characteristically
stained by ferric oxides (Sarnthein et al., 1982; Coudé-Gaussen et al.,
1987; Grousset et al., 1992b). On the other hand, the composition of
the dust from the northernmost area (North Morocco and the Atlas
Chain) exhibits minor or no iron content (pale quartz and illite)
(Lange, 1982; Sarnthein et al., 1982; Bergametti et al., 1989).
Therefore, magnetic susceptibility and Fe element concentration are
proxies especially for the dust of Saharan and Sahelian origin
Several studies found that enhanced precession-driven insolation
controlled monsoon and related wind systems, which are dynamic in
112
Chapter 5. Storminess control over African dust
the equatorial and subtropical eastern Atlantic (Kutzbach, 1981; Prell
and Kutzbach, 1987; McIntyre et al., 1989; Molfino and McIntyre,
1990; deMenocal et al., 1993). Spectral results clearly document that
precession is the most obvious and dominant signal in our dust proxy
records.
This suggests a strong link between low-latitude climate
mechanisms and dust contribution to the Northeast Atlantic. Enhanced
ferric and magnetic terrigenous inputs occurred during interglacial
stages 5 and 7 and at odd substages. Similar results have been
achieved in a parallel study performed in a nearby area (Moreno et al.,
2001). Here, the Al concentration and the Fe/Al ratio are used as
proxies for dust input. Both geochemical markers display cycles at
23kyr periodicity and are in phase with minima in the precession
index.
These results differ from what has been reported in the literature
(Hooghiemstra, 1989; Matthewson et al., 1995), where increased
values of pollen (Hooghiemstra, 1989) and enhanced Al fluxes
(Matthewson et al., 1995), used as proxies of wind vigour, show their
maxima during Substages 5b and 5d. Here, the complexity and
variability of the dynamical features of the atmospheric circulation in
the area may help to explain the apparent disagreement. Cores located
in the equatorial east Atlantic (as is the case for the quoted studies)
record the terrigenous material transported by the zonal component of
the trade winds, which is more intense during maxima in precession
index and nearly suppressed by monsoon incursions during minima
(McIntyre et al., 1989).
We have previously introduced the great variability in dust source
areas and track paths when it deals with short-time series studies
(Section 5.2). Therefore, when we move to a long time-series
approach, it is reasonable to think that we are not able to sharply
distinguish between the different source areas. Moreover, the
terrigenous material found in the marine sediment cores is mostly the
output of the most extreme dust outbreaks (Rea, 1994). The
identification of a precise source area is in this case even more
difficult because the region travelled over by major dust hazes is quite
large, and mixing processes involving different types of continental
dusts probably occur (Coudé-Gaussen et al., 1987).
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Part III. Results
Nevertheless, the estimation of goethite and hematite is a good
approximation in dust series analysis because these minerals are the
principal trace components of the dust coming from the lateritic
Sahelian soils (goethite) and arid Sahara regions (hematite) (Balsam et
al., 1995). In addition, they greatly influence the output of magnetic
susceptibility, colour reflectance lightness and element intensity
records. Therefore, although we are unable to distinguish between the
two minerals and to speculate about a precise dust source area, we can
reasonably assert that our parameters are a good set of proxies for
tracing the arid African regions-derived dust inputs. High magnetic
susceptibility and Fe values from one side and low sediment colour
reflectance and Ca intensity records from the other, portray the relative
concentration of terrigenous material in the sediment.
5.5.2. Precessional forcing of stormy weather conditions
The obvious result of research on our core is that variations in the
sediment composition are related to climatic changes forced by the
orbital precessional component. The idea of a precessional control
over the African climate is not new. Records from eastern tropical and
equatorial Africa that contain a 23kyr periodicity have been widely
acknowledged (Pokras and Mix, 1985; 1987; Bloemendal and
deMenocal, 1989, Molfino and McIntyre, 1990; Tiedemann et al.,
1994; Wolff et al., 1999b). The striking result of the present study is
the correspondence between major concentrations of terrigenous
material and minima in the precession index (maxima in boreal
summer insolation).
According to studies based on general circulation model (GCM)
experiments (Kutzbach, 1981; Prell and Kutzbach, 1987) and on
paleoclimatic data (Street and Grove, 1979; Rossignol-Strick, 1983;
Pokras and Mix, 1987; deMenocal et al., 2000a), precession-driven
solar radiation maxima correlate with periods of increased strength of
the African summer monsoon and with the longer rainy season in the
Sahara and Sahel regions. As we have already stated, these are the
principal dust source areas and they need to be arid in order to
generate the typical dust outbreaks. Thus, our results apparently do not
fit the general and widely accepted understanding. However,
precipitation is one of the factors that may control the variability in the
atmospheric dust cycle: also other meteorological factors, such as
114
Chapter 5. Storminess control over African dust
large-scale dynamical features of the atmosphere, need to be taken into
consideration (Prospero, 1996; Moulin et al., 1997).
Nowadays, dust storm activity is concentrated in the early
summer months and occurs just before the onset of the rainy season
(Middleton, 1985). Arid and immature soils of North Africa are thus
easily deflated by the strong and deep convective mixing winds
(Prospero, 1996), which are possibly associated with the summer
monsoon. Model simulations show that an increased northern
hemisphere solar radiation in June, July and August provokes warmer
land relative to the ocean surface. The consequences are the intensified
summer monsoon (Kutzbach, 1981); the stronger winds due to the
larger land-ocean pressure gradient (Prell and Kutzbach, 1987); and
the harder seasonal thermal contrast with warmer summers and colder
winters (Kutzbach, 1989; deNoblet et al., 1996). Model experiments
also indicate that increased intensity of summer monsoon circulation is
associated with enhanced strength of the subtropical anticyclone and
with an increased wind speed in the subtropical easterly jet over North
Africa (Kutzbach, 1989). The small-scale atmospheric features (such
as the squall lines), which are associated with the monsoon-trade front,
may also have been more frequent and/or more intense during periods
of maxima in solar radiation. Therefore, we propose the hypothesis
that stronger monsoon events were accompanied by more intense
stormy and turbulent conditions capable of raising a great amount of
dust.
deMenocal et al. (1993) report that maxima in dust transport occur
during intensified dust source area destabilisation rather than absolute
aridity. Ruddiman (1997) indicates that dust flux to the Atlantic Ocean
is related to changes in the wind transport more than to aridity and
dust availability on the continent. More precisely, he suggests a
combined effect of increased wind strength and more powerful
mechanisms generating the storm, which propel the dust into the lower
troposphere. Furthermore, from modern observations we know that
dust generation occurs in those areas where seasonally variable
conditions prevail and declines in hyperarid environments (Pye, 1987;
1989). A strong association between storms in the Atlantic and
western Sahel monsoon rainfall has been observed for at least four
decades of the twentieth century (Landsea and Gray, 1992). The
weather feature that would link the two phenomena is recognised in
the amplitude of the easterly waves. These, in fact, both seem to
115
Part III. Results
contribute to the rainfall in the Sahel and serve as the nucleus of
tropical hurricanes.
As a conclusion, we suggest that aridity of the source region is a
necessary condition for dust availability, but not enough to explain its
generation. Extensive conditions of storminess and turbulence
occurring at the monsoon-trades front may also be efficient trigger
mechanisms for dust uplift and injection into the troposphere.
According to our results, this mechanism was much more efficient
during periods of maxima in solar radiation and acted at a 23kyr
tempo during the last 220kyr. At the time of minima in the precession
index, the enhanced seasonal thermal contrast (colder winters, warmer
summers) and the increased intensity of summer monsoon circulation
could provoke vigorous and prolonged turbulence conditions along the
monsoons-trades front. Also, wind speed in the subtropical easterly
flow and the amplitude of the easterly waves could be stronger. Such a
scenario for the dust transport and deposition is very similar to today's
summer conditions. The destabilisation causes the dust to rise to midtropospheric levels. Here, the Saharan Air Layer, blowing from east to
west at 3-5km aloft, intercepts the dust haze and pushes it towards the
Atlantic Ocean. The dust moves northward through the discontinuity
of the African high-pressure cell within the hook-shaped branch of the
SAL flow. Afterwards, haze reaches the Moroccan margin, where dry
removal mechanisms finally cause the dust to deposit.
Acknowledgements
This work is part of the European CANIGO project (MASTIII-CT9-0060).
Additionally, it received further funding from the MAYC project (CICYT-AMB950196). G.B.'s research activity was also supported by a Marie Curie PhD Fellowship,
TMR Programme. H.K. acknowledges the funding by the Deutsche
Forschungsgemeinschaft Project We922/41-4. Thanks are due to the participants of
the Meteor 37/1 cruise and to V. Diekamp for technical assistance. M. Hüls
(Geomar, Kiel) and H. Meggers (University of Bremen) are greatly thanked for
running the isotope measurements and for providing the colour spectrophotometry
data. We are indebted to S. Lebreiro (IGM, Lisbon), J. Villanueva (CID, Barcelona),
M. Hüls and J. Schönfeld (Geomar, Kiel), and R. Zahn (University of Cardiff) for
helpful discussions and many valuable suggestions for improving this study. The
constructive comments of J.M. Prospero, P. De Deckker, and an anonymous
reviewer are gratefully acknowledged.
116
Chapter 6.
SIGNIFICANCE OF ORGANIC CARBON IN
THE NORTHEAST SUBTROPICAL ATLANTIC: PRODUCTIVITY AND LATERAL ADVECTION
117
118
Chapter 6. Organic carbon: productivity and lateral advection
SIGNIFICANCE OF ORGANIC CARBON IN
THE NORTHEAST SUBTROPICAL ATLANTIC: PRODUCTIVITY AND LATERAL ADVECTION
Abstract
The total organic carbon (TOC) values found in the down-core
sediments from the NW African margin (northeast subtropical Atlantic
Ocean) range from 0.3 to 1.7% during the last 220kyr, with peaks
occurring during low boreal summer insolation. These values are fairly
unusual as the core was recovered from an open-ocean environment
that is currently oligotrophic. Three processes are discussed: (1) the in
situ primary production associated with the extension of the Cape Ghir
upwelling filament, (2) the bottom water conditions that may favour the
organic carbon preservation and (3) lateral organic carbon advection.
Primary production is evidenced by the high abundance of the
planktonic foraminifer G.bulloides. The site occasionally experienced
more eutrophic conditions, especially during Terminations (I and II);
however, given the absence of correlation between TOC and
G.bulloides records, high TOC storage is not exclusively attributed to
primary production. Preservation factors such as bottom water
ventilation are also ruled out. Lateral TOC advection seems to be the
most plausible process. Today, lateral advection and offshore transport
of nutrients and organic matter have been observed in the study region.
However, it has never been investigated whether and how these
mechanisms were operating in the past. Different controlling factors of
the mobilization and advection of organic carbon from coastal
upwelling regions to the deep basin are discussed. Due to the
correlation between lateral advection and minimum insolation, sealevel drops seem to be the most effective mechanism that drives
nepheloid layers detachment and seaward material transport.
119
Part III. Results
6.1. Introduction
Coastal upwelling is the subject of a wide range of research
fields of local and global significance. The growth of ecosystems and
fish communities as well as the role of ocean-atmosphere CO2
exchange in the global climate change are closely related to coastal
upwelling expansion (e.g. Summerhayes et al., 1995). Large upwelling
areas, favoured by the prevailing northeasterly Trade winds exist in the
Northeast Atlantic Ocean along the Iberian and the African margins.
Comprehensive studies on the NW African and the Iberian margins
have improved our understanding of the upwelling system in these
regions (e.g. Thiede and Suess, 1983; Ruddiman et al., 1989; Parrilla et
al., 2002a,b). More specific works have compared glacial (especially
the Last Glacial Maximum, LGM) and interglacial (modern) scenarios,
and have reached the conclusion that upwelling during glacial periods
(LGM) is enhanced by stronger Trade winds (Hooghiemstra et al.,
1987; Sarnthein et al., 1988; Abrantes, 1991; Marret and Turon, 1994).
On the other hand, recent results obtained from sediment cores
located in the NW African upwelling system do not support the classic
view of enhanced upwelling during glacial periods (Bertrand et al.,
1996; Martinez et al., 1999). According to these authors, the combined
effect of shelf width, sea level change, and wind stress gives rise to an
ample spatial heterogeneity of the whole system. This would result in a
larger productivity during interglacial periods than during the LGM and
Stage 2. Thus, given its complexity, NW African coastal upwelling is a
convoluted system in which currents, winds, bottom topography and
coastline shape interact and create a considerable temporal and spatial
variability (Van Camp et al., 1991; Bertrand et al., 1996; Martinez et
al., 1999; Zhao et al., 2000; Sicre et al., 2001).
The productivity signal stored on the sea floor may be altered by
other factors such as sediment lateral advection, down slope transport,
resuspension and bottom currents winnowing (e.g. Fütterer 1983;
Jahnke et al., 1990; Jahnke and Shimmield, 1995). Among these, lateral
advection of nutrients and organic matter into the open ocean is
currently taking place in the study region (Neuer et al., 1997;
Freudenthal et al., 2001; Abrantes et al., 2002; Neuer et al. 2002;
Pelegrí et al., 2004). However, it has never been investigated whether
and how lateral advection occurred in the past. Moreover, it is still to be
determined which is the prevailing path for organic matter advection,
120
Chapter 6. Organic carbon: productivity and lateral advection
whether it is exported to the oligotrophic subtropical gyre by the eddy
field or it is transported downward to the deep ocean (Pelegrí et al.,
2004).
In this study, we discuss the factors controlling the organic carbon
deposition in an oligotrophic region of the northeast subtropical
Atlantic during the last 220kyr. We seek to differentiate between the
influence of the local productivity and the impact of the allochthonous
supplies derived from the continent. To this end, we combine the
analyses of the organic carbon content with the micropaleontological
evidences in a sediment core located off the Moroccan margin (north of
the Canary Islands). We also compare our results with the organic
carbon data in the literature on equivalent cores in the Canary Basin
and off the Portuguese margin.
6.2. The study area
The location, persistence and intensity of the coastal upwelling off
the Moroccan and Portuguese margins are associated with the seasonal
latitudinal migration of the Trade winds (e.g. Van Camp et al., 1991;
Nykjær and Van Camp, 1994). The upwelling season in the study area
is summer, when the Trades are well-established (Mittelstaedt, 1991).
On occasions, the interaction between wind systems, coastline shape
and bottom topography causes the coastal upwelled nutrient-rich water
to be trapped by eddy-shaped mesoscale features and injected offshore
through filaments that extend as far as 200 km offshore (e.g. Van
Camp, 1991; Haynes et al., 1993). Today, filaments are easily
recognised through satellite images given that these structures present a
higher pigment concentration and a colder temperature than the
surrounding environment (e.g. Hagen et al., 1996; Davenport et al.,
1999). Well developed filaments have been observed, mainly
associated with the upwelling season, off the Moroccan coast (Cape
Ghir, at 31ºN) and along the Portuguese margin (Van Camp et al.,
1991; Gabric et al., 1993; Haynes et al., 1993; Nykjær and Van Camp,
1994; Hagen et al., 1996; Kostianoy and Zatsepin, 1996).
The complex bottom topography is incised by the Agadir canyon at
approximately 31ºN, whereas it is enlarged to the north by the presence
of the Cape Ghir Plateau (Fig. 6.1) (Hagen et al., 1996). The study area
is currently in the advection path of the Canary Current (Portugal
Current at the Iberian latitudes), which is a southward flowing surface
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Part III. Results
current that occupies the upper 100-150 meters. Below this current, the
nutrient-rich North Atlantic Central Water (NACW), which is today the
main source water for the coastal upwelling, spreads south with depths
between 150 and 700 meters. Water masses derived by a modification
of the Antarctic Intermediate Water (AAIW) have been traced just
below the NACW. Further, cells of Mediterranean Water (MW) have
been described at a depth ranging between 800 and 1200 meters.
Finally, the North Atlantic Deep Water (NADW), which is
characteristically enriched in oxygen, extends down to the bottom
(Minas et al., 1982; Sarnthein et al., 1982; Mittelstaedt, 1991; Hagen,
2001; Johnson and Stevens, 2000; Knoll et al., 2002; Llinás et al.,
2002).
30ºW
45ºN
40ºN
20ºW
10ºW
Atlantic
Ocean
0ºW
Iberian
peninsula
MD952042
GeoB4205
35ºN
GeoB4216
30ºN
25ºN
Africa
(A)
Canary
islands
34ºN
C.Ghir Plateau
33ºN
20ºN
GeoB4205
32ºN
(B)
Cape Ghir
GeoB4216
16ºW
14ºW
12ºW
31ºN
Agadir
10ºW
30ºN
8ºW
Fig. 6.1. A, left panel: Location of core GeoB4205 and the other two NE Atlantic sites
discussed in the present study. B, right panel: Close up of the bathymetry next to
GeoB4205 and GeoB4216 location indicating the complexity of the bottom
topography. Dashed-line runs along the Agadir canyon axis.
122
Chapter 6. Organic carbon: productivity and lateral advection
6.3. Materials and methods
The present study is mainly based on core GeoB4205 recovered off
the Moroccan margin, north of the Canary Islands (Table 6.1; Fig. 6.1).
Moreover, data from two other cores are included in the elaboration of
our results: core MD952042 and core GeoB4216 (Fig. 6.1; Table 6.1).
The three cores have distinct settings with respect to the current coastal
upwelling location.
Core name
Latitude
Longitude
depth
source
GeoB4205
32º10.8N
11º38.9W
3296m
This study (also Bozzano et
al., 2002)
GeoB4216
NE subtropical Atlantic
30º 37.8N 12º 23.7W
2324m
Freundenthal et al., 2002
Moreno et al., 2002a
MD952042
NE subtropical Atlantic
37.8ºN
10.17ºW
3146m
Cayre et al., 1999; Pailler and
Bard, 2002
Iberian Margin
Table 6.1. List of cores discussed in this study.
Core GeoB4205, despite its proximity to the Cape Ghir filament,
does not seem to be under the main influence of its chlorophyll-rich
waters in accordance with current satellite images for this site
(Davenport et al., 2002).
Core GeoB4216, from the Canary Basin, was recovered from the
offshore development of the Cape Ghir filament, and is therefore
suitable for characterising filament (and upwelling-related) modes.
Freundenthal et al. (2002) monitored the filament activity in the past
and performed geochemical analyses on this core. The findings
demonstrate that the Cape Ghir filament was controlled by the intensity
of the Trade winds, at least during the last 250kyr, and achieved
maximum extensions during glacial periods, at times of minima in the
precession index.
Core MD952042 was obtained from an oligotrophic site off the
Iberian margin. Studies based on foraminiferal census counts and
organic carbon accumulation indicate that this site exhibited more
vigorous upwelling conditions, especially during glacial periods (Cayre
123
Part III. Results
et al., 1999; Pailler and Bard, 2002).
6.3.1. Stratigraphy
Stratigraphic information on core GeoB4205 is based on stable
oxygen isotope measurements carried out on planktonic foraminifera
(G.ruber white). The core spans the last 220kyr, back to the end of
stage 7, with an average sedimentation rate of 2.3cm kyr-1 (Bozzano et
al., 2002).
The stratigraphic information on the comparative cores was
gathered from the original works and from the PANGAEA server
(http://www.pangaea.de). Core GeoB4216 spans the last 250kyr, back
to early stage 7, with an average sedimentation rate of 4-5cm kyr-1
(Freundenthal et al., 2002) whereas core MD952042 covers the last
glacial cycle, back to late stage 6, with an average sedimentation rate of
21 cm kyr-1 (Cayre et al., 1999).
In order to construct a common age scale for the three cores, we
tuned GeoB4205 calcium and GeoB4216 carbonate records to the
CaCO3 curve available for core MD952042 (Pailler and Bard, 2002).
The choice was attributed to the quality of the age model (first 160kyr)
of core MD952042, well-constrained by Shackleton et al. (2000), and
by the high sedimentation rate of this core allowing the resolution of
the finest climatic fluctuations. Calcium (Ca) element intensities for
core GeoB4205 (the original data are from Bozzano et al., 2002) were
acquired using an X-ray fluorescence core scanner (Jansen et al., 1998;
Röhl and Abrams, 2000) at a 1-cm resolution at the University of
Bremen. CaCO3 data for core GeoB4216 were available at the
PANGAEA server (Freundenthal et al., 2002).
6.3.2. Total organic carbon
The total organic carbon (TOC) content of core GeoB4205 was
analysed at 5cm sample interval resolution using two methods applied
to the same set of samples.
Method I. The first routine was performed on an aliquot of 6-7 mg
of powdered sediment. The samples, placed in small stainless steel
cups, were repeatedly leached with drops of 1M solution of HCl until
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Chapter 6. Organic carbon: productivity and lateral advection
total removal of carbonates. The samples were then burned in a Carlo
Erba CHN analyser. Replicate analyses showed that the measurement
reproducibility, calculated as the deviation from the mean, fell within
±0.07. However, as the values obtained were higher than expected, we
assumed that this method provided only a partial removal of CaCO3,
which could then be computed as TOC. In order to ensure the reliability
of the measurements, a second run of analyses was performed and a
different method applied.
Method II. Sediment samples (0.3-0.4g) were first homogenized.
The carbonate fraction was removed by adding HCl solution of 7%
concentration (4-5ml in volume) and heated at 50ºC in a bain-marie
overnight. The resulting suspension was centrifuged for 10 minutes at
3,000 rpm to avoid loss of fine particles. The decarbonated sediment
was neutralized by washing with ultra-pure water before being freezedried. TOC was determined with a Carlo-Erba (NA 1500) Elemental
Analyzer, and percentages were calculated after correction for the
carbonated fraction. Measurement reproducibility of replicate analyses
fell within ±0.1.
The mass accumulation rate (MAR) of the total organic carbon was
calculated according to Stein et al. (1989), using the linear
sedimentation rate (LSR) [cm kyr-1], the wet bulk density (WBD) [gr
cm-3] and the porosity (P) [%]. WBD and P were completed at the
University of Bremen using a GEOTEK Multi Sensor Core Logger.
6.3.3. Foraminiferal census counts and G. bulloides identification
Counts of the planktonic foraminiferal fauna assemblage on core
GeoB4205 were conducted on samples collected every 5 cm; a
minimum of 300 specimens were identified within the ∅>150 µm size
fraction. Total abundance of benthic foraminifera was also counted.
Splitting and counting procedures are described in Pflaumann et al.
(1996). Planktonic foraminiferal species and morphotypes were
distinguished following the taxonomy of Saito et al. (1981) and of
Kennett and Srinivasan (1983).
G.bulloides abundance was used as an indicator of surface water
eutrophic conditions. Although this planktonic foraminifer is typical of
the subpolar and transitional faunal provinces (e.g. Hemleben et al.,
1989), it is also found in tropical and subtropical nutrient-rich waters
125
Part III. Results
with the result that it is widely used as a proxy indicator for upwelling
(e.g. Prell and Curry, 1981; Vincent and Berger, 1981; Peterson et al.,
1991; Guptha et al., 2003). Off SW Africa, G.bulloides, associated with
right-coiling N.pachyderma, is found in the mixing zone between the
Benguela upwelling system cells and the oligotrophic open ocean
(Giraudeau, 1993).
In the Canary Basin, G.bulloides abundance in the surface
sediment shows a good correlation with surface water chlorophyll
distribution, which is associated with the extension of the upwellingfilaments off Cape Ghir and Cape Yubi (Meggers et al., 2002). These
results, which corroborate the link between G.bulloides distribution and
the NW African upwelling signal, validate the use of G. bulloides as an
approximation of productivity signals. However, as G bulloides is not a
particularly dissolution-resistant species, the preservation state of the
foraminiferal shells was also investigated. Three parameters were
combined: the fragmentation index of planktonic foraminifera
[fragments / (whole + fragments) *100], the ratio of benthic to
planktonic foraminiferal tests, and the percentage of CaCO3 (e.g. Parker
and Berger, 1971; Luz and Shackleton, 1975; Thunell, 1976; Thunell
and Honjo, 1981).
6.4. Results
6.4.1. Total organic carbon
TOC results are shown as the composite curve of the point-to-point
average of the values obtained from the two methods applied (Method I
and Method II) (Fig. 6.2A). The values, which range between 0.3 and
1.7%, may occasionally exceed 2%. A characteristic glacial/interglacial
pattern does not exist. Stage 4 and most of stage 6 are especially
depleted in TOC whereas stage 5 (especially 5.2 and 5.4) is punctuated
by the highest values. Terminations (I and II) do not record any
substantial increase in TOC. The mass accumulation rate (MARTOC)
was calculated in order to determine the possible effect of dilution by
terrigenous material. The resulting curve (Fig. 6.2B) does not diverge
significantly from that of the TOC relative concentrations.
126
Chapter 6. Organic carbon: productivity and lateral advection
6.4.2. Foraminiferal preservation and G.bulloides abundance
Today, core location is above the depth of the lysocline (Knoll et
al., 2002). In the ocean, the lysocline depth is related to the carbonate
ion concentration (CO3-2) in the deep water and in the Atlantic Ocean,
the CO3-2 distribution is determined by the mixing between NADW and
AABW (Broecker and Takahashi, 1978). Today the limit between
NADW and AABW in the northeast Atlantic ranges between 3500m
and 4000m, although it could have shoaled during the past glacial times
(Sarnthein et al., 1994). However, according to Curry and Lohmann
(1990), calcite dissolution during glacials was severe only for those
cores located below 4000 m.
3
4
6
5
7
Termination II
TOC (%wt)
2
MIS
(A)
1
0.20
10
0.15
0
(B)
8
0.10
5
0.05
MAR (TOC)
LSR (cm/kyr)
2
Termination I
1
3
0
0
100
200
300
400
0.00
500
Core depth (cm)
Fig. 6.2. A. Total organic carbon (TOC) content in GeoB4205. Vertical bars indicate
the error determined as the deviation from the mean. B. TOC mass accumulation rate
(MAR) (shaded area) and linear sedimentation rate (LSR) (solid line) for core
GeoB4205. Marine isotope stages (MIS) from 1 to 7 are shown in the upper part of
the panel. Terminations I and II are indicated by two dotted lines.
127
Part III. Results
In core GeoB4205, two possible dissolution events are identified at
core depths of about 50cm and 140cm (Fig. 6.3). Here, low calcium
values, high fragmentation index, and high benthic/planktonic ratio
take place simultaneously. These two dissolution events characterise
stages 2 and 4. Thus they are interpreted as glacial enhanced calcite
dissolution episodes, in accordance with similar results obtained in the
North Atlantic (Crowley, 1983; Curry and Lohmann, 1990; Verardo
and McIntyre, 1994). The rest of the core length does not seem to be
affected by other dissolution processes that could have adverse effects
on foraminiferal counting.
As for G.bulloides abundance, this foraminifer is never the
dominant species of the assemblage, accounting for only 5-20% of the
total planktonic foraminiferal assemblage (800-15000 G.bulloides
specimens per gram of dry sediment). Distinctive peaks occur at
Terminations (I and II), and at the transition from stage 4 to stage 3.
Tendencies to high values also characterise the warm interglacial
substages (5.1, 5.3, 5.5, 7.1, and 7.3) whereas low values mainly
distinguish the cold glacial stages 2, 3 and 4, and the cold interglacial
substages 5.2 and 5.4 (Fig. 6.3). Dissolution proxy profiles compared
with G.bulloides abundance suggest that the latter is mostly
independent from calcium dissolution effects.
6.4.3. NW Africa and Iberian Peninsula: integrated results
In order to compare the data obtained from core GeoB4205 with
those from the other two cores, GeoB4216 from the Canary Basin and
MD952042 from the western Iberian margin, we first constructed a
common age scale for the three cores. For the tuning, we used the
calcium and carbonate profiles shown in figure 6.4. The reference age
model is taken from core MD952042 (Shackleton et al., 2000). Core
GeoB4205 (3296 m water depth) and core GeoB4216 (2324m water
depth) show the same range in calcium carbonate content variability
(∼from 20% to 70%). By contrast, core MD952042 (3146m water
depth) displays a smaller CaCO3 range (from 20% to 40%) (Fig. 6.4).
The parallel CaCO3 fluctuations observed along the three cores
suggest a common mechanism shaping the sediment composition, i.e.
the dilution due to continental-derived lithogenic material on the one
hand and the dissolution during glacial periods (Stages 2 and 4) on the
other. Dissolution during glacial periods has been shown to be a
128
Chapter 6. Organic carbon: productivity and lateral advection
common feature in the Atlantic (Crowley, 1983).
3
4
5
5.1 5.3 5.5
6
MIS
7
7.1
7.3
7500
5000
0.45
Calcium (cps)
T- II
2
T-I
1
2500
0.2
0.1
0.1
10
28
0.0
G.bulloides (103/gr)
5
4.8
15
5
0
Bent./Planct forams index
Fragm. index
0.35
10
5
0
50 100 150 200 250 300 350 400 450 500
Core depth ( cm)
Fig. 6.3. Foraminiferal preservation proxies (calcium element intensity, fragmentation
index and benthic/planktonic foraminifera ratio) and G.bulloides occurrence (number
of specimens per gram). Only two outstanding dissolution events (at 50cm and
140cm) occur when low calcium intensity coincide with high fragmentation index and
high benthic/planktonic foraminifera ratio. At 430cm another dissolution event, of
less intensity, is observed. Numbers in boxes indicate the value of dissolution events.
For the rest, G.bulloides abundance is not influenced by dissolution effects.
129
Part III. Results
1
2
3
4
5.1
5.3
5.5
6
7
40
MD952042
MD952042
GeoB4205
20
6
CaCO3
(20%-70%)
Calcium (103 cps)
8
4
CaCO3( % wt.)
30
GeoB4216
10
80
GeoB4205
70
50
40
GeoB4216
CaCO3( % wt.)
60
30
20
0
50
100
150
200
250
Age (kyrs)
Fig. 6.4. Calcium carbonate contents of the three cores considered in the present
study. The range of calcium element intensities for core GeoB4205 corresponds to 2070% amplitude in calcium carbonate (Bozzano et al., 2002). The good fit of the three
curves suggests a common mechanism for CaCO3 changes at the three sites. Right
upper panel shows, as a reference, the location of the three cores. Marine isotope
stages (from 1 to 7) are shown in the upper part of the panel.
130
Chapter 6. Organic carbon: productivity and lateral advection
1
2
3
4
5.1
5.3
5.5
6
7
GeoB4205
1
GeoB4216
2
δ18O (G.bulloides)
0
MD952042
MD952042
3
GeoB4205
0
-1
1
0
GeoB4216
2
1
δ18O (G.bulloides)
δ18O (G.ruber) white
-1
2
0
50
100
150
200
250
Age (kyrs)
Fig. 6.5. Stratigraphy of the three cores considered in the present study. Oxygen
isotope curves are based on G.ruber (white) for core GeoB4205 (Bozzano et al.,
2002) and on G.bulloides for core GeoB4216 and core MD952042 (Freudenthal et al.,
2002; Shackleton et al., 2000). Core GeoB4205 and core GeoB4216 have been tuned
to the age model of MD952042. Marine isotope stages (from 1 to 7) are shown in the
upper part of the panel.
131
Part III. Results
The compatibility of the three cores is further reflected by the good
match of the oxygen isotope curves based on G.ruber (core GeoB4205)
and G. bulloides (core GeoB4216 and core MD952042) (Fig. 6.5). All
the main stages (from Stage 1 to Stage 7) are readily identified and
correlated in the records. Core GeoB4205 and core GeoB4216 exhibit
the same δ18O amplitude (from 2 to -1); core MD952042 record is
shifted toward heavier δ18O values (from 3 to 0).
Finally, we evaluated the fluctuations in the TOC content of the
three cores. We used TOC percentages rather than TOC mass
accumulation rates (MAR) for two reasons: (a) TOC percentages do not
differ significantly from TOC MAR in core GeoB4205 and in core
GeoB4216; (b) TOC MAR in core MD952042 could be controlled by
changes in the sedimentation rate, which could be an artefact of the
core stretching (Cayre et al., 1999). Comparison of the three TOC
records (Fig. 6.6) highlights a striking difference in the absolute values.
In core GeoB4205, TOC percentages range between 0.3% and 1.7 %.
In core GeoB4216, TOC values are lower (0.05% - 0.9%) (Freudenthal
et al., 2002) whereas TOC percentages in core MD952042 (0.4% 1.2%) (Pailler and Bard, 2002) cover an intermediate range between the
two other cores. As for the down-core patterns, similar trends in TOC
percentages characterise both core GeoB4205 and core MD952042,
whereas an opposite trend distinguishes core GeoB4216 (Fig. 6.6).
High TOC values in cores GeoB4205 and MD952042 match the
minima in the northern hemisphere summer insolation, whereas high
TOC values in core GeoB4216 correspond to the maxima of insolation.
TOC peaks at Terminations (I and II), displayed by core GeoB4216 and
core MD952042, are absent in core GeoB4205.
6.5. Discussion
Only a small proportion of the organic carbon produced in the
photic zone is exported to the deeper water column. The final
concentration of the total organic carbon stored in the marine sediment
is the net effect between the rate of supply, remineralization in the
water column, and preservation after burial (Müller and Suess, 1979;
Pedersen and Calvert, 1990; Jahnke and Shimmield, 1995; Rühlemann
et al., 1999; Tyson, 2001).
In the following sections we discuss the mechanisms that regulate
the organic carbon deposition/preservation in core GeoB4205. First, we
132
Chapter 6. Organic carbon: productivity and lateral advection
focus on the abundance of the supplies to the sea floor, i.e. marine
plankton productivity. Then, we discuss the factors that influence the
organic carbon degradation, i.e. sedimentation rate, organic carbon
degradation rate and oxic/anoxic bottom water conditions. Finally, we
suggest that allochthonous advections may constitute the mechanism
for TOC supply to the seafloor.
1
2
3
4
5.1
5.3
5.5
6
7
MD952042
MD952042
TOC (% wt.)
GeoB4216
2
0.50
GeoB4205
TOC (% wt.)
1.0
GeoB4205
1
1.0
GeoB4216
Insolation (W/m2)
TOC (%wt.)
0.5
500
65ºNJul
0.0
450
400
0
50
100
150
200
250
Age (kyrs)
Fig. 6.6. Total organic carbon (TOC) content of the three cores considered in the
present study. The 65ºN June insolation curve (Berger and Loutre, 1991) is shown at
the bottom. A good parallelism exists between TOC curves of cores GeoB4205 and
MD952042; by contrast, the GeoB4216 TOC profile shows an opposite pattern. TOC
peaks at terminations (I and II) are observed only in cores GeoB4216 and MD952042.
Marine isotope stages (from 1 to 7) are shown in the upper part of the panel.
133
Part III. Results
6.5.1. Marine plankton productivity
TOC values found in core GeoB4205 are too high for an openocean oligotrophic setting since they are more typical of an upwelling
eutrophic zone. Within the Cape Blanc and Benguela-Namibia
upwelling systems, TOC values range between 0.5% and 5% (Stein et
al., 1989; Martinez et al., 1999; Berger et al., 2002), whereas TOC
values usually range between 0.05% and 0.5% in a non upwelling deepsea sediment (e.g. Stein et al., 1989; Pflaumann et al., 1998). Moreover,
high proportions of TOC in core GeoB4205 parallel the minima of
boreal summer insolation. This would suggest a direct response of TOC
storage in the deep-ocean to summer insolation, which in turn is linked
to low-latitude atmospheric dynamics (monsoons/trade winds systems)
(e.g. Kutzbach, 1981). Taken together, these two observations would
lead to the assumption that TOC abundance in our core reflects the
enhanced coastal upwelling favoured by more vigorous Trade winds at
times of minimum insolation. However, this contrasts with the results
obtained in the Canary Basin (Freundenthal et al., 2002; Moreno et al.,
2002a) where several productivity proxy records match the maxima of
summer insolation. These authors show that total organic carbon and
other productivity proxies (barium and marine diatoms) increase during
glacial periods and at Terminations (I and II) and decrease during
interglacial periods. Primary productivity would be stimulated by a
reinforcement of the coastal upwelling and the associated Cape Ghir
filament as a result of enhanced Trade winds, which are governed by
the dynamics of the Azores cell.
In sharp contrast to core GeoB4216, core GeoB4205 does not
reveal any glacial/interglacial pattern and does not record any TOC rise
along Terminations I and II. Rather it shows high proportions of
organic carbon during interglacial stage 5. Our results are in agreement
with the TOC percentages found in core MD952042 off the Iberian
margin (Fig. 6.6) (Pailler and Bard, 2002), where the authors conclude
that TOC fluxes are correlated with enhanced upwelling events caused
by the intensification of the subtropical atmospheric circulation over
the North Atlantic. Few examples of productivity proxies associated
with periods of minima insolation can be found in the North Atlantic
subtropical gyre (C37 alkenone flux, Villanueva et al., 1998) and in the
Indian Ocean (authigenic metals, TOC and alkenons, Pailler et al.,
2002).
Thus, are the TOC percentages in core GeoB4205 real signs of
134
Chapter 6. Organic carbon: productivity and lateral advection
productivity? To answer this question, we determined the abundance of
G.bulloides, a typical upwelling-indicative planktonic foraminifer. We
then compared the G.bulloides abundance in our core with that of the
organic carbon in core GeoB4216 (Fig. 6.7). In core GeoB4216,
Moreno et al. (2002a) found a good correlation between organic carbon
and other productivity proxies, such as barium and marine diatoms,
especially at Terminations (I and II) and at stage 4/3 transition. Figure
6.7 shows a good match between the TOC peaks (and therefore diatoms
and barium) in core GeoB4216 and the G. bulloides proportions in core
GeoB4205 at Terminations (I and II) and at the transition from stage 4
to 3. These findings have three implications: (1) G.bulloides is a good
indicator of productivity in the study region; (2) the oligotrophic
environment from which core GeoB4205 was retrieved received signals
of coastal upwelling, probably advected by the Cape Ghir filament at
times of its maximum extension; (3) the G.bulloides records are
consistent with the results of Moreno et al. (2002a), suggesting
enhanced productivity within Terminations. However, the high rates of
G. bulloides do not match the high TOC percentages in core GeoB4205
(Fig. 6.7). This finding, together with the high absolute TOC values,
suggest that high proportions of TOC in core GeoB4205 are caused by
factors other than eutrophic conditions in surface waters.
6.5.2. Organic carbon preservation
In order to discuss possible degradation/preservation imprints on
our records, we compare the TOC fluctuations of core GeoB4205 with
the results obtained from core GeoB4216 in the Canary Basin. This
core was recovered at about 220-230 km from the coast, 150 km further
south, at a water depth that was 970m shallower than the GeoB4205
location (Table 6.1). Core GeoB4216 has a higher sedimentation rate
(SR) than core GeoB4205 (SR=4cm ky-1 for GeoB4216 and SR=2.3cm
kyr-1 for GeoB4205). As a general rule, the higher the sedimentation
rate, the better the organic carbon preservation (e.g. Müller and Suess,
1979), although a very high SR may have an adverse effect (i.e.
dilution) on TOC concentrations in the sediment (Tyson, 2001).
According to Kasten et al. (2001), burial efficiency is effective in
preventing organic carbon oxidation when SR is higher than 5-6cm kyr1
. When SR is as low as 1-2cm kyr-1, the preservation of a large amount
of organic carbon is improbable (Jung et al., 1997; Kasten et al., 2001).
Given that core GeoB4205 has a lower SR than core GeoB4216, we
could expect parallel TOC patterns in the two cores and lower values in
core GeoB4205. By contrast, the two cores occasionally show opposite
135
Part III. Results
TOC patterns, and TOC values in GeoB4205 are even higher than those
of core GeoB4216 (Fig. 6.6). Therefore, in our opinion, the difference
in the sedimentation rates does not account for the discrepancy in the
two TOC records.
1
2
3
4
6
7
GeoB4205
2.0
TOC (%wt.)
5
GeoB4205
GeoB4216
1.5
1.0
0.5
10
TOC (% wt.)
1.0
5
G.bulloides (N 103/ gr)
15
GeoB4205
GeoB4216
0.5
0.0
0
50
100
150
200
250
Age (kyr)
Fig. 6.7. TOC and G.bulloides records of core GeoB4205 contrasted with TOC values
in core GeoB4216. G.bulloides (productivity proxy) is in accordance with TOC
fluctuations in GeoB4216, especially at Terminations and transition of stages 3/4
(dotted lines in this figure). No correlation is observed between G.bulloides and TOC
in core GeoB4205. Marine isotope stages (from 1 to 7) are shown in the upper part of
136
Chapter 6. Organic carbon: productivity and lateral advection
the panel.
The presence or absence of anoxia in the bottom water column is
not considered to be the primary control of the accumulation of
organic-rich sediments in the modern oceans (Pedersen and Calvert,
1990). However, bottom water oxygen content could regulate organic
carbon preservation especially when the sedimentation rate is low
(Tyson, 2001). Regardless of other factors, organic carbon is preserved
in O2-depleted environments. Today, the well-oxygenated NADW
occupies the deep eastern tropical Atlantic between 1500 and 3500 m
water depth (Sarnthein et al., 1994), which includes the depth range of
both GeoB4205 and GeoB4216 core locations. Past changes in
thermohaline circulation altered the NADW production with the result
that, during glacial periods, the deep NADW formation weakened,
deepwater ventilation decreased and the O2-depleted Antarctic Bottom
Water (AABW) incursions were more extended. During interglacial
periods, the rates of the NADW production and deepwater ventilation
reverted to conditions that are comparable to the present (e.g. Boyle
and Keigwin, 1982; Curry et al., 1988; Duplessy et al., 1988; Curry and
Lohmann, 1985; 1990). If TOC concentrations in core GeoB4205 had
been affected by bottom water oxygenation, the organic carbon would
have been consumed by the combined effects of low sedimentation rate
and well-ventilated water masses during interglacial periods. By
contrast, we find high TOC values especially during interglacial
periods.
6.5.3. Allochthonous advection
Owing to the proximity of the continental margin and the Canary
Islands scarps, sediment gravity-flows (slides, debris flows, etc) and
turbidite deposition are dominant processes in shaping the morphology
of the NW African margin (e.g. Weaver and Kuijpers, 1983; Weaver
and Rothwell, 1987; Wynn et al., 2000). Nevertheless, these processes
did not significantly affect our core site. Few biogenic fragments,
typical fauna from the shelf environment and lithic particles were
observed under the stereomacroscope. Moreover, the sedimentation
rates remain fairly constant along the whole length of the core. The
terrestrial or marine nature of the organic matter in the study area has
been investigated by Freudenthal et al. (2002) and Meggers et al.
(2002), who concluded that this organic matter was mostly of marine
origin. High concentrations of TOC in our core may be attributed to the
existence of allochthonous sources of organic carbon at the coring site.
137
Part III. Results
Lateral advection of organic carbon comes from the eutrophic photic
zone of the nearby Cape Ghir filament (30-31ºN).
Accordingly, advection can proceed in two ways: on the sea
surface, directly advected by the filament, or through the water column,
as a near-bottom or mid-depth lateral transport. Both processes are
currently active along the NW African margin. Filament export has
been described in the Cape Blanc upwelling system (Gabric et al.,
1993), but in our case this mechanism seems unlikely because the time
of TOC storage in core GeoB4205 does not coincide with the times of
maximum extension of the filament inferred by Freudenthal et al.
(2002). The near-bottom or mid-depth lateral transport is a more
reasonable option. The finest fraction of the sediment would be
conveyed within nepheloid layers that descend the shelf-break and
continental slope and are advected hundreds of kilometres from the
source. Lateral transport has been invoked to account for lithogenic
particle fluxes in the Panama Basin (Honjo et al., 1982), phytoplankton
carbon fluxes off SE United States continental shelf (Yoder and
Ishimaru, 1989), particulate material fluxes off the European margin
(McCave et al., 2001; Fagel et al., 2004) or particulate organic carbon
fluxes off the central California continental shelf (Jahnke et al., 1990).
In the Canary Islands, recent investigations on deep-water particle
fluxes indicate that these increase with depth (Neuer et al., 1997).
According to these authors, sediment and living matter are laterally
advected from the high-productive NW African coastal upwelling zone
(especially the Cape Ghir filament region) toward the oligotrophic open
ocean through the deep water column (Neuer et al., 1997; Davenport et
al., 1999). Subsequent studies from the same area and based on a
different set of geochemical and micropaleontological proxies have
reinforced the lateral advection hypothesis and have confirmed the
Cape Ghir filament as the potential source area (Freudenthal et al.,
2001; Abrantes et al., 2002; Meggers et al., 2002; Neuer et al., 2002).
The results of the present study demonstrate that organic matter
was laterally advected also in the past. However, two issues still need
to find an explanation: it is necessary to account for the enhanced
advection of TOC during interglacial periods, which are considered to
be of lower productivity along the coast, and for the lateral advection
connected to minimum insolation.
The first issue was addressed by Bertrand et al. (1996), who
138
Chapter 6. Organic carbon: productivity and lateral advection
proposed that during interglacial periods of high sea-level stand,
production was extended onto the entire and broad shelf. By contrast,
during glacial periods of low sea-level stand, productivity, although
enhanced, affected the reduced area formed by the outer shelf and
continental slope. At our location, this explanation is even more
convincing as coastal productivity may have been displaced along the
very wide submarine Cape Ghir Plateau (Hagen et al., 1996) (Fig. 6.1),
making advection a very plausible mechanism.
As for the mobilization of the sediment stored on the shelf edge
and activation of nepheloid layers, different factors can be invoked:
wave storms action, tide effect, bottom morphology and sea-level
fluctuations. At the outer shelf, the material within bottom nepheloid
layers is maintained into suspension by the effect of the waves and
significant amounts of material are resuspended from the bed especially
during storms (McCave, 2002). In addition, the tides generating high
bottom stress drive to the resuspension and transport of sediments and
possibly induce the export of freshly deposited material to the slope and
rise (Wollast, 2002; Thomsen, 2002). Finally, major submarine
canyons may intercept the alongshore transport of sediment and divert
it to the deep-sea. Hence, the effect of wave storms and tide floods as
well as the presence of a submarine canyon may favour sediment lateral
advection. However, the link between these mechanisms and minimum
insolation need further investigations. Alternatively, the correlation
between lateral advection and minimum insolation, could be attributed
to the effect of sea-level drops occurring at times of very low northern
hemisphere summer insolation (Cutler et al., 2003). According to these
authors, sea level descended approx. 40-60 meters in less than 10kyr.
Such abrupt sea-level drops may have mobilized the sediment stored on
the shelf and induced the down-slope matter transfer.
6.6. Conclusions
In an oligotrophic environment off the NW African margin we
found high values of TOC partly associated with the minima in the
northern hemisphere summer insolation. In situ productivity, organic
carbon preservation and lateral advection were considered as potential
factors shaping TOC accumulation in the open ocean. The possible
influence of filaments from coastal upwelling and the eutrophic
conditions in the surface water were investigated through the study of
G.bulloides. This planktonic foraminifer proved to be a suitable proxy
139
Part III. Results
for primary productivity. Its variability parallels other productivity
proxies used in the same study area. Thus, it seems that eutrophic
conditions existed in this region, which is currently oligotrophic,
especially during Terminations and the stage 3/4 transition, and that the
coastal upwelling probably proceeded from the Cape Ghir filament.
However, the peaks of G.bulloides do not coincide with those of the
TOC records. Therefore, the high values of TOC are not only due to
productivity. Other factors inducing organic carbon preservation/
degradation, such as the bottom water ventilation, did not significantly
shape the organic carbon record. Substantial amounts of organic carbon
could have been laterally advected from the highly-fertile coastal zone
around Cape Ghir and then deposited into the deep ocean 200km
seaward. The correlation between lateral advection and low northern
hemisphere summer insolation suggests that sea-level changes could be
the trigger mechanism. Other factors, such as the action of waves,
storms and tide floods, associated to the presence of submarine
canyons, deserve further investigations. We lend support to the idea
that the NW African upwelling is a complex and heterogeneous
structure. Regional studies are useful in understanding the coastal
upwelling heterogeneity, which plays such an important role in the
global carbon cycle.
Acknowledgements
This study forms part of the European CANIGO project (MASTIII-CT9-0060). The
work also received the support of the MARSIBAL project (REN2001-3868-CO3-03).
We are indebted to the technicians that carried out the first run of the organic carbon
analyses at the Scientific Technical Services of the University of Barcelona, and to
those that performed the second run at the IIQAB-CSIC laboratory in Barcelona.
Special thanks are due to Joan Grimalt for allowing us to use the IIQAB-CSIC
laboratory and to Constancia Lopez and Montse Ferrer for logistic help. We are
grateful to Ana Moreno for the revision of the first draft of the manuscript. The
comments of Tim Freundenthal and Enrique Isla are also acknowledged. The paper
was significantly improved after incorporating the suggestions made by Rainer Zahn.
George von Knorring reviewed the English style of the paper.
140
Chapter 7.
A 220 KYR HISTORY OF SEA SURFACE
HYDROLOGICAL
CHANGES
IN
THE
NORTHEAST SUBTROPICAL ATLANTIC: A
PLANKTONIC FORAMINIFERA APPROACH
141
142
Chapter 7. Sea surface hydrological changes
A 220 KYR HISTORY OF SEA SURFACE
HYDROLOGICAL
CHANGES
IN
THE
NORTHEAST SUBTROPICAL ATLANTIC: A
PLANKTONIC FORAMINIFERA APPROACH
Abstract
Surface hydrological conditions of the northeast subtropical Atlantic
changed significantly along the last two glacial-interglacial cycles. The
interglacial periods were as warm as today, whereas the glacial periods
were 3-4ºC colder. Stage 2 and Stage 6 were occasionally punctuated
by further sea surface temperature drops (∼2ºC). Salinity curve shows a
see-saw pattern of gradual salinity enrichment during the glacial periods
that culminates before the beginning of terminations I and II. Both
terminations coincide with a dramatic drop toward low-salinity values.
Density estimations for the last interglacial Stage 5e are similar to
modern values during summer; by contrast density values of glacial
periods exceed the modern seasonal fluctuation. This reconstruction of
the northeast subtropical Atlantic water column structure, corroborated
by G.truncatulinoides records, point to glacial density gradient smaller
than today. This result contrasts with the northwest Atlantic, on the
other side of the Atlantic Ocean, where density gradient during the Last
Glacial Maximum (LGM) have been reported to be steeper than today.
143
Part III. Results
7.1. Introduction
Today, warm and saline subtropical waters are transported to the high
latitudes by the North Atlantic Current (NAC) (Fig. 7.1). As it moves
northward, this water cools down and becomes dense enough to sink
and form the North Atlantic Deep Water (NADW). This large-scale
Thermohaline Oceanic Circulation (THC) greatly influences global
climate through the transport of heat and salt (Broecker, 1991).
Modifications of the temperature and salinity patterns in the highlatitudes of the North Atlantic have been considered primary drivers of
major climate changes of the Pleistocene (Seidov and Maslin, 1999).
However, because the tropics are the source regions of the heat
transported within the NAC, a link between low-latitude and highlatitude climate changes can be expected. Sea surface temperature
(SST) and density changes at subtropical latitudes influence the tropical
thermocline structure and eventually the thermal characteristics of the
source waters of tropical upwelling (Luyten et al., 1983; Liu and
Pedloski, 1994; Billups and Schrag, 2000). Also, the variability in
thermocline structure and position shapes the mixed layer thickness,
which in turn affects the ocean’s heat capacity and the oceanatmosphere vapour exchange, whose alteration in the past brought
about dramatic global climate changes.
Surface hydrological conditions (temperature, salinity, density and
thermocline position) of the eastern edge of the North Atlantic
subtropical gyre for the last two glacial-interglacial cycles will be
addressed in the present study. Specific objectives deal with the
following issues:
•
•
•
144
sea surface temperature (SST) and salinity (SSS) evolution.
sea surface density conditions, presented for selected time slices
scenarios: modern time, Last Glacial Maximum (LGM),
Heinrich events and interglacial Stage 5e.
upper water vertical structure and thermocline depth changes
during the LGM.
Chapter 7. Sea surface hydrological changes
NAC
GS
CC
NEC
Fig. 7.1. Surface circulation in the North Atlantic Ocean (simplified). GS: Gulf
Stream; NAC: North Atlantic Current; CC: Canary Current; NEC: North Equatorial
Current.
To accomplish these objectives,
1. downcore records of SST and SSS were obtained combining
planktonic foraminiferal census counts and Globigerinoides
ruber (white) oxygen isotopes analyses.
2. potential density values were calculated accordingly (i.e. from
SST and SSS).
3. thermocline depth was instructed from the variable proportions
of the planktonic foraminifer Globorotalia truncatulinoides,
which has a wide application as a paleo-indicator of vertical
water column structure, mixed layer thickness and thermocline
base depth1 (Lohmann and Schweitzer, 1990; Mulitza et al.,
1997; Martínez, 1997).
1
The use of G.truncatulinoides as paleoindicator is due to its ecological behaviour.
The right-coiling form of this foraminifer spans the first 600 m of the water column
during its life cycle. The juvenile specimens travel toward the surface and there
increase their size by encrusting. A strong vertical density gradient in the water
column is thought to hamper their migration to the surface (Lohmann and Schweitzer,
1990; Mulitza et al., 1997).
145
Part III. Results
7.2. The oceanographic setting
The region is currently bathed by the cold Canary Current (36.5psu;
19º-20ºC at our coring site) (Stephens et al., 2002; Boyer et al. 2002).
North Atlantic Central Water (NACW), which ranges from 150m to
about 700m (Knoll et al., 2002), is characterized by a nearly straight
line in the T-S diagram, connecting the T-S point 7°C, 35.0psu with the
points 18°C, 36.7psu (Emery and Meincke, 1986; Poole and Tomczak,
1999; Tomczak and Godfrey, 2003). The mid-depths of the region also
receive the influence of Mediterranean Outflow Water (MOW) that
leaves the Strait of Gibraltar with a temperature of about 13.5°C and a
salinity of 37.8psu, but quickly spreads isopycnally across the ocean,
mixing gradually with deep water and crossing the North Atlantic as a
salinity and temperature anomaly (Tomczak and Godfrey, 2003). At
deeper depths, North Atlantic Deep Water (NADW) has salinity of
about 34.9psu and temperature ranges from 1.5º to 4ºC. (Fig 7.2).
Fig. 7.2. Temperature-Salinity plot showing the characteristics of the principal water
masses of the study area (Pfannkuche et al., 2000). Points a and b indicate the two
extreme values that define NACW (see text). NACW: North Atlantic Central Water;
MOW: Mediterranean Outflow Water; NADW: North Atlantic Deep Water.
146
Chapter 7. Sea surface hydrological changes
7.3. Material and methods
The material used for the present study (Fig. 7.3) is:
•
•
•
Gravity core GeoB4205 retrieved off the Moroccan margin at
32º10.8’N, 11º38.9’W, 3296m water depth. This location
belongs to the oligotrophic section of the Northeast Atlantic
subtropical gyre and does not receive additional influences of
the NW African coastal upwelling (see Chapter 6). Stable
oxygen isotope data on G.ruber (white) are originally from
Bozzano et al. (2002) (see Chapter 5, Section 5.3.4).
Core-tops of multicorer cores GeoB4202, (32.48ºN 13.66ºW,
4289 m) and GeoB4204 (32.02ºN 11.95ºW, 3213m), from the
Moroccan margin, are additionally used for SST calibration.
Core MD952042 (Cayre et al., 1999; Eynaud et al., 2000;
Shackleton et al., 2000), from the Iberian margin, off Portugal
(37.8ºN 10.17ºW, 3146m), is used for comparison. The
common stratigraphic frame is based on the age model from
Shackleton et al. (2000) and was performed through graphical
correlation using AnalySeries software (Paillard et al., 1996).
45º
40º
MD952042
35º
GeoB4202
GeoB4205
GeoB4204
30º
25º
20º
10º
0º
Fig. 7.3. Location of the four sediment cores used in this study. GeoB4202,
GeoB4204, GeoB4205, MD952042. Note that the location of cores GeoB4205 and
GeoB4204 are very proximate so that they are represented by one spot only.
147
Part III. Results
7.3.1. Planktonic foraminifera
Counts of the planktonic foraminiferal assemblage on core
GeoB4205 were conducted on samples collected every 5cm; a
minimum of 300 specimens was identified within the >150 µm size
fraction in all cases. Splitting and counting procedures are described by
Pflaumann et al. (1996). Planktonic foraminiferal species and
morphotypes follow the taxonomy concepts of Saito et al. (1981) and
of Kennett and Shrinivasan (1983).
7.3.2. Sea surface temperature
Sea surface temperature (SST) calculations are based on 27 species
after
combining
right-coiling
N.pachyderma
and
N.pachyderma/N.dutertrei intergrades, right and left-coiling
G.truncatulinoides. The contribution of three rare species (T.humilis,
G.uvula and S.dehiscens) was omitted (Table 7.1).
Three approaches were applied for temperature estimations:
1. MAT (Modern Analog Technique; Prell, 1985).
2. SIMMAX, a variant of the analog technique based on the
similarity index (Pflaumann et al., 1996, modified equation
version 28).
3. RAM (Revised Analog Technique; Waelbroeck et al., 1998).
The reference data set for the modern surface faunal assemblage is an
enlarged and updated database compiled by Pflaumann et al. (1996),
which contains 916 core tops in the Atlantic Ocean, embracing 56ºS 86ºN latitude range. Boreal hemisphere warm season (July-September)
and cold season (January-March) temperature values were estimated.
Reported average standard errors are in the range of ±1.0 ºC for
temperature estimations made at 0-75m water depth, but they can
increase to more than ±2ºC when estimations are done at 0-10m
(Pflaumann et al., 1996). The mean annual values were calculated as
the average of the four seasonal temperatures (summer, autumn, winter
and spring).
148
Chapter 7. Sea surface hydrological changes
GENERA
SPECIES
Globigerina
bulloides
calida
falconensis
pachyderma (right)
p/d intergrades
pachyderma (left)
quinqueloba (right)
quinqueloba (left)
siphonifera
(aequilateralis)
rubescens
conglobatus
ruber (pink)
ruber (white)
tenellus
trilobus trilobus
trilobus sacculifer
Neogloboquadrina
Turborotalita
Globigerinella
Globigerinoides
GENERA
SPECIES
Globigerinita
glutinata
uvula*
crassaformis
hirsuta
inflata
truncatulinoides
(right)
truncatulinoides
(left)
scitula
humilis*
dutertrei
universa
obliquiloculata
dehiscens*
Globorotalia
Turborotalia
Neogloboquadrina
Orbulina
Pulleniatina
Sphaeroidinella
Table 7.1. List of the planktonic foraminiferal species used for SST estimations in
sediment core GeoB4205. Species labelled with * were omitted in calculations.
7.3.3. Salinity and density
Foraminiferal δ18O (δ18Oc) reflects sea surface temperature and
isotopic composition of the seawater; on the other hand, seawater δ18O
(δ18Ow) depends on the combined effects of evaporation and fresh
water input (E-P balance) plus the global effect of ice volume changes
occurred along the geological times. Salinity and seawater isotopic
composition are strictly correlated because they are both influenced by
local hydrological regime and by the global ice-volume effect during
the glacial-interglacial transition. The linear relationship between these
two parameters depends on the local climatic regimes (GEOSECS,
1987; Duplessy et al., 1991).
Sea surface salinity (SSS) and water masses density have been
reconstructed through the estimation of past values of δ18Ow, using the
δ18O of G.ruber (white) and the faunal-derived SSTs. The modern SSSδ18Ow relationship was derived from the Global Seawater Oxygen- δ18O
Database (Schmidt, 1999a), selecting a region limited between 10º50ºN and 0º-60ºW (Fig. 7.4) (values concerning the Mediterranean
basin were deleted from the data set) and obtained equation 1 as follow:
149
Part III. Results
18
SSS=34.4+1.58 δ Ow
(in SMOW2)
(1)
There are several paleotemperature equations linking sea-surface
temperatures (SST) and seawater oxygen isotope (δ18Ow) (see Bemis et
al., 1998 for a review). In this study, we used the equation from Erez
and Luz (1983) generated from laboratory-grown specimens of G.
sacculifer in the temperature range of 14º-30ºC (Eq.2).
T=17.0-4.52(δ18Oc – δ18Ow)+0.03(δ18Oc – δ18Ow)2
(2)
We used this equation because both G.sacculifer and G.ruber are
spinose, symbiont –bearing foraminiferal species, with comparable
ranges of temperature and salinity tolerance, dwelling at mixed layer
depths in the tropical to subtropical regions (Bijma et al., 1990).
37
36
Salinity (ppt)
35
34
33
32
31
30
-2
0
-1
1
18
Seawater δ O (‰)
18
Fig. 7.4. Modern salinity- seawater δ O relationship for the region limited between
10º-50ºN and 0º-60ºW (Schmidt et al., 1999). (See www.giss.nasa.gov/data/
o18data/). The correlation is based on 129 samples and has a linear coefficient
r2=0.815.
2
SMOW: standard mean ocean water
150
Chapter 7. Sea surface hydrological changes
In our case, δ18Oc is δ18OG.ruber, measured in Pee Dee Belemnite
(PDB) scale (see Chapter 5, Section 5.3.4). As δ18Ow on SMOW =
δ18Ow on PDB + ε, an adjustment of ε =0.27 is required (Hut, 1987).
Finally, we solve equation (2) and calculate the value of δw as follow:
δ18Ow=0.27+ δ18Oc +75.34+ 5108.45 + 33.34T
(in SMOW)
(3)
This equation was obtained after laboratory experiments and is
calculated for an ideal foraminiferal species. In reality, each foraminifer
calcifies its shell during a preferred season of the year (optimum
temperature range) and in general not in equilibrium with ambient sea
water δ18Ow and temperature (Duplessy et al., 1991; Wolff et al.,
1999a). For this reason, the so called "vital effects" or "habitat effects"
have to be taken into consideration through a core-top calibration for
the selected foraminiferal species. Wang et al. (1995) found that in the
low-latitude Atlantic, δ18O values of G.ruber correspond to summer
SST and SSS of the upper 50m of the water column, although
foraminifera growth season is the whole year. We chose therefore two
calibration equations from Wang et al. (1995) as follow:
Tiso=3.147+0.963Tm
(4.1)
Tiso=7.020+0.833Tm
(4.2)
calculated for summer season and 0-50m water depths
and
averaged on an annual basis and 0-50m water depths
where:
Tiso is the calculated isotopic temperature and
Tm is the actual temperature.
Equation (3) is finally transformed into:
δ18Ow =0.27+ δ18Oc +75.34+ 5108.45 + 33.34(3.147+ 0.963Tm )
and
δ18Ow =0.27+ δ18Oc +75.34+ 5108.45 + 33.34(7.020+ 0.833Tm )
(5.1)
(5.2)
There is still the ice-volume global contribution (δ18Oice) that has to
be subtracted from the δ18Oc. In the literature, a number of δ18Oice
curves have been produced (e.g. Labeyrie et al., 1987; Vogelsang,
1990; Waelbroeck et al., 2002; Siddall et al., 2003). We use the curve
obtained by Vogelsang (1990), partly derived after that of Labeyrie et
151
Part III. Results
al. (1987), and the curve calculated by Waelbroeck et al. (2002). Each
of these approaches is based on assumptions, such as the maintenance
of the deep water temperature through geological time or the linear
relationship between δ18Oice and sea level change (δ18Ow enrichment of
1.1‰ corresponds to 130 meters of sea-level drop). However, they both
fulfil the global mean δw enrichment during the LGM to be ∼1.0±0.1‰
(Schrag et al., 1996; Schrag et al., 2002).
Finally, salinities should be calculated applying the equation (1) to
the reconstructed δ18Ow. This approach brings the assumption that the
modern SSS- δ18Ow relationship is applicable also to the past (SchäferNeth, 1998; Schmidt, 1999b). Therefore, we first describe the
hydrological changes inferred only from δ18Ow. Then, we calculated sea
surface salinity applying the SSS- δ18Ow equation proposed by SchäferNeth (1998).
 (δ 18Ow − ∆ ice ) 
 +∆S∆H
SSS= SSSrec0 1 −


δ 18O f


(6)
where SSSrec0 is the salinity factor of equation (1), δ18Ow is the output
of equations (5), ∆ice is the global ice volume effect, ∆S∆H is the global
salinity increase due to sea-level change and δ18Of is the freshwater
oxygen isotope ratio, which globally ranges from -10‰ (recent rain) to
below -40‰ (precipitation over glacial ice caps).
The original equation of Schäfer-Neth (1998) was designed to resolve
salinities during the LGM. As we apply the equation downcore, we
introduce some modifications: instead of ∆ice =1, we used both
Vogelsang and Waelbroeck δ18Oice curves; instead of ∆S∆H=1.07psu,
we calculated ∆S∆H =∼0.01*sea-level change (m) (Adkins et al., 2002),
the curves of sea-level changes are from Vogelsang (1990) and
Waelbroeck et al. (2002). As for δ18Of, we use -35‰ for glacial
periods, as originally suggested by Schäfer-Neth (1998). Salinity values
for Substage 5e were reconstructed applying equation (1). Errors
associated to salinity reconstructions range from ±0.3‰ (Rostek et al.,
1993) to ±0.6÷1.8‰ (Schmidt, 1999b).
Finally, density values were obtained solving the Equation of State of
Sea Water with the reconstructed values of temperature and salinity
152
Chapter 7. Sea surface hydrological changes
(Fofonoff and Millard, 1983). The online seawater density calculator
was used.3
7.4. Results
7.4.1. Planktonic foraminifera
In core GeoB4205, six species dominate the foraminiferal fauna,
representing 63%-92% of the total assemblage: N.pachyderma (rightcoiling
form),
G.inflata,
G.ruber
(white),
G.bulloides,
G.truncatulinoides (both left and right –coiling forms) and G.trilobus.
N.pachyderma (right-coiling) percentages, ranging from 5% to 65%,
distinguish the glacial (>27%) from the interglacial periods (<27%).
Maximum and minimum values characterize Stage 6 and 5e
respectively (Fig. 7.5a). Polar species such as N.pachyderma (left
coiling) and T.quinqueloba, which today are virtually absent from the
faunal association, occur with small percentages (2-3%) during Stages
2-4 and 6, often, albeit not always, associated with N.pachyderma
(right-coiling) (Fig. 7.5b-c).
G.inflata, ranging from 8% to 52%, seems not to follow any special
tendency or any glacial-interglacial pattern (Fig. 7.5d). G.bulloides and
its correlation to enhanced nutrient-rich conditions have already been
described in Chapter 6.
As for the subtropical species, values of G.ruber (white) greater than
20% are concomitant with peaks of G.trilobus and of G.ruber (pink)
during the Holocene and the warm Substages 5e, although relative high
abundances characterize also part of Stage7 (Fig 3e-g). Right-coiling
G.truncatulinoides (0-19%) prevails over the left-coiling variety (012%). Right-coiled specimens increase (9-10%) during glacial periods
(LGM, Stage 3 and Stage 6) and reaches even higher values (12-15%)
during the interglacial stadials 5.2. and 7.2. Specimens of
G.truncatulinoides left-coiled increase at the substages 5.1, 5.4, 6.4 and
7.2 (Fig. 7.5h).
3
See http://gaea.es.flinders.edu.au/~mattom/Utilities/density.html
153
Part III. Results
1
3
4
5
6
MIS
7
(a)
60
N.pachyderma (d)
2
50
40
30
20
3
2
0
N.pachyderma (s)
POLAR SPECIES
(b)
1
G.quinqueloba
10
0
2
(c)
1
0
(d)
40
G.ruber (white)
30
0
(e)
20
10
3
(f)
1
10
G.ruber (pink)
2
15
WARM SPECIES
0
G.trilobus
G.inflata
20
0
5
(g)
20
0
15
5
G.truncatulinoides
10
(h)
0
0
50
100
150
200
250
300
350
400
450
500
depth (cm)
Fig. 7.5. Planktonic foraminiferal association in sediment core GeoB4205. (a)
N.pachyderma (r); (b) T.quinqueloba; (c) N.pachyderma (s); (d) G.inflata; (e) G.ruber
(white); (f) G.ruber (pink); (g) G.trilobus; and (h) G.truncatulinoides, right-coiled
bold line, left-coiled thin line. MIS stand for Marine Isotope Stages.
154
Chapter 7. Sea surface hydrological changes
7.4.2. Sea surface temperature
Results of SST estimations are described first for the summer season,
then for the winter season. Description is focussed on the glacialinterglacial contrast; thus, first, values for core tops and interglacial
Stage 5e are reported, then values for glacial Stages 2-4 and 6 are
described. Finally, SST changes at Terminations are depicted.
Summer Season (July-September).- In GeoB4205, temperature
estimation of the core top is ∼23ºC (MAT, RAM, and SIMMAX) and
results from the last interglacial Substage 5e show temperature values
equivalent to those computed for the core tops (Fig. 7.6).
Temperature estimation of the core-top sample exceeds the summer
mean temperatures (21.3 ºC) depicted from World Ocean Atlas 2001
(Stephens et al., 2002) by more than 1.5ºC (Fig. 7.7). This may occur
for three reasons: (1) because the core top sample corresponds to the
Holocene Climatic Optimum (4-7kyr before present, when
temperatures were 2ºC higher than present), (2) for an error of the
method, or (3) for errors associated to the measured value. The core top
sample of gravity core GeoB4205 (3cm) actually dates back to 7kyr
B.P (see Chapter 5). However, SST results are confirmed by
calculations made on the surface samples of two neighbouring
multicorers (GeoB4202 and GeoB4204), which are designed to
preserve the sediment-water interface, without any loss of surface
sediment. Calculations of modern temperatures are within the range of
the ±2ºC standard error of the method reported for SST estimations
made at 0-10m (see section 7.3.2). Deviations between estimated and
measured SST may also derive from the lack of data in the reference
data set, which forces to select a pool of ten best analogs located to the
south of the study region, i.e. in the tropical Atlantic between 20ºN and
28ºN latitude (Table 7.2). Finally, errors associated to the measured
values of SST depicted from NOAA ATLAS could range ±1ºC. During
a cruise performed in the study region in late October (Pfannkuche et
al., 2000), the in situ measured SST was slightly higher than that
reported by NOAA Atlas (Fig. 7.7).
As for the glacial periods, temperature estimations for Stage 6 are
colder than for Stages 2-4, ranging from 20ºC to 21ºC in Stages 2-4 and
from 18ºC to 21ºC during Stage 6. However, during Stage 2, two events
(28cm and 53cm) register further temperatures drops by 1.5º-2ºC. Both
terminations I and II record SST gradients of about 3-4ºC (Fig. 7.6).
155
Part III. Results
24
1
2
3
4
7
6
5
23
SST(ºC)
22
21
20
19
18
17
MAT
16
24
23
22
20
19
SST (ºC)
21
18
17
RAM
16
24
23
SST (ºC)
22
21
20
19
18
17
SIMMAX
16
0
50
100
150
200
250
300
350
400
450
500
Depth (cm)
Fig. 7.6. Sea surface temperature (SST) reconstructions for sediment core GeoB4205
at the warm season (July- September) obtained from the three approaches
implemented: MAT, RAM and SIMMAX. The two solid lines indicate SST drops at
28cm and 53cm. Grey bars highlight the glacial Stages 2, 4 and 6.
156
Chapter 7. Sea surface hydrological changes
Analogs
-V16-37
+A180-39
+V30-67
+MIKS07
+V27-162
+MIKS14
+V30-68
+V30-58
+V29-170
+V30-65
Latitude
-21.33
25.83
26.60
20.53
34.20
28.88
26.91
25.71
22.46
25.91
Longitude
-8.95
-19.30
-15.15
-29.45
-16.86
-25.40
-19.13
-20.880
-20.06
-19.05
Winter temp.
21.21
19.75
18.89
22.14
17.08
19.68
19.5
20.12
19.9
19.67
Summer temp.
24.15
22.7
21.6
24.75
22.05
23.54
22.65
23.11
23.02
22.63
Table 7.2. The best 10 core analogs selected for calculating SST of the core top (3cm).
Note that the majority of the cores are from warm tropical environments. For this
reason, estimated modern temperatures are higher than those extracted from the
NOAA Atlas (Stephens et al., 2002).
Winter season (January-March) (Fig. 7.8).- Both in core GeoB4205
and in the two neighbouring cores GeoB4202 and GeoB4204, SST
estimations are about 19.5º-20ºC. This value exceeds the mean winter
modern SST observations (17.5ºC) depicted from NOAA Atlas
(Stephens et al., 2002) by 1.5º-2ºC (Fig. 7.7). Results from the last
interglacial Substage 5e show temperature values (19ºC) slightly lower
than those computed for the core tops.
As for the glacial periods, temperature estimations of glacial Stage 6
are lower than those of Stage 2-4. SST records range from 15º to 16º
during Stage 2-4 and from 11º-12ºC to 16ºC during Stage 6. Two
temperature drops by 2-3ºC are found during Stage 2 at 28cm and 53
cm. Terminations I and II show SST falls of 4º-5ºC.
157
Part III. Results
23
Modern Values
Summer mean = 21.3ºC
22
21
SSS
36.65
Annual SST mean= 19.4ºC
Salinity
36.60
20
19
Winter mean = 17.5ºC
18
Temperature (ºC)
SST
17
36.55
Annual SSS mean = 36.5
16
36.50
36.45
Dec Jan
Feb Mar Apr May Jun
Jul
Aug Sep
Oct Nov Dec
Fig. 7.7. Seasonal temperature and salinity evolution at the coring site (data from
NOAA Atlas, Stephens et al., 2002; Boyer et al., 2002). Diamonds (on the upper right
side of the panel) indicate temperatures depicted during a cruise performed in the
study region in late October (Pfannkuche et al., 2000).
As a general overview of the SST reconstructions performed in this
study, two fundamental results were achieved:
1. the three methods implemented (MAT, SIMMAX and RAM)
give comparable results. However, on occasion (e.g. 93-98cm,
113cm, 188cm, 453cm) RAM estimations provide considerable
temperature drops that don’t find a parallel in the other two
methods. From here on, calculations and discussion are based
on MAT and SIMMAX only.
2. the annual average SST evolution (Fig. 7.9c), show that the
interglacial periods were as warm as today, whereas the glacial
periods were 3-4ºC colder. Occasional further SST drops by
∼2ºC characterized Stage 2.
158
Chapter 7. Sea surface hydrological changes
20
1
2
3
4
5
6
7
19
18
SST (ºC)
17
16
15
14
13
12
11
MAT
10
20
19
18
17
15
14
SST(ºC)
16
13
12
11
RAM
10
20
19
18
SST (ºC)
17
16
15
14
13
12
11
SIMMAX
10
0
50
100
150
200
250
300
350
400
450
500
Depth (cm)
Fig. 7.8. Sea surface temperature (SST) reconstructions for sediment core GeoB4205
at the cold season (January-March) obtained from the three approaches implemented:
MAT, RAM and SIMMAX. The two solid lines indicate SST drops at 28cm and
53cm. Grey bars highlight the glacial Stages 2, 4 and 6.
159
Part III. Results
7.4.3. Salinity and density
Seawater δ18O (δ18Ow) (Fig. 7.9d) is calculated combining the SST
estimates (SIMMAX and MAT) (Fig. 7.9c) and the δ18O measurements
of G.ruber (Fig. 7.9a). Subtraction of the ice volume effect estimated
by Vogelsang (1990) and by Waelbroeck et al. (2002) results in the
isotopic composition of seawater regulated only by local hydrological
changes in precipitation and evaporation (P-E) and by local surface
advection (Duplessy et al., 1992). We did not observe any significant
difference between the use of Vogelsang’s curve and of Waelbroeck’s
curve (Fig. 7.9b). We computed δ18Ow both for the summer season (eq.
5.1) and for annual average (eq. 5.2), finding no other difference than a
systematic offset of 0.1-0.2‰ between the two, being the records of the
summer season higher than those of the annual average.
Seawater δ18O (annual average values).-In core GeoB4205, seawater
δ18Ow core top reconstruction is 1.45‰, which is similar to the δ18Ow
values of last interglacial Stage5e (∼1.5‰). As for glacial periods,
δ18Ow values for Stage2 (∼2.2‰ in average) are higher than for Stage6
(1.8‰ in average). Along Terminations, δ18Ow values decrease by 0.9‰
(Termination I) and by 0.6‰ (Termiantion II). Both terminations
culminates in δ18Ow values (1.2‰-1.3‰) that are lighter than the core
top value (1.45‰). (Fig. 7.9d).
Salinity (annual average values).-To estimate the absolute values of
salinity variations in core GeoB4205 (Fig. 7.9e), we used annual δ18Ow
(both Vogelsang and Waelbroeck curves). Salinity of core top and
Eemian samples were computed by introducing δ18Ow values into the
modern SSS- δ18Ow relation (eq. 1).
Estimated core-top reconstruction (36.7‰) well agree with the
modern values (36.5-36.7‰) observed in the region (Wefer and Müller,
1998, p: 93; Pfannkuche et al., 2000, p: 178-179) and with the salinity
values calculated for the interglacial Stage 5e (36.7‰). Salinities of the
other downcore samples were calculated by using the formula of
Schäfer-Neth (1998) (eq. 6). The resulting curve shows a pattern of
gradual salinity enrichment during the glacial periods that culminates
before the terminations I and II. Glacial LGM salinity values (37.5‰ 37.8‰) were higher than today by 0.8-1.1psu. During glacial Stage 6,
salinity values are slightly lower and do not exceed 37.6‰. The glacial
“high-salinity” pattern is interrupted by short salinity drops of ∼0.6‰
in amplitude at 23cm and 58cm. Both terminations coincide with a
160
Chapter 7. Sea surface hydrological changes
dramatic drop toward low-salinity values, with ∆S=1.3‰ (Termination
I) and ∆S=0.9‰ (Termination II) (Fig.7.9e).
0
2
3
4
5
6
7
(a)
1
0
2
(b)
0.4
22
20
0.8
(c)
1.2
18
16
14
2.5
(d)
2.0
1.5
Salinity (‰)
38.0
37.5
1.0
(e)
δ18O seawater (SMOW)
SST (ºC)
MIS
RSL (‰)
δ18O (G.ruber) (PDB)
1
37.0
36.5
36.0
35.5
0
25
50
75
100
125
150
175
200
Age (kyr)
Fig. 7.9. Downcore salinity reconstruction in core GeoB4205. (a) G.ruber stable
oxygen isotopes; (b) Relative Sea Level (RSL) in ‰, Vogeslang’s curve (cross) and
Waelbroek’s curve (dot); (c) Annual average sea surface temperatures (SST),
SIMMAX (open circle) and MAT (solid rhombus); (d) seawater oxygen isotopes,
derived from SIMMAX (open circle) and by MAT (solid rhombus); (e) Salinity,
derived by SIMMAX (open circles) and by MAT (solid rhombus). Dotted lines
indicate Terminations 1 and 2.
161
Part III. Results
Estimated values in core GeoB4205 are close to the glacial sea
surface salinities (37.25-37.75‰) calculated by Duplessy et al. (1991)
and to the records of the low-latitude Atlantic (∼37.8‰ at LGM)
obtained by Wang et al. (1995).
Density values.- In core GeoB4205, density estimation of the core top
is 25.8σ. This value is within the modern seasonal variation, which
ranges from a minimum of 25.4σ in September to a maximum of 26.6σ
in February (Fig.7.10A). Density estimation of the last interglacial
Stage 5e is 25.4σ, i.e. similar to modern values during summer (Fig.
7.10B).
Estimated density values of the LGM (∼27.5σ) exceed the modern
winter values by 0.9kg m-3. This estimation is consistent with the
values obtained in the north subtropical Atlantic by Billups and Schrag
(2000), where LGM surface water density was found to be 0.75kg m-3
to 1.5kg m-3 higher than today.
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
-2
GLACIAL
tion
25
ctua
al flu
n
o
s
Sea
top 26
Core
26
5e
27
LGM
27
5d
28
H
29
NACW
MOW
29
28
NACW
30
30
NADW
31
34.5 35.0 35.5 36.0 36.5 37.0 37.5 38.0
Salinity (psu)
35.0
36.0
37.0
38.0
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
-2
Temperature (ºC)
Temperature (ºC)
TODAY
39.0
Salinity (psu)
Fig. 7.10. Temperature-Salinity-Density plot for the study region. A). Today main
water masses (NACW, MOW and NADW) profiles are from Pfannkuche et al.
(2000). Seasonal fluctuations are calculated from NOAA Atlas (Boyer et al., 2002;
Stephens et al., 2002). Core top SST and SSS are reconstructed in this study. B)
Water masses profiles for a glacial scenario: LGM, Stage5d (cold spell) and Heinrich
type events (H). Today profile (grey) is shown for reference. Interglacial Substage 5e
samples (green crosses) are also shown. See text for the assumptions made on NACW
during LGM.
162
Chapter 7. Sea surface hydrological changes
7.5. Discussion
7.5.1. Heinrich fingerprints
In sediment core GeoB4205, two drops in temperature of 2ºC,
accompanied by salinity decreases of ∼0.6‰, are observed at 28cm and
53cm (see Section 7.4.2 and 7.4.3). These episodes are placed in Stage
2 at ∼16kyr and ∼26kyr (Fig.7.11). Other three similar episodes, of less
magnitude, characterize Stage 3 at ∼35ky, 39kyr and ∼46ky (Fig. 7.11).
All these episodes are here postulated to represent the fingerprints of
the Heinrich events H1, H2, H3, H4 and H5, whose description can be
found in Bond et al. (1993), Manighetti et al. (1995), Zhao et al. (1995),
and Chapman et al. (2000) among others.
North to the study area, off the Iberian margin (southern Portugal and
Gulf of Cadiz), evidence of Heinrich events have been recognized
(Lebreiro et al., 1996, 1997; Zahn et al., 1997; Cayre et al., 1999; Bard
et al., 2000; Cacho et al., 2001) by temperature decreases of 5-10ºC
(e.g. Lebreiro et al., 1997; Zahn, 1997; Cayre et al., 1999). To the south
of the study region, off the African margin, in the subtropical eastern
Atlantic, SST drops of 2.5ºC in magnitude have been observed
(Kudrass and Thiede, 1970; Kudrass, 1973; Englinton et al., 1992) and
later associated with meltwater Heinrich events (Wang et al., 1995;
Zhao et al., 1995; Pflaumann et al., 1998). In addition, Wang et al.,
(1995) reported salinity lows, with anomaly of 0.35‰, during Heinrich
events H1 and H2.
In order to demonstrate that SST and SSS drops in core GeoB4205
are fingerprints of meltwater discharges at time of Heinrich events,
sediment core MD952042 from the Iberian margin was compared to
sediment core GeoB4205. In core MD952042, Heinrich events have
been firmly recognised by magnetic susceptibility peaks and lithogenic
grains accumulation (Cayre et al., 1999) and age model has been
revised by Shackleton et al. (2000). We adjusted our age model tuning
GeoB4205 calcium record to the CaCO3 curve available for core
MD952042 (Pailler and Bard, 2002) (see Chapter 6). SSTs drops in
core MD952042 (Fig. 7.11a) match SST decreases in our core (Fig.
7.11b). These drops are well pronounced for H1 and H2 (2ºC±0.5º), and
less resolved for H3, H4, and H5 (<1ºC). Furthermore, peaks of
foraminiferal polar species, such as N.pachyderma (s) and
T.quinqueloba, parallel the most significant temperature and salinity
drops (Fig. 7.11c-f).
163
Part III. Results
20
MD952042
(a)
YD (?)
H1
H2
H3 H4
H5
18
H6
16
24
14
23
(b)
23
SST (ºC)
24
12
SST (ºC)
22
22
10
21
21
8
20
20
38
19
19
37
pachyderma (d)
36
40
(d)
35
0
1
0
pach. (s)
2
(e)
quinqueloba
Salinity (per mil)
(c)
18
2
(f)
1
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
Age (kyr)
Fig. 7.11. Correlation between core MD952042 and core GeoB4205 to infer the
occurrence of Heinrich events in the study region. (a) core MD952042: SST
(Shackleton et al., 2000). (b-f) core GeoB4205: several proxies (this study) (b) SST
(c) salinity (SIMMAX, open circle; MAT, solid rhombus) (d-f) cold water planktonic
foraminifera.
164
Chapter 7. Sea surface hydrological changes
These results show that the concurrent temperature and salinity
decreases in core GeoB4205, accompanied by parallel polar species
peaks, which are correlated with Heinrich events occurring in the
northern core MD952042, may well be the expression of cool and fresh
surface water advections transported by the Canary current at times of
massive meltwater discharges in the North Atlantic. These conclusions
seem more robust for Heinrich events H1 and H2, whereas the other
three events (H3, H4 and H5) are not well resolved due to the
limitations of low sedimentation rate in core GeoB4205 (2.5cm/kry)
(Bozzano et al., 2002; see Chapter 5) and of the effects of bioturbation.
7.5.2. Seawater δ18O and salinity
Surface salinity depends on the extent to which freshwater is added,
by precipitation and run-off, or removed by evaporation. Also,
advection and mixing processes can influence the total salinity budget.
In the study region, evaporation and precipitation are mostly driven by
the prevailing wind regimes associated with the low-latitude insolation
(e.g. Rossignol-Strick, 1983; Kutzbach and Street-Perrott, 1985; Pokras
and Mix, 1987).
Continental runoff is of secondary importance because our core site
lacks of relevant rivers nearby. The most important source of advection
is the entrance of the fresh and cold Canary current that brings its
hydrological imprint in the region. Finally, mixing processes with the
subsurface water NACW, associated with strong African coastal
upwelling events, usually make salinity to drop by 0.2-0.3 psu (Knoll et
al., 2002).
According to the 220kyr-reconstruction of SSS performed in core
GeoB4205 (Fig. 7.9e), waters changed from more saline to less saline
than today.
Waters were more saline than today during glacial periods and
especially during the LGM. This scenario can be explained by a
sluggish Canary current advection, as suggested by Wang et al. (1995),
who found high glacial salinities in the tropical Atlantic exceeding
Holocene values by 0.5-1.0‰. Furthermore, high salinities during
LGM may be imputed to the arid glacial conditions, as documented by
several authors (e.g. Sarnthein, 1978; Sarnthein et al., 1982; Pokras and
Mix, 1985; Hooghiemstra et al., 1992).
165
Part III. Results
In the study region, waters were less saline than today in two periods:
at the Terminations I and II, and occasionally in Stage 2 and in the
early Stage 6 (∼180ky). Low salinities during terminations I and II may
have resulted from the ascending colder and less saline NACW.
Regional studies have in fact demonstrated that upwelling-driven
productivity was enhanced during terminations (Freundhental et al.,
2002; Moreno et al., 2002a). If this was the case, we should expect
concomitant salinity and temperature drops. By contrast, decreasing
salinities match increasing temperatures. Rather, low salinities can be
ascribed to the enhanced humid conditions that characterized NW
Africa during deglaciations. Wang et al. (1995) have observed extreme
low salinity values (by up to 1‰ lower than today) at 5.5-9 kyr B.P, at
the same time as the African humid period and the monsoon activity
(Street and Grove, 1979; Lézine and Casanova, 1991; Kutzbach, 1981;
Kutzbach and Liu, 1997; deMenocal et al., 2000a,b; Rohling et al.,
2002).
Low salinities in Stage 2 and early Stage 6, further accompanied by
SST drops, suggest a direct influence of the oceanic circulation, as
already mentioned in the previous section 7.5.1. Low-salinity signals of
the meltwaters surges are advected into the middle and low latitudes of
the eastern Atlantic by the Canary current (Wang et al., 1995; Bard et
al., 2000; Chapman et al., 2000), which, at glacials, was under the polar
front influence. Henrich-type events in Stage 6 have been widely
described off the Iberian Margin (Cayre et al., 1999; Eynaud et al.,
2000; Moreno et al., 2002).
7.5.3. Structure of the upper water column: LGM snapshot
Today, the permanent thermocline in the study region is bathed by
the NACW in the upper section and influenced by mixing processes
with Mediterranean Outflow Water (MOW) cells at deeper depths (Fig.
7.12). The density gradient between surface and subsurface waters is
well pronounced during summer and less pronounced during winter
(Fig. 7.10a). This is reflected in the enhanced surface layer mixing that
in fact occur during the winter season due to the action of cold and
strong winds.
Glacial scenarios differed greatly from today due to the concomitant
change of water temperature and salinity values which affected the
overall water density distribution. As for the temperature changes,
166
Chapter 7. Sea surface hydrological changes
during the LGM, surface waters were ∼4ºC colder than today, as shown
by results on core GeoB4205 (see section 7.4.2). As for the scenario
about glacial changes of the subsurface waters, we do no have direct
data of the study area. Slowey and Curry, (1992) have performed LGM
reconstruction in the western subtropical Atlantic and have reported
that thermocline water temperatures were colder than today by 1º-4ºC.
Modelling simulations for the whole North Atlantic have confirmed
these findings (Paul and Schäfer-Neth, 2003) and the results from a
recent study that combines paleoclimatic data and modelling efforts
have pointed to a slight thermocline waters cooling (∼1ºC) in the
northern hemisphere (Rühlemann et al., 2004). Thus, in the light of
these data published in the literature, we assumed, at least, 1ºC cooling
of the subsurface waters also in our study region.
As for the salinity changes, LGM surface salinity values increased by
up to 0.8-1.1psu according to our results. As for the subsurface waters,
we may allow for a potential influence of the MOW. However, we
would exclude it for the glacial epochs because reconstructions based
on benthic foraminifera off the Iberian margin have shown that during
glacial periods the MOW became denser (e.g. Rohling and Gieskes,
1989; Zahn, 1997) and sank ∼700m deeper than today (Schönfeld and
Zahn, 2000). Furthermore, although model reconstructions have shown
indeed a stronger influence of the MOW during LGM, this would be
more effective for water depths below 650m (Paul and Schäfer-Neth,
2003). So we just assume an average 1psu salinity increase for water
depths above 600m, which corresponds to the reported global salinity
increase during LGM in the North Atlantic (Adkins et al., 2002; Schrag
et al., 2002).
The results obtained in this study and the assumptions made on the
base of informations retrieved from the literature are combined in
Figure 7.10B where it can be observed that the density of surface LGM
waters was similar to the density of the upper part of the NACW
(27.5σ). This means that, during the LGM, the density gradient
between surface and subsurface waters was reduced. This result has
direct implications on the thermocline depth: a gentle density gradient
between surface and subsurface waters means that the base of the
thermocline was deeper than today (Fig. 7.10B). This scenario would
be imputed to the concurrent surface temperatures drops and salinity
rises that characterized the surface waters in a larger degree than the
subsurface waters.
167
Part III. Results
Surface
NACW
MOW
NADW
Fig. 7.12. Oxygen (Ox), Temperature (T), and Salinity (Sal) profiles for a core located
close to our study region (32º41.8N; 13º32.9W) (after Wefer and Müller, 1998, pg.
21). On the right side of the figure, water masses vertical distribution is shown. At
approx. 1000 water depth, the increase in temperature and salinity, in conjunction
with oxygen reduction are ascribed to the influence of cells of MOW. Legend:
NACW is North Atlantic Central Water; MOW is Mediterranean Outflow Water; and
NADW is North Atlantic Deep Water.
This interpretation about thermocline deepening is supported by the
results obtained from G.truncatulinoides proportions, which fluctuate
in parallel with Earth’s orbital obliquity, with higher amount of the
planktonic foraminifer during glacial periods, i.e. at times of low boreal
summer insolation at high latitudes (Fig. 7.13). G.truncatulinoides is a
subtropical, deep-dwelling species that calcifies the last stage of the test
at 600m water depth or deeper, mostly in winter (Lohmann and
Schweitzer, 1990; Mulitza et al., 1997). It is well known that high
proportions of G.truncatulinoides in the subtropics indicate low density
gradient between surface and subsurface waters and a deepening of the
thermocline (e.g. Mulitza et al., 1997).
168
Chapter 7. Sea surface hydrological changes
1 2
3
4
5
6
7
G.truncatulinoides
26
20
Obliquity
LGM
24
10
G.truncatulinoides %
Oliquity
0
22
0
25
50
75
100 125 150 175 200 225
Age (kyr)
Fig.7.13. Changes in G.truncatulinoides percentages parallel the Earth’s obliquity
curve, with higher proportion of the foraminifer at times of high obliquity. LGM: Last
Glacial Maximum. Marine isotope stages from 1 to 7 are shown in the upper section
of the panel.
Our results contrast with scenarios reconstructed from the western
part of the Atlantic. In the NW Atlantic, there is evidence for a strong
vertical density gradient, both at high and subtropical latitudes (Slowey
and Curry, 1992; de Vernal et al., 2002). Studies performed between
the Labrador Sea and the Iceland basin have shown that during the
LGM the difference between surface and subsurface water density
ranged from 1.5 σ units, in winter, to 3 σ units, in summer (de Vernal
et al., 2002). At subtropical latitudes, it has been demonstrated that
glacial thermocline waters were colder and that the thermocline base
was shallower by 50-100m than today (Slowey and Curry, 1992).
In summary, during the LGM thermocline waters of the North
Atlantic were colder, at least by 1ºC (Slowey and Curry, 1992). Colder
thermocline waters presumably resulted from the winter cooling at the
region of thermocline ventilation, that during the LGM was 2º-4ºC
colder than today (CLIMAP, 1981). Thermocline cooling apparently
occurred both the east and the west side of the north Atlantic basin
(Paul and Schäfer-Neth, 2003; Rühlemann et al., 2004; this study). The
different vertical water structures that are observed at the two sides of
169
Part III. Results
the Atlantic are probably driven by the different hydrological changes
experienced by the surface waters. In the west, the glacial water column
was well stratified with a strong pycnocline between dense subsurface
waters and light surface waters. Low surface salinity resulted from
dilution by seasonal meltwaters from Laurentide ice sheet (de Vernal et
al., 2002). In the east Atlantic, the water column was well mixed with a
weak pycnocline caused by high density surface waters. The lack of
sources of freshwaters, both by precipitation and by runoff, and the
cold water temperatures were the responsible for such peculiar
hydrological characteristics.
7.6. Conclusions
During the last 220 kyr, the northeast subtropical Atlantic underwent
significant changes both in temperature and salinity. Temperature
fluctuations of 3-4ºC during glacial/interglacial transition fit the results
obtained by previous studies on different proxies (see Crowley, 2000).
Salinity increases by up 1psu respect to today are ascribed to the lack of
runoff and precipitations, in alliance with the well-known glacial
aridity and suppression of the monsoonal rains. Superimposed to this
general scenario, brief low temperature and salinity drops are registered
both in Stage 2-4 and in Stage 6. These events are concomitant with
massive ice surges in the higher latitudes and imputed to the direct
influence of the Canary current that would transport the Heinrich
hydrological effects southward, as already postulated by other authors
(Zhao et al., 1995; Wang et al., 1995; Bard et al., 2000).
The cold and saline – and thus dense- surface waters that occupied
the northeast subtropical Atlantic during the LGM generated a low
density gradient with the underlying subsurface waters. This result,
further confirmed by the abundance of G.truncatulinoides, contrasts
with the counterpart western Atlantic, where density gradient during
the LGM seem to have been steeper than today (Slowey et al., 1992;
1995; de Vernal et al., 2002). Our hypothesis is that surface density and
ultimately surface salinity made this difference. Thus salinity patterns,
and not only temperature distribution, might have driven changes in
thermocline depth and mixed layer thickness, which have great
paleoclimatic implications.
170
PART IV
DISCUSSION AND CONCLUSIONS
171
172
Chapter 8.
DISCUSSION
173
174
Chapter 8. Discussion
DISCUSSION
Past dynamics of the study region was the result of complex
interplays among several fluctuating components. The understanding of
their modes and changing rates is necessary to aim a global knowledge
of the paleoclimatic and paleoceanographic history of the region.
In this last part of the thesis, the results and discussions already
presented in the previous Chapters 5, 6, and 7 are further discussed and
combined in a composite portrait of the study region.
Fig. 8.1. Figurative representation of the pieces of information used in this thesis for
paleoclimatic and paleoceanographic reconstructions. Improvements and further
research are symbolized by the white piece of the puzzle. Legend. Forams, planktonic
foraminifera; SST, sea surface temperature; SSS, sea surface salinity; Corg, organic
carbon; CaCO3, calcium carbonate.
175
Part IV. Discussion and Conclusions
8.1. Dominant sedimentary processes
Different sedimentary processes distinguish the study area
although two of them seem to have been more effective in shaping the
late Quaternary sediment deposits: hemipelagic settling, which consists
in in situ vertical deposition of biogenic and terrigenous particles
trough the water column and lateral advection within nepheloid layers,
which represents the main mode of lateral matter transfer from the
continent to the deep sea.
In the hemipelagic settling through the water column, the particles
are mainly made of planktonic organisms, among which planktonic
foraminifera are the most abundant. Also terrigenous particles of
African wind-blown dust make a consistent contribution to the marine
sediment composition, whereas wind-blown volcanic ashes, probably
originated from past volcanic eruptions in the Canary Islands, occur
occasionally (Wefer et al., 1997; Kuhlmann et al., 2004).
Lateral advection, by means of nepheloid layers that travel near the
bottom or at mid depth, is thought to transfer organic rich material,
originated within the NW African coastal upwelling, from the coast to
the deep ocean. This process would explain the high organic carbon
contents of the marine sediment underlying the study area, which today
is an open-sea oligotrophic region. Turbiditic processes, although
reported in the study region to route the Agadir canyon (Ercilla et al.,
1998; Wynn et al., 2000), did not influence the analyzed sediment core,
which is located in the interfluve of the canyon. However, due to the
proximity of the canyon, it is not excluded that nepheloid layers, and
their organic-rich suspended load, could be associated with turbidite
events. In this case, they would bring only the finest fraction of the
turbidite distal portion (Stow and Wetzel, 1990).
8.2. Controlling factors
Hemipelagic settling and lateral advection are the principal
sedimentary processes acting at the NW African margin in the study
area. Further attention is now posed to the controlling factors that
regulated the variability of these processes through geological times.
Triggering factors can be distinguished into two categories:
regional (coastline shape, continental shelf width, bottom topography,
176
Chapter 8. Discussion
Cape Ghir filament) and global (solar radiation, sea-level fluctuations,
bottom waters changes, wind system dynamics).
Regional factors comprehend the region’s features whose geometry
interacts with the oceanographic and atmospheric circulation
determining some of the specific characteristics of the study region.
Coastline shape and the bottom topography are responsible, together
with the persistent NE Trade winds blowing, of filament formation at
the Cape Ghir (Hagen et al., 1996). Relevant bottom topography
features, such as submarine seamounts, the Agadir canyon and
secondary channels, mostly control the path of the sediment transfer
from the continent to the deep sea. In principle, the geometry of these
features remained constant during the last 220 kyr. However, climate
induced changes, such as sea level fluctuations or variable strength of
the blowing winds, built different interplays with these features. For
example, the shelf width expanded and shrank according to sea level
fluctuations, determining a glacial-interglacial shift of the principal
upwelling cells (see Chapter 6). Variable wind strength is supposed to
determine the growth and extension of the filament seaward,
influencing the sediment composition at orbital cycle pace.
Global factors include the already mentioned climate-driven
changes in sea level and the variability of wind system dynamics
(Trade winds and Monsoons), as well as global changes in ocean
bottom water chemistry. Sea level drops can mobilize organic-rich
sediments accumulated at the shelf edge and make them descend down
to the deep basin. It is not excluded that storm surges could be another
concurrent factor in provoking the sediment destabilization. Global
changes in bottom water chemistry and lysocline fluctuations influence
the sediment carbonate content at glacial-interglacial intervals. In the
oceans, the lysocline depth is related to the carbonate ion (CO3-2)
concentration in the deep water. In the Atlantic Ocean, the CO3-2
distribution is determined by the mixing between North Atlantic Deep
Water (NADW) and Antarctic Bottom Water (AABW). The lysocline
becomes shallower when the waters of Antarctic origin bathe the
sediment (Broecker and Takahashi, 1978). Although today the limit
between NADW and AABW in the Northeast Atlantic ranges between
3500m and 4000m, this shoaled during the past glacial times (Sarnthein
et al., 1994). Changes in bottom water characteristics actually
determined decreased sediment CaCO3 content at the obliquity cycle
pace. However, the sediment composition changes observed at the
precessional band are due to variable inputs of terrigenous dust, which
177
Part IV. Discussion and Conclusions
were driven by a different set of global factors: solar radiation and large
scale wind system dynamics (see Chapter 5).
8.3. Dust
8.3.1. Dust generation and transport
Dust content within the sediment is the result of two main factors:
dust production and dust transport. In turn, these two factors are driven
and influenced by a complex mixture of climatic components.
Dust production mainly depends on the climatic regime at the
source region, which is governed by local hydrological cycle, rate of
precipitation, length of xeric conditions, vegetation cover, and land
surface modification (e.g. Middleton, 1985; Littmann, 1991; Tegen and
Fung, 1995). Dust transport is controlled by the climatic variability of
the transport agents, i.e. the wind system dynamics, which is driven by
land-ocean and Pole-Equator thermal gradients (Goudie and Middleton,
2001, pg. 194), large-scale features affecting the subtropical high
pressure anticyclone cell, and the North Atlantic Oscillation (NAO)
(Moulin et al., 1997).
Today, dust production seem to be abundant in areas of Sahara
where annual precipitation rates are below 100mm (Goudie and
Middleton, 2001) and in the Sahel, where prolonged droughts and
intense land use have increased deflation and wind erosion (Prospero,
1996). Dust events in core GeoB4205, reflected by high Fe element
intensity, high magnetic susceptibility, and low colour reflectance
(lightness L*), originated from Sahara and Sahelian regions. However,
more than source region aridity, other climate components seem to play
a significant role:
1. the seasonal shift of the subtropical high pressure centre, which
would govern the variability in the dust source areas and
transport paths.
2. the wind speed of the subtropical easterly jet and the amplitude
of the easterly waves, which would contribute to unstable and
stormy conditions that favour squall lines generation and dust
mobilization.
178
Chapter 8. Discussion
8.3.2. Glacial dust
Several contributions have provided evidence for increased dust
events during glacial phases in general and during LGM in particular
(Kolla et al., 1979; Sarnthein and Koopmann, 1980; Tetzlaff and
Peters, 1986; Grousset et al., 1998). These findings found further
support from the results of investigations performed on polar ice cores,
where high dust loads were found to characterize cold phases, both on
glacial-interglacial and on millennial time scales (Bory et al., 2004).
By contrast, the results of this thesis show that dust inputs to the
northeast subtropical Atlantic punctuated both the glacial and the
interglacial phases. In addition, favourable conditions for dust
generation and transport are found to be more effective during periods
of maxima boreal insolation. Our findings are largely supported by
similar results obtained in the study region (Freundhental et al., 2001;
Moreno et al., 2002) and in other low-latitude regions where
precessional control over dust emission is also reported (deMenocal et
al., 2000a).
The discrepancy between dust deposition in polar and subtropical
regions can be attributed to the differences in dust source regions.
Recent investigations, based on a geochemical approach of Sr and Nd
isotopic ratio that traces dust provenance (Grousset et al., 1992a; Basile
et al., 1997), have found that African desert can be excluded as source
region of polar ice core dust (Bory et al., 2003; Delmonte et al., 2004).
During cold phases, the dominant source of Antarctica ice cores
(Vostok and Dome C) could be the southern tip of South America
(Pampas, Patagonia and Argentine exposed continental shelf)
(Delmonte et al., 2004) whereas Eastern Asian desert would provide
mineral dust to the Greenland ice cores (Bory et al., 2003).
Further, it is important to account for the complexity of the factors
that determine dust production and deposition. In polar regions, it has
been pointed out that high dust fluxes (during glacial phases) may have
resulted from reductions in precipitation (Yung et al., 1996) more than
enhanced wind strength. At low-latitudes, changes in vegetation cover
(Mahowald et al., 1999) soil moisture and types of landforms (Prospero
et al., 2002) can play an important role together with source region
aridity. The results of this thesis also show that atmospheric instabilities
and land-ocean thermal gradients can be added to the list of the
179
Part IV. Discussion and Conclusions
important mechanisms that determine dust deposition, either during
glacial and interglacial periods.
8.3.3. Dust contribution to climate change
Some debate remains on the effective role exerted by atmospheric
dust on global climate (e.g. Maher and Dennis, 2001). Model
simulations have accepted the challenge to investigate the interaction
between dust cycle and climate system (Overpeck et al., 1996;
Mahowald et al., 1999; Tegen, 2003).
In this thesis, cross-spectral analysis between dust proxies (Fe and
Ca element intensities, magnetic susceptibility, and colour reflectance)
and Earth’s orbital parameters (ETP curve) (see Chapter 5) was applied
with the aim to identify lead and lags of these two climate components.
Results show that dust inputs lag minimum of precession index by 1.5
kyr (magnetic susceptibility) and 0.2 kyr (Fe). Iron content provides to
be the most consistent proxy for spectral analyses as sediment colour
and magnetic properties can be altered by post-deposition processes.
However, taken in mind the errors associated to the age model and the
low sedimentation rate of core GeoB4205, there are few chances for a
robust assessment of the interrelationships between dust events and
solar radiation. At this stage, we are not able to resolve the question
whether dust is a forcing-agent or a consequence of the climate change.
8.4. Sea surface temperatures
Paleo sea surface temperature (SST) evolution results from data of
planktonic foraminifera census counts and of G.ruber (white) oxygen
isotope. Afterwards, SST values enter the formula of sea surface
salinity and density reconstruction. Thus, reliable SST values are
instrumental for a correct computation of other paleoclimatic series.
Moreover, the increased use of model simulations of past and future
climate scenarios requires SST pale reconstructions as accurate as
possible.
The quality of SST results strictly depend on the reliability of the
planktonic foraminifera census counts. It is thus important to assure
that foraminiferal assemblage is not biased by the effects of calcite
dissolution caused by the shoaling of the local lysocline. If this
180
Chapter 8. Discussion
occurred, preferential dissolution would induce the fragile tests (e.g.
G.aequilateralis, G.sacculifer, G.ruber, G.bulloides) to disappear and
the resistant species (e.g. G.truncatulinoides) to be preserved.
Eventually, SST results would be altered. Higher probability for
lysocline shoaling exists during glacial periods when the local lysocline
has been documented to vary in depth (Sarnthein et al., 1994). In core
GeoB4205, fragmentation index is on average 8% and reaches 45%
only during Stages 2 and 4, in conjunction with high benthic/planktonic
ratio, which is also used as a dissolution proxy (see Chapter 6).
Nevertheless, SST results should not be biased by dissolution. In fact,
relative proportions of the species are not significantly modified until
fragments represent 20% of the total (Thunell and Honjo, 1981) and the
assemblage is seriously altered only when fragments exceeds 80%
(Conan et al., 2002). So, in general core GeoB4205 results are safe
although some cautions need to be paid for those samples where
fragmentation index is higher than 20%.
Due to the proximity to the African continent, SST changes in core
GeoB4205 could have been driven either by injection of cool coastal
upwelling waters or by changes in global ocean circulation within the
subtropical gyre. Modern satellite observations at the coring site show
that the area is currently oligotrophic and not under the influence of the
Cape Ghir filament (chlorophyll concentration are low and satellitederived SSTs are high). However, the area could have been influenced
by the exceptional filament extensions in the past, when wind systems
dynamics was rougher than today. This hypothesis is tested in Chapter
6 combining different set of data in core GeoB4205 and comparing the
results with data from other sediment cores located nearby. Results
show that the influence of the coastal upwelling waters, trapped by the
filament and injected seaward, is possible during Terminations I and II,
but unlikely during the rest of time. Thus, SST evolution at the coring
site is part of a global scenario of temperature changes within the
subtropical gyre.
8.5. Time slices scenarios
With the intent to offer a global vision of past climatic and
oceanographic scenarios of the study region, selected time slice are
here described. Holocene and last interglacial are the periods that
mostly resemble modern conditions. The study of these time intervals
instructs on the current climatic variability and help to make better
181
Part IV. Discussion and Conclusions
prediction on future changes. LGM is the most extreme climatic event
of the recent climatic history of the Earth. Terminations are important
because they represent the change between glacial and interglacial
states. Currently, high resolution and detailed data on these time laps
are necessary to elucidate the triggering factors of the
glacial/interglacial switch.
Holocene (Stage 1) (∼7-8kyr B.P.) and Last Interglacial
(Substage 5.5) (125kyr).- During these intervals of time, hemipelagic
settling trough the water column was the dominant sedimentary
process; sediment composition was mostly characterized by calcium
carbonate derived by the accumulation of calcareous biogenic
components. North Atlantic Deep Water (NADW) resided the bottom
depths, thus ensuring a good preservation of the carbonate content.
Surface water temperature, salinity and density values were similar to
the modern conditions. Sea surface temperatures were as warm as
today, as demonstrated by the foraminiferal association among which
warm subtropical species like G.ruber, white and pink forms, and
G.trilobus were the dominant forms. The surface mixed layer, like
today, was characterized by large seasonal fluctuations, with
stratification well established during summer and surface mixing
enhanced during winter, in response to the stronger winds of this
season.
Last Glacial Maximum, LGM (Substage 2.2) (17-23kyr).During LGM, hemipelagic deposition of calcareous planktonic
organisms was associated with lithic particles derived from the nearby
African continent, either transported by winds or mobilized by the large
sea-level change that characterized this cold epoch. Bottom waters
showed evidence of enhanced influence of masses of southern origin
(AABW, Antarctic Bottom Water) that may have induced partial
calcium carbonate dissolution. On the surface, temperatures were 3º4ºC lower than during the Holocene and were inhabited by foraminifera
species typical of cold waters, such as N.pachyderma right-coiling. The
sub-polar species N.pachyderma left-coiling and T.quinqueloba, which
today are not present at subtropical latitudes, were part of the
foraminiferal assemblage during the LGM. Surface salinity and density
were higher than today by 1‰ and 1.5kg m-3 respectively. These
conditions resulted from the shift of the polar front toward southern
latitudes, which induced sea surface temperatures in the study region to
decrease. In addition, xeric conditions prevailed on the continent
determining a lack of freshwater input to the ocean. The glacial surface
182
Chapter 8. Discussion
mixed layer was thicker than today, enhancing the water vertical
mixing and the ocean-atmosphere exchange.
Terminations I and II (Stages 2/1 and 6/5) (∼10kyr and 128130kyr). - These transitional phases between glacial and interglacial
states were characterized by the most powerful changes affecting both
the atmosphere and ocean circulations. At the study region, wind
systems intensified greatly leading to enhanced coastal upwelling and
Cape Ghir filament extension. Sea surface temperature values raised by
up to 4ºC, while salinity dropped to values lower than today caused by
increased humidity resulted from enhanced monsoon activity.
183
Chapter 9.
CONCLUSIONS
185
186
Chapter 9. Conclusions
CONCLUSIONS
The study of the northeast subtropical Atlantic presented in this
thesis aimed to reconstruct scenarios of past atmospheric and
oceanographic circulation as well as of past climate variability. The use
of different proxies in one sediment core from the NW African margin
and the integration with data belonging to other four sediment cores
located in nearby areas, allow us to obtain a comprehensive
understanding of the interrelationships existing between climatic
factors and natural systems.
9.1. Major achievements
The major achievements obtained during the fulfilment of this
thesis can be summarized into three points:
•
This thesis offers a different scenario to explain dust supply
variability to the northeast subtropical Atlantic. Factors
controlling dust generation and transport are more and more
complex than the simple aridity of the source region and wind
strength. Dust generation in the source areas is by itself a
complex mechanism controlled by factors other than lack of
precipitation. Additionally, other atmospheric processes need to
be active to uplift dust from the soil and transport dust hazes
over large distances. In this thesis, thermal gradients,
atmospheric instability and storminess associated to the
monsoon activity are regarded as the most important factors.
•
This thesis completes current observation of matter export from
the productive shelf to the oligotrophic ocean province of the
study area, extending the analyses into the past, back to the last
two glacial-interglacial cycles. This allows to recognize
frequency patterns of the processes as well as to identify
possible controlling factors.
187
Part IV. Discussion and conclusions
•
This thesis fills a gap in the characterization of the surface water
properties variability of the region during the last two glacial
interglacial cycles. Sea surface temperature and salinity
reconstructions of the Canary Basin and Agadir regions, as well
as investigations on the water vertical density profile and the
thermocline depth fluctuations in the northeast subtropical
Atlantic had never been attempted before.
9.2. Major results
The general conclusions of this thesis are fully presented here in
two groups. The first one (1-5) is mostly focussed on the used
methodology and the other one (6-11) relates on the reconstructed
paleoclimatic and paleoceanographic scenarios.
1. The sediment core used in this thesis is suitable for
paleoclimatic and paleoceanographic reconstructions. This
conclusion stems from the consideration that hemipelagic
settling is the dominant sedimentary process affecting the core
site. Other processes related to gravity flows and turbidity
currents, which elsewhere are described to be common in the
region, do not influence our core location.
2. Automatic devices, such as the Multi Sensor Core Logger, the
Spectrophotometer and the XRF Core Scanner are very helpful
in producing series of excellent quality data.
3. Sediment colour, magnetic susceptibility and Fe element
intensity are unquestionable proxies for tracking Saharan and
Sahelian provenance of the iron-rich dust found in the marine
sediment of the northeast subtropical Atlantic.
4. The planktonic foraminifer G.bulloides proved to be a suitable
proxy for primary productivity in the study area. Its abundance
concurs with peaks of other micropaleontological and
geochemical paleoproductivity proxies, which distinguish other
sediment cores located within the same study region.
5. The planktonic foraminifer G.truncatulinoides (right coiling) is
confirmed to be a good proxy to infer thermocline depth and
vertical density changes in the water column.
188
Chapter 9. Conclusions
6. Variations in the sediment composition, i.e. the variable content
in lithogenic or carbonate fraction, are mostly related to climatic
changes forced by the orbital precessional component. Major
concentrations of terrigenous material occur at times of maxima
in boreal summer insolation.
7. Terrigenous material, proceeding from the nearby African
continent, is mobilized and carried by the winds associated to
monsoon activity. Enhanced monsoon activity and intense
thermal gradients at times of maxima summer insolation would
provoke intense stormy and turbulent conditions, which could
lead great amount of dust to be raised up to the atmosphere.
8. Aridity of the source region is a necessary condition for dust
availability, but not enough to explain its generation. Other
climatic factors play complementary roles, and atmospheric
instability seems to be very important.
9. Although the study area is currently oligotrophic, eutrophic
conditions characterized past scenarios of the region. Coastal
upwelling fingerprints proceeded from the Cape Ghir filament,
which probably extended as far as the core location during
Terminations, due to stronger Azores-cell circulation and Trade
winds.
10. Substantial amounts of organic carbon found in the deep
sediment of the core location are laterally advected from the
highly-fertile coastal zone around Cape Ghir. Likely, the
transport occurs by means of nepheloid layers that travel at
intermediate and near-bottom depths. This process is currently
active in the study region and has been described by modern
studies. In the past, the variability of this process seems to be
linked to that of northern hemisphere summer insolation.
11. During the last 220kyr, surface hydrological conditions of the
study region changed significantly:
11a . Sea surface temperature fluctuations are not driven or
influenced by the adjacent coastal upwelling; rather,
SST changes are part of the global variability of the
subtropical North Atlantic gyre; more specifically, they
189
Part IV. Discussion and conclusions
are imputed to the direct influence of the Canary
Current.
11b. Salinity changes are driven by local variations in
hydrological cycle, which in turn depend on the
dynamics of the wind systems. Glacial salinity increases
are ascribed to the lack of runoff and precipitation, in
alliance with the continental glacial aridity. During
deglaciation periods, low salinity values are attributed to
enhanced precipitations associated to monsoon activity.
11c . Changes in thermocline depth and mixed layer
thickness, inferred from sea water density variation and
varying abundance of G.truncatulinoides, seem to be
controlled by local salinity distribution, more than by
sea surface water temperature pattern.
9.3. Future lines of investigation
The realization of this thesis highlights some questions as worthy
to be further investigated and completed by additional data.
190
•
As a general remark, future investigations of the study area need
additional data gathered from sediment cores characterized by
high sedimentation rate. This would ensure better insight into
the sub-Milankovitch, high frequency millennial climatic
changes. Also, high resolution time series would firmly
constrain the leads and lags among the different climatic
components analysed in this thesis, which would help in
differentiating between forcing and responding factors.
•
Sea surface reconstructions of the study area would certainly
profit from additional SST data derived by other proxies, as for
example the alkenone UK37 or Mg/Ca. The convenience of this
approach is twofold: (1) temperature reconstructions would be
better
constrained
and
(2)
the
comparison
of
micropaleontoloical and geochemical proxies would be very
instructive.
•
In order to reduce the errors of SST estimations, planktonic
foraminiferal census counts of the core tops of the entire
Chapter 9. Conclusions
CANIGO region could be performed and introduced into the
database used by transfer function and analog methods for SST
calculations. This would ensure a better selection of the pool of
analog samples used by MAT, SIMMAX and RAM methods.
•
As the stable oxygen isotope (δ18O) difference between
shallow- and deep-living planktonic foraminifera is a proxy for
the stratification of surface water (Mulitza et al., 1997), a robust
estimation of the thermocline depth in the study area would
profit from additional data of δ18O of the deep-dwelling
G.truncatulinoides. These data could be easily combined with
the already performed δ18O of the surface-dwelling G.ruber.
•
Further geochemical analyses performed on the lithogenic
fraction of the marine sediment of the study area, especially on
those samples enriched in African dust, would ensure a better
constrain of the geographical provenance of the dust and, in
addition, would allow a better reconstruction of past air masses
tracks.
191
References
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