QUATERNARY EVOLUTION OF THE BENGUELA COASTAL UPWELLING SYSTEM

- RESPONSES TO LOCAL AND GLOBAL CLIMATE CHANGES -

 

 

19 -21 April 2000

 

 

Résidence ESPACOTEL, Carcans, France

 

 

 

 

A workshop co-sponsored by IMAGES (IGBP/PAGES), the University of Bordeaux I, and the "Centre National de la Recherche Scientifique".

 

 

 

Programme and Abstracts

 

 

 

 

Compiled and edited by Jacques Giraudeau

Département de Géologie et Océanographie, UMR 5805 CNRS, Université Bordeaux I

Foreword and ACKNOWLEDGMENTS

 

This workshop aims at getting closer to an integrated view 1) of the productivity history of the Benguela upwelling system, 2) of the interrelationships between dynamics, biology and sedimentology at regional scale, and 3) of the interrelationships between the evolution of this

regional ecosystem and global changes: climate, sea-level, thermo-haline circulation and atmospheric CO₂.

 

The Benguela upwelling system was recently the focus of two major research cruises within the framework of the Ocean Drilling Program and IMAGES (PAGES/IGBP) during the years 1996 (NAUSICAA/IMAGES 2; Chief Scientist: P. Bertrand) and 1997 (ODP Leg 175; Chief Scientists: W. Berger and G. Wefer). Material recovered during both cruises is of exceptional quality and will allow, for the first time, to document the Neogene evolution of this system at orbital and sub-orbital time-scales.

Initial results from Leg 175 and IMAGES 2 were presented at the last International Conference on Paleoceanography (Lisbon, Aug. 1998). More recently, the Benguela and other related upwelling systems were the focus of a very successful session co-chaired by B. Christensen and P. Meyers at the Fall 1999 AGU meeting.

The present workshop offers the opportunity to young and senior paleoceanographers involved in the study of the Benguela realm  to discuss exciting up-to-date developments, and to initiate future fruitful collaborations.

 

We acknowledge, with sincere thanks, the following for their sponsorship of this workshop: IMAGES (IGBP/PAGES), the University of Bordeaux I, and the "Centre National de la Recherche Scientifique". Thanks are extended to the key-scientists Wolfgang Berger, Gerold Wefer, Tom Pedersen, Phil Meyers, and John Rogers, for having accepted to chair the various sessions, as well as for their continuous enthusiasm and involvement in the study of the Benguela upwelling region and other related systems. Finally, we are indebted to the manager and staff of ESPACOTEL for their availability and efficiency in making our stay in Carcans as enjoyable as possible.

 

We sincerely hope that the relaxing and friendly atmosphere in Carcans will inspire all scientists through a constructive and successful meeting.

 

 

                                                                                    Jacques Giraudeau and Philippe Bertrand
Programme outline

 

The three days meeting has been divided into five half-day sessions starting on the morning of April 19th, and ending April 21st at lunchtime.

Oral contributions will be limited to 30 min. each (including 5-10 min. for questions).

Poster sessions will take place on April 19th and 20th, from 4.30 to 6.30 pm. All posters will be on show for the whole duration of the meeting.

 

Tuesday, April 18th:

 

            Arrival at ESPACOTEL (Afternoon bus shuttles from airport and station).

 

 

Wednesday, April 19th:       

 

            AM:            9.00: Welcome + infos (J. Giraudeau, P. Bertrand)

                        9.30 - 10.30: Session 1 - Chair W. Berger.

                        10.30 - 11.00: coffee break (+ hanging up of posters)

                        11.00 - 12.00: Session 1 - Chair W. Berger

            Lunch

            PM            2.00 - 4.15: Session 2 - Chair P. Meyers

                        4.15 - 4.30: coffee break

                        4.30 - 6.30: poster session

            Dinner (barbecue party)

 

 

Thursday, April 20th:

 

            AM            9.00: Infos (proceedings, etc...)

                        9.30 -10.30: Session 3 - Chair G. Wefer

                        10.30 - 10.45: coffee break

                        10.45 - 12.15: Session 3 - Chair G. Wefer

            Lunch

            PM            2.00 - 4.15: Session 4 - Chair J. Rogers

                        4.15 - 4.30: coffee break

                        4.30 - 6.00: poster session

            Wine and dinner treat at "Chateau Aney" (Haut Médoc).

 

 

Friday, April 22:

 

            AM            9.30 - 10.30: Session 5 - Chair T. Pedersen

                        10.30 - 10.45: coffee break

                        10.45 - 11.15: Session 5 - Chair T. Pedersen

            Lunch

            PM:            Bus shuttle to airport and station.

 

Saturday, April 22:

 

            AM            Bus shuttle to airport and station

Lectures

 

Wednesday, April 19th, morning:             Session 1 - "Exploring the sedimentary record of biogenic production: upwelling intensity, diagenetic processes, relation with orbital cycles" Part I. - Chair: W. Berger.

09.30   Philippe Bertrand, Tom Pedersen, Ralph Schneider, Graham Shimmield, Elisabeth Vergès, Jean-Robert Disnar, Delphine Massias, Juan Villanueva, Nicolas Tribovillard, Alain Huc, Xavier Giraud, Organic-rich sediments in ventilated deep-sea environments: climate, sea-level and trophic changes.

10.00   Beth A. Christensen, Mark MASLIN, Megan Ellershaw, and James L. Kalbas, Orbital Variations in the Pliocene Cape Basin Sites 1085 and 1087.

11.00   Joan Villanueva, and Philippe Bertrand, Sea-Surface-Temperature and Productivity changes in the Luderitz area.

11.30   Karin Zonneveld, Oliver Esper and Helmut Willems, Relationship between surface water bioproductivity, preservation of organic matter and Late Quaternary climate change.

 

Wednesday, April 19th, afternoon:             Session 2 - "Alongshore modulation/latitudinal migration of the upwelling system and associated atmospheric and circulation changes". - Chair: P. Meyers.

14.00          Lieselotte Diester-Haass, Phil A. Meyers, Thorsten Bickert, Laurence Vidal, and Gerold Wefer, Late Miocene onset of high productivity in the Benguela Current Upwelling System: local or global control?

14.30     J. H. Fred Jansen, SE Atlantic trade-wind zonality, a high-resolution CORTEX record of eolian silts in Walvis Ridge sediments (ODP Site 1081).

15.00     Philip A. Meyers, Pliocene-Pleistocene evolution of carbon production and burial in the Benguela Current upwelling system.

15.30     Jan-Berend W. Stuut, Maarten A. Prins, Ralph R. Schneider, Gert Jan Weltje, J.H. Fred Jansen and George Postma, Late Quaternary climate variability in SW Africa: grain size of terrigenous sediments from Walvis Ridge as a proxy for wind strength and aridity.

 

Thursday, April 20th, morning:             Session 3: "Exploring the sedimentary record of biogenic production: upwelling intensity, diagenetic processes, relation with orbital cycles" Part II. - Chair: G. Wefer

9.30       Min-Te Chen, Y.-P. Chang, C.-C. Chang, K.-L. Jwo, L.-W. Wang, C.-H. Wang, and E.-F. Yu, Planktonic Foraminiferal Sea Surface Temperature Variations in the Southeast Atlantic Ocean: A High-Resolution Record MD962085 of the Past 400,000 Years from the IMAGES II - NAUSICAA Cruise.

10.00          Jeremy R Marlow, Carina B Lange and Toni Rosell-Melé, A 4.5 M.Y. Record of Benguela Upwelling at ODP Site 1084: A Combined Biomarker and Micropalaeontological Approach.

10.45     Mark Maslin, V. Ettwein, E. Laurie, A. Rosell-Mele, C. Stickley, L. Vidal, E. Platzman and G. Wefer, De-coupling of Upwelling Intensity and Productivity of the Benguela Current System after the Intensification of Northern Hemisphere Glaciation at 2.54 Ma.

11.15          Antoni Rosell-Mele, Mark Maslin, Ralph Schneider and Laurence Vidal, Effects of the culmination of the Onset of Northern Hemisphere Glaciation in SE Atlantic productivity.

11.45          Catherine Pierre and Marie Thérèse Venec-Peyre, Surface and deep ocean variability during the last 330 kyrs in the Lüderitz upwelling system.

 

Thursday, April 20th, afternoon:             Session 4: "Dynamics of the Agulhas retroflection and Subtropical Convergence at the polar end of the Benguela ecosystem" - Chair: J. Rogers

14.00     Ruth Acheson, Dick Kroon, Ralph Schneider, and Hans-Stefan Niebler, Interglacial conditions in the southern extension of the Benguela system during the past 500 ky with special emphasis on Oxygen Isotope Stage 11.

14.30     Oliver Esper, Ralph Schneider, Karin Zonneveld and Helmut Willems, Palaeoceanological reconstruction of the Late Quaternary Agulhas Current (South Atlantic) based on calcareous and organic-walled dinoflagellate cysts.

15.00          Jacques Giraudeau, Catherine Pierre, and Laurence Herve, Late Quaternary evolution of the interocean leakages south of Africa: preliminary results from ODP Site 1087.

15.30     John Rogers, Amanda Rau, and Min‑Te Chen, Evidence of Latitudinal Fluctuations of Watermasses in the South Atlantic from Ice‑Rafted Detritus off the West Coast of South Africa.

 

Friday, April 21st, morning:             Session 5: "Interhemispheric and interoceanic relationships in coastal upwelling process and upper deep water circulation changes" - Chair: T. Pedersen

9.30       Dyke H. Andreasen, A. Christina Ravelo, and Michael W. Wara, Pliocene upper deep water circulation in the South Atlantic.

10.00     Raja .S. Ganeshram, D. Kroon, T. F. Pedersen, G. L. Cowie, S. E. Calvert, R. R. Schneider and P. J. Müller, A comparative study of the timing of high-frequency oscillations (sub-Milankovitch) in upwelling records of the southern Atlantic (Benguela current), the eastern Pacific and the Arabian Sea margins.

10.45          Laurence Vidal, Gerold Wefer, Ralph Schneider, and  Thorsten Bickert, Upper deep water circulation in the South Atlantic over the last 1,5 M.Y.

INDEX: Author and Abstract titles

Presenting authors are listed first, unless otherwise specified (underlined)

 

 

Ruth Acheson, Dick Kroon, Ralph Schneider, and Hans-Stefan Niebler, Interglacial conditions in the southern extension of the Benguela system during the past 500 ky with special emphasis on Oxygen Isotope Stage 11.

 

Dyke H. Andreasen, A. Christina Ravelo, and Michael W. Wara, Pliocene upper deep water circulation in the South Atlantic.

 

Philippe Bertrand, Tom Pedersen, Ralph Schneider, Graham Shimmield, Elisabeth Vergès, Jean-Robert Disnar, Delphine Massias, Juan Villanueva, Nicolas Tribovillard, Alain Huc, Xavier Giraud, Organic-rich sediments in ventilated deep-sea environments: climate, sea-level and trophic changes.

 

Babette Boeckel and Karl-Heinz Baumann, Sedimentation of coccoliths in the SE-Atlantic during the last 250 ka: implications for surface-water conditions and carbonate accumulation.

 

Min-Te Chen, Y.-P. Chang, C.-C. Chang, K.-L. Jwo, L.-W. Wang, C.-H. Wang, and E.-F. Yu, Planktonic Foraminiferal Sea Surface Temperature Variations in the Southeast Atlantic Ocean: A High-Resolution Record MD962085 of the Past 400,000 Years from the IMAGES II - NAUSICAA Cruise

 

Beth A. Christensen, Mark MASLIN, Megan Ellershaw, and James L. Kalbas, Orbital Variations in the Pliocene Cape Basin Sites 1085 and 1087.

 

Lieselotte Diester-Haass, Phil A. Meyers, Thorsten Bickert, Laurence Vidal, and Gerold Wefer, Late Miocene onset of high productivity in the Benguela Current Upwelling System: local or global control?

 

Lydie Dupont, Barbara Donner, and Britta Jahn, Early to Mid-Pleistocene terrestrial input in and marine paleoproductivity off the Congo River (ODP Site 1075).

 

Emma Durham, Mark Maslin, Ellen Platzman, Antoni Rosell-Melé, Jeremy Marlowe, Dave Mattey, Steve Burns, and the Leg 175 Shipboard Scientific Party, Reconstructing the climatic history of the western coast of Africa over the past 1.5 million years. A comparison of proxy records from the Congo Basin and the Walvis Ridge and the search for the Mid-Pleistocene Revolution.

 

Oliver ESPER, Ralph SCHNEIDER, Karin ZONNEVELD and Helmut WILLEMS, Palaeoceanological reconstruction of the Late Quaternary Agulhas Current (South Atlantic) based on calcareous and organic-walled dinoflagellate cysts.

 

Raja .S. Ganeshram, D. Kroon, T. F. Pedersen, G. L. Cowie, S. E. Calvert, R. R. Schneider and P. J. Müller, A comparative study of the timing of high-frequency oscillations (sub-Milankovitch) in upwelling records of the southern Atlantic (Benguela current), the eastern Pacific and the Arabian Sea margins.

 

Xavier Giraud, Philippe Bertrand, Véronique Garçon, and Isabelle Dadou, Sensitivity of the NW African and Benguela upwelling systems to the sea level change: a modelling approach using nitrogen isotopes.

 

Jacques GIRAUDEAU, Catherine PIERRE, and Laurence HERVE, Late Quaternary evolution of the interocean leakages south of Africa: preliminary results from ODP Site 1087.

 

Ute Güntner, S. Gebhardt, and J. Rullkötter, Organic and isotopic geochemical analysis of lipids in sediments from the southwest African margin (ODP Leg 175, Holes 1075A and 1079A): Changes in the variation of organic matter sources.

 

Britta Jahn, Barbara Donner, Peter J. Mueller, Ursula Roehl, Ralph R. Schneider, and Gerold Wefer, Pleistocene Paleoclimatology in the Benguela Upwelling System for the last 1.5 Ma: Results from ODP Site 1082.

 

J. H. Fred Jansen, SE Atlantic trade-wind zonality, a high-resolution CORTEX record of eolian silts in Walvis Ridge sediments (ODP Site 1081).

 

Junghyun Kim, Ralph R. Schneider, Carsten Rühlemann, Stefan Mulitza, and Gerold Wefer, Evolution of past sea surface temperature in the subtropical eastern South Atlantic over the past 22 kyr: implications for oceanic heat transport.

 

Jeremy R Marlow, Carina B Lange and Toni Rosell-Melé, A 4.5 M.Y. Record of Benguela Upwelling at ODP Site 1084: A Combined Biomarker and Micropalaeontological Approach

 

Mark Maslin, V. Ettwein, E. Laurie, A. Rosell-Mele, C. Stickley, L. Vidal, E. Platzman and G. Wefer, De-coupling of Upwelling Intensity and Productivity of the Benguela Current System after the Intensification of Northern Hemisphere Glaciation at 2.54 Ma.

 

Philip A. Meyers, Pliocene-Pleistocene evolution of carbon production and burial in the Benguela Current upwelling system.

 

Gesine Mollenhauer, Balancing C_org deposition in the Benguela upwelling region for glacial and interglacial periods.

 

Maria E. Pérez and Wolfgang H. Berger, Late Quaternary productivity fluctuations in the mid-Angola Basin: evidence from benthic foraminifera.

 

Jean-Jacques PICHON, Jacques GIRAUDEAU, Gabriel VARGAS, Luc ORTLIEB, and Joan VILLANUEVA, Paleoceanographic changes in the Benguela Upwelling System (Lüderitz cell) as indicated by abundances of Chaetoceros resting spores - Comparisons with upwelling data off northern Chile (23°S).

 

Catherine PIERRE and Marie Thérèse VENEC-PEYRE, Surface and deep ocean variability during the last 330 kyrs in the Lüderitz upwelling system.

 

Amanda RAU, Julia LEE-THORP, John ROGERS, and Jacques GIRAUDEAU, Late Quaternary fluctuations in the circulation patterns of the Southern Benguela region.

 

Rebecca S. Robinson and Philip A. Meyers, Late Pliocene records of upwelling intensity and biogeochemical cycling from the Benguela and California Current systems (2.3- 2.0 Ma).

 

John Rogers, Amanda Rau, and Min‑Te Chen, Evidence of Latitudinal Fluctuations of Watermasses in the South Atlantic from Ice‑Rafted Detritus off the West Coast of South Africa

 

Antoni Rosell-Mele, Mark Maslin, Ralph Schneider and Laurence Vidal, Effects of the culmination of the Onset of Northern Hemisphere Glaciation in SE Atlantic productivity.

 

Ulrich STRUCK, Kay-Christian EMEIS, Jürgen ALHEIT, and Ralph SCHNEIDER, Holocene variability of upwelling from sediments in the diatomaceous mud bemt offshore Namibia.

 

Jan-Berend W. Stuut, Maarten A. Prins, Ralph R. Schneider, Gert Jan Weltje, J.H. Fred Jansen and George Postma, Late Quaternary climate variability in SW Africa: grain size of terrigenous sediments from Walvis Ridge as a proxy for wind strength and aridity.

 

SandraVaqueiro, Catherine Pierre, Delphine Massias, and Fatima Abrantes, The 185 kyr "C_org event": A diatom perspective.

 

Gabriel Vargas, Luc Ortlieb and Michel Pujos, Paleo-wind reconstruction from sedimentological and climatological studies in an upwelling area off northern Chile (23ºS): First results and perspectives

 

Laurence Vidal, Gerold Wefer, Ralph Schneider, and  Thorsten Bickert, Upper deep water circulation in the South Atlantic over the last 1,5 M.Y.

 

Joan Villanueva, and Philippe Bertrand, Sea-Surface-Temperature and Productivity changes in the Luderitz area.

 

Andrea Volbers, H.-S. Niebler, J. Giraudeau, H. Schmidt, and R. Henrich, Paleoceanographic changes in the northern Benguela upwelling system over the last 245 kyrs as derived from planktic foraminifera assemblages.

 

Claudia Wenzel, and Jurgen Rüllkötter, Organic geochemical analysis on organic matter in sediments from the Walvis Bay area (ODP Leg 175 Site 1082).

 

Bas West and J. H. Fred Jansen, Planktonic foraminifera as tracers of the position of the Angola  Benguela Front.

 

Karin Zonneveld, Oliver Esper and Helmut Willems, Relationship between surface water bioproductivity, preservation of organic matter and Late Quaternary climate change.

 Ruth Acheson¹, Dick Kroon¹, Ralph Schneider², and Hans-Stefan Niebler²

¹ Geology and Geophysics, University of Edinburgh, Edinburgh, U.K.

² Fachbereich Geowissenschaften, Universität Bremen, Bremen, Germany

 

racheson@glg.ed.ac.uk

 

Interglacial conditions in the southern extension of the Benguela system during the past 500 ky with special emphasis on Oxygen Isotope Stage 11.

 

The most marked feature of records of past climate change is the rapid warming which occurs after long intervals of gradual cooling. These abrupt warming episodes of the past five transitions from glacial to interglacial conditions are examined in the Benguela region offshore Cape Town. Uniquely, in the mid latitude Southern Atlantic there is a strong tropical ocean component to the records, derived from the Agulhas Current eddies which pass from the Indian Ocean around the Cape into the southern extension of the Benguela Current system. Here,  we present faunal estimates of SST, planktonic and benthic foraminiferal d ¹⁸O records along with foraminiferal abundance counts and fragmentation analysis, in order to examine the sensitivity of the system to changes in tropical insolation by focusing on these interglacial periods with emphasis on MIS 11 (duration 420 to 360 ka). Analysis of planktonic foraminiferal assemblages from the interglacial periods (MIS 1, 5, 7, 9 and 11) shows both similarities and differences in the assemblages. This data suggests that conditions were different in each interglacial period. MIS 11 is apparently the warmest and longest interglacial of the past 500 ky, characterised by warm SST and high abundances of G. ruber , and N. dutertrei (in addition to higher than average assemblage content of other subtropical-tropical species). In MIS 7 and 9 inflow from the Indian Ocean appears to have been more limited. MIS 5e is almost as warm but of a shorter duration than MIS 11.

 

Dyke H. Andreasen¹, A. Christina Ravelo², and Michael W. Wara²

¹ Department of Earth Sciences, University of California Santa Cruz, Santa Cruz, USA

² Ocean Sciences Department, University of California Santa Cruz, Santa Cruz, USA

 

andreasn@es.ucsc.edu

 

Pliocene upper deep water circulation in the South Atlantic

 

Global climate models predict average surface temperatures were 3.6°C warmer during the mid-Pliocene (ca. 3 Ma), a value comparable to the model estimated future 2XCO₂ world. However, mid-Pliocene CO₂ estimates are modest, roughly 100 ppm above pre-anthropogenic values suggesting that more vigorous poleward heat transport was instrumental for maintaining Pliocene warmth. Compelling evidence indicates deep Atlantic thermohaline circulation play a critical role in maintaining and distributing global warmth during the mid-Pliocene by advecting relatively warm northern component water southward at roughly 3,000m depth. At depths of 1000-2000 m the Caribbean Basin is fed by a mixture of relatively corrosive southern component water and by a younger (less corrosive) northern component water mass. Enhanced foraminifera preservation and a gradual monotonic benthic d¹³C enrichment in the Caribbean basin since 4.6 Ma indicate a progressive strengthening of the upper northern component water relative to the southern component. Further south, however, no continuous high-resolution, chronologically constrained Pliocene time series exists in the South Atlantic which monitors upper deep water circulation. Here we present a 3 Myr high resolution (2.5kyr sample interval) benthic Cibicidoides wuellerstorfi isotopic record (1438 analyses) of Pliocene upper deep water circulation changes (ODP Leg 175, Site 1085, 29°22S, 13°59E, 1724 m)  in the Mid-Cape Basin of the South Atlantic. Today the Site 1085 is situated primarily in water of northern origin (Upper North Atlantic Deep Water) just below Upper Circumpolar Deep Water. Results show Site 1085 benthic d¹³C values were heavier throughout the Pliocene than the deep Pacific (Site 846) with differences ranging from roughly 0.6$  in the early Pliocene and 0.4$ in the late Pliocene. Between 3.5-3.3 Ma, a step towards lighter benthic carbon isotopes at Site 1085 is coeval with a step towards heavier values in the deep Pacific and at ODP Site 704 (2500 m depth) in the Southern Ocean. From 3.5-2.7 Ma interval, Site 704 was isotopically heavier in d¹³C the Site 1085 indicating northern component water shifted deeper in the water column while Southern Ocean sources increased at Site 1085. Coupling Site 1085 with new high resolution benthic isotope data from Site 607 in the North Atlantic (2.5-3.1 Ma) we investigate the history of North and South Atlantic deep water circulation during the onset of Northern Hemisphere glaciation. Using (Cross)Bi-spectral analysis at Site 1085, we investigate whether non-linear couplings exist between variance in the Milankovitch band and variance at higher (sub-orbital, <1/19kyr) frequencies for which no direct forcing is known to exist.

 

Philippe Bertrand¹, Tom Pedersen², Ralph Schneider³, Graham Shimmield⁴, Elisabeth Vergès⁵, Jean-Robert Disnar⁵, Delphine Massias¹, Juan Villanueva¹, Nicolas Tribovillard⁶, Alain Huc⁷, Xavier Giraud¹

¹ Departement de Géologie et Océanographie, UMR-CNRS 5805, Université de Bordeaux I, Talence, France

² Oceanography, Earth and Ocean Sciences, University of British Columbia, Vancouver, B.C. Canada

³ Fachbereicht Geowissenschaften, Universität Bremen, Bremen, Germany

⁴ Dunstaffnage Marine Laboratory, Oban, Argyll, Scotland, UK

⁵ Sédimentologie et Diagenèse de la Matière Organique, UMR 6531, Université d’Orléans, Orléans, France

⁶ Sédimentologie et Géodynamique, UMR-CNRS 8577,  Université de Lille 1, Villeneuve d’Ascq, France

⁷ Institut Français du Pétrole, Rueil Malmaison, France

 

bertrand@geocean.u-bordeaux.fr

 

Organic-rich sediments in ventilated deep-sea environments: climate, sea-level, and trophic changes.

 

Classical views for oil-prone and organic-rich sediment deposition in deep-sea environments invoke mainly two kinds of oceanographic and sedimentological settings. A first one is confined basins leading to water stratification and anoxic preservation of organic matter in the water column. A second one is open ocean where episodic mass transfers due to slope sediment instability may lead organic matter, originated from outer shelves and upper slopes, to be rapidely buried within the redeposited sediment and hence preserved from either oxic or anoxic degradation. However, different works showed that organic matter in modern deep-sea sediments may be concentrated where oxygen is not significantly depleted. Moreover, studies on jurassic anoxic sediments showed that productivity more than anoxia may have been the primary control for organic content variability. Here, we present results from southeastern Atlantic which show that bioturbated pelagic sediments at depths ranging from 1,000 down to more than 3,500 meters are highly enriched in hydrocarbon-prone organic matter. Such sedimentation has occurred for more than 2 millions years and is geographically distributed over hundreds of kilometers along the namibian margin, so that the sediments of this region contain a huge concentrated stock of organic carbon. A further insight in the anoxia/productivity paradigm comes from the fact that most of the organic content variability is due to relative concentration or dilution by carbonates. Such modern occurrence of potential source-rock in a present open ocean, such as the well-ventilated South Atlantic Ocean, is thus discussed in terms of local wind-driven productivity, trophic conditions, transfer efficiency, diagenetic processes, climate-related sea level and deep circulation. The probability of past occurrences of such oceanographic settings, at the edge of large oceanic basins should be carefully considered in deep offshore exploration.

 

Babette Boeckel and Karl-Heinz Baumann

FB 5 Geowissenschaften, Universitaet Bremen, Bremen, Germany

 

bboeckel@uni-bremen.de

 

Sedimentation of coccoliths in the SE-Atlantic during the last 250 ka: implications for surface-water conditions and carbonate accumulation

 

Coccoliths retrieved from 50 surface sediment samples off SW-Africa and from two sediment cores from the continental slope offshore Cape Town (GeoB 3603) and central Namibia (GeoB 1710) have been examined for changes in distribution and abundance. Furthermore biometric measurements were carried out in order not only to determine the carbonate contribution from coccoliths but also to study the degree of species-level variation and microevolution.

Located in the SE-Atlantic the study area offers an ideal opportunity of monitoring variations in warm- and coldwater transfer to the Benguela current system. Core GeoB 3603 underlies the northern branch of the Agulhas Current which is one of the world´s major western-boundary currents responsable for a substantial leakage of tropical and subtropical water from the South Indian Ocean into the S-Atlantic. The northward advection of Agulhas filaments even influences the Benguela upwelling region where core GeoB 1710 is located. Vertical and lateral shifts in the current´s movements and in the upwelling-intensity, especially during glacial intervals, would effect inter- and intra-ocean heat transfer and thus affect the thermohaline circulation.

Fluctuations in coccolith abundances are interpreted in terms of changing ecological parameters in the upper water column thus reflecting variations in surface-water dynamics and the degree of carbonate dissolution. The distribution of certain species corresponds relatively well to the hydrography of theoverlying surface-water masses but is also affected by selective dissolution. By means of different coccolith assemblages distinct oceanic environments can be distinguished. Calculations of the carbonate derived from coccoliths reveal a mean carbonate contribution of 20%.

In addition, analysis of the two sediment cores provides further information on variations in surface-water circulation and productivity in the SE-Atlantic during the past 250 ka. Both cores exhibit highest coccolith abundances during the interglacials 7 and 5. In either core the coccolith assemblages are dominated by G. ericsonii, G. muellerae, and E. huxleyi. Subtropical and tropical species play a minor role. The interglacial nannofloral assemblage consists of G. ericsonii, E. huxleyi, C. leptoporus, F. profunda and G. oceanica, indicating higher productivity and warmer surface water conditions. Whereas during the glacials G. muellerae is the dominant species, hinting towards colder surface waters. F. profunda, a species adapted to the lower photic zone exhibiting a sensitivity towards lower temperatures, shows fairly continuous abundances in core GeoB 3603 throughout the studied interval. This can be interpreted as a clue to incessant warm-water influx via the Agulhas Current.

 

Min-Te Chen, Y.-P. Chang, C.-C. Chang, K.-L. Jwo, L.-W. Wang, C.-H. Wang, and E.-F. Yu

Institute of Applied Geophysics, National Taiwan Ocean University, Keelung, Taiwan

 

mtchen@mail.ntou.edu.tw

 

Planktonic Foraminiferal Sea Surface Temperature Variations in the Southeast Atlantic Ocean: A High-Resolution Record MD962085 of the Past 400,000 Years from the IMAGES II - NAUSICAA Cruise

           

A high-resolution (~ 4 - 5cm / kyr) giant piston core record (MD962085) retrieved during an IMAGES II - NAUSICAA cruise from the continental slope of the southeast Atlantic Ocean reveal striking variations in planktonic foraminifer faunal abundances and sea-surface temperatures (SST) during the past 400,000 years.  The location and high-quality sedimentary record of the core provide a good opportunity to assess changes in the intensity and position of the Benguela Current System and the Subtropical Convergence, two key features of the ocean-climate system in the south Atlantic.  This record can be also used to evaluate the possible influence of Agulhas Current from the throughflow of the Indian Ocean into the South Atlantic.  The planktonic foraminifer faunal abundances of the core are dominated by three assemblages: (1) N. pachyderma (right coiling) + N. dutertrei, (2) G. bulloides, and (3) G. inflata.  The assemblage of N. pachyderma (right coiling) + N. dutertrei shows distinctive abundance changes which are nearly in-phase with glacial-interglacial variations.  High abundances of this assemblage are associated with major glacial conditions, possibly representing low SST / high nutrient level conditions in the southwestern Africa margin.  In contrast, the assemblages of G. bulloides and G. inflata show more high-frequency abundance change patterns, which are not well-parallel to glacial-interglacial changes.  These patterns may indicate rapid oceanic frontal movements from the south, and a rapid change in the intensity of Benguela upwelling system from the east.  A set of statistically-accurate paleoecological transfer functions of planktic foraminifers is necessary for reconstructing the paleoceanographic parameters of this core.  To meet this objective, we collected 446 published planktic foraminifer assemblage coretop data of the South Indian - Atlantic Ocean as well as the modern observation data of surface oceans to rewrite a new set of South Indian - Atlantic Ocean paleoecological transfer functions.  The correlation coefficients of the transfer functions for cold and warm season SST are 0.97 and 0.96 respectively, indicating that the transfer functions estimate the paleotemperature with good precision.  A winter-season SST estimate using transfer function techniques for this record shows primarily glacial-interglacial variations.  The SST reaches maxima during the transitions from the major glacial to interglacial stages (Termination II, III, IV), and is associated with the abundance maxima of a warm water species indicator G. ruber.  The relationship shown by the SST and planktonic foraminifer d¹⁸O implies that the SST maxima lead the d¹⁸O minima by approximately 3-5 kyr.  Preliminary results on the extraction of faunal SST gradients between core MD962085 and another IMAGES II core MD962077 will be presented and discussed.

 

Beth A. Christensen¹, Mark MASLIN², Megan Ellershaw², and James L. Kalbas¹

¹ Furman University, Greenville, USA

² Environmental Change Research Centre, Department of Geography, University College London, London, UK

 

Beth.Christensen@furman.edu

 

Orbital Variations in the Pliocene Cape Basin Sites 1085 and 1087.

 

In order to look at the response of the both the deep and surface water systems between 5 and 2 Ma., we analyzed sediments and well logs from Sites 1085 (29°22'S, 13°59'E,1713 m w.d.) and 1087 (31°28’S, 15°19’E, 1372 m w.d.). Today, these sites are bathed by AAIW which is underlain by NADW. At 5 Ma., however, NADW was just developing as global circulation reorganized in response to the progressive closure of the Isthmus of Panama.  These sites lie beneath the modern Benguela Current system, a major surface water  system which transports heat and salt across the equator into the Northern Hemisphere via the North Brazilian Coastal Current.  Site 1085 is in the zone of open ocean upwelling and is not affected by coastal upwelling.

We developed an age model by tuning the magnetic susceptibility logs to the eccentricity curve, basing the correlation on the shipboard biostratigraphic framework.  Spectral analysis indicates a dominant periodicity of 100 k.y.  We then applied this age model to other records to investigate the geochemical response of the surface and deep water.  Recent work suggests that the Onset of Northern Hemisphere Glaciation (ONHG) was preceded by a series of false starts at glaciation until changes in the orbital variation allowed for the full ONHG (Haug and Tiedemann, 1998; Driscoll and Haug, 1998). We find a similar pattern in the % sand, %TOC, C:N, and %CaCO3 records between 4 and 2.2 Ma. at Site 1085, suggesting a response in concert with preservational changes in the Caribbean Basin at Site 999.  Although Site 1087 has a coarser sampling resolution than 1085, Milankovitch periodicities are also suggested.  A 400 ky cyclity is also apparent in productivity indicators (e.g., benthic to planktonic ratios; G. bulloides size variations) in the period between 3.3 and 4.2 Ma.  Furthermore, oxygen isotope ratios at Site 1087 correspond with other Atlantic records of change preceeding the ONHG, indicating the Cape Basin is a good site for investigation of AAIW response to thermohaline circulation changes.

 

Lieselotte Diester-Haass¹, Phil A. Meyers², Thorsten Bickert³, Laurence Vidal⁴, and Gerold Wefer³

¹ Zentrum fuer Umweltforschung der Universitaet, Saarbruecken, Germany

² Department of Geological Sciences, The University of Michigan, Ann Arbor, USA

³ Fachbereich Geowissenschaften der Universitaet Bremen, Bremen, Germany

⁴ CEREGE, Universite Aix-Marseille, Aix en Provence, France

 

A.L.Haass @t-online.de

 

Late Miocene onset of high productivity in the Benguela Current Upwelling System: local or global control?

 

The Benguela Current area off SW Africa has very strong coastal and shelf edge upwelling and associated elevated primary productivity. Questions remain about the history of the high productivity. Is its onset and evolution related to changes in global climate, to intensification of regional trade wind circulation, to local changes in subsurface watermasses, or to changes in the local strength or position of the Benguela Current? We studied these questions in late Miocene sediments from ODP Site1085 in the middle Cape Basin (1700 m water depth) to explore the early history of the Benguela Current. High resolution benthic foram oxygen isotope values and a continuous XRF Fe profile were used to construct a precise time scale for the time interval 7-4.8 Ma based on orbital tuning. Biostratigraphy from Hole 1085A provided an age model prior to 7 Ma based on sedimentation rate. Sediments accumulate slowly before 7.5 Ma. Carbonate dissolution is strong in this early part of the record and creates a minimum in CaCO₃ content centered between 10 and 9Ma. Marine biological productivity was low as inferred from accumulation rates of benthic foraminifers and organic carbon. At 6.7 Ma the inferred paleoproductivity dramatically increases six-fold. Cyclic changes in productivity are well-developed from 6.7-4.5 Ma. Biogenous opal (radiolaria)appears for the first time at 5.4-5.0 Ma.This general pattern is comparable to findings in the tropical easternPacific, where a "carbonate crash" (Farrell et al.,1995) centered on 9.5 Ma occurs between 11.2 and 7.5 Ma and is followed by a dramatic increase in paleoproductivity at 6.7 Ma ("biogenic bloom": op.cit.) that is mirrored by similar increases in the western and northern Pacific and the Indian Ocean.The similarity between the records from the Pacific and Indian Oceans and the Site 1085 record indicates that the elevated carbonate dissolution prior to 7.5 Ma and the dramatic productivity increase at 6.7 Ma are responses to global processes.

 

Lydie Dupont, Barbara Donner, and Britta Jahn

Geosciences, University Bremen, Bremen, Germany

 

dupont@uni-bremen.de

 

Early to Mid-Pleistocene terrestrial input in and marine paleoproductivity off the Congo River (ODP Site 1075)

 

Strong carbonate dissolution causes sediments of the Congo deep-sea fan, in particular at the deeper Site 1075, to be very poor in foraminifers. Sediments of Site 1077, taken ca. 600 m upslope, contain more foraminifers but some interglacial sediments of this site are also barren because of large terrestrial input. Consequently the oxygen isotope curve shows gaps, but otherwise correlates well to the magnetic susceptibility. The latter is high during interglacial and low during glacial times, because the magnetic susceptibility in the Congo deep-sea fan largely depends on the terrestrial input of material either directly discharged by the Congo river or redeposited from the shelf. Therefore, in constructing the age model, we can use the continuous susceptibility curves to correlate between Sites 1077 and 1075 and to bridge the gaps in the oxygen isotope stratigraphy of Site 1077.

The pollen record of Site 1075 shows substantial change in the vegetation of west equatorial Africa taking place between 1.1 and 0.9 Ma. At 1.05 Ma there is a strong increase in the representation of the Afromontane forest by pollen of Podocarpus indicating lower

temperatures on the continent. Afterwards, the pollen record indicates repeated extension of the Afromontane forest during glacial stages and shrinking during rather short interglacial intervals. The high values for montane forest in Early Pleistocene glacial stages indicates that

these stages were less arid than the Late Pleistocene ones, when the highest representation of Podocarpus is found during the interstadials of Stage 5. Prior to 0.9 Ma (MPR after Berger & Jansen) no strong glacial-interglacial variation is found. At ca. 0.93 Ma the grass

pollen record declines to a lower level and after that date, records of grasslands, lowland forest, and mangroves show extensions during interglacial periods. Around the same time, the dinoflagellate cyst record changes from a Spiniferites-dominated association to a more

diverse one including small amounts of I. aculeatum. It probably indicates a reduction of river discharge by the Congo.

The same conclusion can be drawn from TOC records and stable isotopes  of bulk organic carbon. High TOC and high isotope values correlate  positively. Therefore, TOC contents can be used as an indicator for  marine productivity which is enhanced at the end of interglacial

periods. Between 2 and 1.1 Ma the d¹³C_org curve shows a tendency to  lighter isotope values

(-23 %), indicating an increase in terrigenous  input by the Congo. From 1.1 Ma until the Holocene, the d13Corg  record changed its mode and we observe a trend to heavier marine

values (-19%), indicating a reduction in river transport. These  long-term trends testify to a climate shift in west equatorial Africa already beginning about 1.1 Ma.

 

Emma Durham¹, Mark Maslin¹, Ellen Platzman², Antoni Rosell-Melé³, Jeremy Marlowe⁴, Dave Mattey⁵, Steve Burns⁶, and the Leg 175 Shipboard Scientific Party.

¹ ECRC, Department of Geography, UCL, UK

² Department of Geology, UCL, UK

³ Department of Geography, University of Durham, UK

⁴ FFEG, Newcastle Research Group, University of Newcastle, UK

⁵ Geology Department, Royal Holloway, University of London, UK

⁶ Stable Isotope Laboratory Geological Institute, University of Berne, Switzerland

 

edurham@geog.ucl.ac.uk

 

Reconstructing the climatic history of the western coast of Africa over the past 1.5 million years. A comparison of proxy records from the Congo Basin and the Walvis Ridge and the search for the Mid-Pleistocene Revolution

 

Two areas drilled by ODP Leg 175, sites 1076 and 1077 located at 5⁰S in the Congo Basin, and site 1081 at 17⁰S on the Walvis Ridge, are being investigated to reconstruct the history of oceanic and continental climate, sea-surface temperature, and productivity on the western coast of Africa over the last 1.5 million years. In particular, the effects of orbital forcing and the Mid-Pleistocene Revolution (MPR) on this area are being considered, including how the MPR manifests itself (sudden, smooth, “flickering”?), and how it alters the local ocean circulation, productivity and terrigenous inputs.

The MPR is the name given to the complex change in climate which occurred within the last 1 million years and led to the characteristic features of the late Pleistocene ice-ages, including increased mean ice volume, rapid terminations of glacial intervals, and a strong 100,000 year (100-Ky) periodicity. Prior to this, the climate system was warmer and more ice-free, and was dominated by a lower amplitude 41-Ky periodicity. The MPR has been established as being a global event, yet it is still not clear exactly when the event did occur, whether it was gradual or abrupt, or what mechanism was responsible for its cause (Mudelsee & Stattegger, 1997).

Analyses include stable isotopes, magnetic characterisation measurements, and total organic carbon and calcium carbonate concentrations. In addition, several organic geochemistry techniques provide evidence of sea surface temperature (U^k₃₇), productivity (pigments), and terrestrial vegetation and climate.

Preliminary results from both the Congo Basin and the Walvis Ridge support the hypothesis that productivity increases during glacial periods (Meyers, 1992), which may be the result of increased wind strength causing increased intensity of upwelling and hence nutrient availability. In the Congo Basin there is also evidence of a higher frequency cyclicity within the proxy productivity records, which is possibly in phase with the precession orbital forcing factor (Vidal et al., 1998). Records of terrigenous inputs to the site demonstrate that the Congo Basin receives an especially complex signal mostly from the Congo River whilst continentally-derived material which reaches the Walvis Ridge is predominantly wind-blown.

 

Mudelsee, M. & Stattegger, K. 1997: Exploring the structure of the Mid-Pleistocene Revolution with advanced methods of time-series analysis. Geologisch Rundsch. Vol. 86, 499-511.

Vidal, L., Bruchert, V., and Shipboard Scientific Party: 1998. Regional and stratigraphic patterns in colour reflectance of sediments from Leg 175. Proceedings of the Ocean Drillling Program, Initial Reports, Vol. 175.

Meyers, P. 1992. Organic matter variations in sediments from DSDP sites 362 and 532: evidence of changes in the Benguela Current upwelling system. In: Summerhayes, C. P., Prell, W. L., & Emeis, K. C. 1992. Upwelling systems: Evolution Since the Early Miocene. Geological Society Special Publication. No. 64, pp 323-329.

 

Oliver ESPER, Ralph SCHNEIDER, Karin ZONNEVELD and Helmut WILLEMS

Fachbereich 5 - Geowissenschaften, Universität Bremen, Bremen, Germany.

 

esper@uni-bremen.de

 

Palaeoceanological reconstruction of the Late Quaternary Agulhas Current (South Atlantic) based on calcareous and organic-walled dinoflagellate cysts

 

In the last decades it has become apparent that climatic change is greatly influenced by changes in the global  ocean current systems. One of the most important systems is the thermohaline circulation  of the Atlantic Ocean, which transports cold, higher saline water from the northern Atlantic to the southern hemisphere and in return  transports warmer water of the South Atlantic to the north, positively influencing the northern continental climate. To understand the fluctuations in the heat flux through glacial/interglacial changes in the Late Quaternary, it is necessary to study several key areas. One of such areas is the eastern South Atlantic, where warmer Indian Ocean water is carried around the cape of South Africa through the Agulhas Current into the Atlantic Ocean. The aim of the present study is to reconstruct the palaeoceanology of this region.

Changes in ocean currents can be detected through the analysis of different constituents of the seafloor sediments. One useful tool are dinoflagellate cysts. These micro-organisms possess a life cycle with a planktonic thecate stage, sensitive to environmental parameters like water temperature, salinity, and nutrient supply, and a corresponding resting cyst stage. Depending on the species, there are two different groups of fossilisable dinoflagellate cysts (dinocysts) distinguishable through the nature of their wall material: organic walled and calcareous dinocysts.

In this study, an approach is made to correlate and compare results of the analysis of organic walled dinocyst assemblages with calcareous walled dinocyst assemblages and the calcareous vegetative stage of Thoracosphaera heimii. Samples are taken from sediments of core   GeoB 3603-2 located in the southern Cape Basin offshore South Africa, and cover the time interval from oxygen isotope stage 6 (150.000y BP) to the Recent, thereby including the last three deglaciations. The results are used to reconstruct the palaeoenvironment and palaeoceanography of the Agulhas Current during glacial and interglacial times in order to gain more knowledge on the role of  the thermohaline circulation on global climatic change.

 

Raja .S. Ganeshram¹, D. Kroon¹, T.  F. Pedersen², G. L. Cowie¹, S. E. Calvert², R. R. Schneider³ and P. J. Müller³

¹ University of Edinburgh, Geology and Geophysics Dept., Edinburgh, UK

² University of British Columbia, Department of Earth and Ovean Sciences, Vancouver, Canada

³ Universitat Bremen, Fachbereich Geowissenschaften, Bremen, Germany.

 

raja.ganeshram@glg.ed.ac.uk

 

A comparative study of the timing of high-frequency oscillations (sub-Milankovitch) in upwelling records of the southern Atlantic (Benguela current), the eastern Pacific and the Arabian Sea margins

 

Recent studies report rapid changes in upwelling, productivity and oxygenation/

denitrification in coastal upwelling areas of the eastern Pacific and the Arabian Sea in sub-Milankovitch frequencies during the last 120 kyr (Schulz et al., 1998; Pedersen et al., 1999). These oscillations recorded variously as rapid changes in organic carbon and opal contents, in the presence or absence of laminations, and in d¹⁵N of mid-slope sediments correlate with Dansgaard-Oscheger climatic cycles recorded in Greenland ice cores (GISP 2). Warm interstadials in Greenland are matched by intensified upwelling, higher surface water productivity and poorly-oxygenated/denitrifying upper-intermediate waters off Pakistan, Northwestern Mexico and Southern California.  These northern hemisphere records will be compared with high-frequency changes in cores collected from the Benguela upwelling system of the South Atlantic (Little et al., 1997). In this area, upwelling events are recorded as cold SST and increase in organic carbon contents during the last 140 kyr. The phase relationships between Millenial-scale upwelling events of the South Atlantic and those of the eastern Pacific and the Arabian Sea margins will be discussed.

 

References:

Pedersen, T.F., K. Cannariato, J. Kennett, and R. Tada, 1999. Rapid Changes in SST, Productivity and Oxygenation on the Open Southern California Margin Correlate with Climate in Greenland During the Interval 30-60 kyr B.P. Abstract, EOS, AGU Fall Meeting, San Francisco.

Schultz, H., U. V. Rod and H. Erlenkeuser, 1998. Correlations between Arabian Sea and Greenland climate oscillationsof the past 110,000 years, Nature, 393, 54-57.

Little M.G. et al., 1997. Trade wind forcing of upwelling, seasonality and Henrich events as a response to sub-Milankovitch climate variability, Paleoceanography, 12, 568-576.

 

Xavier Giraud¹, Philippe Bertrand¹, Véronique Garçon², and Isabelle Dadou²

¹ Dept. de Géologie et Océanographie, UMR 5805 EPOC, CNRS, Université de Bordeaux I, Talence, France

² UMR 5566 LEGOS, CNRS, Toulouse, France

 

x.giraud@geocean.u-bordeaux.fr

 

Sensitivity of the NW African and Benguela upwelling systems to the sea level change: a modelling approach using nitrogen isotopes.

 

Coastal upwelling areas are places of a significant primary productivity and ocean-atmosphere exchanges as carbon fluxes. As the upwelling circulation and the biological development are strongly linked, any modification in the climatic conditions may have an impact on the entire upwelling system and on the carbon cycle. Here we investigate the sea level change effects on the circulation patterns and the biological response during the glacial-interglacial stages.

Our study deals with the NW African and Benguela coasts. In both cases, the continental shelves are large and shallow, inducing shelf recirculation cells that are responsible of the primary productivity increase. However, because the shelf break is deeper for the Benguela coast, the modifications of the circulation may be different during the sea level changes for the two upwelling systems.

In order to consider the complex effects of the sea level changes, we are using a physical and biogeochemical coupled model, simulating the upwelling circulation features and the biological development associated. The biological model simulates the evolution of four biological components (Nutrients, Phytoplankton, Zooplankton and Detritus). Our particularity is to model the nitrogen isotope ratio along the trophic food chain and in the sedimentary output, which is a proxy of the nitrate utilization level and therefore of the entire upwelling system evolution in time and space. Comparisons between sedimentary data and model output allow us to judge the accuracy of the palaeoclimatic scenarios we test.

 

Jacques GIRAUDEAU¹, Catherine PIERRE², and Laurence HERVE¹

¹ Département Géologie et Océanographie, UMR CNRS 5805 EPOC, Université Bordeaux I, Talence, France.

² Laboratoire d'Océanographie Dynamique et de Climatologie, Univ. Pierre et Marie Curie, Paris, France

 

giraudeau@geocean.u-bordeaux.fr

 

Late Quaternary evolution of the interocean leakages south of Africa: preliminary results from ODP Site 1087.

 

The southern Benguela region is one of the most complex hydrological system of the South Atlantic. There, three distinct processes are influencing the nature of surface and intermediate waters: upwelling of cold nutrient-rich South Atlantic central water, variable admixture of southern ocean waters induced by instabilities in the region of the Subtropical Convergence, input of warm, salty Indian thermocline waters and rings which are shedded from the Agulhas Current retroflection area. This last phenomenon is thought to be a key-component of the Atlantic heat conveyor. It also allows the transfer of allochtonous planktonic fauna from Indian subtropical water masses within a region otherwise characterized by populations adapted to the Benguela upwelling regime. The last ten years have seen a series of field, remote sensing and modeling studies aiming at documenting and understanding the mechanisms of Agulhas water input into the SE Atlantic.

Site 1087, which was drilled in 1371 m deep water, on the continental slope off Namaqualand, lays slightly outside (shoreward) the present main path of Agulhas shedded rings past the Cape of Good Hope. We may however surmise that, in the past, exceptional volume of Agulhas intrusion, as well as inshore advections of these warm waters under reduced south-easterlies winds,  might have affected the mixed layer at the location of Site 1087.

We are investigating the downcore distribution of planktonic foraminiferal assemblages to look at possible micropaleontological signature of such warm water pulses in the southern Benguela. Of special interest is the tropical species Globorotalia menardii, whose reseeding in the tropical Atlantic during several interval of the late Quaternary is thought to be tight to the dynamics of the Indian-Atlantic leakage. Preliminary data suggests that peak accumulation of G. menardii at Site ODP 1087 are synchronous with the stratigraphic position of the tropical zonal boundaries U-V, W-X and Y-Z, therefore supporting the assumed re-seeding mechanism of this species in the tropical Atlantic. Beside these short term events, marine isotopic stages 11 and 12 are characterized by the highest concentrations of G. menardii ever recorded at Site 1087 throughout the last 460 k.y.r. These patterns are discussed in view of other foraminiferal proxies of warm  water, and considering potential scenarios of atmospheric and oceanic circulation changes in nearby areas. 

 

Ute Güntner, S. Gebhardt, and J. Rullkötter

Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, Oldenburg, Germany.

 

guentner@icbm.de

 

Organic and isotopic geochemical analysis of lipids in sediments from the southwest African margin (ODP Leg 175, Holes 1075A and 1079A): Changes in the variation of organic matter sources

 

ODP Site 1075 is located in the deep-sea fan of the Congo River in the eastern Angola Basin. The depositional environment is influenced by freshwater supply from the Congo River, seasonal coastal upwelling, and incursions of open-ocean waters. In contrast to that sediments from Site 1079 should be unaffected by coastal upwelling and therefore provide information on open ocean background sedimentation.

In sediments from Hole 1075A the carbonate contents of the last 260 ka average around 3.0% whereas in sediments from Hole 1079A they are approximately five times higher. The total organic carbon (TOC) content in sediments of the last 260 ka from both holes ranges from 1.2% to 4.7%, with an average value of 2.8% (n_1075A=22; n_1079A=56). Carbon isotopic values of the organic matter in the range from ‑21.4 ‰ to ‑19.5 ‰, with an average value of ‑20.3 ‰ at both locations, are also similar to each other. At first sight these values indicate a high amount of marine organic material (d¹³C values of ‑20 ‰⁽¹⁾). The d¹³C values of terrestrial organic matter vary around ‑27 ‰⁽¹⁾, assuming that a contribution of C₄ plants to the bulk terrestrial organic matter is negligible. In a recent study, Muzuka interpreted very heavy d¹³C_org values in coastal marine sediments of East Africa as an indication for a substantial contribution of C₄ plants⁽²⁾. The d¹³C_org values in sediments from the Angola Basin determined by Müller et al. also could not be explained by a simple mixing of marine and C₃-type terrestrial organic matter⁽³⁾, whereas in Congo Fan deposits this model was applied successfully⁽⁴⁾.

In sediments from Hole 1075A and 1079A the d¹³C_org values show different trends in relation to the TOC contents. In the Congo Fan region heavier d¹³C_org values were observed with higher TOC values and in the Angola Basin sediments the opposite occurs. If these findings are at least partly due to different amounts of C₄-type terrestrial organic matter there should also be evidence on a molecular level.

The n‑alkanes in the lipid extracts of the sediments range from C₁₆ to C₃₅ with a strong predominance of C₂₉ and C₃₁ homologues (CPI_27-33 > 3), a distribution which is ascribed to higher plant leaf waxes. At Site 1079 these n‑alkanes are probably mainly transported by winds, but at Site 1075 a major amount could be supplied by the Congo River system. The d¹³C values of the individual homologues, measured by GC/irm‑MS, afford an estimate of the proportions of C₃ vs C₄ vegetation in the hinterland, as it varied through time^(5,6). Conspicuously heavy values are obtained for the C₃₃ and C₃₅ n‑alkanes, which may reflect enhanced C₄ biomass for these high carbon numbers. At Site 1079 the d¹³C values of the odd-numbered long-chain n‑alkanes follow the downcore trend of the d¹³C_org values over the last 160 ka indicating a significant contribution of C₄-type (e.g. grasses) terrestrial organic matter. Off the mouth of the Congo river organic matter from C₃-type (e.g. most trees in the Congo region⁽¹⁾) vegetation is probably more pronounced. No correlation between the d¹³C values of the long-chain n‑alkanes with the bulk d¹³C_org values could be observed there during the last 260 ka. It will be interesting to see if these results will also be found in older sediments (OIS 10 to 12).

Further preliminary lipid analyses have concentrated on the quantification of polar components such as n‑alcohols, n‑fatty acids, sterols, triterpenols and long-chain alkenones.

 

References

1. Mariotti A., et al., 1991. Chemical Geology (Isotope Geoscience Section) 86, 345-357.

2. Muzuka A. N. N., 1999.  Journal of African Earth Sciences 28, 757-766.

3. Müller P. J., et al., 1994. Nato ASI Series, Carbon Cycling in the Glacial Ocean: Constraints on the Ocean's Role in Global Change, Vol. 117 (ed. R. Zahn, et al.). Springer-Verlag.

4. Schneider R. R., et al., 1996. The South Atlantic: Present and Past Circulation (ed. G. Wefer, et al.), pp. 527-551. Springer-Verlag.

5. Rieley G., et al., 1993. Rapid Communications in Mass Specrometry 7, 488-491.

6. Bird M. I., et al., 1995. Geochimica et Cosmochimica Acta 59, 2853-2857.

 

Britta Jahn, Barbara Donner, Peter J. Mueller, Ursula Roehl, Ralph R. Schneider, and Gerold Wefer

Universitat Bremen, Fachbereich Geowissenschaften, Bremen, Germany.

 

bjahn@uni-bremen.de

 

Pleistocene Paleoclimatology in the Benguela Upwelling System for the last 1.5 Ma: Results from ODP Site 1082

 

Detailed records of foraminiferal oxygen isotopes, mass accumulation rates of CaCO₃ and TOC in combination with XRF-core-scanner analyses of elemental Ca and Fe for ODP Leg 175, Site 1082 from the Walvis Basin reveal the timing of paleoclimatic events in the northern Benguela system over the past 1.5 Ma. The most pronounced climatic changes detected in our records correspond to the Mid Pleistocene Revolution (Berger & Jansen, 1994) and the mid-Bruhnes carbonate maximum event (Jansen et al., 1986).

Stable oxygen isotopes have been measured on the planktonic foraminifera Globorotalia inflata to obtain a chronology for the last 1.5 Ma. A comparison between oxygen isotope data from Site 1082 with those from benthic foraminifera in the central equatorial deep Atlantic (ODP Site 664 after Raymo et al., 1997) show a good correspondence for glacial-interglacial cyclicity. In contrast to the benthic oxygen isotopes with a more or less constant glacial-interglacial range of isotopic values, planktonic isotope ratios at Site 1082 show a tendency to lighter values by about 1 per mil over the last 0.65 Ma.

General sedimentation rates are remarkably constant over the time period from 0.70 to Holocene, with pronounced drop from about 13.5 cm / ka before 0.70 Ma toward 10 cm / ka after 0.70 Ma, which is about 0.20 Ma after the MPR event after Berger & Jansen (1994).

In the Northern Benguela upwelling system carbonate fluxes and preservation were enhanced during interglacial intervals and show a long-term variability with maxima roughly every 400 ka.

Organic carbon accumulation taken as a paleoproductivity indicator and the input of terrigenous material by eolian transport (Fe counts measured by XRF) are also higher during interglacials compared to glacials with a pronounced amplitude in sediments older than 0.55 Ma BP. From 0.55 Ma BP to the Holocene TOC and Fe indicate an abrupt shift to a low-amplitude variability at a periodicity of 100 ka. The obtained sedimentary records indicate that the Benguela Upwelling system had been stable between 0.55 and 1.5 Ma. Since 0.55 Ma BP the upwelling intensity decreased, while warming took place in surface waters. This may have occurred in response to an increase in northern hemisphere glaciation.

 

Netherlands Institute for Sea Research, Texel, The Netherlands.

jansen@nioz.nl

 

SE Atlantic trade-wind zonality, a high-resolution CORTEX

record of eolian silts in Walvis Ridge sediments (ODP Site 1081)

 

The eolian minerals in ODP core 1081 from the eastern Walvis Ridge have two major continental igneous sources, the granitic igneous rocks of the Kalahari Craton in southern

Namibia and the gabbros of the Congo Craton in Angola. These two sources are reflected in the feldspar composition of the silt grains, microcline and albite representing the Kalahari and Congo Craton respectively.

Non-destructive XRF measurements with our CORTEX scanner produced high-resolution (250 y) records of the major elements (K to Sr) over the last 2.2 My. A preliminary

stratigraphy is developed by correlation of the Fe record with those of the nearby cores GeoB 1028-5 and MD 962094 for the last 400 000 y and extrapolation beyond.

A positive correlation between the Ti-Fe and microcline-albite ratios suggests that Ti is supplied as titanite by the southern granites. If this is true, the Ti-Fe ratio can be used as a proxy for the origin of the eolian dust on Walvis Ridge and, therefore, for the zonality of the trade winds.

 

Junghyun Kim, Ralph R. Schneider, Carsten Rühlemann, Stefan Mulitza, and Gerold Wefer

Fachbereich Geowissenschaften, Universität Bremen, Bremen, Germany

 

e-mail: jungkim@allgeo.uni-bremen.de

 

Evolution of past sea surface temperature in the subtropical eastern South Atlantic over the past 22 kyr: implications for oceanic heat transport

 

We investigated sediment core GeoB 1023-5 recovered in the eastern Angola Basin (17°09.5´S, 11°00.5´E) from a water depth of 1978 m (Wefer et al., 1988) in order to estimate past sea surface temperature (SST) for the transition from the Last Glacial Maximum (LGM) to the Holocene (Termination 1), using the alkenone method. According to our age model based on ¹⁴C determinations, the SST record in sediment core GeoB 1023-5 covers the last 22 cal. kyr B.P.. With 5 cm sampling interval, the average temporal resolution of sediment core GeoB 1023-5 achieves about 115 years. SST values increase from about 17.5°C in the LGM to almost 22.5°C in the Holocene. This warming trend at Termination I is interrupted by a cooling event for about 1000 years between 12 cal. kyr B.P. and 13 cal. kyr B.P., although the temperatures do not decrease again to glacial levels, as is often observed in North Atlantic records for this period. The timing of this cooling event corresponds to the "Younger Dryas (YD)" period (11.5 cal. kyr B.P. - 12.7 cal. kyr B.P.) observed in North Atlantic records. In contrast, for the "Heinrich Event 1 (H1)" (15 cal. kyr B.P. - 17 cal. kyr B.P.), the eastern South Atlantic record shows warming opposite to the North Atlantic where significant cooling is observed for this period. We compared this high temporal resolution SST record from the subtropical eastern South Atlantic with that of sediment core M35003-4 from the tropical western North Atlantic (Rühlemann et al., 1999). Apart from the H1, subtropical eastern South Atlantic SST variations are essentially out of phase with those from the tropical western North Atlantic. One possible explanation for the warming during the H1 is a reduced northward heat transport into the northern North Atlantic linked to the slowdown of North Atlantic Deep Water formation (e.g. Stocker et al., 1992). In contrast, for other time intervals including the YD period, the comparison reveals a fairly asynchronous SST response in the subtropical eastern South Atlantic relative to that in the tropical western North Atlantic. This indicates that the atmospheric mechanism associated with alterations of subtropical water vapour concentrations (e.g. Pierrehumbert, 1999) could have played a main role in the subtropical eastern South Atlantic surface circulation. This assumption is supported by comparison of SST records from subtropical eastern South Atlantic and tropical North Atlantic regions with climatic records from GRIP ice core (e.g. Blunier et al., 1998) and tropical eastern North Atlantic sediment cores (e.g. Bard et al., 1990; Zhao et al., 1995).

 

References

Bard, E., Labeyrie, L. D., Pichon, J.-J., Labracherie, M., Arnold, M., Duprat, J., Moyes, J., and Duplessy, J.-C., 1990. The last deglaciation in the southern and northern hemispheres: a comparison based on d¹⁸O, sea surface temperature estimates and accelerator ¹⁴C dating from deep sea sediments. NATO ASI Series. Series C: Mathematical and Physical Science, 308, 405-415.

Blunier, T., Chappellaz, J., Schwander, J., Dällembach, A., Stauffer, B., Stocker, T. F., Raynaud, D., Jouzel, J., Clausen, H. B., Hammer, C. U., and Johnsen, S. J., 1998. Asynchrony of Antarctic and Greenland climate change during the last glacial period. Nature, 394, 739-743.

Pierrehumbert, R. T., 1999. Subtropical water vapor as a mediator of rapid global climate change. In: Clark, P. U., Webb, R. S., and Keigwin, L. D. (eds) Mechanisms of global climate change at millennial time scales. 339-361.

Rühleman, C., Mulitza, S., Müller, P. J., Wefer, G., and Zahn, R., 1999. Warming of the tropical Atlantic Ocean and slowdown of thermohaline circulation during the last deglaciation. Nature, 402, 511-514.

Stocker, T. F., Wright, D. G., and Broecker, W. S., 1992. The influence of high-latitude surface forcing on the global thermohaline circulation. Paleoceanography, 7 (5), 529-541.

Wefer, G., Bleil, U. et al. 1988. Bericht über die METEOR-Fahrt M6-6, Libreville-Las Palmas, 18.2.1988-23.3.1988. Berichte, Fachbereich Geowissenschaften, Universität Bremen, 3.

Zhao, M., Beveridge, N. A. S., Shackleton, N. J., Sarnthein, M., and Eglinton, G., 1995. Molecular stratigraphy of cores off northwest Africa: Sea surface temperature history over the last 80 ka. Paleoceanography, 10 (3), 661-675.

 

Jeremy R Marlow¹, Carina B Lange² and Toni Rosell-Melé³

¹ Department of Fossil Fuels and Environmental Geochemistry, University of Newcastle, Newcastle upon Tyne, Newcastle, UK.

² Scripps Institution of Oceanography, University of California at San Diego, Geosciences Research Division, La Jolla, USA.

³ Department of Geography, University of Durham, Durham, UK.

 

J.R.Marlow@ncl.ac.uk

 

A 4.5 M.Y. Record of Benguela Upwelling at ODP Site 1084: A Combined Biomarker and Micropalaeontological Approach

 

ODP Site 1084 provides a continuous record of Benguela coastal current (BCC) upwelling off the coast of Namibia for the past 4.5 m.y. We have taken samples at a temporal resolution of approx. 50 k.y. to reconstruct past variations in sea-surface temperature (SST) and productivity to assess the timing and magnitude of the changes in upwelling that took place as a response to the cooling associated with the Pliocene-Pleistocene climate transition. The shift from relatively stable mid-Pliocene global warmth to cooler glacial-interglacial 100 k.y. cycles of the late Quaternary has been ascribed to a combination of tectonic and orbitally induced factors. However, quantifying the absolute decline in temperatures has been problematic owing to limitations in the micropalaeontological assemblage techniques previously employed. We have adopted the alkenone palaeothermometer technique (UK37') to reconstruct SSTs at Site 1084 and our results suggest that there has been a cooling by as much as 10 deg.C since 3.2 Ma with major fluctuations coincident with variations in the benthic oxygen isotope records from ODP Sites 659 and 846. By comparing our SST curve with micropalaeontological (diatoms) and geochemical reconstructions of palaeoproductivity at we conclude that the magnitude of this deterioration can be attributed to a combination of declining global

temperatures and intensification of upwelling. This is supported by a UK37' reconstruction at Site 1087 that lies further south and away from the upwelling region and shows a far smaller decline in SST over the Pliocene-Pleistocene epochs. These findings imply that Atlantic Ocean

surface circulation during the mid-Pliocene was more 'sluggish' than today and that the development of the bi-polar cryosphere promoted eastern boundary upwelling with consequences for global and regional climate.

 

Mark Maslin¹, V. Ettwein¹, E. Laurie¹, A. Rosell-Mele², C. Stickley¹, L. Vidal³, E. Platzman⁴ and G. Wefer³

¹ Department of Geography, University College London, London UK

² Department of Geography, University of Durham, Durham, UK

³ Fachbereich Geowissenschaften, Universität Bremen, Bremen, DE

⁴ Department of Geology, University College London, London UK

 

mmaslin@geog.ucl.ac.uk

 

De-coupling of Upwelling Intensity and Productivity of the Benguela Current System after the Intensification of Northern Hemisphere Glaciation at 2.54 Ma

 

ODP Leg 175 (Benguela Current) provides an opportunity to investigate the response of the South Atlantic to the intensification of Northern Hemisphere Glaciation and to identify potential positive feedbacks which may have enhanced this glacial process.  The Benguela Current not only transports heat and salinity across the S. Atlantic into the N. Hemisphere

but it is also one of the three most productive upwelling systems in the world, hence a major sink of atmospheric carbon dioxide.  Site 1083 is situated in the middle of the Benguela current in the open ocean upwelling zone and is not affected by coastal upwelling.  We have attempted to reconstruct; surface water conditions, wind intensity and productivity for the key period between 2.4 -2.7 Ma when the Laurentide ice sheet expands dramatically.

Site 1083 detailed age model between 2.4 and 2.7 Ma is based on benthic foraminifera oxygen isotopes which have been correlated to Site 607.  Surface water temperature, reconstructed using UK37-biomarkers, shows strong cooling during glacial stages 100, 98 and 96 (see Rosell-Mele et al., presentation for further details).  We suggest this reflects both regional cooling and an intensification of upwelling.  Reconstructed wind intensity, using magnetic parameters and geochemistry, also shows increases during these glacial periods providing the mechanism for the observed increase in upwelling intensity.  Surface water productivity is notoriously difficult to reconstruct, thus we have used a multi-proxy approach using:

pigments, total organic carbon, diatom assemblages, and nitrogen and organic carbon isotope.  The paleo-productivity peaks occur only during the initial cooling at the beginning of glacial stages 100, 98 and 96.  During the full glacial and interglacial conditions and the terminations the productivity is very low.  These peaks in productivity correspond to peaks in the accumulation of Antarctic diatoms and lighter nitrogen isotopes, suggesting either more or more fertile upwelling of Antarctic Intermediate Water at this site.  Hence it is not the amount of water that is upwelled but is nutrient content which is controlling productivity in the Benguela Current after 2.54 Ma.

 

Philip A. Meyers

Department of Geological Sciences, The University of Michigan, Ann Arbor, U.S.A.

 

pameyers@umich.edu

 

PLIOCENE-PLEISTOCENE EVOLUTION OF CARBON PRODUCTION AND BURIAL IN THE BENGUELA CURRENT UPWELLING SYSTEM

 

Comparison of the amounts of inorganic and organic carbon in Pliocene-Pleistocene sediments from ODP Sites 1081, 1082, 1084, 1085, and 1087 and DSDP Site 532 on the margin of southwest Africa provides insights into the evolution and oscillations of enhanced paleoproductivity associated with the Benguela Current upwelling system. Concentrations of

CaCO3, which record coccolith production in this system, are consistently about 75% at Sites 1085 and 1087 in the Cape Basin. In contrast, concentrations in sediments near the Pliocene-Pleistocene boundary drop to 25% at Sites 532, 1082, and 1084 in the Walvis Basin and on the Walvis Ridge because of greater opal production at these areas during this period.

Organic matter is predominantly marine in origin at all locations; its accumulation started to increase in the late Miocene and peaked in the Pleistocene. Total organic carbon (TOC) concentrations range between 0.1% and 18% and are highest sediments deposited at Site 1084 in the Walvis Basin. TOC concentrations are especially elevated in dark parts of prominent light-dark color alternations that record varying combinations of elevated productivity, downslope transport of organic-carbon-rich sediments from the shelf edge, and dissolution of CaCO3 in the seafloor. Both the production and preservation of organic matter appear to have fluctuated regularly during the evolution of this upwelling system. Moreover, the correspondence of higher Rock-Eval Hydrogen Index values, higher elemental C/N ratios, and higher TOC concentrations suggests that the rate of export production and the degree of preservation of marine organic matter in bottom sediments are related and have increased since the Miocene.

 

Gesine Mollenhauer

Fachbereich Geowissenschaften, Universität Bremen, Bremen, Germany;

 

mollenh@uni-bremen.de

 

Balancing C_org deposition in the Benguela upwelling region for glacial and interglacial periods

 

In this study, an attempt is made to regionalise data on organic carbon accumulation at different time slices. The aim is to quantify the amount of organic carbon being deposited in the Benguela Upwelling Region at glacial and interglacial times.

Based on several gravity core transects across the continental slope, accumulation rates are calculated for the respective time periods. Supplementing the core data, sediment thickness maps derived from sediment echograph data will be used.

In order to calculate reliable accumulation rates, the stratigraphy for the cores investigated must be very precise. Even for high resolution cores, it may be difficult to exactly determine sediment levels representing distinct short time slices, such as the LGM. Therefore it was decided to first integrate over longer time periods in order to gain a general picture of the processes and to minimise possible sources of errors. Representing glacial conditions the period including oxygen isotope stages 2, 3 and 4 was chosen. Interglacial conditions are expected to be exemplified by sediments from stage 5.

For three gravity core transects, stratigraphic correlation between the cores using multi-parameter approaches have been established. Additional data on a fourth transect is presently measured. At sites of very high sedimentation rates, the core recovery from simple gravity coring does not reach back to stage 6.0. Therefore, including data ODP cores and piston cores from the IMAGES programme in the study will be required.

 

Maria E. Pérez and Wolfgang H. Berger

Scripps Institution of Oceanography, UCSD, La Jolla, U.S.A.

 

meperez@ucsd.edu

 

LATE QUATERNARY PRODUCTIVITY FLUCTUATIONS IN THE MID-ANGOLA BASIN: EVIDENCE FROM BENTHIC FORAMINIFERA

 

The abundance and faunal composition of benthic foraminifera are good tracers of environmental changes at the seafloor and provide information about the productivity of the surface water. ODP Site 1079 has been analyzed for its benthic foraminifera content and isotopic composition to reconstruct late Quaternary productivity changes. This site, located in the Mid-Angola Basin (11°55.78'S, 13°18.54'E, water depth 749 m), is of special interest to document variations in the Angola Dome upwelling and the position of the Angola-Benguela Front.

To date, our results show a good agreement between the benthic foraminifera accumulation rates (BFAR) and other paleoproductivity proxies (namely, planktic foraminifera and G. bulloides accumulation rates). Increase in BFAR values and dominance of infaunal species occur during glacials reflecting high flux of organic matter and low oxygen bottom water conditions. In addition, the occurrence of opportunistic species suggests that seasonality had a significant impact on the benthic community. Interglacials are characterized by the presence of epifauna and decrease of infauna and BFAR, which suggests lower productivity and more oxygenated conditions. Spectral analyses indicate that benthic foraminifera in the Mid-Angola Basin varied especially in response to changes in eccentricity and precession.

 

Jean-Jacques Pichon¹, Jacques Giraudeau¹, Gabriel Vargas^1,2,3, Luc Ortlieb³ and Joan VILLANUEVA⁴

¹  Département de Géologie et Océanographie, UMR CNRS 5805, Université Bordeaux I, Avenue des Facultés, 33405 Talence, France

²  Departamento de Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile

³ Institut de Recherche pour le Développement, 32 Avenue Henri-Varagnat, F-93143 Bondy-Cédex, France

⁴ Institut d'Investigacions Quimiques i Ambientals de Barcelona.(CSIC), Barcelona, Spain.

 

pichon@geocean.u-bordeaux.fr

 

Paleoceanographic changes in the Benguela Upwelling System (Lüderitz cell) as indicated by abundances of Chaetoceros resting spores - Comparisons with upwelling data off northern Chile (23°S)

 

A sediment core from the continental slope off Lüderitz, MD96 2087 (25°48’S - 12°7.7’E, 1026 m. water depth), reveals striking variations in diatom abundances. These fluctuations vary between total absence and 3 x 10⁹ valves.g^-1. This variability is investigated to assess changes in the intensity of the upwelling process off central Namibia. The downcore distribution of diatoms documents a series of episodes with respect to the dominant pennate and centric diatom orders, and Chaetoceros resting spores. The episodes of high Chaetoceros resting spores are interpreted as evidence of increased upwelling stress, and associated increase in nutrient input. The graphic representation shows similarities with a  glacial-interglacial pattern of isotope stages. This time scale, if taken at face value, would indicate that large changes in upwelling intensity occurred between warm and cold periods  (high abundances of Chaetoceros corresponding  to glacial conditions and substages 7.4, 7.2, 5.4, 5.2; the minima to interglacial periods). However, this preliminary biostratigraphy must be regarded as tentative in the absence of AMS dates and isotopic studies. Other proxies (see Villanueva et al., this volume) suggest  that this sedimentary sequence could provide an exceptional high-resolution record for the last 190,000 years in the Benguela Upwelling System.

This study opens the possibility of detailed comparisons between the Benguela Upwelling System and the upwelling processes off the coast of northern Chile at the same latitude (Mejillones Bay,  23°S). This bay is dominated by biogenic siliceous sedimentation as a result of the high productivity of the upwelled waters. An exceptionally well-preserved, undisturbed laminated sedimentary sequence encompasses the last 1,000 years (core 33C) with sedimentation rates of 50-330 cm/10³ years. The diatom records is caracterized by high variations  of Chaetoceros resting spore abundances (between 1 and 400 x 10⁶ valves g^-1). Some laminae, as thin as 0,01 mm, may correspond to monospecific blooms. Episodes of intensive upwelling are documented by successive peaks in abundance of productivity indicators such as Chaetoceros resting spores.

Further investigation on timing and correlation of these events is necessary to understand the complexity of the upwelling systems and inter-ocean teleconnections.

 

Catherine PIERRE¹, and Marie-Thérèse VENEC-PEYRE²,

¹ Université Pïerre et Marie Curie, LODYC, Paris, France

² Muséum National d’Histoire Naturelle, Laboratoire de Paléontologie, Paris, France

 

Catherine.Pierre@lodyc.jussieu.fr

           

SURFACE AND DEEP OCEAN VARIABILITY DURING THE LAST 330 KY IN THE LUDERITZ UPWELLING SYSTEM

 

The Lüderitz upwelling cell is one of the most active of the upwelling system which borders the West coast of South Africa. During the Images 2-Nausicaa cruise, a transect of cores was realized in the Lüderitz cell to estimate the variability of the upwelling activity related to the climate changes. The deepest core of this transect, MD 96-2086 (25°48.8 S/12°7.7 E) retrieved at 3606 meters of water depth lies presently in the North Atlantic Deep Water (NADW). This core has been studied down to 20 meters at a sampling resolution of 10 cm, for measuring the oxygen and carbon isotopic compositions of planktonic (Globigerina bulloides, Globorotalia inflata) and benthic (Cibicides wuellerstorfi) foraminifers and for counting of planktonic foraminifers.

The base of the 20 meters-thick sedimentary sequence corresponds to the transition between isotope stages 9 and 10. The isotope record is given at a resolution of 2 ky to 4 ky, but it is interrupted in the interval between 14.2 m and 14.8 m due to the quite complete carbonate dissolution; this interval corresponds to isotope stage 6.6 and it is characterized by a very high organic carbon content (max 7.24%).

The oxygen isotope variations of the two planktonic foraminifer species and fluctuations of the foraminiferal assemblages are controlled both by the variations of global ice volume and sea surface temperature during the glacial-interglacial cycles. The D¹⁸O gradient between G. inflata and G. bulloides displays important fluctuations which are interpreted as specific responses to the surface water circulation in the area. Episodes of warm water influx are inferred at the MIS.3/MIS.4 transition and during MIS 5.5, while massive influx of cold water occurred during MIS 8 and probably at the MIS 6/MIS 7 transition; a significant increase of the warm water species (i.e. G. ruber and N. dutertrei) is also evidenced during MIS 5.5. The D¹⁸O gradient between the surface and deep foraminifers may help to deduce variations in the upwelling activity: periods of intense upwelling are evidenced during all glacial stages although short periods of upwelling relaxation occurred during MIS 2 and 6.

The d¹³C values of benthic foraminifers record the variations in the NADW ventilation : high d¹³C values during interglacials where the ventilation was high and low d¹³C values during glacial stages where ventilation was low. In the surface dwelling foraminifers, the d¹³C records are more complicated probably due to latitudinal and zonal migrations of the frontal structures. This is well documented by episodes of sharp decreases of the d¹³C values of G. bulloides as at the MIS 3/MIS 4, MIS 5/MIS 6 , MIS 9/MIS 10 transitions and during MIS 5.5. The D¹³C gradient between surface and deep foraminifers indicates the permanency of high productivity levels for the last 330 ky, except a few short periods of major decrease of productivity during MIS 5.5, 7.3, 7.5.

 

Amanda RAU¹, Julia LEE-THORP², John ROGERS¹, and Jacques GIRAUDEAU³

¹ Department of Geological Sciences, University of Cape Town, Rondebosch, South Africa

² Archaeometry Laboratory, University of Cape Town, Rondebosch, South Africa

³ Departement Geologie et Oceanographie, University of Bordeaux I, Talence, Cedex, France

 

amanda@beattie.uct.ac.za

 

LATE QUATERNARY FLUCTUATIONS IN THE CIRCULATION PATTERNS OF THE SOUTHERN BENGUELA SYSTEM

 

Marine sediments deposited on the continental margins off southern Africa encode the history of water column conditions, dynamics and productivity and hold the key to understanding regional responses and global climatic shifts.  The importance of the relationship between marine productivity in the Benguela upwelling system and regional climate is well known. The history of the northern Benguela upwelling system is relatively well understood, but little is known of the southern Benguela system.  It is expected that the southern Benguela had a more complex history due to greater sensitivity to latitudinal shifts in the South Atlantic Anticyclone, as well as fluctuating heat and salt transfer from the Indian Ocean.  Changes in circulation patterns of this system would affect the position and intensity of upwelling cells, climate on the subcontinent and mirror wider changes in atmospheric and ocean circulation.

A giant gravity core, MD962080, taken on the Agulhas Bank Slope was sampled at 10 cm intervals.  Subsamples were prepared and analysed for stable nitrogen and carbon isotopes to provide a history of productivity.  Weight percentage calcium carbonate was acquired through treatment of samples prior to carbon isotope analysis.  Stable carbon and oxygen isotope analyses were undertaken on the planktonic foraminifera Globorotalia inflata and the benthic species Cibicides wullerstorfi in order to establish a time scale.

 Planktonic foraminiferal assemblages have been examined for fluctuations in abundance.  Planktonic foraminifera reflect sea surface conditions and hence monitor temporal shifts in circulation patterns.  Fluctuations in abundance provide insight into the effect of warm, saline Indian Ocean water input into the South Atlantic via the Auglhas retroflection.  The mixture of subtropical, transitional and subantarctic fauna suggests that during glacial periods there is decreased input of warm, saline Indian Ocean water into the Benguela system, possibly due to the northward migration of the Subtropical Convergence and early retroflection of the Agulhas Current.

Comparison of preliminary results of stable nitrogen and carbon isotopes and carbonate content from cores MD962080 and MD962084 suggest that the southern Benguela has experienced greater fluctuations in productivity and upwelling than the northern Benguela.  Latitudinal movements of the upwelling system in response to circulation changes are indicated by variations in the isotopic ratios. Core MD962080 indicates lower nitrate availability on the Agulhas Bank than in the main Benguela upwelling system.  Periods of upwelling have been confined and constrained spatially and chronologically as indicated by the consistently depleted isotope values in core MD962080.  Strong upwelling episodes in the southern-most extremity of the Benguela system are the exception, whilst in the central and northern Benguela periods of weak or no upwelling are anomalous. Similarities in the downcore fluctuations in the two cores suggest that changes in the earth’s orbital forcing are a fundamental cause of major climate changes during the Late Quaternary.  The data point to a change in orbital forcing from 100 kyr to 41 kyr as well as to a major change in local and regional oceanography.

 

Rebecca S. Robinson and Philip A. Meyers

Department of Geological Sciences, University of Michigan, Ann Arbor, USA

 

rsrobins@umich.edu

 

Late Pliocene records of upwelling intensity and biogeochemical cycling from the Benguela and California Current systems (2.3- 2.0 Ma)

 

The complete sedimentary records drilled during Legs 175, along the Southwest African margin, and 167, from the California margin, both show regional maxima in upwelling intensity during the Late Pliocene.  In the Benguela record, it is manifested by elevated opal deposition centered at ~2.2 Ma (Shipboard Scientific Party, 1998).  The upwelling peak in the California record is marked by a broad (3.5- 2.0 Ma) regional high in carbonate accumulation that varies in time along the margin (Ravelo et al., 1997).  Two sites from each of the major upwellings were chosen to compare the Late Pliocene upwelling peaks at higher resolution.  Sites 1082 and 1084 from the Benguela are compared to Sites 1012 and 1014, drilled from two California Borderland basins.  The sites were selected because they contain cyclic variations in organic carbon and carbonate concentration and their respective upwelling peaks appear to be approximately synchronous. 

High-resolution carbonate, opal, and organic carbon concentrations and C and N isotope results, within the broader productivity maxima, show variability at shorter time scales.  The different proxy data sets do not co-vary and therefore do not provide a clear picture of climate-driven upwelling intensity and productivity changes.  The C and N isotope records suggest that the two nutrients are recording different processes within the upwelling system, perhaps due to the relative sizes of the nutrient pools or geochemical alteration at the seafloor. The differences in the surface productivity proxy records illustrate the complexity of the biogeochemical processes at play in major upwelling systems.  The comparison of the California and Benguela Current Upwelling Systems at 2.3 to 2.0 Ma provides a unique opportunity to evaluate global (?) climate-driven upwelling changes while gaining fundamental information about upwelling systems in general.

 

References

Ravelo, A. C. et al., 1997. Pliocene carbonate accumulation along the California margin

Paleoceanography, v. 12, 729-741.

Shipboard Scientific Party., 1998. Introduction: Background, scientific objectives, and principal  results for Leg 175 (Benguela Current and Angola-Benguela Upwelling Systems).  In: Wefer, G., Berger, W. H., and Richter, C., et al., Proceedings of the Ocean Drilling Program Initial Reports, 175, Ocean Drilling Program, College Station, TX, p. 7-23.

 

John Rogers¹, Amanda Rau¹, and Min‑Te Chen²

¹ Department of Geological Sciences, University of Cape Town, South Africa.

² Institute of Applied Geophysics, National Taiwan Ocean University, Keelung, Taiwan

 

jrogers@geology.uct.ac.za

 

Evidence of Latitudinal Fluctuations of Watermasses in the South Atlantic from Ice‑Rafted Detritus off the West Coast of South Africa

 

During the 19th Century there were several well‑documented reports of icebergs sighted in latitudes as low as 36^o S, immediately south of the Cape of Good Hope. Subsequently, faceted and striated ice‑rafted detritus (IRD), including gneiss boulders, was trawled from the continental slope west of Cape Town (34^o S).  Routine research‑trawls for demersal fish on sandy bottoms on the outer shelf recovered geologically exotic boulders of amphibolite and granite as close to the equator as 32^o S off Namaqualand, SW of the Orange River.  During surveys of manganese nodules on the continental rise, SW of Cape Town (37^o S), bottom photographs revealed IRD cobbles and the sampled sediment was rich in IRD granules, as were the younger, outer parts of the manganese nodules.  These regional observations are now being complemented by detailed studies of a giant piston core, the 22m‑long MD962080 from the western continental slope of the southern tip of the continental margin of Africa in latitude 36^o 16' S and longitude 19^o 28' E in 2488m of water.  Although the sediment is primarily a foraminiferal ooze, there is a significant abundance of sand‑size quartz, mainly very fine sand, with traces of angular very coarse sand, interpreted as ice‑rafted detritus.  This quartzose sand has been concentrated by wet‑sieving, dissolving calcareous microfossils with dilute hydrochloric acid, floating off siliceous sponge spicules and radiolaria with sodium polytungstate and magnetically separating the slightly magnetic glauconite pellets from the quartz.  Oxygen‑isotope determinations on a single species of planktonic foraminifera, Globorotalia inflata, have shown that the abundance of quartzose very fine sand increases with the  ¹⁸O value in the calcite of the foraminifera.  This is interpreted as a cooling of Sea Surface Temperature (SST) due to the preferential loss of the lighter ¹⁶O isotope during evaporation, its temporary entrapment in the polar icesheets during glacial periods (polar and subpolar Ice Ages) and the consequent enrichment of surface water in the ¹⁸O isotope.  In addition, the abundance of right‑coiling Neogloboquadrina pachyderma increases in sympathy with that of the IRD sand, suggesting that, off South Africa, this species is an indicator of colder water.  This implies that the SST over the southern tip of the continental margin of Africa cooled significantly during glacial periods and that icebergs were able to drift to lower latitudes than is the case in the modern interglacial Holocene epoch. This may also mean that, during Late Pleistocene hypothermals, the Subtropical Convergence shifted significantly equatorwards and that the Agulhas Current was unable to exchange heat with the South Atlantic Ocean to the same degree as is the case today.

 

Antoni Rosell-Mele¹, Mark Maslin², Ralph Schneider³ and Laurence Vidal³

¹ Department of Geography, University of Durham, Durham, UK

² Department of Geography, University College London, London UK

³ Fachbereich Geowissenschaften, Universität Bremen, Bremen, Germany

 

antoni.rosell@durham.ac.uk

 

Effects of the culmination of the Onset of Northern Hemisphere Glaciation in SE Atlantic productivity

 

We have measured the concentrations of biomarkers and alkenones in  ODP Site 1083 from the Southeastern Atlantic, during the interval 2.7 - 1.8 Ma at a resolution of 2.5kyrs. The agreement of the sea surface temperatures as inferred from UK37’ with planktonic d18O is remarkable, and a marked cooling in the Benguela Current is evident during oxygen

isotope stage 96 / 98 / 100. The shape and form of the cooling of UK37' is also almost exactly the same as the ice volume curve (d18O). Large productivity peaks, as inferred from chlorin pigments, correlate with the rapid cooling at the beginning of 96 / 98 / 100, and maintain high values during the cold periods.  As soon as there is the first sign of warming the productivity values plummet. We argue that the increase productivity is due to changes in upwelling strength. The data shows that there is a strong Southern hemisphere response to the final stages of the intensification of the northern hemisphere glaciation. It could be speculated that the observed productivity changes could have increased down-drawing of atmospheric carbon dioxide which would have contributed to the intense cool period that lasted for 120 ka.

 

Ulrich STRUCK¹, Kay-Christian EMEIS¹, Jürgen ALHEIT¹, and Ralph SCHNEIDER²

¹ Baltic Sea Res. Institute Wärnemunde, Rostock, Germany

² Fachbereich Geowissenschaften,, Universität Bremen, Bremen, Germany

 

kay.emeis@io-warnemuende.de

 

Holocene variability of upwelling from sediments in the diatomaceous mud belt offshore Namibia.

 

We investigated a kastencore and multicores taken from the rapidly accumulating diatomaceous muds on the Namibian shelf to trace decadal-scle fluctuations of coastal upwelling by sedimentological, isotopic, geochemical, and paleo-ecological methodss. One specific objective is to reconstruct the fluctuations of fish populations (indicative of oceanographic and ecologic conditions) using the occurrence of fish scales in varved sediment layers and to correlate these fluctuations with paleo-environmental conditions. The partly laminated core was dated by ¹⁴C AMS and subsampled at high resoltuion (1cm intervals), yieldinga resolution of about 3 to 10 years per sample. The interval for which data are available covers the last 4000 years. The core contains scales of all fish species which are associated with a typical upwelling community: sardines, anchovies, mackerel, jack mackerel, hake, and myctophids. Accumulation rates of organic matter and fish scales fluctuate considerably. The variability in abundance and type of fish scales at different depths indicates drastic fluctuations of fish abundance in the Benguela Current during the last 4000 years, as well as changes in the fish community structure. The paleo-environmental conditions associated with the different phases of fish population abundance are reflected in the frequency of laminations (indicative of anoxic conditions on the shelf), variations of stable isotopes of organic matter (d¹³C_TOC, range from -22.7%o to -17.9%o, and d¹⁵N range from 4.78 to 8.7 %o) and alkenone unsaturated ratios (a proxy of paleo sea-surface temperature, range from 14.7°C to 17.0°C).

It is unclear, whether the data set from one location is affected by spatial inhomogeneities of sedimentation; this will be checked by analysing additional cores from an upcoming expedition (August 2000). When assembled, the multidisciplinary data set will contribute to a better understanding of causes for and amplitudes of natural fluctuations in the proximal upwelling sector of the eastern boundary current, particularly when compared with on-going work in the California and Humboldt Currents off North and South America.

 

Jan-Berend W. Stuut^1,2, Maarten A. Prins³, Ralph R. Schneider⁴, Gert Jan Weltje⁵, J.H. Fred Jansen¹ and George Postma²

¹ Netherlands Institute for Sea Research (NIOZ), Den Burg, The Netherlands

² Faculty of Earth Sciences, Utrecht University, Utrecht, The Netherlands

³ Faculty of Earth Sciences, Free University of Amsterdam, Amsterdam, The Netherlands

⁴ Fachbereich Geowissenschaften, Universität Bremen, Bremen, Germany

⁵ Delft University of Technology & Netherlands Institute of Applied Geoscience TNO, Delft, The Netherlands

 

jbstuut@nioz.nl

 

Late Quaternary climate variability in SW Africa: grain size of terrigenous sediments from Walvis Ridge as a proxy for wind strength and aridity

 

Terrigenous sediments recovered from a piston core from Walvis Ridge, SE Atlantic Ocean, reveal a history of abrupt changes in SW African climate during the last 240 kyr. Application of an end-member modeling algorithm (Weltje, 1997) to 277 grain-size distributions, results in three distinct end member grain-size distributions. The two coarsest end members are interpreted as eolian dust, the third less sorted end member is interpreted as hemipelagic sediment. The ratio of the two eolian over the hemipelagic end member is interpreted in terms of variations in aridity. The ratio of the two eolian end members reflects the eolian grain size and is interpreted in terms of variation in the intensity of the SE trade wind. The aridity record shows a good correlation with the d¹⁸O-record of G. ruber (w), reflecting relatively arid climate conditions during interglacial stages compared to relatively humid climate conditions during glacial stages. The anticorrelation of the aridity record with records from the Arabian Sea confirms the predicted antiphase sensitivity to precessional-insolation forcing of climate on both hemispheres. The wind-strength record shows a high-frequency, sub-Milankovitch signal during the last 80 kyr, whereas from 300-80 kyr a lower-frequency signal is observed. The similarities between the records of wind strength and upwelling intensity, the latter derived from the cold-water species N. pachyderma (s)-records in core GeoB 1711 (Little et al., 1997b), emphasizes the importance of the trade-wind system, controlling the oceanographic setting in the SE Atlantic Ocean.

 

SandraVaqueiro¹; Catherine Pierre² , Delphine Massias³ , and Fatima Abrantes¹

¹ Instituto Geológico e Mineiro, Departamento de Geologia Marinha, Portugal

² Université de Bordeaux I, Département de Geologie et Océanographie, France

³ Université Pierre et Marie Curie, Laboratoire d’Oceanographie Dynamique et de Climatologie, France

 

sandra.vaqueiro@igm.pt

 

The 185 kyr "C_org event": A diatom perspective

 

The Benguela region is one of the great coastal upwelling systems of the world, extending from Cape Frio (18^oS) to Cape Town (34^oS). In this area, upwelling occurs preferentially at seven cells (Cunene, Namibia, Walvis Bay, Lüderitz, Namaqua, Columbine and Peninsula). The central upwelling cell (Lüderitz, 27^o S) registers the most intense values (> 180 gC/m²/yr) To the south of Lüderitz, the cells are less intense but equivalent between them while to the north, the intensity decreases towards the equator.

In order to better understand productivity variations associated to the Benguela upwelling system during glacial stage 6, diatom accumulation rates (DAR) and assemblages composition were determined and compared to the existing C_org data on two cores, IMAGES MD96-2086 (Lüderitz cell) and ODP-175-1083 (Namibia cell).

In core MD96-2086 DAR agrees well with the C_org record, revealing stage 6 as a productive one. However, at 185 kyr, an inverse behaviour is observed between the two proxies; C_org shows a maximum while DAR decreases to 1/3 of the flux observed for the rest of stage 6. The fact that this phenomena was also observed on core M1710 (Walvis Bay cell) (Abrantes, F. et al., 1994), points to an “event” with expression, at least, in the central region of the Benguela system..

 

References:

Abrantes, F; Moreno, J.; Pinto, J.; Winn, K.; Muller, P.J.; Schneider, R., 1994.  Productivity variations due to Upwelling Oscillations off SW Africa during the last 180 kyr. South Atlantic Symposium. Berichte, Fachbereich Geowissenschaften, 52, 10.

 

Gabriel Vargas^1,2,3, Luc Ortlieb³ and Michel Pujos¹

¹  Département de Géologie et Océanographie, Université Bordeaux I, Talence, France

²  Departamento de Geología, Universidad de Chile, Santiago, Chile

³  Institut de Recherche pour le Développement (IRD, ex-ORSTOM), Bondy, France

 

Gabriel.Vargas@bondy.ird.fr

 

Paleo-wind reconstruction from sedimentological and climatological studies in an upwelling area off northern Chile (23ºS): First results and perspectives

 

The study area is located on the coastal fringe of the core of the hyper-arid Atacama Desert in northern Chile. Climate is characterized by a high stability in the low atmosphere, due to the inter-seasonal permanency of  the high-pressure cell of the South-Eastern Pacific. This synoptic feature is responsible for the predominance of SW winds which drive strong upwelling processes along the coast of northern Chile and southern Peru. Mejillones Bay (23°S) is situated close to one of the major upwelling centers, to the north-east of Mejillones Peninsula. This geographical situation protects the embayment from the regional northbound longshore drift. Regional and local morphological particularities of the continental margin associated to oceanographic circulation patterns explain that exceptionally low (<0.1 ml/l) oxygen concentration are found at the bottom of the water column within the bay, at a 80 to 120-m depth. Besides, this region is episodically affected by the ocean-climatic anomalies of the El Niño-Southern Oscillation system.

Mejillones Bay acts as a depocenter of primary productivity material with sedimentation rates of 15-50 cm/10³ y. The sediment is a diatomaceous ooze (60-80% of diatom frustules) which also includes foraminifers, fish scales, and a relatively high content of organic matter. Petrographical, mineralogical and grain size analyses of the terrigenous fraction (5-10% of the total sediment) show that quartz and feldspaths grains are almost exclusively of aeolian origin.

High-resolution stratigraphical analyses (SCOPIX image analysis, thin section microscopy), sedimentological characterization of layers (IRTF, microscopy, SEM, X-ray diffraction) and first geochronological results (AMS radiocarbon, ²¹⁰Pb) of gravity and box cores aim to reconstruct manifestations of the ocean-climatic variability in the course of the last 2,000 years. Inter-annual variability can be studied, at least in part of the cores, at a sub-mm scale. Inter-decadal to secular  variability is depicted by an alternance of clear and dark mm to cm-thick layers. Infra-mm lenticular laminae are interpreted as indicators of strong upwelling events. Layers with a high content of biogenic material generally also contain a higher density of terrigenous sediment. This co-variation of the biologic and lithic remains suggests that these layers correspond to episodes of  enhanced upwelling processes with stronger aeolian inputs.

The exceptional conditions of sedimentation (no alluvial input, very high primary productivity) and of preservation (no bioturbation and limited diagenetic alteration) of the sediment accumulated in the center of Mejillones Bay offers an unusual opportunity to reconstruct variations of aeolian particle supply, and thus of paleo-wind intensity, during the last few millennia. The down-core study of the aeolian input variations can be constrained by a parallel study of the phytoplankton variations.

The Eolos experiment is designed to quantify the modern aeolian particle flux from the coastal plain located south of Mejillones Bay. Sediment traps coupled to wind measurements already allow to monitor present-day (inter-seasonal) aeolian fluxes. Longer time-series of climatological data and a study of AVHRR and SeaWifs imageries will also be used to understand present-day inter-annual and inter-decadal variations of wind intensity and induced impacts on oceanographic circulation patterns. Finally, the correlation between the modern flux data will be compared to the recent variations observed at the top of the sedimentary cores.

 

Laurence Vidal (*), Gerold Wefer, Ralph Schneider, and  Thorsten Bickert

University of Bremen, Geosciences Dpt., Bremen, Germany

* now at : CEREGE, Europôle de l'Arbois, Aix en Provence, France

 

vidal@cerege.fr

 

Upper deep water circulation in the South Atlantic over the last 1,5 M.Y.

 

The Mid-Pleistocene Revolution dated at about 0.9 Ma marks the transition from the dominance of the 41-ka cycle to the dominance of the 100 ka-cycle in climatic records. This transition spanning over 0.3 M.y. coincides with an increase in the size of the Northern hemisphere ice sheets. Feedbacks to larger ice sheets involve modifications of land morphology in the high northern latitudes (Berger and Jansen, 1994) as well as effect of lowering atmospheric CO₂ (Raymo et al. 1997). During the same time interval significant changes in the mean ocean d¹³C are recorded, probably caused by the addition of terrestrial carbon to the ocean-atmosphere reservoirs as global aridity increased (Raymo et al. 1997).

We present high resolution stable isotope records from ODP Site 1085 (29°S, 13°E, 1713m) spanning the last 1.5 M.y. to study hydrological changes during the Mid-Pleistocene Revolution and the onset of the 100-kyr cycle world. Special emphasis is placed on surface and subsurface circulation related to changes in the strength of the Benguela Current and upwelling system and upper deep water circulation in the South Atlantic .

Stable isotope (d¹⁸O and d¹³C) measurements were performed on benthic (Cibicides wuellerstorfi) and planktonic (G. bulloides and G. inflata) foraminifera every 10 cm and 20 cm, respectively. The age model at Site 1085 is mainly based on benthic d¹⁸O stratigraphy.

Hydrographic changes in the South Atlantic circulation are inferred from major changes documented in the long-term trends of the planktonic and benthic isotope records between 0.8 and 0.55 Ma. The benthic d¹³C values show a continuous increase from about 0.55 Ma ending with Holocene values on average 0.7 ‰ higher than before MIS 16. This increase is neither observed in deep North Atlantic nor in deep Pacific d¹³C records. The observed trend in the d¹³C values shows that Site 1085 experienced an increased contribution of UNADW during interglacials as well as during glacials periods. Glacial/Interglacial d¹⁸O variations for the planktonic records show increasing amplitudes and reduced differences in d¹⁸O values between G. inflata and G. bulloides  during glacials periods from about 0.75Ma. This latter point may be linked either to stronger mixing of the thermocline waters or enhanced upwelling intensity.

 

Joan Villanueva¹, and Philippe Bertrand²

¹ Institut d'Investigacions Quimiques i Ambientals de Barcelona.(CSIC), Barcelona, Spain.

² DGO - UMR 5805 EPOC. Université Bordeaux I, Talence, France

 

jvrqam@cid.csic.es

 

Sea-Surface-Temperature and Productivity changes in the Luderitz area.

 

We have determined the lipid composition and the TOC/CaCO₃ content along the IMAGES cores MD962086 (25°36'S,  13°22.8'E, 3606 m. water depth) and MD962087 (25°48'S, 12°7.7'W, 1026 m. water depth). These two cores represent a bathymetric transect off Luderitz that will enable us to compare the temporal variability recorded at different water depths over the last 200 kyrs.

The TOC and carbonate profiles recorded in both cores show a different temporal pattern. The deeper core shows a marked response to the glacial-interglacial global change. We also found a linear relationship between the sedimentary TOC content and the Uk'37-derived SST estimates (r2=0.69, N=52) This linear relationship could not be observed in the shallower core, where low SST values were not correlated with high TOC values.  We argue that the TOC profile is the result of changes in the degree of preservation/dissolution of carbonates. These changes can be related to both productivity changes in surface waters or to changes in the bottom water chemistry.

From a qualitative point of view, the lipid composition of the samples present within one sediment core has remained constant over the last 200 kyr.  From the quantitative point of view, the concentration of individual lipids is linearly related to the TOC content. The main exception to this general trend is the total pigments and alkenone compounds. The total

pigment concentration show an exponential decrease in downcore concentration superimposed to the glacial-interglacial pattern, which we assume, is related to slow post-depositional diagenetic processes. The one-step-change in the relative alkenone concentrations observed during stage 4 in both cores coincides with a major change in the coccolithophorid populations observed in the Atlantic Ocean.

 

Andrea Volbers¹, H.-S. Niebler¹, J. Giraudeau², H. Schmidt¹, and R. Henrich¹

¹ Dept. of Geoscience, University of Bremen, Germany

² DGO - UMR 5805 EPOC, Université Bordeaux I, Talence, France

 

avolbers@mail.sedpal.uni-bremen.de

 

PALEOCEANOGRAPHIC CHANGES IN THE NORTHERN BENGUELA UPWELLING SYSTEM OVER THE LAST 245 KYRS AS DERIVED FROM PLANKTIC FORAMINIFERA ASSEMBLAGES

 

Planktic foraminiferal records from several sediment cores recovered from the Walvis Ridge and the northern Cape Basin indicate changes in the areal extent and intensity of northern Benguela upwelling cells during the past 245 kyrs. From changes in distribution of characteristic planktic foraminiferal assemblages three hydrographic regimes (open-oceanic,

intermediate, upwelling) can be distinguished downcore by factor analysis. During periods of intensified upwelling, northern Benguela upwelling cells are displaced westward and increase in size, covering areas at least three times as large as at the present day. Several distinct upwelling events are recognized during OIE 7.4, 6.5/6.4, 6.2, OIE 5.53, OIE 5.4, 5.2, 4.2 and

during OIS 3 and 2; during OIE 5.4 the maximum upwelling extent is recorded. During OIE  5.1 upwelling was at its minimum. Paleo sea surface temperature (SST) reconstructions derived from planktic foraminifera using the classical tranfer function approach document lower SSTs during periods of intensified upwelling whereas SSTs derived from alkenons show glacial to interglacial temperature shifts. A good correlation between upwelling events in the northern Benguela region and increases in equatorial seasonality implies that both regions respond to the same mechanism, probably changes in the trade wind intensity.

 

Claudia Wenzel, and Jurgen Rüllkötter

Institute of Chemistry and Biology of the Marine Environment (ICBM) Carl von Ossietzky University of Oldenburg, Oldenburg, Germany.

 

C_Wenzel@ogc.icbm.uni-oldenburg.de

 

Organic geochemical analysis on organic matter in sediments from the Walvis Bay area (ODP Leg 175 Site 1082)

 

ODP Site 1082 lies close to the major upwelling centers along southwest Africa with year-round upwelling activity. It is a part of transect that is central to the reconstruction of the history of the Benguela Current upwelling System off Namibia. Site 1082 is closest to the coast and is expected to contain a direct record of upwelling history in the Walvis Bay area. The core obtained span ca. 5.7 Ma and the sedimentation rates range from 7 to 20 cm/k.y (Wefer et al., 1998).

Their total organic carbon (TOC), calcium carbonate and sulphur contents are variable with TOC values between 2 and 18% and total sulphur contents between 0,2 and 4%. Dark-colored sediments have higher TOC and sulphur values than light-colored layers. The high concentrations are a consequence of the elevated paleoproductivity of the Benguela Current upwelling system (Wefer et al., 1998).

Preliminary lipid analysis have been taken in a small number from the first meters of the core. The UK`37-reconstructed sea surface temperatures vary in the range of 17 and 22°C.

The n-alkanes range from C16 to C35 with the predominance of C29 and C31 homologues, a distribution, which is, ascribe to higher plant waxes.

The aim of the work is a high resolution study to reconstruct environmental changes in the Benguela Current System and upwelling history. It will be necessary to investigate the UK`37-reconstructed sea surface temperatures, a comparison of the distribution pattern and the stable isotope signature of the aliphatic hydrocarbon fraction as well as the analysis of the molecular lipid composition of the heterocomponent-fraction, primarily sterols and alcohols.

 

Reference

Wefer, G., Berger, W. H., Richter, C., et al., 1998. Proceedings of the Ocean Drilling Program, Init. Repts, 175, 273-312.

 

Bas West and J. H. Fred Jansen

Netherlands Institute for Sea Research, Den Burg – Texel, The Netherlands

 

basw@nioz.nl

 

Planktonic foraminifera as tracers of the position of the Angola  Benguela Front

 

In the South East Atlantic the Angola Benguela Front (ABF)  separates warm Angola Current water from the north and cold  Benguela Current water from the south. The front is very closely  tied to the position of the atmospheric high-pressure cell over the  South Atlantic. The During ODP leg 175 site 1079 was drilled  on the SW African Margin, just north of the present location of  the ABF. Our main objective is to study the position of the ABF  through time. According to biostratigraphic data the oldest  sediments in this core are about 700 ka old. Piston cores taken  in the region have reached sediments of 200 ka old.

Planktonic foraminiferal assemblages from this site were  analyzed, focussing on the lower part of the core (50-120 mbsf),  which covers the Mid-Brunhes climatic event 300- 400 ka ago.  Planktonic foraminiferal assemblages in the sediments show large  differences with assemblages found in the water column today.  Abundant species include dextral coiling N. pachyderma, G.  bulloides and G. inflata. Nowadays these species occur in high  concentrations in ABF waters. Relative abundances of sinistral  coiling N. pachyderma show large peaks, although nowadays  this species is only present south of the ABF and is associated  with coastal upwelling. These peaks are probably a result of  northward shifts of the ABF. Warm water species like G. ruber  and G. sacculifer are characteristic for the Angola Current, so  high abundances of these species may indicate a relative  southward position of the front.

A difficulty that occurs is that dissolution of forminiferal tests is  also a process that determines the composition of planktonic  foraminiferal assemblages. This mechanism may also be  important at site 1079.

 

Karin Zonneveld, Oliver Esper and Helmut Willems

FB 5-Geowissenschaften, Universitat Bremen, Bremen, Germany

 

zonnev@uni-bremen.de

 

Relationship between surface water bioproductivity, preservation of organic matter and Late Quaternary climate change.

 

By reconstructing the bioproductivity in surface waters it is often a matter of debate to what extent the used proxy reflects bioproduction or diagenetic processes such as the decay of organic material related to oxygen in bottom sediments. Here we present a method to estimate bioproductivity and diagenesis separately, by using organic walled dinoflagellate cyst associations.

Recent studies show how organic walled dinoflagellate associations are modified by oxygen availability in bottom sediments. Moreover, the dinoflagellate cyst distribution in surface sediments from the Benguela upwelling region clearly reflects the present day bioproductivity in surface waters. By combining this information we can estimate the degradation induced association change and create independent palaeoproductivity and diagenesis proxies. This is demonstrated for the Late Quaternary record of core GeoB 1710, which is situated along the margin of the Namibia/Lüderitz upwelling cell. The obtained records provide new insights in the relations between productivity, preservation and climate change.[G1]