Cenozoic deep-water circulation at Walvis Ridge, eastern South Atlantic

Dr. D. C. Leuschner
Prof. Dr. W. Ehrmann


International Cooperation:
Leg 208 Scientific Party




Scientific Background

In addition to the long-term Cenozoic climatic changes, a several short but extreme changes occurred. The impact of the extreme climates on the oceanic deep-water circulation will be reconstructed in a project within the framework of the "Ocean Drilling Program (ODP)". The sediment cores used in this project were recovered with the drill ship "Joides Resolution" from the South Atlantic at the Walvis Ridge, off Namibia, in water depths between 2550 and 4750 m.

 
map

Fig.1: Bathymetric map of the eastern South Atlantic with core locations of the ODP Leg 208 depth transect. The sediment cores, that were selected for this study are marked in red. The core image on the top left shows the significant and abrupt change in the sediment composition at the Paleocene - Eocene transition.


 
Aims of the study:

Paleogene to Neogene sediments recovered during ODP Leg 208 at Walvis Ridge allow detailed reconstructions of the timing and variability in the changes and reorganization of the deep- and bottom water masses over a range in water depth of more than 2000m. The aim of the project is to reconstruct the circulation of deep- and bottom-water masses during the Cenozoic. Particular attention will be laid on the nature and behaviour of the deep-water masses during extreme climatic situations and on their response to abrupt environmental and climatic changes.

Our study will combine sedimentological and clay mineralogical investigations. The intention is to reconstruct the long-term trend of the oceanic circulation associated with the transitions from an intermediate Paleocene climate to the Eocene greenhouse, the subsequent cooling into the Oligocene icehouse and Miocene cooling events. Additionally, we will focus on the late Paleocene / early Eocene interval, which is characterized by short climatic excursions overriding the long-term warming trend.


First results:

The clay mineral assemblages indicate that environmental conditions in the eastern South Atlantic underwent significant changes during the late Paleogene. These changes were in particular associated with the Early Eocene Climatic Optimum and the Eocene/Oligocene boundary. Differences in the relative abundances of smectite, illite, kaolinite and chlorite, as well as in the illite crystallinity, reflect temporal changes in the sediment provenance and transport mechanisms. The late Palaeocene and early Eocene sediments underwent a significant diagenetic overprint, as indicated by the occurrence of clinoptilolite. This overprint, however, does not obliterate the palaeoclimatic information of the clay mineral assemblages.

In the late Paleogene, abrupt climatic events like the Palaeocene-Eocene Thermal Maximum (PETM) at about 55.5 Ma ago and other hyperthermals, with the most pronounced  the Eocene Thermal Maximum 2 (ETM2 or "ELMO"-event) at about 53.5 Ma ago, are of highest interest in recent climate research, in particular, because of their strong impact on oceanic and atmospheric temperature, oceanic chemistry and environment and therefore on the biosphere. It is interesting that these events are documented in the clay mineral assemblages at Walvis Ridge, but that they do not exceed significantly the overall variations. This has major implications. We have no indication for a major change in the oceanic circulation and/or the provenance of the clay minerals deposited at the Walvis Ridge associated with these short events, despite of the intense changes in temperature and ocean water chemistry, that are resulting in a shoaling of the CCD of more than 2000 m in the South Atlantic.

The most striking feature in the late Eocene and Neogene record is the occurrence of strong carbonate dissolution phases, as indicated by the increase of the terrigenous silt and clay fraction of the bulk sediment. The dissolution is stronger in the deeper basin and contemporaneous with ephemeral Antarctic ice sheets, implying that the variability of the ice sheets result in major perturbations of the carbon cycle and the shoaling of at least the South Atlantic CCD.

.