Paleogene sedimentation changes along a depth transect at the northern flank of the Walvis Ridge (ODP Leg 208), South Atlantic Ocean

We analysed a large number of sediment samples recovered from a depth transect along the northern flank of the Walvis Ridge (ODP Leg 208) for their grain size distribution and clay mineral composition. The clay mineral assemblages at Sites 1265, 1266 and 1267, spanning a depth range of almost 1300 m, 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 Paleocene and early Eocene sediments underwent a significant diagenetic overprint, as indicated by the occurrence of clinoptilolite. This overprint, however, does not obliterate the paleoclimatic and paleoceanographic information of the clay mineral assemblages. In the late Paleogene, abrupt climatic events like the Paleocene-Eocene Thermal Maximum (PETM), at about 55.5 Ma ago, and other hyperthermals, with the most pronounced being 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. It is remarkable that these events are documented by minor changes in the clay mineral assemblages at Walvis Ridge, but that these changes do not exceed significantly the overall variations in the individual clay mineral concentrations. 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. This implying that the variability of the ice sheets is associated with major perturbations of the carbon cycle and the shoaling of at least the South Atlantic CCD.