Abstract:
Ocean currents play an important role in shaping the climate of the earth. Studying sediments deposited by ocean currents help us understand the development of these currents and their influence on palaeoclimate. The South Atlantic Ocean is a place where dominant water masses as the Antarctic Bottom Water and the North Atlantic Deep water meet. Seismic data over the Agulhas Ridge, collected by the Alfred Wegener Institute, and sediment core data from Sites 1088, 1089 and 1090 of the Ocean Drilling Program Leg 177 are connected to understand the palaeo-oceanographic history of the south Atlantic ocean after the Gondwana break-up. The physical properties measured on the sediment cores are used to create a link between structures observed in reflection seismic data to layers and hiatuses in corresponding sediment bodies. This way, an extrapolation of the observations at the drilling locations over a wider area is made possible. An hiatus present in the early Oligocene section of the cored sediments at ODP Site 1090 can be traced over a wide area in the seismic data of the Cape Basin. This hiatus corresponds to a change in sedimentation in the early Oligocene, from rapidly accumulating sediment sheets to the build-up of distinct drift structures. Large scale drift structures are observed in the seismic data parallel to the Agulhas Ridge. There is still a controversy about the timing of the opening of Drake Passage as a gateway for deep water masses in the Oligocene, and its role as a critical gateway in the development of the Antarctic Circumpolar Current as a prerequisite for the formation of deep water near Antarctica. The data from the Cape Basin adds additional insights in this debate, as it shows that deep water influx from the Antarctic region into this area has existed since the early Oligocene. The accumulation of contourite sediment structures found in the Cape Basin indicate a sustained influx of sediment particles by a deep water current, interrupted by periods of erosion or non-deposition in the middle Miocene and early Pliocene. The south-westward direction of the bottom current transporting the sediments, following the bathymetric contours of the Agulhas Ridge in the Cape Basin, is inferred from the geometry of the layers within the contourite drifts. This direction is similar to the presently active bottom current consisting of Circumpolar Deep Water.