Seismic evidence for bottom current activity at the Agulhas Ridge

The South Atlantic is a region where water masses from the Atlantic andIndian Oceans meet Antarctic water masses.The Agulhas Ridge in the eastern South Atlantic is a pronouncedelevation of the ocean bottom which witnessed major changes inoceanic circulation since the Cretaceous.It has acted as a barrier for deep oceanic currents of Atlantic originlike the North Atlantic Deep Water (NADW) and Antarctic origin like theAntarctic Bottom Water (AABW), or water masses derived from AABW such asCircumpolar Deep Water (CDW).The history of these currents is recorded in the sedimentary sequencein the adjacent Cape and Agulhas Basins.Seismic profiles over the Agulhas Ridge show sediment packagesin the Cape Basin which are identified as contourite sheets.These consist of thick, seismically predominantly low reflectivesequences interrupted by widespread hiatuses.ODP Leg 177 cores date hiatuses linked with widespread discontinuitiesin the Early Oligocene, the Middle Miocene, around the Miocene/Plioceneboundary and in the early Pleistocene.These hiatuses are clear markers inside contourite drift bodies and thisis used to date an elongate contourite drift in the Cape Basin.This drift shows sediments deposited by a westward current, implicatingthat the bottom current in the Oligocene followed the same trajectory aspresent-day CDW does