History of the Benguela Current off Sw Africa (DSDP Sites 362 and 532)

A coarse fraction analysis of Miocene-Pliocene-Quaternary sediments of Site 532 and 362 from the Eastern Walvis Ridge off SW Africa has been performed. Radiolaria and diatom contents show short-term and long-term cyclic variations. They can be related to carbonate dissolution variations, expressed by benthos/plankton foraminiferal ratios. Carbonate dissolution is strong in glacial, and weak in interglacial periods. In the Late Miocene (10-6 MY) opal maxima are related to carbonate dissolution maxima, that means to glacial periods. In the interval 5.2 MY to the present the opposite relation is observed: opal maxima correspond to carbonate dissolution minima, thus to interglacial periods. The following history of the Benguela Current is reconstructed from these findings. During interglacial periods of the 10-6 MY interval the Benguela Current turned from the SE-NW direction to the west within the Cape Basin. During glacial periods of this 10-6 MY interval the current system shifted northward. The Benguela Current then turned to the west over the Walvis Ridge and transported opal from the near-coastal upwelling zone to the Walvis Ridge Terrace. During the interval 6-5.2 MY the Benguela Current shifted southward and northward over the Walvis Ridge and finally, at 5.2 MY, the present and Late Quaternary situation has been attained. The Benguela Current shifted equatorwards in glacial periods and polewards in interglacial periods. Thus the westward-turning which is responsible for the opal supply from the shelf upwelling area to the continental slope, occurred over the Walvis Ridge in interglacial periods and in the Angola Basin in glacial periods. The long-term cyclic changes in opal content can be related either to an on/offshore displacement of the Current or to variations in the velocity of the Benguela Current. The Late Pliocene/Early Quaternary opal maximum can be related to a northward shift of the polar front in the South Atlantic. 56