Plio-Pleistocene sedimentation on the Walvis Ridge, Southeast Atlantic (DSDP Leg 75, Site 532) — Influence of surface currents, carbonate dissolution and climate

Abstract

An investigation of the quantitative composition of the coarse (> 40 μm) and clay (<2 μm) fraction of HPC 532, DSDP Leg 75, in 1300 m water depth on the eastern Walvis Ridge off Southwest Africa yielded the following results: 1. (1) The sediments reflect a complete Latest Miocene to Recent depositional history. Sedimentation rates vary between 2.3 and 7.8 cm 10 −3 years. 2. (2) Preservation of calcium carbonate is subject to strong variations: short-term (<100,000 years) and long-term (about 1 m.y.) cycles in carbonate dissolution have been observed, with strongest dissolution occurring during periods of lowered sea level. 3. (3) Upwelling influence from the near-coastal upwelling centre has been detected by means of the opal content: interglacial periods show high opal contents, because the Benguela Current turned westward at about 20°S and carried opal-laden upwelled water to the west. Sediments from glacial periods, however, show opal minima. Besides these short-term cyclic variations in opal content, long-term cycles have been found, with maximum upwelling influence in the latest Pliocene/early Quaternary. 4. (4) Each CaCO 3 dissolution minimum (maximum) is correlated with an opal maximum (minimum) throughout the sediment sequence. 5. (5) The oceanographic system off southwest Africa remained essentially unchanged since the latest Miocene: sea level rose and fell periodically on a small and on a large scale, and the Benguela Current flowed southeast-northwest and turned to the west at the latitude of Site 532 during interglacial periods, when sea level was high. 6. (6) The climate in the near-coastal area of southwest Africa in the latitude of Site 532 has probably been arid throughout the investigated period.