Sea level changes, carbonate dissolution and history of the Benguela Current in the Oligocene-Miocene off Southwest Africa (DSDP Site 362, Leg 40)

Abstract

A coarse fraction analysis has been performed on sediments from DSDP Site 362 (Leg 40), in a water depth of 1325 m at the Eastern Walvis Ridge. The influence of carbonate dissolution has been revealed by benthos/plankton foraminifera ratio variations and by fragmentation of planktonic foraminifera. Carbonate dissolution shows strong variations: An Early Oligocene preservation spike, followed by a Middle Oligocene dissolution pulse, an Early Miocene preservation maximum, a dissolution increase in the Middle Miocene (14–15 m.y. ago) and a strong increase in dissolution at 10 m.y. ago, with several small-scale cyclic changes, with a maximum in dissolution between 10 and 9.5 m.y. ago and between 6 and 5.2 m.y. ago. These dissolution variations can be related to sea level fluctuations and ice extent in Antarctica. The history of the Benguela Current can be recreated by means of the opal content of the sediments. The W-turning Benguela Current transports opal from the near-coastal upwelling area to the continental slope. The Benguela Current first reached the Walvis Ridge 10 m.y. ago. During glacial periods in the interval 6–10 m.y. ago, the Benguela Current turned to the west over the Walvis Ridge and produced high opal contents. During interglacial periods in the interval 6–10 m.y. ago, the Benguela Current turned to the west within the Cape Basin. Opal contents in interglacial sequences of Site 362 in the interval 6–10 m.y. ago are lower than the glacial opal contents. Beginning at 5.2 m.y. ago, the reverse relationship has been found: The highest opal contents occur in interglacial sediments, and the smallest contents occur in glacial sequences. This observation reflects the northward shift of the Benguela Current.