The Benguela Current and associated upwelling on the southwest African Margin: a synthesis of the Neogene-Quaternary sedimentary record at DSDP sites 362 and 532

Abstract

Sediments cored at DSDP Sites 362 and 532 on the Walvis Ridge provide a Neogene-Quaternary history of the development of the upwelling system on the southwest African margin. Upwelling occurs principally on the landward side of the Benguela Current. The upwelling centres have shifted northward since the Middle Miocene as the current has intensified and has flowed farther to the north. Changes in productivity are recorded in the types, proportions and preservation of foraminifera, radiolaria, diatoms, organic matter, and clay minerals in the sedimentary record. Prior to the Late Miocene (10 Ma), the Benguela Current did not reach the Walvis Ridge, and enhanced productivity is not evident in the sediments at this location. Between 10 to 5.2 Ma, upwelling was recorded in the DSDP sites in glacial periods, indicating that the Benguela Current intensified during glacial periods and transported evidence of upwelling to Sites 362/532 from near-coastal areas. During interglacial periods the current was not as strong and did not reach the Walvis Ridge, turning instead to the west within the Cape Basin. Strengthening of the current continued such that by the Pliocene and Quarternary the upwelling signal is contained in interglacial sediments. Sediments deposited in these more recent glacial times contain a weak or absent upwelling signal because glacial intensification shifts the Benguela Current system northward to reach the Angola Basin before it turns westward away from its coast-parallel direction.

The radiation of the Cape flora, southern Africa

“…So with increased glaciation of Antarctica, the South Atlantic high-pressure cell becomes stronger, the trade winds increase in strength, and the upwelling on the southern African west coast is accentuated. This is corroborated by regular fluctuations in the strength of the system, and the northward penetration of the cold upwelling waters, which are correlated to the extent of the Antarctic glaciation (Diester-Haass et al, 1992). This leads to the postdiction that the Oligocene should also have been a period of enhanced trade winds, with their associated coastal upwelling, since Antarctica was also glaciated during the Oligocene (Zachos et al, 2001), indicating a steeper latitudinal temperature gradient than either during the Eocene or the Early Miocene.…”

“…Although it is assumed that the initiation of the Benguela upwelling was caused by the formation of sea-ice around Antarctica (Diester-Haass et al, 1992), and ultimately by the thermal isolation of the southern continent subsequent to the opening of the Drake Passage approximately 28.5 Mya (Barker & Burrell, 1977), and the establishment of the circum-Antarctic westerly drift, the association is not immediately evident. Upwelling is the result of the northerly long-shore drift resulting from the persistently strong tradewinds, and the strength of the upwelling is related to the strength of the trade winds (Shi et al, 2001).…”

Sediment deposits in the Cape Basin: Indications for shifting ocean currents?

“…Siesser (1980) dated the onset of modern Benguela Current circulation at 10 Ma, associating enhanced productivity with the stepwise increase in organic-carbon accumulation observed at site 532. Diester-Haass et al (1992) suggested a progressively northward shift of the Benguela Current since the middle Miocene, initiating upwelling in the Walvis ridge area at about 10.5 Ma.…”

“…Giraudeau et al, 2001b). The variations in TOC concentration probably result from downslope transport of organic-rich sediments from the shelf edge that is related to glacial-interglacial variations in trade-wind activity and in sea level (Diester-Haass et al, 1992;Giraudeau et al, 2001b).…”

Palaeogeography of radiolarite and organic-rich deposits in Mesozoic Tethys

“…South Atlantic coast off Africa, Walvis Ridge; Diester-Haass et al 1992;Hay and Brock 1992). The height of the sea level itself is apparently relatively unimportant.…”