Site 1081, in 794 m water depth on the Walvis Ridge, is the shallowest of the sites drilled on the Walvis Ridge/Walvis Basin transect. The transect consists of DSDP Sites 532 and 362 (Legs 75 and 40) in ~1300 m water depth and ODP Sites 1081–1083. The DSDP sites are seaward of the upwelling center but contain an upwelling signal that has been transported by the Benguela Current. At DSDP Sites 362 and 532, the effects of glacial/interglacial cycles appear as carbonate dissolution, productivity, and continental sedimentation cycles. Both sea-level and climatic changes are recorded in these cycles. Site 1081, at the other end of the transect, will give a better record of the upwelling itself. Located above the CCD in a passive-margin area of high sedimentation rates, the transect can provide high-resolution records of these important processes and add important new dimensions to the records now available. Site 1081 is situated on the only topographic high over which the Benguela Current passes; thus, it is central to the reconstruction of the history of the current.
The upwelling off southwest Africa is centered, at present, on the inner shelf and at the shelf edge. The Benguela Current flows roughly parallel to the coast and stays within ~180 km of it south of 25°S; then it turns west over the Walvis Ridge between 23° and 20°S (Fig. 1). At about 20°S, warm, tropical water masses from the north meet the cold Benguela Current and generate eddies. Eddies of cold, upwelled water contain radiolarian and diatom skeletons that are transported from the upwelling area to the northern part of the Walvis Ridge, where they have been sampled at DSDP Sites 532 (Hay, Sibuet, et al., 1984) and 362 (Bolli, Ryan et al., 1978).
During the Last Glacial Maximum (LGM), the eddy formation took place farther north, and the Benguela Current flowed parallel to the coast and over the Walvis Ridge to reach the Angola Basin, finally bearing to the west at about 17°S. Sediments deposited at DSDP Site 532 during the LGM confirm the absence of upwelling eddies by containing very few or no opal skeletons (Hay, Sibuet, et al., 1984; Diester-Haass, 1985). Upwelling may have continued to occur on the African shelf, but the Benguela Current did not transport that upwelling signal to the Walvis Ridge. In contrast, from the distribution of foraminiferal assemblages at DSDP Site 532, it was suggested that the northeastern Walvis Ridge was characterized by intensified upwelling and a westward expansion of coastal upwelling cells at glacial periods during the last 500 k.y. (Oberhänsli, 1991).
Judging from opal sedimentation, the influence of the Benguela Current first makes itself felt on the Walvis Ridge about 10 m.y. ago (Bolli, Ryan et al., 1978; Diester-Haass, 1988; Diester-Haass et al., 1990; see Fig. 2). From 10 to ~6 Ma, maximum opal contents occurred during glacial periods, and opal minima coincided with interglacials. According to Diester-Haass and Rothe, (1987), the pattern of the opal signal changed at 5.2 Ma to become what has continued into the Quaternary: high opal concentrations during interglacial times and low concentrations during glacial intervals. The results from DSDP Sites 362 and 532 have been used to reconstruct the evolution of the Benguela Current during the past 10 m.y. (Fig. 2). Apparently, the 5.2-m.y. transition between the two upwelling regimes marks an important event. At this time (presumably) the dynamics dominating at present were established, perhaps in response to the simultaneous migration of the polar front to its modern position.
Another explanation for the transient increase and decrease in organic carbon and opaline silica-rich sediments at DSDP Sites 362 and 532 (Walvis Ridge) and DSDP Site 530 (Southern Angola Basin) is proposed by Hay and Brook (1992). They suggest that the enhanced upwelling during interglacials is a response to intensification of the Angola Dome, a cyclonic gyre off the Angola coast at 10°S that brings Antarctic Intermediate Water (AAIW) to within 50 m of the surface.
The shift in correlation from high opal deposition during glacials before 5.2 Ma to increased deposition during interglacials is thought by Hay and Brook (1992) to be related to changes in the production of AAIW. Although AAIW has a low salinity, the rate of production and the silicate content of AAIW presumably depends on the upwelling of North Atlantic Deep Water (NADW). If the production of NADW is reduced, the production of AAIW might also weaken because its salinity would become too low for vigorous sinking from the surface. If the production of AAIW diminished, sterile North Atlantic Intermediate Water formed from mixing of North Atlantic and Mediterranean water masses could become the dominant source of thermocline water in the northern South Atlantic during glacials. Thus, water upwelled from the Angola Dome would be nutrient poor, and hence there would be no record of enhanced biological productivity in the sediment, even if upwelling increased.
Clay mineralogical results at DSDP Site 362 give indications for a precursor to the modern Benguela Current in the middle Miocene (14 Ma), which was too weak to produce upwelling but reached the Walvis Ridge during glacial periods and transported montmorillonite northward from the Orange River. In interglacial periods of the middle Miocene, the local source of illite (the Namib Desert) overwhelmed the distant montmorillonite supply (Diester-Haass et al., 1990).
The upwelling signal recorded in the opal content in the Pliocene–Pleistocene section at DSDP Site 532 is not only driven by glacial/interglacial cycles; large-scale changes also occur (Diester-Haass and Rothe, 1987). These changes can be explained either by east–west migrations of the axis of the Benguela Current or by changes in the intensity of the current. A strong increase in the upwelling signal at 2.4 Ma is synchronous with both a northward shift of the polar front in the South Atlantic and a lowering of the sea level.
Two ~11-m-long cores (Geosciences Bremen [GeoB] 1705-1, water depth 642 m and GeoB 1706-2, water depth 980 m) taken near Site 1081 showed sedimentation rates of 4–7 cm/k.y.