REGIONAL SETTING

The ABC-system is one of the five or six great upwelling regions in the world. It extends over a considerable portion of the western margin of South Africa, with productivity values >180 gC/m2/yr (black areas in Fig. 6). It is characterized by organic-rich sediments containing an excellent record of productivity history, which, in turn, is closely tied in with the regional dynamics of circulation, mixing, and upwelling, as seen in the oxygenation of thermocline waters (Fig. 7). In addition, this environment provides an excellent setting for "natural experiments" in diagenesis, especially concerning the genesis of economically important resources such as petroleum and phosphate.

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 within ~180 km of it south of 25°S, and then turns to the west over the Walvis Ridge (WR) between 23°and 20°S (Stramma and Peterson, 1989; Fig. 8). At about 20°S, warm, tropical water masses from the north meet the cold Benguela Current water. Eddies of cold, upwelled water contain radiolarian and diatom skeletons, which are transported from the upwelling area to the northern slope of the WR where they have been sampled at Deep Sea Drilling Project (DSDP) Sites 532 (Hay, Sibuet, et al., 1984) and 362 (Bolli, Ryan, et al., 1978).

According to previous studies, eddies formed farther north during the Last Glacial Maximum (LGM). The Benguela Current stayed close to the coast and flowed over the WR to reach the Angola Basin, only bearing to the west at ~17°S. Sediments deposited at Site 532 during the LGM apparently confirm the postulated absence of upwelling eddies by showing very low abundances of opal skeletons (Hay, Sibuet, et al., 1984; Diester-Haass, 1985). Upwelling may have continued to occur near the African shelf, but the Benguela Current then did not transport that upwelling signal to the WR. However, from the distribution of foraminiferal assemblages at Site 532, it appears that, under glacial conditions, the northeastern WR was, in fact, characterized by intensified upwelling and a westward expansion of coastal upwelling cells during the last 500 k.y. (Oberhänsli, 1991). The issue of contrasting models of glacial/interglacial upwelling dynamics in this region is unresolved. It hinges on the question of why opaline fossils show contrary abundance variations with respect to the productivity record from other proxy indicators.

The results from Sites 362 and 532 can be used to reconstruct, tentatively, the evolution of the Benguela Current during the past 10 m.y. This evolution is characterized, on the whole, by increasing rates of accumulation of organic carbon (Corg). In addition, there are indications from changing correlations among percent carbonate, percent Corg, and diatom abundance that the dynamics of the system undergo stepwise modifications. In this connection, as well, a distinct opal maximum centered near the Pliocene/Pleistocene boundary is of great interest (Fig. 9). The nature of this opal maximum is not clear; perhaps, it is a response to the migration of the polar front to a more northern position.

The evolution of the climate of the Northern Hemisphere, particularly that of northern Europe, is linked to the exchange of heat between the South Atlantic and the North Atlantic Oceans (Fig. 10). Operating over long distances, this energy transport is involved in the control of global climate patterns including the growth and wasting of polar ice caps. In today's world, a net heat transfer from the South Atlantic to the North Atlantic exists in currents above the thermocline (Fig. 11). A part of the heat contribution from the South Atlantic is believed to originate from the Indian Ocean via the Agulhas Current. Water masses from the Agulhas Current—warm eddies produced within the Agulhas Retroflection—are swept up by the Benguela Current and moved northward. Northward and southward shifts of the Southern Ocean polar front constrict or expand, respectively, the interchange of heat from the Indian Ocean to the South Atlantic (McIntyre et al., 1989). This interchange presumably has a drastic impact on the heat budget of the Benguela Current system and, consequently, that of the entire Atlantic Ocean. Such variations in heat transfer should appear as changes in the course and intensity of currents and productivity regimes and should be recorded in the sediments accumulating along the southwest African margin.

An important element of the heat transfer dynamics is the deep-circulation pattern. Traditionally, the focus in reconstructing this pattern has been on the properties and boundaries of NADW-related water masses, as seen in the δ13C of benthic foraminifers. The emphasis has been on glacial-to-interglacial contrast (Fig. 12). This contrast shows that NADW production was greatly reduced during glacial periods (also reflected in the pattern of carbonate preservation). More recent studies have added much detail to this story (summarized in Bickert and Wefer, 1996; see Fig. 13). It appears that the strength of the NADW is reflected in the differences between eastern and western basins and in gradients within the eastern basin. Information on associated changes at depths above the NADW has been sparse. It must be assumed that the strength of the nutrient maximum underlying the Benguela upwelling regions (Fig. 14) is somehow coupled to the evolution of NADW, which, in turn, influences the dynamics of intermediate water-mass formation to the south. At this point, we do not know how the different cycles are related, so little or nothing can be said about causal relationships.

Paleoceanographic interpretations regarding the history of the Benguela Current are derived mainly from a single location off the southwestern coast of Africa (Site 532) and must be considered preliminary. Given the indications that the axis and the intensity of the Benguela Current have changed over the past 15 m.y. and that productivity has fluctuated with glacial/interglacial cycles, confirmation and refinement of these ideas are needed. Although sites in the Cape and Angola Basins and on the WR were occupied during DSDP Legs 40, 74, and 75, these sites are situated too far offshore to provide the required information regarding processes in the eastern boundary system. Sites 362 and 532 on the WR receive an indirect record of near-coastal upwelling from material transported to their location by the Benguela Current. Furthermore, modern coring technology (APC and XCB) allows for high-resolution studies by avoiding much of the drilling disturbance present in the Leg 40 cores. Such high-resolution work is crucial if the dynamics of upwelling are to be captured back to the Miocene on a scale of glacial/interglacial cycles. Information from the Leg 175 array of sites situated in the Southern and Mid-Cape Basins (SCB and MCB, respectively), on the WR, and in the Southern Angola Basin (SAB) allow the construction of a coherent picture.

NEXT