The Angola-Namibia upwelling system is one of the five or six great upwelling regions of the world. It extends over a considerable portion along the western margin of southern Africa, with productivity values reaching, and sometimes exceeding, 180 gC/m2/y (Berger, 1989). The upwelling system is characterized by organic matter–rich sediments and contains an excellent record of productivity history, which, in turn, is closely tied to the regional dynamics of circulation, mixing, and upwelling of subsurface waters (Berger and Wefer, 1996b; Fig. 1).
Upwelling of cold water 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, staying close to it (within ~180 km) south of 25°S. Over the Walvis Ridge, between 23° and 20°S, the Benguela Current turns to the west. At about 20°S, warm tropical water masses moving in from the north meet the cold Benguela Current and form eddies. Cold-water eddies with upwelled water contain radiolarians and diatoms and transport their load to the northern parts of the Walvis Ridge. Here, the deposits have been sampled by the Deep Sea Drilling Project (DSDP; Site 532, Hay, Sibuet, et al., 1984; Site 362, Bolli, Ryan, et al., 1978).
As reconstructed from DSDP Sites 362 and 532, the evolution of the Benguela Current during the past 10 m.y. is characterized, on the whole, by increasing rates of accumulation of organic carbon. In addition, there is evidence from changing correlations between percent carbonate, percent Corg, and diatom abundance, that the dynamics of the system underwent stepwise modification during the late Neogene. In this context, a distinct opal maximum in the late Pliocene to early Quaternary is of special interest. The nature of this maximum is not clear; it may be related to a migration of the polar front to its modern position, to changes in silicate content within subsurface waters, or both. The results from DSDP Sites 362 and 532 suggested that there has been a general northward migration of the Benguela Current upwelling system during the last 14 m.y. (Hay, Sibuet, et al., 1984; Diester-Haass et al., 1990). Because the shape of the South Atlantic has not changed appreciably during this time, the changes in the upwelling system must reflect large-scale changes in climate and ocean circulation.
Leg 175 focused primarily on the paleoceanographic and paleoclimatic aspects of the area. In addition, this environment provides an excellent setting for "natural experiments" in diagenesis, especially regarding the precipitation of calcite and dolomite (Baker and Kastner, 1981; Baker and Burns, 1985; Garrison et al., 1984; Murray et al., Chap. 20, this volume) and the genesis of hydrocarbons (e.g., methane; Meyers et al., Chap. 21, this volume) and phosphate (Calvert and Price, 1983). It will be of great interest to compare results with those obtained during Leg 64 in the Gulf of California (Curray, Moore, et al., 1982), during Leg 112, off Peru (Suess, von Huene, et al., 1990), and for Leg 167, off California (Lyle, Koizumi, Richter, et al., 1997).
A host of questions arise in connection with the conditions of formation of the dolomite layers and concretions. Some of the findings of Leg 175 bear on these questions, but much has to be deferred for laboratory work on shore. Here, we describe the drilling strategy and regional setting and then turn to the presence of authigenic minerals. For this purpose, we summarize smear-slide analyses and visual core descriptions. For defining the presence of dolomite layers, the logging results are invaluable. The chemical environment is defined by the pore-water analyses. We also compare our results with those found elsewhere, with special focus on dolomite produced by early diagenesis in other depositional regimes. The layers are hard and often hinder the advanced hydraulic piston corer (APC). Drilling by extended core barrel (XCB) is necessary to penetrate the layers so that, in many cases, APC coring can then be resumed.
Thirteen sites were occupied during Leg 175 off the western coast of Africa, from off the mouth of the Congo River to off the Cape of Good Hope (Fig. 2). Coring was by APC from the seafloor to refusal and by XCB below refusal, where appropriate. The overall goal is to reconstruct the late Neogene history of the Benguela Current and the associated upwelling regime between 5° and 32°S.
Within this goal, the evolution of the Benguela Current system and its relationship to the onset of glacial cycles in the Northern Hemisphere is of central importance. Most of the sites show high sedimentation rates (~100 m/m.y.), offering an opportunity to develop detailed paleoceanographic records (see Berger et al., Chap. 17, this volume). The holes drilled at these sites will greatly extend and refine the partial record of the paleoceanographic and paleoclimatic changes for the late Neogene that was provided by DSDP Sites 362 and 532. Sediments are largely nannofossil clays or oozes, commonly diatomaceous, and with generally high but variable organic carbon contents (a few percent to as much as 20%).
In general, holes are shallow north of the Walvis Ridge (200 m or less) because this region has salt tectonics and is the target of active hydrocarbon exploration. On and south of the Walvis Ridge, most sites reach depths of several hundred meters below the seafloor, with the deepest site at 605 m.
As mentioned, in addition to microfossils and terrigenous materials, the sediments contain a rich assemblage of diagenetic products. There are finely disseminated euhedral dolomite crystals and other carbonates, iron sulfides of various habits ("pyrites"), and glauconites, all found in smear slides. Furthermore, up to 75-cm-thick dolomite and/or calcite layers or concretions were found in the cores or were detected with sensor tools during logging. Black, odorous, organic-rich layers are seen in places and are especially abundant at Site 1084, where they reached thicknesses of a decimeter or so. These layers also are recognizable in the logging profiles.