The eastern South Atlantic plays an important role in the global heat transport system (Berger and Wefer, 1996). It contains one of the largest upwelling systems worldwide and receives clastic and organic material from the second largest river in the world. The oceanographic environments include a tropical region strongly affected by river discharge, a tropical oceanic region, and a region influenced by coastal upwelling. It should thus be possible to distinguish all these depositional settings by specific assemblages of terrigenous and marine components. Lipid biomarker compositions provide information on different types of terrigenous lipid sources and a detailed view on marine plankton groups contributing to the sedimentary organic matter. Furthermore, the biomarkers in marine sediments can supply information on early diagenetic reactions in the sediments and paleoceanographic conditions (Brassell, 1993).

For these purposes, a detailed molecular organic geochemical overview study was performed on two to three samples each from Ocean Drilling Program (ODP) Sites 1076, 1077, 1079, 1082, 1084, and 1085 in the southeast Atlantic Ocean (Fig. F1). The study is intended to be solely inventory; samples were chosen randomly from different sediment depths in each core (Table T1). Therefore, only restricted paleoceanographic interpretations can be drawn from the results. The concentrations of single compounds and lipid classes are used to compare the different oceanographic environments with respect to terrigenous, phytoplanktonic, and zooplanktonic lipids. An attempt is made to give an overview for each site about the lipid biomarkers present and their potential use as paleoceanographic indicators. Earlier organic geochemical investigations of sediments from this particular ocean area (e.g., Boon, 1978) provided valuable background information.

Oceanographic Setting

In the present-day eastern South Atlantic, the South Equatorial Countercurrent in the north feeds the warm southward-flowing waters of the Angola Current (AC) (Fig. F1). At ~15°S-17°S, these waters converge with the cold northward-flowing Benguela Coastal Current and are deflected to the northwest. A sharp frontal zone, the Angola-Benguela Front (ABF), is established and extends to a distance of up to 1000 km off the coast (Meeuwis and Lutjeharms, 1990). The ABF effectively forms a barrier for surface ocean transport, as does the Walvis Ridge for deep water.

The cyclonic gyre in the Angola Basin is overlain by the low-salinity plume of the Congo River at ~5°S. Sites 1076 and 1077 are located below the Congo plume. Riverine input, coastal upwelling, and incursions of open ocean waters influence this environment. Site 1076 is the shallowest location and should record the strongest interaction of river discharge and coastal upwelling. Site 1077 is at larger water depth and is presumably more influenced by open-ocean conditions.

In the eastern Angola Basin, the cyclonic circulation causes shoaling of the thermocline and supply of nutrients to the photic zone. Site 1079 is located on the Angola margin and is neither directly influenced by rivers nor situated under a coastal upwelling cell (Lutjeharms and Stokton, 1987). Productivity benefits from a seasonal supply of nutrients from the Angola Dome.

In the south, the Benguela Current is fed by the South Atlantic Current and the Agulhas Current. At ~30°S, the Benguela Current splits into the Benguela Oceanic Current, the main current in northwestern direction, and the Benguela Coastal Current, flowing as a sluggish, wide current along the continental margin. These currents are driven by the predominantly southerly and southeasterly winds (Shannon, 1985). Off the coast of South Africa and Namibia, these winds drive coastal upwelling of cold, nutrient-rich South Atlantic Central Water (Shannon, 1985). Productivity reaches high values of >180 g C/m2/yr (Berger, 1989). Site 1082 is located in the Walvis Basin, outside recent upwelling activity. Since filaments of cold, nutrient-rich waters extend up to 600 km offshore (Lutjeharms and Stokton, 1987) where they mix with low-productivity waters sustaining intermediate productivity, Site 1082 most probably records an upwelling signal transported by filaments and eddies. Site 1084 is situated close to the upwelling cell in Lüderitz Bay. At this site, persistent upwelling of South Atlantic Central Water (Shannon, 1985) causes high productivity and high rates of sedimentary accumulation of phytoplankton biomass (Brown et al., 1991). Site 1085 is located in the southern part of the Cape Basin near the continent. Besides seasonal upwelling activity, this site may also record terrigenous supply from the Orange River.