RESULTS

Concentrations of calcium carbonate are low in sediments from Sites 1075, 1076, and 1077 near the mouth of the Congo River (Fig. F1). Their maximum is 15.2 wt%, and most sediment samples contain <5 wt% CaCO₃ (Table T1), which generally agrees with the paucity of coccolith microfossils and the abundances of opaline and continental clastic material reported in these sediments (Wefer, Berger, Richter, et al., 1998). Sediments from Sites 1078, 1079, and 1080 on the Angola margin contain more CaCO₃ than those near the Congo River, probably because of diminished clastic dilution. Their concentrations typically fall between 10 and 20 wt% (Fig. F1). The highest calcium carbonate concentrations in our survey exist in sediments from Sites 1081, 1082, and 1084, where they vary between 13.2 and 80.9 wt% (Table T1). In particular, closely spaced and well-developed variations that are related to light-dark color changes and more gradual downhole increases and decreases in CaCO₃ concentration occur at Sites 1082 and 1084 in the Northern Cape Basin (Fig. F1).

The organic carbon concentrations in sediments from the nine locations that we studied are generally higher than in most deep-sea sediments from the South Atlantic (~0.3 wt%) (Premuzic et al., 1982; Keswani et al., 1984). The concentrations are typically between 1 and 4 wt%, except for Sites 1081, 1082, and 1084, in which average values are >5 wt% (Table T1). In addition, the amount of variation is sometimes >5 wt% between adjoining samples at Sites 1082 and 1084 (Fig. F2).

Organic carbon stable isotopic compositions (¹³C_org) differ among sediment samples of different age from the nine sites (Fig. F3). Dashed lines at -23 that are positioned between the typical ¹³C_org values of continental and tropical/temperate marine organic matter (about -26 and -20, respectively) (Sackett, 1989; Meyers, 1994) provide a reference for comparison. The ¹³C_org values (<-23 for most of the samples) at Sites 1076, 1078, and 1079 are notably more negative than at the other sites, which have values around -21 (Fig. F3). Of special interest is the sharp decrease in ¹³C_org values found in the uppermost samples at Sites 1075, 1076, and 1084 (Fig. F3). Of particular note, the 3.5 negative excursion at Site 1084 was confirmed by triplicate analyses.

Concentrations of biogenic opal are relatively elevated in sediments from Sites 1075 and 1077 near the mouth of the Congo River than others (Fig. F4). Their maximum is 24.8 wt%, although most sediment samples contain between 15 and 20 wt% opal (Table T1). Concentrations are lower in sediments from Site 1076 (7%-11%), which is closer to the mouth of the river (Fig. F4). These patterns agree with the abundant but variable amounts of opaline and continental clastic material reported in the sediments of these three sites (Wefer, Berger, Richter, et al., 1998). Sediments from Sites 1078 and 1079 on the Angola margin contain low opal concentrations (2.4-5.5 wt%), whereas those at Site 1080 are relatively enriched in this biogenic sediment component. Values at this location in the southern Angola Basin are generally between 10 and 20 wt%, although one sediment sample (175-1080A-6H-4, 125-130 cm) contains 31.3 wt% opal (Table T1). The highest concentrations of opal in our survey exist in sediments from Sites 1082 and 1084, where they reach 36.0 wt% (Table T1). However, most of the sediments from these Northern Cape Basin locations, like those from Site 1081 on the Walvis Ridge, contain <10 wt% opal (Fig. F4) because of their elevated concentrations of CaCO₃ (Fig. F1).