Concentrations of CaCO3 in Site 1087 sediments range between 91.0 and 55.9 wt% (Table T1). Sediments at this site are divided into two lithostratigraphic units (Shipboard Scientific Party, 1998) that contain nearly the same amounts of carbonate. Unit I, an upper middle Miocene-Pleistocene nannofossil-foraminifer ooze, averages 76 wt% CaCO3. Unit II is an interrupted upper Eocene-middle Miocene foraminifer-rich nannofossil ooze that averages 73 wt% CaCO3. Marked variations in concentrations occur within each unit (Fig. F1) and reflect varying combinations of changes in delivery of calcareous material, dilution by noncalcareous components, and carbonate dissolution fueled by oxidation of organic matter. The variations in Unit I mimic similar oscillations in carbonate concentrations that exist in Miocene-Pleistocene sediments from other Leg 175 sites under the Benguela Current (Wefer, Berger, Richter, et al., 1998).
TOC concentrations of the samples of Site 1087 sediments vary between 3.13 and 0.06 wt% (Table T1). Concentrations in sediments shallower than 240 mbsf are larger and more variable than in deeper sediments (Fig. F2). This sediment depth corresponds to the latest Miocene (Shipboard Scientific Party, 1998), which has been identified as the time when the Benguela Current upwelling system began to develop (Siesser, 1980; Meyers et al., 1983; Berger et al., 1998). Maximum TOC concentrations appear between 80 and 60 mbsf, which is equivalent to about 2 Ma and therefore much like similar maxima observed at Deep Sea Drilling Project Sites 362 and 532 on the Walvis Ridge (Siesser, 1980; Meyers et al., 1983) and at other Leg 175 sites farther north in the Cape Basin than Site 1087 (Berger et al., 1998). These maxima suggest that upwelling once induced greater biological productivity than recent times. However, Pliocene-Pleistocene sediments at Site 1087 are not nearly as rich in organic carbon as those closer to the Walvis Ridge (Wefer, Berger, Richter, et al., 1998), indicating that upwelling in the southern Cape Basin has never created similarly elevated levels of productivity as at the more northerly locations.
Variations in TOC concentrations in the upper 240 m of sediment are typically ~1% (Fig. F2). Similar, but more strongly expressed, variations in TOC concentrations are present in Pliocene-Pleistocene sediments at Leg 175 Sites 1082, 1084, and 1085 (Wefer, Berger, Richter, et al., 1998). These variations resemble the cyclic variations in TOC concentrations in sediments at Sites 362 and 532 on the Walvis Ridge that Diester-Haass et al. (1992) interpret to represent glacial-interglacial changes in the intensity and locus of the Benguela Current.
Organic C/N values were calculated for sediment samples from Site 1087 using TOC and total nitrogen concentrations (Table T1). The C/N values vary from 15.8 to 5.4 (Fig. F3). Most of these atomic ratios are intermediate between unaltered algal organic matter (5-8) and fresh land-plant material (25-35) (Emerson and Hedges, 1988; Meyers, 1994). Many of the low C/N values occur in sediment that is poor in organic carbon; these values may be biased by the tendency of clay minerals to absorb ammonium ions generated during the degradation of organic matter (Müller, 1977). Because of their setting off shore of a coastal desert, it is likely that these sediments contain mostly marine-derived organic matter. However, the initiation of the Namib Desert and consequent paucity of land-derived organic matter is linked to the onset of coastal upwelling in the late Miocene (van Zinderen Bakker, 1984). The elevated C/N values of organic carbon-lean Eocene-Oligocene sediments near the base of the core (Fig. F3) may record contributions of land-derived organic matter from a less arid and consequently more vegetated continental paleoenvironment.
C/N values are higher in sediments richer in TOC than in those lean in TOC (Fig. F4). Many of the values are higher than in fresh algal organic matter and indicate that selective loss of nitrogen-rich proteinaceous matter and consequent elevation of the elemental ratios occurred during sedimentation of marine organic matter. Similar evidence of early diagenetic alteration of C/N values is commonly seen under areas of elevated marine productivity such as upwelling systems (Meyers, 1997). The existence of the diagenetically elevated C/N values is evidence that organic matter preservation was enhanced during accumulation of the Pliocene-Pleistocene sediments at Site 1087, presumably because of the presence of a strongly developed oxygen minimum zone along the continental margin of the Cape Basin.