Calcium carbonate and organic carbon concentrations were measured on sediment samples from Hole 1082A (Table 12). Organic matter atomic carbon/nitrogen (C/N) ratios and Rock-Eval pyrolysis analyses were employed to determine the type of organic matter contained within the sediments. Elevated amounts of gas were encountered, and routine monitoring of the sedimentary gases was done for drilling safety.
Concentrations of carbonate carbon in Site 1082 sediments range between 10.3 and 0.1 wt%, corresponding to 85.8 and 1.0 wt% CaCO3 (Table 12). The carbonate concentrations vary in two ways: (1) closely spaced changes related to light–dark color fluctuations and (2) a general downhole decrease followed by an increase in concentrations (Fig. 22). Sediments at this site are divided into an upper lithostratigraphic unit, which has three subunits, and a lower unit (see "Lithostratigraphy" section, this chapter). Subunit IA, a Pleistocene nannofossil- and foraminifer-rich clay, averages 42 wt% CaCO3. Subunit IB is a Pliocene–Pleistocene diatom-rich clay that averages 33 wt% CaCO3. Subunit IC is a Pliocene nannofossil clay that contains an average of 52 wt% CaCO3. Unit II, a Miocene–Pliocene nannofossil ooze, averages 67 wt% CaCO3. The variations in concentrations reflect varying combinations of changes in delivery of calcareous material, dilution by noncalcareous components, and carbonate dissolution fueled by oxidation of organic matter.
TOC determinations were done on selected samples of Hole 1082A sediments to estimate the amounts of organic matter in the different lithostratigraphic units (Table 12). Like CaCO3 concentrations, TOC concentrations change in both short-term and longer term patterns (Fig. 23). Dark-colored sediments have higher TOC values than light-colored layers. TOC concentrations also differ in Hole 1082A lithostratigraphic units, averaging 6.16 wt% in Subunit IA, 4.07 wt% in Subunit IB, 2.31 wt% in Subunit IC, and 0.71 wt% in Unit II. The high TOC concentrations in the subunits of Unit I are a consequence of the elevated paleoproductivity of the Benguela Current upwelling system, which has delivered abundant organic matter to the sediments, and the high accumulation rate of sediments (see "Biostratigraphy and Sedimentation Rates" section, this chapter), which enhances preservation of the organic matter.
Organic C/N ratios were calculated for sediment samples from the different Site 1082 lithostratigraphic units using TOC and total nitrogen concentrations (Table 12). The C/N ratios vary from 18.2 to <1 (Fig. 24). Most of these C/N ratios are intermediate between unaltered algal organic matter (5–8) and fresh land-plant material (25–35; e.g., Emerson and Hedges, 1988; Meyers, 1994). The low C/N ratios occur in samples that are 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). The means of the C/N ratios in each lithostratigraphic unit are as follows: Subunit IA, 14.9; Subunit IB, 14.1; Subunit IC, 11.8; and Unit II, 6.8. Because of their setting under a major upwelling system and offshore from a coastal desert, it is likely that these organic carbon–rich sediments contain mostly marine-derived organic matter with only a minor contribution of detrital continental organic matter. The C/N ratios that are higher than fresh algal organic matter indicate that preferential loss of nitrogen-rich, proteinaceous matter and consequent elevation of C/N ratios occurred during settling of organic matter to the seafloor. Such early diagenetic alteration of C/N ratios is often seen under areas of elevated marine productivity such as upwelling systems (Meyers, 1997).
A Van Krevelen–type plot of the hydrogen index (HI) and oxygen index (OI) values indicates that the sediments contain type II (algal) organic matter (Fig. 25) that has been altered by microbial processing during early diagenesis. Well-preserved type II organic matter has high HI values (Peters, 1986); these values can be lowered by microbial oxidation (Meyers, 1997). In general, Hole 1082A sediments having lower Rock-Eval TOC values also have lower HI values (Fig. 26). This relationship confirms that the marine organic matter has been subject to partial oxidation, which simultaneously lowers TOC and HI values (Meyers, 1997). Further evidence of substantial amounts of in situ organic matter degradation exists in the large decreases in sulfate and increases in alkalinity and ammonia in the interstitial waters of Site 1082 sediments (see "Inorganic Geochemistry" section, this chapter).
The sediment samples have low Rock-Eval Tmax values (Table 13), showing that their organic matter is thermally immature with respect to petroleum generation (Peters, 1986) and therefore is unlikely to contain much detrital organic matter derived from erosion of ancient sediments from Africa.
Relatively high amounts of hydrogen sulfide, methane, and CO2 were found in sediments from Site 1082. The odor of hydrogen sulfide was noted in Cores 175-1082A-2H through 13H (1.5–120 mbsf), and detectable concentrations of this gas were occasionally found throughout Hole 1082A (Table 14). Total gas pressures became great enough in sediments below Core 175-1082A-2H (17 mbsf) to require perforating the core liner to relieve the pressure and prevent excessive core expansion.
Methane (C1) first appears in headspace gas samples of Hole 1082A sediments at 4.5 mbsf. Concentrations become significant in sediments below 20 mbsf (Fig. 27). As at Site 1081, high methane/ethane (C1/C2) ratios and the absence of major contributions of higher molecular weight hydrocarbon gases (Table 14) indicate that the gas is biogenic, as opposed to thermogenic, in origin. A biogenic origin of methane is supported by the disappearance of interstitial sulfate at approximately the same sub-bottom depth where methane concentrations begin to rise (see "Inorganic Geochemistry" section, this chapter), inasmuch as Claypool and Kvenvolden (1983) observe that the presence of interstitial sulfate inhibits microbial methanogenesis in marine sediments.
Natural gas analyses determined that the most abundant gas was CO2 and that headspace concentrations of this gas remained high deep in Hole 1082A (586 mbsf; Fig. 28). Cragg et al. (1992) report the existence of viable microbes to depths of ~500 mbsf in the sediments of the Japan Sea. The abundance of biogenic gases in sediments from Site 1082 suggests the presence of viable microbial communities to similar sub-bottom depths on the Walvis Ridge.