Calcium carbonate and organic carbon concentrations were measured on sediment samples from Hole 1083A (Table 11). 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 1083 sediments range between 9.9 and 2.4 wt%, corresponding to 82.8 and 19.8 wt% CaCO3 (Table 11). The carbonate concentrations display closely spaced changes related to light–dark color fluctuations (Fig. 16). Sediments at this site are divided into an upper lithostratigraphic subunit and a lower unit (see "Lithostratigraphy" section, this chapter). Subunit IA, a Pleistocene–Holocene foraminifer-rich clayey nannofossil ooze, averages 57 wt% CaCO3. Subunit IB is a Pliocene–Pleistocene diatom- and foraminifer-rich clayey nannofossil ooze that averages 50 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.
Total organic carbon (TOC) determinations were done on selected samples of Hole 1083A sediments to estimate the amounts of organic matter in the two lithostratigraphic subunits (Table 11). Like CaCO3 concentrations, TOC concentrations fluctuate on various scales (Fig. 17). Dark-colored sediments have higher TOC values than light-colored layers. TOC concentrations differ somewhat in Hole 1083A lithostratigraphic units, averaging 3.12 wt% in Subunit IA and 2.36 wt% in Subunit IB. The elevated TOC concentrations in the subunits are a consequence of the elevated paleoproductivity of the nearby 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 has enhanced preservation of the organic matter.
Organic C/N ratios were calculated for sediment samples from the different Site 1083 lithostratigraphic units using TOC and total nitrogen concentrations (Table 11). The C/N ratios vary from 17.4 to 8.2 (Fig. 18). These C/N ratios are intermediate between unaltered algal organic matter (5–8) and fresh land-plant material (25–35; e.g., Emer-son and Hedges, 1988; Meyers, 1994). The mean C/N ratios of the two lithostratigraphic subunits are virtually identical (12.4 in Subunit IA and 12.6 in Subunit IB). Because of their setting seaward of a major upwelling system and offshore from a coastal desert, it is likely that these sediments contain mostly marine-derived organic matter with only a minor contribution of detrital continental organic matter. The C/N ratios 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. 19) 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 1083A sediments having lower Rock-Eval TOC values also have lower HI values (Fig. 20). 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 1083 sediments (see "Inorganic Geochemistry" section, this chapter).
The sediment samples have low Rock-Eval Tmax values (Table 12), 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 1083. The odor of hydrogen sulfide was noted in Cores 175-1083A-2H through 15H (3.3–130 mbsf). Total gas pressures became great enough in sediments below Core 3H (18 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 1083A sediments at 6.3 mbsf. Concentrations become significant in sediments below 35 mbsf (Fig. 21). As at Sites 1081 and 1082, high methane/ethane (C1/C2) ratios and the absence of major contributions of higher molecular weight hydrocarbon gases (Table 13) indicate that the gas is biogenic, as opposed to thermogenic, in origin. A biogenic origin of the 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 1083A (200 mbsf; Fig. 22). Cragg et al. (1992) isolated viable microbes to depths of ~500 mbsf in sediments of the Japan Sea. The abundance of biogenic gases in sediments from Site 1083 suggests the presence of similarly viable microbial communities throughout the sedimentary sequence at this location.