Calcium carbonate and organic carbon concentrations were measured on sediment samples from Hole 1081A (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. High gas contents were encountered, and routine monitoring of the sedimentary gases was done for drilling safety.
Concentrations of carbonate carbon in Site 1081 sediments range between 6.4 and 0.1 wt%, corresponding to 53.4 and 1.0 wt% CaCO3 (Table 11). 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. 35). Sediments from 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 31 wt% CaCO3. Subunit IB is a Pliocene–Pleistocene diatom-rich clay that averages 8 wt% CaCO3. Subunit IC is a Miocene–Pliocene nannofossil-rich clay that contains an average of 18 wt% CaCO3. Unit II, a Miocene clayey nannofossil ooze, averages 30 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 regularly throughout Hole 1081A sediments to estimate the amounts of organic matter in the different lithostratigraphic units (Table 11). Like CaCO3 concentrations, TOC concentrations change in both short-term and longer term patterns (Fig. 36). Dark-colored sediments have higher TOC values than light-colored layers. TOC concentrations also differ in Hole 1081A lithostratigraphic units, averaging 5.00 wt% in Subunit IA, 5.21 wt% in Subunit IB, 3.34 wt% in Subunit IC, and 2.18 wt% in Unit II. The high TOC concentrations in all units 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 1081 lithostratigraphic units using TOC and total nitrogen concentrations (Table 11). The C/N ratios vary from 16.9 to 7.8 (Fig. 37). 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 means of the C/N ratios are Subunit IA, 13.4; Subunit IB, 14.1; Subunit IC, 11.9; and Unit II, 10.3. 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. 38) 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 1081A sediments having lower Rock-Eval TOC values also have lower HI values (Fig. 39). 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 1081 sediments (see "Inorganic Geochemistry" section, this chapter).
The sediment samples have relatively low Rock-Eval Tmax values (Table 12), showing that their organic matter is thermally immature with respect to petroleum generation (Peters, 1986) and therefore contains little detrital organic matter derived from the erosion of ancient sediments from Africa.
Sediments from Site 1081 had high gas content, including CO2, methane, and hydrogen sulfide. Total gas pressures became great enough in sediments below Core 175-1081A-3H (20 mbsf) to require perforating the core liner to relieve the pressure and prevent excessive core expansion. The odor of hydrogen sulfide was noted in Cores 175-1081A-2H through 5H (1.5–37.9 mbsf), although this gas remained below the detection limits of shipboard instruments (~1 ppm).
Methane (C1) first appears in headspace gas samples from Hole 1081A sediments at 43.4 mbsf. Concentrations become significant in sediments below 60 mbsf (Fig. 40). 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. The origin of the methane is probably from in situ microbial fermentation of the marine organic matter present in the sediments. Similar microbial production of methane from marine organic matter has been inferred from high biogenic gas concentrations in Pliocene–Pleistocene sediments from DSDP Site 532 nearby on the Walvis Ridge (Meyers and Brassell, 1985), Sites 897 and 898 on the Iberian Abyssal Plain (Meyers and Shaw, 1996), and also in middle Miocene sediments from Site 767 in the Celebes Sea (Shipboard Scientific Party, 1990). 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 usually CO2, and headspace concentrations of this gas remain high to the bottom of Hole 1081A (446.5 mbsf; Fig. 41). 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 1081 suggests the presence of viable microbial communities to similar sub-bottom depths on the Walvis Ridge.