Calcium carbonate and organic carbon concentrations were measured on sediment samples from Hole 1085A (Table 11). Organic matter atomic carbon/nitrogen (C/N) ratios and Rock-Eval 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 1085 sediments range between 10.2 and 3.3 wt%, corresponding to 84.8 and 27.8 wt% CaCO3 (Table 11). The carbonate concentrations vary in two ways: (1) closely spaced changes related to light–dark color fluctuations and (2) more gradual downhole increases and decreases (Fig. 24). Sediments at this site are divided into an upper lithostratigraphic unit, which has two subunits, and a lower unit (see "Lithostratigraphy" section, this chapter). Subunit IA, a Pliocene–Holocene nannofossil-foraminifer ooze, averages 69 wt% CaCO3. Subunit IB is a Miocene–Pliocene nannofossil ooze that averages 65 wt% CaCO3. Unit II, a Miocene clay-rich nannofossil ooze, averages 59 wt% CaCO3. The variations in concentrations reflect varying combinations of changes in delivery of calcareous material, dilution by noncalcareous components, and carbonate dissolution.
TOC determinations were done on selected samples from Hole 1085A 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. 25). TOC concentrations are low, averaging 1.42 wt% in lithostratigraphic Subunit IA and 0.66 wt% in Subunit IB. The single TOC determination from Unit II is 0.37 wt%. These TOC concentrations reflect a history of moderate-to-low productivity in this part of the Benguela Current system, which has delivered only modest amounts of organic matter to the sediments, and the low accumulation rate of sediments (see "Biostratigraphy and Sedimentation Rates" section, this chapter), which has not aided preservation of the organic matter.
Organic C/N ratios were calculated for sediment samples from the different Site 1085 lithostratigraphic subunits using TOC and total nitrogen concentrations (Table 11). For those samples having nitrogen concentrations below the limit of reliable measurement (0.05 wt%), C/N values have been excluded. The C/N ratios vary from 17.6 to 3.0 (Fig. 26). 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 are Subunit IA, 11.6; Subunit IB, 9.5; and Unit II, 10.8. Because of their setting offshore from a coastal desert, it is likely that these sediments contain mostly marine-derived 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.
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. 27) that has been heavily 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 1085A sediments having lower Rock-Eval TOC values also have lower HI values (Fig. 28). 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 1085 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 thermally mature sedimentary rocks from Africa.
Moderately high amounts of methane and CO2 were found in sediments from Site 1085 (Table 13). The odor of hydrogen sulfide was noted in Cores 175-1085A-2H through 10H (5.2–84 mbsf). Total gas pressures became great enough in sediments below Core 175-1085A-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 1085A sediments at 17.7 mbsf. Concentrations become significant in sediments below 46 mbsf (Fig. 29). High 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. As at Sites 1081 through 1084, 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 methane, not CO2 as at other Leg 175 sites. Headspace concentrations of methane remained high, whereas those of CO2 gradually diminished with depth in sediments from Hole 1085A (Fig. 30). Cragg et al. (1992) report the existence of viable microbes to depths of ~500 mbsf in the sediments from the Japan Sea. The abundance of biogenic gases deep in sediments from Site 1085 suggests the presence of viable microbial communities to similar sub-bottom depths on the Namibia margin.