Calcium carbonate and organic carbon concentrations were measured on sediment samples from Hole 1076A (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 headspace gas contents were measured, and routine monitoring of the sedimentary gases was done for drilling safety.
Concentrations of carbonate carbon are low in Site 1076 sediments. They vary between 1.9 and 0.1 wt% (Table 11). The maximum carbonate carbon concentration is equivalent to 16 wt% sedimentary CaCO3, and most sediment samples contain <5 wt% CaCO3. These generally low concentrations agree with the paucity of coccoliths and the high abundances of opaline and continental clastic material at this site (see "Biostratigraphy and Sedimentation Rates" section, this chapter). The range in concentrations reflects a varying combination of changes in biological production of calcareous material, dilution by noncalcareous components, and carbonate dissolution fueled by oxidation of organic matter.
TOC determinations were done on a smaller number of Hole 1076A sediment samples than carbonate determinations because of the generally uniform lithology. TOC values range from 4.32 to 0.93 wt% (Table 11) and average 2.56 wt%. The concentrations are ~10 times greater than the average of 0.3 wt% given by McIver (1975) based on DSDP Legs 1-33, a value that can be considered representative of typical deep-sea sediments. The high TOC concentrations at this site may be ascribed to a combination of high supply from elevated paleoproductivities and a high rate of accumulation enhancing preservation of the organic matter.
Organic C/N ratios were calculated for Site 1076 samples using TOC and total nitrogen concentrations to help identify the origin of their organic matter. Site 1076 C/N ratios vary from 17.2 to 4.5 (Table 11). The C/N ratios average 13.9, a value that is intermediate between unaltered algal organic matter (5-8) and fresh land-plant material (25-35; e.g., Emerson and Hedges, 1988; Meyers, 1994). It is likely that these organic carbon-rich sediments contain a mixture of partially degraded algal material and detrital continental organic matter. Preferential loss of nitrogen-rich, proteinaceous matter can elevate the C/N ratios of algal organic matter during settling to the seafloor (Meyers, 1997).
A Van Krevelen-type plot of the hydrogen index (HI) and oxygen index (OI) values indicates that the sediments contain a mixture of type II (algal) and type III (land-derived) organic matter (Fig. 22). This source assignment for the organic matter is consistent with the intermediate C/N ratios for these samples, which also suggests that the organic matter is a mixture of marine and continental material. An equally likely possibility, however, is that the sediments principally contain algal-derived organic matter that has been altered by microbial processing during early diagenesis. Well-preserved type II organic matter has high HI values (Peters, 1986), which can be lowered by microbial oxidation (Meyers, 1997). The low HI values of fresh type III organic matter, however, cannot become elevated by postdepositional alteration. In general, Hole 1076A sediments having higher Rock-Eval TOC values also have higher HI values (Table 12). This relationship is consistent with partial oxidation of algal organic matter. Further evidence of substantial amounts of in situ organic matter degradation exists in the large increases in alkalinity and decreases in sulfate in the interstitial waters of Site 1076 sediments (see "Inorganic Chemistry" section, this chapter).
Variable Tmax values (Table 12) reflect poorly defined S2 peaks and not actual thermal maturities of organic matter. Those samples in which the geometry of S2 peaks was sharp have relatively low Tmax values, showing that organic matter is thermally immature with respect to petroleum generation (Peters, 1986) and therefore contains little detrital organic matter derived from erosion of ancient sediments and transported to this site by the Congo River.
Sediments from Hole 1075A had high gas content. Gas pressures became great enough in sediments below Core 175-1076A-15H (138 mbsf) to require perforating the core liner to relieve the pressure and alleviate core expansion. Natural gas analyses determined that much of this gas was CO2 (Table 13). Hydrogen sulfide could be detected by nose, but not by hydrogen sulfide-sensing instruments having a sensitivity of ~1 ppm, in Cores 175-1076A-3H through 5H (18-39 mbsf).
Methane (C1) first appears in headspace gas samples in Hole 1076A sediments at 28.3 mbsf. Concentrations rapidly increase and become significant in sediments below 35 mbsf (Fig. 23). As at Site 1075, high methane/ethane (C1/C2) ratios and the absence of major contributions of higher molecular weight hydrocarbon gases (Table 13) indicate that the methane 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). As noted by Claypool and Kvenvolden (1983), the presence of interstitial sulfate inhibits methanogenesis in marine sediments.