Calcium carbonate and organic carbon concentrations were measured on sediment samples from Hole 1079A (Table 9). 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 are rather low in Site 1079 sediments (Table 9). The maximum carbonate carbon concentration is equivalent to 17.9 wt% sedimentary CaCO3. These generally low concentrations agree with the paucity of coccoliths and foraminiferal microfossils in these hemipelagic sediments (see "Biostratigraphy and Sedimentation Rates" section, this chapter). The range in concentrations, although small, 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 1079A sediment samples than carbonate determinations because of the generally uniform lithology. TOC values range from 1.98 to 5.25 wt% (Table 9) and average 3.02 wt%. The concentrations are 10 times greater than the average of 0.3 wt% given by McIver (1975) based on Deep Sea Drilling Project (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 elevated paleoproductivities and a high supply of organic matter from high accumulation rate of sediments, enhancing the preservation of organic matter.
Organic C/N ratios were calculated for Site 1079 samples using TOC and total nitrogen concentrations to help identify the origin of their organic matter. Site 1079 C/N ratios vary from 12.4 to 17.7 (Table 9). The C/N ratios average 15.3, which is 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). These organic carbon–rich sediments probably contain a mixture made up mostly of degraded algal material and partly 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 the Angola margin (Meyers, 1997).
A Van Krevelen–type plot of the hydrogen index (HI) and oxygen index (OI) values (Table 10) suggests that the sediments contain a mixture of type II (algal) and type III (land-derived) organic matter (Fig. 21). Such an admixture of organic matter is consistent with the inter-mediate C/N ratios for these samples, which similarly suggest that the organic matter is constituted of marine and continental material. Another possibility—one that seems more likely—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 1079A sediments having lower Rock-Eval TOC values also have lower HI values (Fig. 22). This relationship confirms that the algal organic matter has been subject to much oxidation, which simultaneously lowers TOC and HI values. Further evidence of substantial amounts of in situ organic matter degradation exists in the large decreases in sulfate and increases in alkalinity in the interstitial waters of Site 1079 sediments (see "Inorganic Geochemistry" section, this chapter).
The sediment samples have relatively low Rock-Eval Tmax values, showing that their 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 on the African continent.
Sediments from Site 1079 had high gas content. Gas pressures became great enough in sediments below Core 175-1079A-4H (24 mbsf) to require perforating the core liner to relieve the pressure and prevent excessive core expansion. Natural gas analyses determined that most of this gas was CO2, and headspace concentrations of this gas continued to increase to the bottom of Hole 1079A (120 mbsf; Fig. 23). Hydrogen sulfide could be detected by nose, but not by hydrogen sulfide–sensing instruments having a sensitivity of ~1 ppm, in Cores 175-1079A-1H through 8H (5–33.5 mbsf).
Methane (C1) first appears in headspace gas samples in Hole 1079A sediments at 28.6 mbsf. Concentrations gradually increase and become significant in sediments below 40 mbsf (Fig. 24). As at Sites 1075 through 1078, high methane/ethane (C1/C2) ratios and the absence of major contributions of higher molecular weight hydrocarbon gases (Table 11) 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). As noted by Claypool and Kvenvolden (1983), the presence of interstitial sulfate inhibits methanogenesis in marine sediments.