Calcium carbonate and organic carbon concentrations were measured on sediment samples from Site 1078 (Table 10). 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 low in Site 1078 sediments. They vary between 3.0 and 1.3 wt% (Table 10). The maximum carbonate carbon concentration is equivalent to 25.4 wt% sedimentary CaCO3. These moderately low concentrations agree with the low abundances of coccoliths and other calcareous microfossils in these hemipelagic sediments (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 Site 1078 sediment samples than carbonate determinations because of the generally uniform lithology. TOC values range from 1.06 to 5.35 wt% (Table 10) and average 2.53 wt%. The concentrations are nearly 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 a high supply of organic matter caused by elevated paleoproductivities and a high accumulation rate of sediments enhancing the preservation of organic matter.
Organic C/N ratios were calculated for Site 1078 samples using TOC and total nitrogen concentrations to help identify the origin of their organic matter. Site 1078 C/N ratios vary from 9.3 to 15.7 (Table 10). The C/N ratios average 12.1, 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 that is made up mostly of degraded algal material and partially of detrital continental organic matter. The C/N ratios that are higher than fresh algal organic matter reflect preferential loss of nitrogen-rich, proteinaceous matter and consequent elevation of C/N ratios during settling to the seafloor. This change in C/N ratios often occurs under areas of elevated algal productivity, such as the Angola margin (Meyers, 1997).
A Van Krevelen-type plot of hydrogen index (HI) and oxygen index (OI) values (Fig. 31) similarly indicates that the sediments contain type II (algal) 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. Site 1078 sediments having higher Rock-Eval TOC values also have higher HI values (Fig. 32). This relationship confirms that the algal organic matter has been oxidized. 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 1078 sediments (see "Inorganic Geochemistry" section, this chapter).
Rock-Eval Tmax values are low (Table 11), showing that organic matter is thermally immature with respect to petroleum generation (Peters, 1986). The low thermal maturity is consistent with the measured geothermal gradient of 45.7°C/km at this site (see "Physical Properties" section, this chapter).
Sediments from Site 1078 had high gas content. Gas pressures became great enough in sediments below Core 175-1078A-4H (36 mbsf) to require perforating the core liner to relieve the pressure and alleviate 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 1078C (160 mbsf; Fig. 33). Hydrogen sulfide could be detected by nose, but not by hydrogen sulfide-sensing instruments having a sensitivity of ~1 ppm, in Cores 175-1078A-1H through 3H (1.5-26.5 mbsf).
Methane (C1) first appears in headspace gas samples in Site 1078 sediments at 6 mbsf. Concentrations rapidly increase and become significant in sediments between 20 and 35 mbsf, below which they decrease (Fig. 34). As at Sites 1075, 1076, and 1077, high methane/ethane (C1/C2) ratios and the absence of major contributions of higher molecular weight hydrocarbon gases (Table 12) 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.