Calcium carbonate and organic carbon concentrations were measured on sediment samples from Site 1080 (Table 8). 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. High gas contents were encountered, and routine monitoring of the sedimentary gases was done for drilling safety.
Concentrations of carbonate carbon are low in most Site 1080 sediments. Sample 175-1080A-1H-1, 46–47 cm, contains 3.1 wt% inorganic carbon, but concentrations quickly decrease with depth (Table 8). The maximum carbonate carbon concentration is equivalent to 26.1 wt% sedimentary CaCO3. These moderately low concentrations agree with the low abundances of coccoliths and foraminifer microfossils in these hemipelagic sediments (see "Biostratigraphy and Sedimentation Rates" section, this chapter). The decrease in carbonate concentrations is probably largely a result of dissolution fueled by oxidation of organic matter (see "Inorganic Geochemistry" section, this chapter).
TOC values range from 3.86 to 1.10 wt% (Table 8) and average 2.40 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 sedi-ments. The high TOC concentrations at this site may be ascribed to a combination of a high supply of organic matter from elevated paleo-productivities and a high accumulation rate of sediments, enhancing the preservation of organic matter.
Organic C/N ratios were calculated for Site 1080 samples using TOC and total nitrogen concentrations to help identify the origin of their organic matter. Site 1080 C/N ratios vary from 14.2 to 8.4 (Table 8). The C/N ratios average 11.1, 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). It is probable that these organic carbon–rich sediments contain a mixture that is 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, proteinacous 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 hydrogen index (HI) and oxygen index (OI) values (Fig. 19) 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 1080 sediments having higher Rock-Eval TOC values also have higher HI values (Fig. 20). This relationship confirms that the algal organic matter has been partially oxidized. 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 1080 sediments (see "Inorganic Geochemistry" section, this chapter).
Rock-Eval Tmax values are low (Table 9), 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 1080 had high gas content. Gas pressures became great enough in sediments below the first core to require perforating the core liner to relieve the pressure and minimize 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 1080A (50 mbsf; Fig. 19). Hydrogen sulfide could be detected at a concentration of 4 ppm in Core 175-1080A-1H. A moderate to weak odor of H2S prevailed from near-surface sediments through Core 175-1080A-3H (22 mbsf).
Methane (C1) first appears in significant concentrations in headspace gas samples at 6.3 mbsf. Concentrations rapidly increase and are high in sediments below 7 mbsf (Fig. 20). As at Site 1075 and the other sites off Angola, high methane/ethane (C1/C2) ratios and the absence of major contributions of higher molecular weight hydrocarbon gases (Table 10) 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.