Because of time limitations between coring and entering port, the only organic geochemical measurements that were performed entailed routine monitoring of the sedimentary gases for drilling safety.
Moderately high amounts of methane and CO2 were found in sediments from Site 1087 (Table 11). The odor of hydrogen sulfide was noted in cores from 5 to 250 mbsf. Total gas pressures became great enough in sediments between 15 and 200 mbsf to require perforating the core liner to relieve the pressure and prevent excessive core expansion.
Methane (C1) first appears in headspace gas samples from Site 1087 sediments at 22.2 mbsf. Concentrations become significant in sediments below 60 mbsf (Fig. 17). The rate of methane appearance is slower than in most sites cored during Leg 175. 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. As at Sites 1084 through 1086, the origin of the methane is probably from in situ microbial fermentation of the marine organic matter present in the sediments. A biogenic origin of 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.
The most abundant gas is CO2 in the upper 350 m of Hole 1087C. Concentrations of this gas decrease sharply below this depth (Fig. 18). Methane concentrations gradually increase downhole until methane dominates gas compositions in sediments deeper than 350 mbsf, even though methane concentrations never equal those of shallower CO2 concentrations (Table 11). Cragg et al. (1992) report the existence of viable microbes to depths of ~500 mbsf in the sediments of the Japan Sea. The abundance of biogenic gases deep in sediments at Site 1087 suggests the presence of viable microbial communities to similar sub-bottom depths on the southwest African margin.