Calcium carbonate and organic carbon concentrations were measured on sediment samples from Hole 1086A (Table 10). Organic matter atomic carbon/nitrogen (C/N) ratios were employed to identify the origin of organic matter contained within the sediments. Moderate amounts of gas were encountered, and routine monitoring of the sedimentary gases was done for drilling safety.
Concentrations of carbonate carbon in Site 1086 sediments range between 10.6 and 7.6 wt%, corresponding to 88.7 and 63.0 wt% CaCO3 (Table 10). Despite considerable fluctuations in carbonate concentrations (Fig. 21), the sediments at this site represent a single lithostratigraphic unit (see "Lithostratigraphy" section, this chapter). This unit averages 79 wt% CaCO3.
TOC determinations were done on selected samples of Hole 1086A sediments to estimate the amounts of organic matter in the sediments (Table 10). TOC concentrations vary between 3.56 wt% and values below detection (<0.05 wt%) and generally decrease with sediment depth (Fig. 22). Their average value is 1.11 wt%. These TOC concentrations reflect a history of moderate-to-low productivity in this part of the Benguela Current system, which has delivered only modest amounts of organic matter to the sediments, and the low accumulation rate of sediments (see "Biostratigraphy and Sedimentation Rates" section, this chapter), which is detrimental to preservation of organic matter.
Organic C/N ratios were calculated for sediment samples using TOC and total nitrogen concentrations (Table 10). For those samples having nitrogen concentrations below the limit of reliable measurement (0.05 wt%), C/N values have been excluded. The C/N ratios vary from 13.9 to 4.3 (Fig. 23) and average 8.6. Many of these C/N ratios are intermediate between unaltered algal organic matter (5–8) and fresh land-plant material (25–35; e.g., Emerson and Hedges, 1988; Meyers, 1994). Because of their setting seaward of a major upwelling system and offshore from a coastal desert, however, it is likely that these sediments contain mostly marine-derived organic matter with only a minor contribution of detrital continental organic matter.
Low amounts of methane (C1) and CO2 were found in sediments from Site 1086. Methane first appears in headspace gas samples from Hole 1086A sediments at 6 mbsf, but concentrations never become significant (Table 11). The origin of the methane is probably from in situ microbial fermentation of the marine organic matter present in the sediments, and the low availability of organic matter limits gas production. 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), inasmuch as Claypool and Kvenvolden (1983) observe that the presence of interstitial sulfate inhibits microbial methanogenesis in marine sediments.
The most abundant gas was CO2, and headspace concentrations of this gas remained high deep in Hole 1086A (201 mbsf; Table 11). Cragg et al. (1992) isolated viable microbes to depths of ~500 mbsf in sediments from the Japan Sea. The presence of small amounts of biogenic gases in sediments from Site 1086 suggests the presence of similarly viable microbial communities throughout the sedimentary sequence at this location, but their activity has been limited by the low availability of organic matter.