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doi:10.2204/iodp.proc.314315316.123.2009

Organic geochemistry

Hydrocarbon gas composition

Methane is the predominant hydrocarbon present in all Site C0001 cores. Concentrations of methane and ethane, along with methane to ethane (C₁/C₂) ratios, are shown in Table T19. Minor amounts of ethane (<10 ppmv) were detected in most sections, and no heavier hydrocarbons were detected. The vertical distributions of headspace methane and ethane are shown in Figure F41. Methane concentrations increase rapidly from 3.6 ppmv in the near-surface sample (1.6 m CSF) to 36,330 ppmv at 34 m CSF (Fig. F42). Concentrations decrease to 100 m CSF and remain nearly constant to 231 m CSF. Below this depth, methane concentrations are relatively high; the highest value (43,884 ppmv) occurs at 278 m CSF. Vertical distribution of ethane is similar to that of methane (Fig. F41). The C₁/C₂ ratio ranges from 1065 to 6818 with the exception of a low abnormality (342) at 192 m CSF (Fig. F43). The C₁/C₂ ratio gradually decreases with depth in Unit I and slightly increases with depth in Unit II. The increase of methane concentrations and the C₁/C₂ ratio in Unit II indicates a greater contribution of biogenic methane at depth. The C₁/C₂ ratio is mostly >1000, suggesting that methane found at Site C0001 is biogenic, not thermogenic, in origin. The calculated SMT depth is 27 m CSF (Fig. F41). This inverse correlation suggests that methane results from methanogenesis, which is inhibited in the presence of sulfate.

Sediment carbon, nitrogen, and sulfur composition

A total of 143 mud and mudstone samples were collected at Site C0001 next to whole-round samples for pore water and geotechnical tests. All samples were analyzed for inorganic carbon, total carbon, nitrogen, and sulfur (see “Organic geochemistry” in the “Expedition 315 methods” chapter for analytical procedures). Values of inorganic carbon, calcium carbonate (CaCO₃), total organic carbon (TOC), total nitrogen, organic carbon to total nitrogen (C/N) ratio, and total sulfur are listed in Table T20 and presented in Figure F44.

Calcium carbonate content calculated from inorganic carbon content varies from 0.2 to 31.9 wt%. In Unit I, concentrations are highly variable and range from 0.5 to 31.9 wt% with an average of 12.5 wt%. Concentrations tend to decrease with depth, although absolute values are considerably scattered. In Unit II, concentrations are more consistent and the average value is 3.2 wt%. Low carbonate contents throughout Unit II indicate deposition below the CCD.

TOC content ranges from 0.01 to 0.67 wt% with an average of 0.37 wt%. Total nitrogen contents are <0.15 wt%, and the average value is 0.09 wt%. The C/N ratio shows little variation with depth except for high values at 168 and 184 m CSF; the average value is 4.4. This value is more enriched in nitrogen than the expected value for marine organic matter (6–9) if the fraction of inorganic ammonium in the total nitrogen can be ignored. Clay-bound nitrogen substances, including inorganic ammonium, cause C/N ratios to decrease. This effect may explain the low C/N ratio.

Total sulfur content ranges from 0.12 to 4.85 wt% with an average of 0.52 wt%. In Unit I, concentrations are fairly uniform. Four spike peaks are observed at 85, 103, 181, and 353 m CSF, and their concentrations are >2.0 wt%. Between 200 and 260 m CSF in Subunit IIA, concentrations are relatively higher. Below this depth, concentrations decrease and then remain constant.

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