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doi:10.2204/iodp.proc.314315316.124.2009

Organic geochemistry

Hydrocarbon gas composition

Methane was the predominant hydrocarbon present in all Site C0002 cores. Concentrations of methane, ethane, and propane, along with methane to ethane ratios (C₁/C₂), are shown in Table T15. Ethane was detected in all samples, and propane was also detected in the lower part of Hole C0002B. Depth profiles of headspace methane, ethane, and propane are shown in Figure F24. Methane concentrations rapidly increase from 16.7 ppmv in the near-surface sample to 26,449 ppmv at 16.8 m CSF. In the deeper sections of Hole C0002D, concentrations decrease with depth. In Hole C0002B, methane concentrations remain constant from the top of the coring depth (476 m CSF) to 610 m CSF and increase with depth in the deeper part of Unit II. Methane concentrations decrease with depth in Unit III and increase rapidly in the lower part of Unit IV. Ethane was not detected in Hole C0002D. The depth profile of ethane in Hole C0002B is similar to that of methane (Fig. F24). Below 1000 m CSF, propane is present in four samples, suggesting contribution of thermogenic origin. Thermogenic gas indicates diagenetic production of gas from older, more mature organic matter in the sediments or the migration of gas from greater depths. C₁/C₂ ratios range from 169 to 1868 (Fig. F25). Ratios remain constant at ~1000 above 900 m CSF. Below this depth, ratios gradually decrease with depth to the bottom of the hole. Ratios <1000 below 900 m CSF indicate some contribution of thermogenic hydrocarbons.

Sediment carbon, nitrogen, and sulfur composition

A total of 90 sediment samples were collected from Hole C0002B next to all whole-round samples for interstitial 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 in the sediments are listed in Table T16 and presented in Figure F26.

Calcium carbonate content calculated from inorganic carbon content ranges from 0.66 to 22.98 wt% with an average of 7.28 wt%. Calcium carbonate concentrations are significantly higher in Unit III than in Units II and IV. The difference in concentrations between each unit may be related to the position relative to the CCD during deposition and dilution of biogenic carbonate by terrigenous silt and clay within the Kumano Basin.

TOC content ranges from 0.20 to 0.92 wt%. The depth profile of TOC shows scattered values with an average of 0.52 wt%. Total nitrogen content is similar in range to that found at Site C0001. All values for total nitrogen are <0.15 wt% and concentrations are constant throughout Hole C0002B. The depth profile of C/N ratios shows little variation with depth. C/N ratios range from 2.4 to 8.9 with an average of 5.5, suggesting that the organic matter was mainly derived from marine organisms.

Total sulfur content ranges from traceable amounts to 1.8 wt% with an average of 0.42 wt%. In Unit II, concentrations are fairly uniform. Sulfur concentrations increase with depth in Unit III and remain high with scattered values throughout Unit IV.

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