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doi:10.2204/iodp.proc.314315316.134.2009

Organic geochemistry

At Site C0006, four holes were drilled. In Holes C0006C and C0006D, the mudline was not confirmed. Because of the unknown depths (see “Inorganic geochemistry”), data from these holes are included in tables (Tables T15, T16, T17) but are not shown in figures.

Hydrocarbon gas composition

At Site C0006, the SMT zone is at ~8–12 m CSF, which is a few meters shallower than at Site C0004 (Table T16; Fig. F44). The methane depth profile below 14 m CSF decreases throughout the sediment column (except for two peaks at ~100 and 310 m CSF). These increased methane values are obtained only a few meters below the occurrence of measured chloride anomalies that suggest the occurrence of gas hydrates (see “Inorganic geochemistry”). Thus, the enrichment in methane content at these two intervals could be related to the destabilization of gas hydrates and the subsequent release of methane into the sediment. Ethane was detected in low amounts in the sediment below 76 m CSF. Higher molecular weight hydrocarbons were not detected at Site C0006. The methane/ethane (C₁/C₂) ratio remains high throughout the sediment column, indicating biogenic origin of the hydrocarbon gases.

Sediment carbon, nitrogen, and sulfur composition

The amount of calcium carbonate (CaCO₃) in sediments at Site C0006 is lower than that at Site C0004 (Table T17; Fig. F45). The highest values were obtained in Unit I, with a maximum concentration of 6.2 wt%, followed by a decrease in Subunit IIA. This is consistent with calcite data obtained from XRD measurements (see “Lithology”). A slight increase in CaCO₃ occurs at the Unit II/III boundary. The highest carbonate value was measured at the bottom of Hole C0006F.

Except for a few enrichments, total organic carbon (TOC) remains low throughout the core (average = 0.46 wt%). Total nitrogen (TN) increases throughout Unit III. Because ammonium concentration decreases (see “Inorganic geochemistry”) and total clay content is higher in Unit III (see “Lithology”), the TN enrichment is probably related to clay-bound nitrogen substances. The ratio of TOC to TN (C/N) averages ~7.0, indicating that organic matter at Site C0006 is mainly of marine origin. A few higher values indicate that some terrigenous organic compounds are present in lower concentrations in Units I and II.

Total sulfur (TS) concentration in sediments is generally low (average = 0.25 wt%). In Unit I, TS is higher, showing a maximum of 1.7 wt% at ~17 m CSF. The enrichment is below the SMT and can most likely be associated with the precipitation of fine-grained iron sulfides. This will be evaluated by shore-based analyses of samples. In Unit II, TS concentration remains low (average = 0.2 wt%) and slightly increases again in Unit III to 0.77 wt%. The higher TS concentration below ~500 m CSF can be related to the occurrence of small pyrite nodules (see Fig. F6).

Additionally, three samples from 370.18, 370.67, and 371.05 m CSF were analyzed (data are not included in figures). The samples were selected from wood-bearing sediments in turbidite sequences (see Fig. F6). TS concentrations in these samples are 1.8, 2.66, and 2.32 wt%, respectively. TOC and TN concentrations are up to 10 times higher than in the surrounding sediments, with TOC contents of 3.8, 6.87, and 3.91 wt%, respectively, and TN contents of 0.1, 0.15, and 0.11 wt%, respectively. The mean C/N ratio of the three samples is ~40.

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