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doi:10.2204/iodp.proc.314315316.133.2009

Organic geochemistry

Hydrocarbon gas composition

Methane was detected in all samples at Site C0004 (Table T17; Fig. F45). The estimated SMT zone is at 16–20 m CSF. Below this depth, methane concentration increases sharply and reaches a maximum of 9.7 mM at 35.3 m CSF. In deeper sediments, concentration is highly variable. The changes could potentially reflect changes in lithology and corresponding effects on methane degassing upon core retrieval (including core quality) rather than in situ changes in interstitial water methane concentration (Paull, Matsumoto, Wallace, and Dillon, 2000). Changes in lithology and/or physical properties, such as the occurrence of unconsolidated sand and gravels with greater pore space, can have a strong influence on the degassing of methane. Additionally, the quality of the recovered core or contamination (e.g., by seawater; see “Inorganic geochemistry”) can lead to a higher loss of methane. If there are “real” changes in in situ methane concentration, they should also be visible in related pore water constituents (i.e., sulfate, alkalinity, and chloride), which is not seen (see “Inorganic geochemistry”). However, increased methane concentration below ~260 m CSF (in Unit III) coincides with a fractured zone (fault-bounded unit, see “Structural geology”) coincident with variations in major, minor, and trace element concentrations (see “Inorganic geochemistry”). Thus, it cannot be completely excluded that the higher methane concentration in this zone may be related to flow of methane-enriched fluids. This requires further shore-based analyses.

Low ethane concentration was detected in a majority of the samples with mean concentration of ~2 µM below the SMT (Table T17; Fig. F45). The methane/ethane (C₁/C₂) ratio is constantly high throughout the cores, indicating biogenic origin of the hydrocarbon gases. Only at the SMT is a decrease in the C₁/C₂ ratio found. The additional applied headspace extraction procedure with NaOH solution resulted in higher gas yields compared to the 30 min extraction for safety monitoring purposes (cf. Tables T18, T17). This is consistent with observations from ODP Leg 201 (D’Hondt, Jørgensen, Miller, et al., 2003).

Sediment carbon, nitrogen, and sulfur composition

Calcium carbonate (CaCO₃) content calculated from inorganic carbon concentration ranges from 16.3 to 0.2 wt% (Table T19; Fig. F46). The amount of CaCO₃ is rather high in Unit I and decreases to a mean value of ~3.5 wt% in the lower units. This is in good agreement with calcite data from XRD measurements (see “Lithology”).

Total organic carbon (TOC) content remains low throughout the cores (average = 0.43 wt%). Like TOC concentration, total nitrogen (TN) content is rather uniform in the sediments and averages ~0.07 wt%. The ratio of TOC and TN (C/N) has a mean value of ~6, which indicates that the organic matter in the sediments from Site C0004 is mainly of marine origin (with an expected value for marine organic matter of 6–9). Below 350 m CSF, C/N values increase to a maximum value of 10.0.

Total sulfur (TS) content was generally low, ranging from 0.04 to 0.57 wt% through Units I and II (average = 0.23 wt%). In Units III and IV, the concentration is even lower (average = 0.19 wt%) except for a slight enrichment at ~390 m CSF.

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