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doi:10.2204/iodp.proc.314315316.135.2009

Organic geochemistry

Hydrocarbon gas composition

Headspace gas composition was rapidly analyzed for safety monitoring (Table T15) and by using a more time consuming extraction procedure for scientific interpretation (Table T16; Fig. F47). Methane was detected in all samples at Site C0007. Concentration peaks at ~5.3 mM at ~230 m CSF. Below this peak, methane decreases with depth. Throughout Hole C0007C methane concentration is very low (<60 µM) and sulfate concentration is high (see “Inorganic geochemistry”). At the bottom of Hole C0007C, methane concentration is still very low, whereas at the top of Hole C0007D, concentration increases sharply. Based on these data, the SMT zone is likely above 190 m CSF in Hole C0007D and below 150 m CSF in Hole C0007C, although it was not cored. However, the depth of the assumed SMT at Site C0007 would differ strongly from the depths observed at Sites C0004 and C0006 (16–20 and 8–12 m CSF, respectively).

Ethane was the only other light hydrocarbon detected and was only measured at ~94 m CSF in Holes C0007C and C0007D. The methane/​ethane (C₁/C₂) ratio is constantly high, indicating biogenic origin of the hydrocarbon gases.

Sediment carbon, nitrogen, and sulfur composition

Calcium carbonate (CaCO₃) content calculated from inorganic carbon concentration is generally very low and ranges from 6.99 to 0.21 wt% (Table T17; Fig. F48). In Hole C0007D, CaCO₃ concentration is higher below 300 m CSF.

Total organic carbon (TOC) content remains low throughout the sediment column (average = 0.39 wt%). The highest TOC content (~0.7 wt%) was measured at 82 m CSF. Similar to TOC concentration, total nitrogen (TN) content is rather low in the sediments (average = ~0.055 wt%). TN concentration slightly increases at the top of lithologic Unit II in Hole C0007C. In Hole C0007D, the TN concentration profile generally correlates with the profile of total clay minerals (see “Lithology”). This trend is also reflected in the ratio of TOC and TN (C/N), which has a mean value of ~7, indicating that the organic matter in the sediments from Site C0007 is mainly of marine origin.

The highest concentration of total sulfur (TS) was measured in Unit I, where it reaches a maximum of 1.14 wt%. This indicates the precipitation of authigenic iron sulfides related to the remineralization of organic matter by microbial sulfate reduction (see “Inorganic geochemistry”). At the top of Hole C0007D, an enrichment of TS indicates the presence of iron sulfides. These iron sulfides could be related to the alteration of iron oxides and oxyhydroxides by hydrogen sulfide, which was produced by AOM near the SMT. This relationship will be further investigated on shore. The highest amounts of TS are in Unit III. These enrichments of TS are related to the occurrence of pyrite nodules and pyrite filled burrows (see Fig. F12).

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