Previous   |    Next

doi:10.2204/iodp.proc.314315316.136.2009

Organic geochemistry

Hydrocarbon gas composition

Headspace gas composition was rapidly analyzed for safety monitoring (Table T14) and by using a more time consuming extracting procedure for scientific interpretation (Table T15; Fig. F31). The SMT is located at ~4–8 m CSF in Holes C0008A and C0008C. Methane concentration increases sharply at the SMT to a peak of 9.1 mM at 19.90 m CSF in Hole C0008A and 17.5 mM at 12.17 m CSF in Hole C0008C. Below this depth, methane concentration decreases to ~2 mM at 60 m CSF and remains at this value to the base of the sediment column (except for a localized enrichment that reaches 15 mM at ~140 m CSF in Hole C0008A). This indicates the occurrence of gas hydrates, which also correlates with anomalously low chloride concentration in this interval (see “Inorganic geochemistry”). The ethane concentration profile from Hole C0008A follows the methane profile to ~70 m CSF. Below that depth, ethane concentration increases. This increase is also reflected in the methane/​ethane (C₁/C₂) ratio. The C₁/C₂ ratio increases in the uppermost 15 m CSF and then declines to 372 at ~160 m CSF. Below this depth, the ratio slightly increases again. The ethane concentration profile in Hole C0008C shows the same trend with slightly higher concentration at ~168 m CSF. Because no propane or butane was detected, the lower ratio values are probably not due to thermogenic ethane formation or migration.

Sediment carbon, nitrogen, and sulfur composition

Calcium carbon (CaCO₃) content is relatively high in the upper 50 m of Unit I with concentration up to 23.4 wt%. In the lower portion of the unit, concentration decreases (Table T16; Fig. F32). The lowest concentration is at the Unit I/II boundary. Total organic carbon (TOC) and total nitrogen (TN) concentrations remain low throughout the cores and show a strong positive correlation. Similar to CaCO₃ content, the TOC and TN concentrations are lowest at the bottom portion of Unit I. The TOC to TN (C/N) ratio remains generally low throughout the site, indicating marine origin of the organic matter, although some values exceed 10. The total sulfur (TS) content is rather high in the upper 50 m and in the lower ~60 m of the sediment column, correlating with the presence of iron sulfides (see “Lithology”). At ~180 m CSF, TS concentration increases and is highest throughout the lowest portion of Unit I (Subunit IB), related to pyrite occurrence at this interval (see “Lithology”).

Top of page   |    Previous   |    Next