Geochemistry of deep sea sediments at cold seep sites in the Nankai Trough: Insights into the effect of anaerobic oxidation of methane

Two types of sediment were recovered from “methane seep” sites in the Nankai Trough: “Cold seep” sediment affected by anaerobic oxidation of methane (AOM), and unaffected “Typical oxic marine sediment,” Concentrations of carbon, sulfur, iron, and trace metals were analyzed in these sediments. The results showed that the sulfur contents of “cold seep” sediments are higher than those of typical oxic marine sediments, although the organic carbon contents of the two types of sediment are identical. The high sulfur contents of seep sediments suggest that AOM was the dominant process at the “cold seep” sites; H2S produced as a result of AOM was fixed as reduced sulfur such as iron sulfide.The highest manganese concentrations (approx. 1000 ppm) were found in surface sediments at non-seep sites, whereas a strong enrichment of Mn was not observed in the “cold seep” sediments. Manganese might be released from sediments as soluble Mn2 + ion in the reduced environment produced by methane-bearing seepage. The molybdenum content in “cold seep” sediments (up to 28.3 ppm) is about 30 times higher than that in the typical oxic marine sediments. However, a correlation between Mo and organic carbon contents, which is characteristic of anoxic sediment, was not observed in our samples. “Cold seep” sediments are a potential sink in the global geochemical cycle of Mo.