Anaerobic oxidation of methane and trace-element geochemistry in microbial mat-covered sediments related to methane seepage, northeastern Japan Sea

Abstract

We evaluated the spatial variability of geochemical features, including trace-element cycles, associated with methane seepage in a knoll off Sakata (called Sakata Knoll) in the eastern Japan Sea. Seafloor observations from a remotely operated vehicle and environmental visualization around Sakata Knoll by an optical mapping device revealed large patches of white and gray microbial mats covering sediment in the depression on the top of the knoll. Push core samples were collected inside and a few meters outside microbial mats, and outside the gas hydrate-bearing areas. For these cores, the interstitial sulfate ion (SO42−) concentrations, sedimentary total organic carbon, total sulfur (TS), and trace-element contents, and bulk carbonate minerals δ13C were analyzed. In the entire core samples below the microbial mats and in sediments below 10 cm depth outside microbial mats, TS contents were substantially higher than those at the reference site, and bulk carbonate minerals δ13C was strongly negative (−20‰ to −40‰). These results imply that the formations of sulfide minerals and authigenic carbonates occur in the anoxic sediments via microbial anaerobic oxidation of methane (AOM) coupled with sulfate reduction. Although a rapid and linear decrease of interstitial water SO42− concentration in the sediments collected inside microbial mats supports the occurrence of AOM, there is no fall in SO42− concentration outside microbial mats, which may be explained by the higher upward flow of methane gas inside than outside the mats. The AOM-impacted sediments at Sakata Knoll were characterized by enrichments in Mo and As that were well correlated with TS contents and the authigenic iron fraction, and by strong enrichment of authigenic Mo relative to U, indicated by higher Mo/U ratios than those of seawater. These observations suggest that Mo and As are effectively transferred from the water column to AOM-impacted sulfidic sediments at Sakata Knoll via precipitation of particulate iron–manganese oxyhydroxides in the water column that have adsorbed Mo and As.