Characteristics of geochemistry and benthic communities in microbial mat-covered sediments related to methane seepage, northeastern Japan Sea

We investigated characteristics of benthic macrofaunal communities and geochemical parameters in and around microbial mat-covered sediments associated with a methane seepage on Sakata Knoll in the northeastern Japan Sea. A depression on top of the knoll corresponds to a gas-hydrate-bearing area with seepage of methane-rich fluid, and microbial mats patchily cover the seafloor sediments. Sediment cores were collected at three sites for this study: one within a microbial mat, a second a few meters outside of the microbial mat, and a third from a reference site outside the gas-hydrate-bearing areas.The profile of porewater sulfate ion concentrations below the microbial mat showed linear decreases from near seawater values close to the sediment-water interface to zero concentration at approximately 7 cmbsf. Compared to the reference site, total sulfur (TS) contents were significantly higher and 13C of total inorganic carbon were strongly negative values in the entire core below the microbial mats and in sediments below 10 cm depth outside mat, respectively. These results indicate the presence of the authigenic carbonates and sulfide minerals produced by the anaerobic oxidation of methane (AOM) in these sediments. There is no fall in sulfate concentration in the whole core collected outside microbial mat, which may be explained by the higher upward flow of methane gas inside than outside the mat. Both the 18S rRNA genes and morphological analyses showed that the surface sediment inside the microbial mat noticeably favored annelids, with dorvilleid Ophryotrocha sp. and ampharetid Neosabellides sp. identified as major constituents. The sulfidic sediment conditions with concentrations of H2S up to 121 M resulting from AOM likely resulted in the predominance of annelids with tolerance to sulfide. In addition, the higher contents of Mo in whole cores collected inside the microbial mat than that at the reference site indicate strong enrichments of molybdenum in the sediment inside the mat. The positive correlation of Mo with TS contents suggests that the AOM-derived H2S favored the capture of Mo on sulfide minerals such as pyrite. Because of the absence of enrichments of redox-sensitive trace elements other than Mo in the Sakata Knoll sediments, molybdenum may be efficiently transported into the AOM-induced sulfidic seafloor by absorption on the particulate Fe-Mn hydroxides that are well preserved in oxic water column of the Japan Sea Proper Water. This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).