Keywords:gas chimney, Sulfate Methane Interface
The Hidaka Trough, offshore Hokkaido, has recently been characterized by subsurface gas chimney structures associated with mound-pockmark topographies and methane plume that indicate intensive gas migration from deep sediments to the surface area. In the region where remarkable gas chimneys are developed such as in offshore Joetsu, shallow marine sediments are characterized by the precipitation of authigenic carbonate, massive gas hydrates and development of chemotrophs because of high methane concentration near the seafloor. In these environments, anaerobic oxidation of methane consume both methane and sulfate, resulting sulfate to be depleted in shallow sediment. Inorganic carbon generated by methane oxidation not only incorporates with calcium ion to form carbonate but also is utilized by microbial-mediated methanogenic reaction. Carbon is therefore key substance near the depth of sulfate methane interface (SMI) in high methane flux area. This study aims to figure out the chemical reaction associated with carbon, characterizing geochemical environment in shallow sediments, by analyzing concentrations of dissolved ions and hydrocarbons, and stable isotopic composition (δ13C) of dissolved carbons collected from shallow sediments with gas chimney structures in the Hidaka Trough, offshore Hokkaido.
In the area of calyptogena community, the SMI locates at shallow depth and indicates reductive environment is established because of sulfate depletion. The sulfate is depleted at shallower depth in the area with carbonates near the seafloor, where the concentration of calcium ion is also lower than other areas. These results indicate carbonate formed at higher rate in the area with higher methane flux. Based on the composition of dissolved hydrocarbons and δ13C of methane, although the methane is predominantly of biogenic origin over the research areas in the Hidaka Trough, the δ13C of methane increase rapidly with depth on the mound center. It indicates more thermogenic methane migrate from deeper sediments at the site. Biogeochemical behavior of carbon varies not only among sampling sites (gas chimneys) but also locations on the single gas chimney structure.