2012 年 77 巻 4 号 p. 268-273
We investigated molecular and stable isotope compositions of dissolved gas in pore water in subsurface sediment cores that were retrieved from the eastern margin of Japan Sea during the MD179 cruise onboard R/V Marion Dufresne in June 2010. Hydrate-bearing sediment cores retrieved from Umitaka Spur in the Joetsu Basin showed high 13C concentrations of methane, indicating its thermogenic origin, whereas those at Joetsu Knoll partly contained microbial methane because 13C and deuterium are both depleted. Other sediment cores without gas hydrates showed mainly microbial methane formed via CO2 reduction. The profiles of methane and CO2 in the sediments showed a minimum concentration of 13C at SMI (sulfate-methane interface) depth. The concentration of methane in the sediments increased dramatically beneath the shallow SMI depth in the Joetsu Basin, indicating active microbial methane generation.