The behavior and release of methane related to hydrates in a pockmark area in the eastern margin of the Japan Sea: An approach from chlorine isotope composition in pore water and sea water

Methane released from the seafloor is a strong contributor to the greenhouse gas budget. Some deposits of methane hydrates existing in ocean sediment are linked to plate collision/subduction boundaries and associated tectonic motion. Methane plumes were observed in the pockmark area off Sado, at the end of the eastern margin of the Japan Sea where the North American and Eurasian tectonic plates intersect. Our goal in this study is to investigate the origin of methane and its actual release mechanisms from the seafloor and its behavior and seasonal variation in the water column by using chemical oceanic observations and geochemical analysis of pore water and sea waters. Geochemical data sets are from five cruises over two years and three seasons. The KT05-11 and KT06-26 expeditions were on the R/V Tansei-Maru, NA220 on the T/S Nagasaki-Maru, and the NT05-10 and NT06-19 expeditions using the unmanned submersible HYPER-DOLPHIN and its mother-ship R/V Natsushima. Results of chlorine and oxygen isotope compositions and other water chemical characteristics indicate that methane hydrate is generated over the bottom and is then melted in the shallow water. The possible processes are: 1) In deep water, chlorine isotope composition shows inverse correlation with oxygen, which suggests the fine particles of methane hydrate are adhering to the surface of gas bubbles released from deep sediment together with cold seep; the methane hydrate particles possibly grow and expand above the bottom and rise in water column. 2) In shallower water mass (< 300m depth), the amount of fresh water accumulated hints that fresh water is derived from the melting of methane hydrate and contributes up to 3% of the amount calculated by the decrease in upper-water salinity; this implies that a corresponding amount of methane was transported to ocean surface. The seasonal variations of dissolved methane and other chemical features in shallow water are possibly affected by the methane-oxidation and/or are trapped by chemical synthesis communities requiring more detailed investigation.

toyama.ac.jp/~jzhang/