First measurements of methane and its carbon isotope ratio in the Japan Sea (East Sea)

We measured dissolved methane (CH4) and its carbon isotope ratio (13C/12C or δ13C) from surface to bottom seawaters (maximum depth: 3574 m) for the first time in three major offshore areas of the Japan Sea (East Sea): the eastern Japan Basin, Yamato Basin, and Ulleung Basin. Seawater samples were taken during the KT-07-24 cruise of the R/V Tansei Maru in 2007 as part of a Japan–Korea collaborative study. The CH4 concentration and its δ13CPDB were obtained by continuous flow isotope ratio mass spectrometry. While the CH4 concentrations of surface seawater were 2.6–3.8 nmol kg− 1, which was 43(± 22)% supersaturated compared to the atmospheric equilibrium values, those in the subsurface (D = 30–150 m) showed much higher values up to 9.7 nmol kg− 1 (~ 300% supersaturation). This subsurface CH4 had the minimum δ13C value of − 54.3‰ (less than the atmospheric CH4 value of − 47‰), implying a contribution from in situ microbial CH4 production in some anoxic microenvironments. The CH4 concentration decreased with depth toward minimum values of ~ 1 nmol kg− 1 (δ13C: − 30 to − 40‰) at 1500–2000 m depth. Below a depth of 2000 m, we found a clear difference in CH4 profiles among the three basins; in the eastern Japan Basin the CH4 concentration remained almost constant down to the seafloor, while it gradually increased with depth up to > 2.0 nmol kg− 1 in the Yamato and Ulleung basins. This CH4 increase toward the bottom was accompanied by a decrease of δ13C–CH4 to less than − 50‰, suggesting a microbial CH4 supply from bottom sediment in the Yamato and Ulleung basins. We observed a local CH4 plume over a deep channel east of the Yamato Basin, indicating unknown submarine CH4 seepage with a microbial δ13C value of about − 60‰.