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

Abstract

We measured dissolved methane (CH 4) and its carbon isotope ratio ( 13C/ 12C or δ 13C) 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 CH 4 concentration and its δ 13C PDB were obtained by continuous flow isotope ratio mass spectrometry. While the CH 4 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 CH 4 had the minimum δ 13C value of − 54.3‰ (less than the atmospheric CH 4 value of − 47‰), implying a contribution from in situ microbial CH 4 production in some anoxic microenvironments. The CH 4 concentration decreased with depth toward minimum values of ~ 1 nmol kg − 1 ( δ 13C: − 30 to − 40‰) at 1500–2000 m depth. Below a depth of 2000 m, we found a clear difference in CH 4 profiles among the three basins; in the eastern Japan Basin the CH 4 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 CH 4 increase toward the bottom was accompanied by a decrease of δ 13C–CH 4 to less than − 50‰, suggesting a microbial CH 4 supply from bottom sediment in the Yamato and Ulleung basins. We observed a local CH 4 plume over a deep channel east of the Yamato Basin, indicating unknown submarine CH 4 seepage with a microbial δ 13C value of about − 60‰.