Relationship between atmospheric CH4 concentration above sea surface and gas plumes from sea?oor along the eastern margin of Japan sea and around Hokkaido

Abstract

： The gas seeps from the seafloor were presumed as a potential source of methane （ CH 4 ） to the atmosphere. Numerous gas plumes have also been reported around Japanese archipelago. During spring and summer in each 2014 and 2015, we have conducted the unique high-resolution measurements of CH 4 in the atmosphere above the seas （ eastern margin of Japan sea and sea around Hokkaido ） where the gas hydrate expected to exist. To date, deepwater gas plumes are not assumed to contribute to the atmospheric CH 4 . Although atmospheric CH 4 concentrations along the ship ʼ s track were almost 1.9 ppm, some observation sites with gas plumes were over 2.1 ppm. In addition to the gas plumes observed at depths shallower than 400 m in these areas, the TGHS （ Top of Gas Hydrate Stability ） was estimated to be shallow in contrast to earlier reports. These observations suggested that some gas plumes in the specific environment, where CH 4 was easily transported to the ocean surface layer, contribute to increasing the atmospheric CH 4 concentration.