2020 Volume 85 Issue 6 Pages 309-314
The gas seeps from the seafloor were presumed as a potential source of methane （CH4） 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 CH4 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 CH4. Although atmospheric CH4 concentrations along the shipʼs 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 CH4 was easily transported to the ocean surface layer, contribute to increasing the atmospheric CH4 concentration.
