Distribution of atmospheric CH4 concentration in eastern margin of Japan Sea from 7K14 cruises

Abstract:

Hydrate of natural gas is widely distributed in marine sediments in the eastern margin of Japan Sea. Active gas vents from the seafloor have previously been reported worldwide at gas hydrate fields. Atmospheric methane (CH₄), a major component of seep gases, is an important short-lived climate pollutant. Gas seeps may contribute to increase atmospheric gas concentration above the sea surface. Our objectives were to continuously measure and investigate the distribution of the atmospheric CH₄ concentration over the sea surface at the gas hydrate fields along the eastern margin of Japan Sea. We used the R/V Kaiyo-Maru No.7 (Kaiyo Engineering Co., Ltd., Japan) for the survey in Japan Sea from mid April to early June 2014. Continuous measurement of atmospheric CH₄ was performed on the ship using a wave-length-scanned cavity ring-down spectrometer (WS-CRDS) (model G2201-i, Picarro Inc., USA). Air sample was collected from an air intake at the top deck (approximately 8 m above sea level) of the ship using an air pump placed in the observation room. To our experience, the ship sailed at approximately 6 knot. Location data were obtained with the nautical GPS. Observed atmospheric CH₄ concentration over the sea surface was approximately between 1.8-1.9 ppm in most sea areas, but exceeds 2.0 ppm in some areas. This study was conducted as a part of the METI’s shallow gas hydrate exploration project.