[M-IS18] Gas hydrates in environmental-resource sciences

[MIS18-P07] Distribution and geochemical characteristics of methane dissolved in the seawater over gas chimney structure, off Tottori, Japan Sea

*TIANYOU ZE¹, Hitoshi Tomaru¹, Naoto Ishida² (1.Chiba University, 2.Kochi University)

Keywords:Methane, Gas hydrate, Sea of Japan

Recent marine expeditions have reported the extensive distribution of gas chimney structures below the seafloor in the eastern margin of the Japan Sea, which act as a local aquifer for methane-rich fluids to the seawater. It has been pointed out that the surface of the ejected methane bubble is hydrated and transported efficiently to a shallow depth. However, methane migration through gas chimneys has not been considered in conventional sediment-seawater methane transport models. Thus the dynamics of methane migration in such an environment is poorly understood. In this research, we determine the concentrations and isotopic compositions of methane dissolved in seawater vertically where gas chimney was found and characterize the behavior of methane gas ascending through the water column. The methane concentrations are almost stable at 5 nM between the seafloor and 400 m below the sea-surface (mbss), and those increase to 7 nM at about 300 mbss corresponding to the local upper limit of the methane hydrate stability. This observation suggests that the “hydrated methane bubbles” ascend and liberate methane to the shallow water. In addition, the δ¹³C of methane showed relatively heavy values over gas chimney compared to the values in typical biogenic methane in marine systems. This reflects that the thermogenic methane is preferentially derived through the gas chimney to the overlying water, these thermogenic components may modify and characterize the geochemistry of seawater around the gas chimney.