10:45 〜 12:15
[MIS18-P08] Geochemistry of pore waters in northern Japan Sea and northeastern Okhotsk Sea
キーワード：methane flux, pore water, Okhotsk Sea, Japan Sea, methane hydrate
In the Okhotsk Sea, Bottom Stimulating Reflectors (BSRs) are distributed on the seafloor, western offshore Palamusir Island. In the northern margin of the Japan Sea, active methane plumes are observed on the seafloor of the Tatar Trough southwestern offshore Sakhalin Island. In both areas, methane hydrates have been recovered from just below the seafloor, and solid-dissolved-gaseous methane are highly accumulated near the seafloor. The methane fluxes from the deep sediments to the seafloor are significant in such locations. Still, the areal variation of the methane fluxes and the relationship with the submarine structures have not been discussed well. In this study, the methane fluxes are determined from the chemical analysis of pore water, and the relationship between the distribution of methane fluxes and the seafloor environment is examined using the samples collected from these two marginal seas.
The concentration of sulfate ions in the pore water, indicative of methane flux, decreases with depth at all sampling sites both in the Japan Sea and Okhotsk Sea, reflecting that the methane derived from deep sediment is oxidized by sulfate from the seawater at the sulfate-methane interface (SMI) located near the seafloor. In the Okhotsk Sea, the depths of SMI are relatively shallow in the BSR area, indicating that the near-seafloor methane fluxes reflect the distribution of the subsurface hydrates. This is probably constrained by the migration of methane-rich fluids along the faults often observed in the BSR area. In addition, the igneous activity near this area possibly enhanced methane generation at depths or might dissociate deep-seated methane hydrates and increase methane flux. This indicates that the methane flux is controlled essentially by tectonic factors such as faults and submarine igneous activities. In the Japan Sea, the depths of SMI become shallower near the methane plume sites, where the methane flux is significantly high. Because the methane plume develops at the depths where the BSR reaches the seafloor, the methane gas existing just below the BSR migrates along the BSR and is released into the seawater. Therefore, in the southwestern offshore Sakhalin Island, the tectonic influence on methane activity is relatively small, and the depth distribution of the feather edge of the gas hydrate stability characterizes the methane flux. Methane is highly accumulated in both areas. However, the processes transporting methane from depths to shallow sediments reflect the subsurface geological structure and distribution of methane hydrates.
The concentration of sulfate ions in the pore water, indicative of methane flux, decreases with depth at all sampling sites both in the Japan Sea and Okhotsk Sea, reflecting that the methane derived from deep sediment is oxidized by sulfate from the seawater at the sulfate-methane interface (SMI) located near the seafloor. In the Okhotsk Sea, the depths of SMI are relatively shallow in the BSR area, indicating that the near-seafloor methane fluxes reflect the distribution of the subsurface hydrates. This is probably constrained by the migration of methane-rich fluids along the faults often observed in the BSR area. In addition, the igneous activity near this area possibly enhanced methane generation at depths or might dissociate deep-seated methane hydrates and increase methane flux. This indicates that the methane flux is controlled essentially by tectonic factors such as faults and submarine igneous activities. In the Japan Sea, the depths of SMI become shallower near the methane plume sites, where the methane flux is significantly high. Because the methane plume develops at the depths where the BSR reaches the seafloor, the methane gas existing just below the BSR migrates along the BSR and is released into the seawater. Therefore, in the southwestern offshore Sakhalin Island, the tectonic influence on methane activity is relatively small, and the depth distribution of the feather edge of the gas hydrate stability characterizes the methane flux. Methane is highly accumulated in both areas. However, the processes transporting methane from depths to shallow sediments reflect the subsurface geological structure and distribution of methane hydrates.