[MIS32-P05] Geochemistry of pore water and headspace gas in the Tatar Trough
キーワード：間隙水、ヘッドスペースガス、嫌気的メタン酸化、メタン硫酸境界
During the LV81 and LV85 expeditions, we have collected a number of sediment cores from various sites in the Tatar Trough where the gas chimney, indicative of methane-rich columnar sediment structure, pockmark, gas flare, carbonate nodules, and high methane flux had been reported. In methane-rich marine environment, geochemistry of pore water have been useful to estimate the methane flux using the depth of SMI (sulfate gradient) as well as the distribution of gas hydrate (low chloride anomaly). The headspace gas analysis also reflects the information of the origin of hydrocarbons as well as the methane-oxidation in the shallow sediments. In this research, we analyzed chemical composition of pore water and chemical/isotopic composition of headspace gas to characterize the biogeochemical processes associated with generation, migration, oxidation, and fixation of methane in the Tatar Trough.
The overall depths of SMI is significantly shallow in the northern area (SMI at 1-3 mbsf) compared those in the middle and southern areas (SMI mostly at >3-5 mbsf). In particular, the SMIs of gas hydrate-bearing core in the northern area at which low chloride anomalies due to gas hydrate dissolution were observed locate at <1.5 mbsf, corresponding to the highest values of all sites, high methane flux is essentially required for the accumulation of gas hydrate in shallow sediments. Within the northern area, the SMI depths are relatively shallow at the eastern slope sites, particularly at shallow sites, compared to those at trough floor sites. The SMI depth on the western slope is apparently deep, methane migration system is different between eastern and western slopes in the northern trough. The SMI depths in the middle research area are generally shallower than those in the southern area; higher methane fluxes are found from the gas chimney sites (SMI at <2 mbsf).
All the methane concentrations and C1/C2 ratios in headspace gases increase with depth due to the active upward methane flux although the methane is oxidized at the SMI. These values in the northern area are higher than those in the middle and southern areas. This probably indicates that the biogenic methane inputs are greater in the northern area, distribution and flux of gases reflect well the subsurface gas system in the Tatar Trough.
The overall depths of SMI is significantly shallow in the northern area (SMI at 1-3 mbsf) compared those in the middle and southern areas (SMI mostly at >3-5 mbsf). In particular, the SMIs of gas hydrate-bearing core in the northern area at which low chloride anomalies due to gas hydrate dissolution were observed locate at <1.5 mbsf, corresponding to the highest values of all sites, high methane flux is essentially required for the accumulation of gas hydrate in shallow sediments. Within the northern area, the SMI depths are relatively shallow at the eastern slope sites, particularly at shallow sites, compared to those at trough floor sites. The SMI depth on the western slope is apparently deep, methane migration system is different between eastern and western slopes in the northern trough. The SMI depths in the middle research area are generally shallower than those in the southern area; higher methane fluxes are found from the gas chimney sites (SMI at <2 mbsf).
All the methane concentrations and C1/C2 ratios in headspace gases increase with depth due to the active upward methane flux although the methane is oxidized at the SMI. These values in the northern area are higher than those in the middle and southern areas. This probably indicates that the biogenic methane inputs are greater in the northern area, distribution and flux of gases reflect well the subsurface gas system in the Tatar Trough.