4:00 PM - 4:15 PM
[ACG50-07] Distribution and origin of methane-rich plumes in the shallow water columns, off Boso area
CH4, the major component of natural gases, is an important energy resource. In addition, CH4 is one of the representative greenhouse gases that have significant impacts on global warming. Although the aquatic environment is one of the major sources of atmospheric CH4, we have little knowledge on the origin and behavior of CH4 enriched in coastal waters. While CH4-enriched water (methane spot) has been found in the surface water of off Boso area having the depths of around 30-50 m, studies on the distribution and origin were limited. Furthermore, while a water-soluble natural gas field known as Minami-Kanto gas field is widely distributed just below the ground surface at terrestrial part of Boso peninsula, the relationship between the terrestrial gas field and the methane spots in the coastal ocean area had been unclear.
In this study, seafloor gas seepages were surveyed using a weighing fish finder and a multibeam sonar in and around off Boso methane spot area during Shinsei Maru KS-24-12 cruise (chief scientist: Dr. Yoko Iwamoto). Both concentrations and stable isotope ratios (δ13C- CH4 and δ2H- CH4 values) of dissolved CH4 in the seawater samples taken at 13 stations during the cruise were determined as well to clarify the origin of CH4. Furthermore, both terrestrial natural gas samples of Minami-Kanto gas field and river water samples of Tone River, a major outflow at off Boso area, were collected to determine the stable isotope ratios of CH4.
The stable isotope ratios of CH4 in the terrestrial natural gas samples were ranged from -67 to -65‰ for δ13C and -203 to -178‰ for δ2H, which correspond well with those in the Minami Kanto gas field reported by Kaneko et al. (2001). During the offshore exploration using the weighing fish finder and the multibeam sonar, up to 50 gas seep sites were found on the seafloor at off Boso area. In addition, the seawater samples taken during the cruise showed significant CH4 enrichment ranging from 7.3 to 50.6 nmol/kg, while that of air equilibrated seawater is less than 3 nmol/kg. Furthermore, significant correlations were found between the concentration and the stable isotope ratios of CH4 in the seawater samples. By using the correlations, we estimated the δ13C and δ2H values of the CH4-enriched end-member component to be -65±1‰ and -133±20‰, respectively. The values coincided well with those of CH4 produced through the hydrogenotrophic methanogenesis (Whiticar, 1999), in which CH4 is microbially produced using CO2 as substrate. While they showed slight enrichment from the stable isotope ratios of the terrestrial CH4, such enrichment can be explained by the progress of oxidation. On the other hand, it was difficult to explain CH4 in Tone River water to be the primary source of CH4 in the methane spot because, while δ13C values showed slight 13C-enrichment, δ2H values showed 2H-depletion from those in Tone River water showing δ13C values of -61‰ and δ2H values of -177‰. We concluded that CH4 in the off Boso methane spot had been supplied from the Minami-Kanto gas field extended to off shore area while methane is partially removed through oxidation processes just before it seeps from seafloor, so that both δ13C and δ2H values showed slight increases from the original values.
In this study, seafloor gas seepages were surveyed using a weighing fish finder and a multibeam sonar in and around off Boso methane spot area during Shinsei Maru KS-24-12 cruise (chief scientist: Dr. Yoko Iwamoto). Both concentrations and stable isotope ratios (δ13C- CH4 and δ2H- CH4 values) of dissolved CH4 in the seawater samples taken at 13 stations during the cruise were determined as well to clarify the origin of CH4. Furthermore, both terrestrial natural gas samples of Minami-Kanto gas field and river water samples of Tone River, a major outflow at off Boso area, were collected to determine the stable isotope ratios of CH4.
The stable isotope ratios of CH4 in the terrestrial natural gas samples were ranged from -67 to -65‰ for δ13C and -203 to -178‰ for δ2H, which correspond well with those in the Minami Kanto gas field reported by Kaneko et al. (2001). During the offshore exploration using the weighing fish finder and the multibeam sonar, up to 50 gas seep sites were found on the seafloor at off Boso area. In addition, the seawater samples taken during the cruise showed significant CH4 enrichment ranging from 7.3 to 50.6 nmol/kg, while that of air equilibrated seawater is less than 3 nmol/kg. Furthermore, significant correlations were found between the concentration and the stable isotope ratios of CH4 in the seawater samples. By using the correlations, we estimated the δ13C and δ2H values of the CH4-enriched end-member component to be -65±1‰ and -133±20‰, respectively. The values coincided well with those of CH4 produced through the hydrogenotrophic methanogenesis (Whiticar, 1999), in which CH4 is microbially produced using CO2 as substrate. While they showed slight enrichment from the stable isotope ratios of the terrestrial CH4, such enrichment can be explained by the progress of oxidation. On the other hand, it was difficult to explain CH4 in Tone River water to be the primary source of CH4 in the methane spot because, while δ13C values showed slight 13C-enrichment, δ2H values showed 2H-depletion from those in Tone River water showing δ13C values of -61‰ and δ2H values of -177‰. We concluded that CH4 in the off Boso methane spot had been supplied from the Minami-Kanto gas field extended to off shore area while methane is partially removed through oxidation processes just before it seeps from seafloor, so that both δ13C and δ2H values showed slight increases from the original values.