3:30 PM - 3:45 PM
[MIS26-06] Geochemical processes in surface sediments in a shallow gas hydrate reserve area off Sakata, eastern margin of the Japan Sea
Keywords:Methane, Japan Sea, Anaerobic oxidation of methane, trace elements
Seepage of subsurface methane into shallow sediments and water column has a substantial impact on geochemical and biogeochemical processes via various aerobic and anaerobic reactions. In a knoll in a shallow gas hydrate reserve area off Sakata, eastern margin of the Japan Sea, the potential areas bearing massive shallow methane hydrates have been identified by seismic exploration. During the survey cruise SS20-1 in July 2020, the ROV observation found patchy distribution of strongly anoxic areas covered with bacterial mats-like structures in the knoll caldera. Approximately 30-cm-long push core samples were collected within the anoxic area as well as just outside the area (a few meters away), which was covered with apparently oxic sediments. As a reference, a push core sample was collected outside the gas hydrate-bearing areas. The core samples from the anoxic site consisted of dark blackish sediments in the top 6 cm with strong hydrogen sulfide odor and below the depth sediments was consisted of dark gray silt. Top 10 cm sediments of the apparently oxic site had a grey color and below the depth sediments was dark in color likely under anoxic condition. The reference sample exhibited a gray color throughout the collected depth. The core samples were sliced into sections and subjected to various analyses of interstitial water sulfate, sedimentary total organic carbon, total sulfur (TS), trace elements and δ13C of total inorganic carbon.
In sediments under anoxic condition including the entire core samples at the anoxic site and the sediments below 10 cm depth at the oxic site, TS contents were substantially high compared with those at the reference site, and the values of δ13C of total inorganic carbon showed strongly negative values between –20‰ and –40‰ equivalent to those of subsurface methane. These results infer that via microbial anaerobic oxidation of methane (AOM) formation of sulfide minerals and authigenic carbonates occurs in the anoxic sediments. This is supported by the results of pore water sulfate distribution. While pore water sulfate concentrations were stable from the top to the bottom of the core sample collected from the reference site, a rapid and linear decrease was observed from the value equivalent to the seawater concentration to zero within the top 10 cm at the anoxic site. It is most likely that active AOM rapidly utilizes sulfate supplied from the overlying seawater through diffusion.
From the results of some geochemical analyses, the active AOM and sulfide mineral formation seems to have a considerable impact on the behaviors of some trace elements in the gas hydrate-bearing area. For example, strong enrichment of Mo and As was found in the anoxic sediments. Positive correlations of Mo and As contents with TS contents suggest that the AOM-derived sulfide favored the capture of Mo and As into sulfide minerals such as pyrite.
This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).
In sediments under anoxic condition including the entire core samples at the anoxic site and the sediments below 10 cm depth at the oxic site, TS contents were substantially high compared with those at the reference site, and the values of δ13C of total inorganic carbon showed strongly negative values between –20‰ and –40‰ equivalent to those of subsurface methane. These results infer that via microbial anaerobic oxidation of methane (AOM) formation of sulfide minerals and authigenic carbonates occurs in the anoxic sediments. This is supported by the results of pore water sulfate distribution. While pore water sulfate concentrations were stable from the top to the bottom of the core sample collected from the reference site, a rapid and linear decrease was observed from the value equivalent to the seawater concentration to zero within the top 10 cm at the anoxic site. It is most likely that active AOM rapidly utilizes sulfate supplied from the overlying seawater through diffusion.
From the results of some geochemical analyses, the active AOM and sulfide mineral formation seems to have a considerable impact on the behaviors of some trace elements in the gas hydrate-bearing area. For example, strong enrichment of Mo and As was found in the anoxic sediments. Positive correlations of Mo and As contents with TS contents suggest that the AOM-derived sulfide favored the capture of Mo and As into sulfide minerals such as pyrite.
This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).