11:30 AM - 11:45 AM
[MIS22-10] Environmental impact assessment for the development of methane hydrates: chemical properties of shallow surface sediments on Joetsu Knoll, Japan Sea
Keywords:methane hydrates, environmental impact assessment, surface sediment, pore water, elution test, heavy metals
Shallow methane hydrates exist beneath the seafloor of the Japan Sea, and investigations are underway to develop the resources. Additionally, an assessment of the impact of the development on the environment is also a matter for investigation, and acquisition of baseline data is in progress. This study addresses environmental assessment from chemical perspective. In development on the submarine resource, sediments are swept up into seawater by drilling, and mixing with seawater may change the chemical properties of the seawater. In this study, we analyzed the chemical composition of sediments and pore water from core samples collected from the methane hydrate-bearing area to clarify the environmental characteristics of the methane hydrate-bearing area and conducted elution tests of sediments using surface seawater to evaluate the leaching characteristics of elements from the sediments.
Push core samples (~28 cm length) were taken in 2021 from methane hydrate-bearing area in Joetsu Knoll, Japan. Sampling was conducted at different sites: Inside (IBM) and outside (OBM) of microbial mats, and outside of methane hydrate-bearing area on Joetsu Knoll (REF-1 site) and outer the Knoll (REF-2 site) as reference sites. Total organic carbon, total sulfur, total nitrogen composition, methane concentration and trace elements (Mg, Al, Ca, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Rb, Sr, Mo, Cd, Ba, Pb, U, REEs, etc.) in sediments, and nutrients, sulfate ion, hydrogen sulfide, dissolved inorganic carbon composition and trace elements in pore water were measured to obtain baseline data. Assuming that the sediment is rolled up into the seawater, elution test was conducted in which sediments and seawater were stirred at 1:100 for 24 hour and trace elements in seawater were measured after the test was completed.
The results of the chemical analysis in pore water indicated the occurrence of anaerobic oxidation of methane (AOM) at IBM and OBM, with sulfate-methane interface shallower than 5 cm below the seafloor in the IBM and deeper than 15 cm below the seafloor in the OBM. At the reference sites (REF-1 and 2), nitrate reduction, manganese reduction, and iron reduction occurred along the depth, and sulfate reduction was also thought to occur at depths greater than 20 cm at REF-1, but no AOM was observed at either site. The concentration of trace elements in pore water at IBM and OBM was lower that at reference sites, except for As and Ba. At REF-1, iron concentrations in the pore water decreased at depths greater than 10 cm, while As, Mo, V, and U increased, but decreased in the layer deeper than 20 cm. The high content of CaCO3 and high concentration of total sulfate of sediments in IBM and OBM suggest that authigenic carbonate and sulfate minerals formed associated with AOM. This is consistent with the high concentrations of elements (Cu, Mo, Cd) in sulfide minerals and elements (Ca, Sr) coprecipitated in carbonate minerals. The concentration of total sulfate increased at depth greater than 10 cm at REF-1, and sulfur isotope ratio decreased along the depth from +3‰ to −30‰. In contrast, sulfur isotope ratios of IBM and OBM ranged from −20 to +9‰, higher than those at REF-1, reflecting differences in sulfate reduction rates.
The results of the elution tests showed that Cd, Mo and Cu were eluted from the sediments of IBM and OBM. Compared to the results of reference sites, these elements have higher concentrations in sediments and consistent with the depth profile. It is important to further investigate the leaching process in the field and methods for predicting the concentration of trace elements in seawater. On the other hand, several elements (V, As, Mn, Fe) which was found to leach from the sediments of reference sites, showed almost no leaching from the sediments of IBM and OBM. In this study, the relation of redox and elemental behaviors at each site and leaching processes will be discussed.
This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).
Push core samples (~28 cm length) were taken in 2021 from methane hydrate-bearing area in Joetsu Knoll, Japan. Sampling was conducted at different sites: Inside (IBM) and outside (OBM) of microbial mats, and outside of methane hydrate-bearing area on Joetsu Knoll (REF-1 site) and outer the Knoll (REF-2 site) as reference sites. Total organic carbon, total sulfur, total nitrogen composition, methane concentration and trace elements (Mg, Al, Ca, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Rb, Sr, Mo, Cd, Ba, Pb, U, REEs, etc.) in sediments, and nutrients, sulfate ion, hydrogen sulfide, dissolved inorganic carbon composition and trace elements in pore water were measured to obtain baseline data. Assuming that the sediment is rolled up into the seawater, elution test was conducted in which sediments and seawater were stirred at 1:100 for 24 hour and trace elements in seawater were measured after the test was completed.
The results of the chemical analysis in pore water indicated the occurrence of anaerobic oxidation of methane (AOM) at IBM and OBM, with sulfate-methane interface shallower than 5 cm below the seafloor in the IBM and deeper than 15 cm below the seafloor in the OBM. At the reference sites (REF-1 and 2), nitrate reduction, manganese reduction, and iron reduction occurred along the depth, and sulfate reduction was also thought to occur at depths greater than 20 cm at REF-1, but no AOM was observed at either site. The concentration of trace elements in pore water at IBM and OBM was lower that at reference sites, except for As and Ba. At REF-1, iron concentrations in the pore water decreased at depths greater than 10 cm, while As, Mo, V, and U increased, but decreased in the layer deeper than 20 cm. The high content of CaCO3 and high concentration of total sulfate of sediments in IBM and OBM suggest that authigenic carbonate and sulfate minerals formed associated with AOM. This is consistent with the high concentrations of elements (Cu, Mo, Cd) in sulfide minerals and elements (Ca, Sr) coprecipitated in carbonate minerals. The concentration of total sulfate increased at depth greater than 10 cm at REF-1, and sulfur isotope ratio decreased along the depth from +3‰ to −30‰. In contrast, sulfur isotope ratios of IBM and OBM ranged from −20 to +9‰, higher than those at REF-1, reflecting differences in sulfate reduction rates.
The results of the elution tests showed that Cd, Mo and Cu were eluted from the sediments of IBM and OBM. Compared to the results of reference sites, these elements have higher concentrations in sediments and consistent with the depth profile. It is important to further investigate the leaching process in the field and methods for predicting the concentration of trace elements in seawater. On the other hand, several elements (V, As, Mn, Fe) which was found to leach from the sediments of reference sites, showed almost no leaching from the sediments of IBM and OBM. In this study, the relation of redox and elemental behaviors at each site and leaching processes will be discussed.
This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).