5:15 PM - 6:45 PM
[MIS22-P06] Anomalously high chloride content in pore water at the methane hydrate field off Joetsu, eastern margin of the Japan Sea
Keywords:pore water, chloride ion, hydrogen and oxygen isotopes
Methane hydrates are attracting attention as an unconventional natural gas resource, and are believed to exist in large quantities around Japan. The eastern margin of the Japan sea is recognized as one of shallow-type methane hydrate fields in Japan. In this study, pore water extracted from sediment cores collected at hydrate sites and reference sites in the Umitaka Spur and Joetsu Knoll by the D/V Chikyu was analyzed to clarify its geochemical characteristics.
The Cl content in pore water at the hydrate sites ranged from 879 mM to 1282 mM between depths of 12 mbsf and 60 mbsf. This is unusually high and approximately twice the bottom water Cl content in the Joetsu Knoll (545 mM). Considering that the sediments at the drilling sites bear methane hydrates, these hypersaline waters could be attributed to residual waters formed by the rapid crystallization of hydrate excluding Cl from the lattice (e.g., Torres et al., 2004). To verify the possibility of the residual waters, hydrogen and oxygen isotope compositions (δD and δ18O) of the pore water were analyzed. The δD and δ18O values of the pore water (δD: -16.4‰ to -9.7‰, δ18O: -2.0‰ to -1.3‰) decreased with increasing Cl content. (879—1282 mM). During the formation of methane hydrates, D and 18O in the pore water are preferentially incorporated into the methane hydrates, so the residual water around hydrates is enriched in H and 16O, leading to lower δD and δ18O values in the residual water. The trend in δD and δ18O values of the pore water at the methane hydrate sites is consistent with this characteristic. As a result, we concluded that hypersaline waters are residual waters around the grown methane hydrates.
Based on the Cl concentration of the residual water, the amounts of water and methane incorporated into the methane hydrates were estimated as follows: Assuming that during methane hydrate formation, pore water with Cl content of (430—550 mM) (the value at the reference sites) was concentrated to generate Cl-rich content residual water. The difference in the pore water Cl content between the hydrate sites and reference sites was used to determine the proportion of Cl enrichment, from which the proportion of water incorporated into the hydrates was calculated. The results indicated that 41-60% of the pore water in the sediment before hydrate formation was incorporated into the hydrates. Subsequently, the amount of water consisting of hydrates per cubic meter of sediment was calculated based on the porosity of the sediment (55—65%) and the proportion of water incorporated into the hydrates. The water consisting of hydrates averaged 2.4—3.7 × 105 g/m3 (1.4—2.2 × 104 mol/m³). Assuming Type I methane hydrates, (molecular formula of CH4-5.75H2O), the methane content of the hydrates ranged 2.5—3.9 ×103mol/m3. Consequently, the amounts of methane contained in hydrates were estimated to be 54-84 m3/m3 (STP). It is thus inferred that at least 54 m3/m3 (STP) of methane exists as hydrates in the sediment surrounding the hydrate sites.
This study was conducted as part of the methane hydrate research project funded by the Ministry of Economy, Trade and Industry (METI), Japan.
The Cl content in pore water at the hydrate sites ranged from 879 mM to 1282 mM between depths of 12 mbsf and 60 mbsf. This is unusually high and approximately twice the bottom water Cl content in the Joetsu Knoll (545 mM). Considering that the sediments at the drilling sites bear methane hydrates, these hypersaline waters could be attributed to residual waters formed by the rapid crystallization of hydrate excluding Cl from the lattice (e.g., Torres et al., 2004). To verify the possibility of the residual waters, hydrogen and oxygen isotope compositions (δD and δ18O) of the pore water were analyzed. The δD and δ18O values of the pore water (δD: -16.4‰ to -9.7‰, δ18O: -2.0‰ to -1.3‰) decreased with increasing Cl content. (879—1282 mM). During the formation of methane hydrates, D and 18O in the pore water are preferentially incorporated into the methane hydrates, so the residual water around hydrates is enriched in H and 16O, leading to lower δD and δ18O values in the residual water. The trend in δD and δ18O values of the pore water at the methane hydrate sites is consistent with this characteristic. As a result, we concluded that hypersaline waters are residual waters around the grown methane hydrates.
Based on the Cl concentration of the residual water, the amounts of water and methane incorporated into the methane hydrates were estimated as follows: Assuming that during methane hydrate formation, pore water with Cl content of (430—550 mM) (the value at the reference sites) was concentrated to generate Cl-rich content residual water. The difference in the pore water Cl content between the hydrate sites and reference sites was used to determine the proportion of Cl enrichment, from which the proportion of water incorporated into the hydrates was calculated. The results indicated that 41-60% of the pore water in the sediment before hydrate formation was incorporated into the hydrates. Subsequently, the amount of water consisting of hydrates per cubic meter of sediment was calculated based on the porosity of the sediment (55—65%) and the proportion of water incorporated into the hydrates. The water consisting of hydrates averaged 2.4—3.7 × 105 g/m3 (1.4—2.2 × 104 mol/m³). Assuming Type I methane hydrates, (molecular formula of CH4-5.75H2O), the methane content of the hydrates ranged 2.5—3.9 ×103mol/m3. Consequently, the amounts of methane contained in hydrates were estimated to be 54-84 m3/m3 (STP). It is thus inferred that at least 54 m3/m3 (STP) of methane exists as hydrates in the sediment surrounding the hydrate sites.
This study was conducted as part of the methane hydrate research project funded by the Ministry of Economy, Trade and Industry (METI), Japan.