[MIS21-P02] バイカル湖ガスハイドレート包接ガスの起源
キーワード：ガスハイドレート、バイカル湖、ガス起源
In the framework of international collaboration between Kitami Institute of Technology (Japan), Limnological Institute (Russia), and Ghent University (Belgium), natural gas hydrates have been recovered from lake-bottom sediments at Lake Baikal. Near-surface gas hydrate was first discovered at the Malenky mud volcano at the southern Baikal basin in 2000. Multi-phase Gas Hydrate Project (MHP, 2009-2018) has revealed distribution of near-surface gas hydrates at the southern and central Baikal basins. We found six new places (Enkhelook, Soukhaya, KIT, LIN, PosolCanyon1, and PosolCanyon2) during the VER18-03 cruise in 2018. The total number of places where near-surface gas hydrates were retrieved is 53. In this report, we summarize characteristics of hydrate-bound gases retrieved from all the sites since 2005.
Gas hydrate crystals were recovered from lake-bottom sediment cores, and hydrate-bound gas samples were stored in 5-mL vials. Total number of hydrate-bound gas samples was 626. Molecular and isotopic compositions of hydrate-bound gas were measured using a gas chromatograph and a stable isotope ratio mass spectrometer.
In the Bernard plot, C1 δ13C and C1 / (C2 + C3) distributes from -70‰ to -44‰ and from 6 to 100,000, respectively, suggesting that the origin of hydrate-bound gas is microbial, thermogenic, and their mixed-gas. Almost all data are plotted on the mixing line between microbial and thermogenic, however, Gorevoy Utes (oil seep site) showed thermogenic C1 with small composition of C2 and C3. The data concentrates in the microbial field (C1 δ13C: -66‰ and C1 / (C2 + C3): 1,000-10,000). C1 δD of thermogenic C1 increases with C1 δ13C. C2 δ13C distributes from -69‰ to -23‰. 17 sites (ex. Solzan, Talanka, Turka, KIT, etc.) showed microbial C1 and C2. 27 sites (Malenky, Oblom, Seep13, Kukuy K6, etc.) showed microbial C1 and thermogenic C2, indicating that small amount of thermogenic gas is mixed with microbial gas. Other sites (Mamay, PosolBank, Gorevoy Utes, Kedr, etc.) showed thermogenic C1 and C2. The Milkov plot (relation between C1 δ13C and C2 δ13C) can express mixing condition between microbial and thermogenic rather than the Bernard plot. C2 δD decreased with decrease in C2 δ13C, therefore microbial C2 is both depleted in 13C and D. We conclude that hydrate-bound gas at Lake Baikal is mainly microbial gas, and thermogenic gas ascends from deep layer at some areas (Kedr, PosolBank, Kukuy, and Gorevoy Utes) and forms the structure II gas hydrate.
Gas hydrate crystals were recovered from lake-bottom sediment cores, and hydrate-bound gas samples were stored in 5-mL vials. Total number of hydrate-bound gas samples was 626. Molecular and isotopic compositions of hydrate-bound gas were measured using a gas chromatograph and a stable isotope ratio mass spectrometer.
In the Bernard plot, C1 δ13C and C1 / (C2 + C3) distributes from -70‰ to -44‰ and from 6 to 100,000, respectively, suggesting that the origin of hydrate-bound gas is microbial, thermogenic, and their mixed-gas. Almost all data are plotted on the mixing line between microbial and thermogenic, however, Gorevoy Utes (oil seep site) showed thermogenic C1 with small composition of C2 and C3. The data concentrates in the microbial field (C1 δ13C: -66‰ and C1 / (C2 + C3): 1,000-10,000). C1 δD of thermogenic C1 increases with C1 δ13C. C2 δ13C distributes from -69‰ to -23‰. 17 sites (ex. Solzan, Talanka, Turka, KIT, etc.) showed microbial C1 and C2. 27 sites (Malenky, Oblom, Seep13, Kukuy K6, etc.) showed microbial C1 and thermogenic C2, indicating that small amount of thermogenic gas is mixed with microbial gas. Other sites (Mamay, PosolBank, Gorevoy Utes, Kedr, etc.) showed thermogenic C1 and C2. The Milkov plot (relation between C1 δ13C and C2 δ13C) can express mixing condition between microbial and thermogenic rather than the Bernard plot. C2 δD decreased with decrease in C2 δ13C, therefore microbial C2 is both depleted in 13C and D. We conclude that hydrate-bound gas at Lake Baikal is mainly microbial gas, and thermogenic gas ascends from deep layer at some areas (Kedr, PosolBank, Kukuy, and Gorevoy Utes) and forms the structure II gas hydrate.