[MIS21-P02] Gas origin of hydrate-bound gas in Lake Baikal
Keywords:gas hydrate, Lake Baikal, gas origin
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.