Keywords:gas hydrate, Lake Baikal, ethane, gas origin
Natural gas hydrate exists in sublacustrine sediments of Lake Baikal. Multi-phase Gas Hydrate Project (MHP, 2009-2019) has revealed characteristics of near-surface gas hydrates retrieved from the southern and central Baikal basins. Recently, we found six new places (Zelen MV, Zelen Seep, Katko MV, BelKamen MV, Melky Seep, and Kukuy K-17 MV) during the VER19-03 cruise in 2019. The total number of places where near-surface gas hydrates were retrieved at Lake Baikal is 60. In this study, we summarize characteristics of hydrate-bound gases retrieved from all the sites since 2005, and focus on hydrate-bound ethane and its gas origin. Hydrate-bearing sediment cores were recovered using a gravity corer and samples of hydrate-bound gas were stored in glass vials with butyl-rubber septum. Total number of hydrate-bound gas samples was 667. Molecular and isotopic compositions of hydrate-bound gas were measured using a gas chromatograph and a stable isotope ratio mass spectrometer. The data plotted in the Bernard plot showed that the gas origin of Lake Baikal gas hydrates distributes microbial, thermogenic, and their mixed-gas. δ13C of ethane distributes from -69‰ (Solzan, gas hydrate mound) to -23‰ (Kukuy K-P, pockmark). The relation between δ13C of methane and ethane showed “L shape”, suggesting that hydrate-bound ethane in Lake Baikal is composed of ethane-rich thermogenic gas and ethane-depleted microbial gas. We propose a new diagram, relation between δ13C and δD of ethane, similar to the Whiticar diagram (relation between δ13C and δD of methane). The diagram showed that light ethane in δ13C also depleted in δD; hydrate-bound thermogenic ethane is plotted around -25‰ in δ13C and -210‰ in δD, whereas those of microbial ethane is plotted around -60‰ in δ13C and -280‰ in δD. These results indicate that the light ethane depleted in 13C and D is generated by a microbial process, and the origin of hydrogen in ethane molecules is thought to be lake water, same as methane.