[M-IS32] ガスハイドレートと地球環境・資源科学

[MIS32-05] バイカル湖の天然ガスハイドレートに包接された微生物起源エタンについて

*八久保 晶弘¹、長谷 優之介¹、池浦 有希¹、Khlystov Oleg²、Kalmychkov Gennadiy³、De Batist Marc⁴、坂上 寛敏¹、南 尚嗣¹、山下 聡¹ (1.北見工業大学、2.ロシア科学アカデミー陸水学研究所、3.ロシア科学アカデミー地球化学研究所、4.ゲント大学)

キーワード：ガスハイドレート、バイカル湖、エタン、ガス起源

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.