[M-IS22_29PO1] Gas hydrates in environmental-resource sciences

[MIS22-P06] Hydrogen isotope of hydrate-bound hydrocarbons at Lake Baikal

*Akihiro HACHIKUBO¹, Hirotoshi SAKAGAMI¹, Hirotsugu MINAMI¹, Satoshi YAMASHITA¹, Nobuo TAKAHASHI¹, Hitoshi SHOJI¹, Oleg KHLYSTOV², Gennadiy KALMYCHKOV³, Marc DE BATIST⁴ (1.Kitami Institute of Technology, 2.Limnological Institute, SB RAS, 3.Vinogradov Institute of Geochemistry, SB RAS, 4.Ghent University)

Keywords:gas hydrate, crystallographic structure, Lake Baikal, methane, stable isotope

Natural gas hydrates exist in sublacustrine sediments of Lake Baikal. Gas hydrates were first obtained from sub-bottom depths of 121 and 161 m in the Baikal Drilling Project well located at the southern Baikal basin. Recently, MHP (Multi-phase Gas Hydrate Project, 2009-2013) revealed distribution of gas hydrate in sub-bottom sediment at the southern and central Baikal basins. We obtained gas hydrate crystals from more than 25 places, and retrieved hydrate-bound gas onboard. We measured molecular and isotopic compositions of hydrate-bound gas. According to the δ¹³C-δD diagram for methane (Whiticar, 1999), high and low methane δ¹³C values indicate thermogenic and microbial origins, respectively, and methane δD provides information on methyl-type fermentation or CO₂ reduction in the microbial field. Kida et al. (2006) and Hachikubo et al. (2010) reported that hydrate-bound methane of Lake Baikal was microbial origin via methyl-type fermentation, because methane δD was about -300‰. We found heavier methane (δ¹³C ranged from -50‰ to -40‰) in the Kukuy Canyon area (central Baikal basin), indicating thermogenic origin. Methane δD was distributed from -330‰ to -270‰. Generally, δD of thermogenic methane of marine gas hydrates is much more heavier (more than -200‰). Methane δD of Lake Baikal gas hydrate seems to be about 100‰ smaller than that of marine gas hydrate. Matveeva et al. (2003) reported that δD of the lake bottom water was about -133‰. Possiblely, methane δD of hydrate-bound methane derives from δD of water. Hachikubo A, Khlystov O, Krylov A, Sakagami H, Minami H, Nunokawa Y, Yamashita S, Takahashi N, Shoji H, Nishio S, Kida M, Ebinuma T, Kalmychkov G, Poort J (2010) Molecular and isotopic characteristics of gas hydrate-bound hydrocarbons in southern and central Lake Baikal. Geo-Mar Lett 30: 321-329. doi:10.1007/s00367-010-0203-1Kida M, Khlystov O, Zemskaya T, Takahashi N, Minami H, Sakagami H, Krylov A, Hachikubo A, Yamashita S, Shoji H, Poort J, Naudts L (2006) Coexistence of structure I and II gas hydrates in Lake Baikal suggesting gas sources from microbial and thermogenic origin. Geophys Res Lett 33: L24603. doi:10.1029/2006GL028296Matveeva TV, Mazurenko LL, Soloviev VA, Klerkx J, Kaulio VV, Prasolov EM (2003) Gas hydrate accumulation in the subsurface sediments of Lake Baikal (Eastern Siberia). In: Woodside JM, Garrison RE, Moore JC, Kvenvolden KA (eds) Proc 7th Int Conf Gas in Marine Sediments, 7-11 October 2002, Baku, Azerbaijan. Geo-Mar Mett 23(3/4): 289-299. doi:10.1007/s00367-003-0144-7.Whiticar MJ (1999) Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chem Geol 161: 291-314. doi:10.1016/S0009-2541(99)00092-3