時間: 5月24日17:15-18:45

会場:コンベンションホール

MIS005-P11

熱分析からみたバイカル湖ガスハイドレートのガス組成不均一性
Heterogeneous molecular composition of Lake Baikal gas hydrate
revealed by a calorimetric measurement
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八久保 晶弘 , 奥田 充 , 坂上 寛敏 , 南 尚嗣 , 山下 聡 , 高橋 信夫 , 庄子 仁 ,23Oleg Khlystov , Jeffrey Poort
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Akihiro Hachikubo , Mitsuru Okuda , Hirotoshi Sakagami , Hirotsugu Minami ,11123Satoshi Yamashita , Nobuo Takahashi , Hitoshi Shoji , Oleg Khlystov , Jeffrey Poort1

北見工業大学, ロシア陸水学研究所, ゲント大学

231Kitami Institute of Technology, Limnological Institute SB RAS, Ghent University

23We got methane and ethane mixed-gas hydrates at southern and central basins of Lake Baikal(Kida et al., 2006; 2009; Hachikubo et al., 2009; 2010). Their ethane composition reached to 15%,
suggested crystallographic structure II. NMR and Raman spectroscopic methods revealed the bothexistence of sI and sII in the same sediment cores, respectively (Kida et al., 2006; Hachikubo et al.,2009). Kida et al. (2006) showed sI and sII mixed samples in the scale of the cell for the NMR
device. However, it has been still unknown how the above different crystallographic structures
and/or gas hydrates of different gas composition are mixed. It is both possible that macroscopic(scale of nodules, from submillimeter to centimeter order) and microscopic (scale of the molecules)
mixtures. In this study, we focused on ethane composition and heterogeneous molecularcomposition of hydrate-bound gas was investigated by using a calorimeter.
The samples were retrieved at Lake Baikal from 2005-2008 and stored in the temperature ofliquid nitrogen. Ethane compositions of gas hydrate samples retrieved at Goloustnoye G-1 wereabout 0.5-1.5%, whereas those at Kukuy K-2 mud volcano were about 14% for sII and sI+sII
mixed type. About 1g of gas hydrate sample was ground up well in liquid nitrogen and put into asmall pressure cell for the calorimeter (Setaram BT2.15). The cell was set into the calorimeter at-1
93K and then heated from 93K to 298K at a rate of 0.15 (K min ). Gas flow system flushed inside-1
the cell by pure N2or He at a rate of 1 [mL min ] to avoid recrystallization of ethane-rich gas
hydrate in the cell. Dissociation gas was sampled every 15 minutes by a gas-tight microsyringe
and measured the gas composition by a gas chromatograph (Shimadzu GC-14B).
Preliminary results indicated that the ethane composition increased as temperature in thedissociation processes. The change of gas composition was relatively small in the case ofGoloustnoye G-1 (sI), whereas that of Kukuy K-2 (sI+sII) increased drastically from several % to18%. Bulk gas composition of this sample was reported about 15% by Kida et al. (2006), however,real gas composition of sII part seemed to be more ethane rich. The ethane composition of sII partwas almost constant. These results supported the hypothesis of double structure gas hydrate(Hachikubo et al., 2009). On the other hand, we also checked the heterogeneity by using Raman
spectrometer (RMP-210, JASCO Corp.) because the device can provide a small laser spot (about 50um in diameter) to the sample surface. We obtained Raman spectra several times from the same
hydrate nodule and found large differences in crystallographic structure and ethane compositioneven in the same sample.
Hachikubo A, Khlystov O, Manakov A, Kida M, Krylov A, Sakagami H, Minami H, Takahashi N,

Shoji H, Kalmychkov G, Poort J (2009) Model of formation of double structure gas hydrates in
Lake Baikal based on isotopic data. Geophys Res Lett 36: L18504. doi:10.1029/2009GL039805
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) Molecularand isotopic characteristics of gas hydrate-bound hydrocarbons in southern and central Lake
Baikal. Geo-Mar Lett: submitted
Kida 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:10.1029/2006GL028296
Kida M, Hachikubo A, Sakagami H, Minami H, Krylov A, Yamashita S, Takahashi N, Shoji H,Khlystov O, Poort J, Narita H (2009) Natural gas hydrates with locally different cage occupancies
and hydration numbers in Lake Baikal. Geochem Geophys Geosyst 10: Q05003. doi:10.1029/2009
GC002473

キーワード:ガスハイドレート,熱分析,バイカル湖
Keywords: gas hydrate, calorimetry, Lake Baikal