[M-IS21] Gas hydrate in environmental-resource sciences

[MIS21-P07] Carbon isotope fractionation of CO₂ during the formation of clathrate hydrate

*Hiromi Kimura¹, Jumpei Matsuda¹, Yuki Kikuchi¹, Akihiro Hachikubo¹, Satoshi Takeya² (1.Kitami Institute of Technology, 2.National Institute of Advanced Industrial Science and Technology)

Keywords:gas hydrate, stable isotope, carbon dioxide, isotopic fractionation

Hydrogen Isotope fractionation in methane during the formation of clathrate hydrate was reported by Hachikubo et al. (2007) that δD of hydrate-bound methane is 4.8±0.4‰ lower than that of residual methane at 274.2K. Although natural methane hydrates distribute widely in the world, natural CO₂ hydrate was only reported at the Okinawa Trough (Sakai et al., 1990). It is also possible that CO₂ hydrate exists in Mars. Isotopic fractionation of guest gas may provide useful information to understand formation processes of gas hydrate. Luzi et al. (2011) revealed that CO₂ δ¹³C in hydrate-bound gas is 0.9‰ lower than that of residual gas, suggesting that light CO₂ molecules prefer to be encaged into clathrate cages. In this study, we checked the temperature effect on the isotopic difference between residual and hydrate-bound CO₂.

Samples of CO₂ hydrate (weight: 0.7 g) were experimentally prepared in a pressure cell (volume: 30 mL), and the temperature was controlled by a liquid bath (258-274 K) and cold rooms (226-254 K). Isotopic compositions of both residual and hydrate-bound CO₂ were measured by an isotope ratio mass spectrometer (IRMS). Samples were formed in the temperature range from 226 K to 274 K. The carbon isotopic differences between hydrate-bound and residual CO₂ distributed between 1.2‰ and 1.5‰, agreed fairly well with the previous report (Luzi et al., 2011). The difference seemed to be large (1.5-2.0 ‰) at 226 K, indicating that the equilibrium pressure of ¹³CO₂ hydrate is slightly larger than that of ¹²CO₂ hydrate, and the difference between them becomes large in lower temperature. We will show these equilibrium data for ¹³CO₂ and ¹²CO₂ hydrates and compare with the results in stable isotope.



Reference

Hachikubo A, Kosaka T, Kida M, Krylov A, Sakagami H, Minami H, Takahashi N, Shoji H (2007) Isotopic fractionation of methane and ethane hydrates between gas and hydrate phases. Geophys Res Lett 34: L21502. doi:10.1029/2007GL030557

Sakai H, Gamo T, Kim ES, Tsutsumi M, Tanaka T, Ishibashi J, Wakita H, Yamano M, Oomori T (1990) Venting of carbon dioxide-rich fluid and hydrate formation in mid-okinawa trough backarc basin. Science: 248, 4959, 1093-1096.

Luzi M, Schicks JM, Erzinger J (2011) Carbon isotopic fractionation of synthetic methane and carbon dioxide hydrates. Proc. 7th International Conference on Gas Hydrates (ICGH2011).