[M-IS22_29PM1] Gas hydrates in environmental-resource sciences

[MIS22-07] Characteristics of natural gas hydrates retrieved off the southeastern and southwestern Sakhalin Island

*Akihiro HACHIKUBO¹, Hirotoshi SAKAGAMI¹, Hirotsugu MINAMI¹, Satoshi YAMASHITA¹, Nobuo TAKAHASHI¹, Hitoshi SHOJI¹, Olga VERESHCHAGINA², Young K. JIN³, Anatoly OBZHIROV² (1.Kitami Institute of Technology, 2.Pacific Oceanological Institute, FEB RAS, 3.Korea Polar Research Institute)

Keywords:gas hydrate, stable isotope, Sea of Okhotsk, Raman spectroscopic analysis, Calorimetry

Gas hydrate samples were retrieved at the southeastern and southwestern Sakhalin Island in the cruises of LV59 and LV62 (R/V Akademik M. A. Lavrentyev). Sakhalin Slope Gas Hydrate (SSGH) project started in 2007, and we retrieved sediment cores including gas hydrates off northeastern Sakhalin Island in 2009-2011. In the recent cruises (2012-2013), we sampled sediment cores at the Terpeniya Ridge and the Tatarsky Trough (SE and SW Sakhalin Island, respectively). We found a lot of gas plumes ascend from the sea bottom and the dissolved methane in sediment pore water was rich. Gas hydrate crystals were recovered from both areas and stored into liquid nitrogen tank. Their dissociation heat and hydration number were measured by a calorimeter and Raman spectrometer, respectively. Dissociation heat of gas hydrates was almost the same as that of pure methane hydrate. Raman spectra showed that the hydrate crystals of both Terpeniya Ridge and Tatar Trough belonged to the structure I, and the hydration number was estimated about 6.0. Molecules of hydrogen sulfide were detected in both large and small cages of the structure I. Therefore, the hydrate crystal is similar to that obtained from NE Sakhalin Island in our previous cruises. We obtained hydrate-bound gas and dissolved gas in pore water on board and measured their molecular and stable isotope compositions. Empirical classification of the methane stable isotopes; δ¹³C and δD indicated that the gases obtained at the Terpeniya Ridge are microbial origin via carbonate reduction, whereas some cores at the Tatarsky Trough showed typical thermogenic origin. We retrieved three sediment cores with gas hydrate at the Tatarsky Trough, and their δ¹³C of hydrate-bound methane were -47.5‰, -44.2‰, and -68.8‰, respectively. Therefore, gas hydrates encaged both microbial and thermogenic gases yield at the Tatarsky Trough. Ethane-rich (up to 1% of the total guest gas) hydrates were found at the Terpeniya Ridge and the Tatarsky Trough, and encaged ethane was also detected in their Raman spectra. Ethane δ¹³C of the all gas samples suggested their thermogenic origin.