The natural gas hydrate studies at gas seepage structures in the Okhotsk Sea and at a Malenky vent structure in Lake Baikal were conducted in 2003 and 2005. The gas hydrate-bearing sediment cores were retrieved from the bottom of the sea/lake floors by using gravity/hydro corers. The pore water sampling was conducted on board by a squeezer. In order to clarify the origin and the composition of water involving the accumulation of gas hydrates, the chemical analyses of sediment cores were conducted. Two types of chemical elements and δ<18>O distribution were observed for the pore water samples obtained from the sediment cores retrieved from Lake Baikal (the first type) and from the Okhotsk Sea (the second type). The first type is as follows: the concentrations of Na, K, Ca, Mg and Cl<-> in the pore water increased with depth. The δ<18>O value of the hydrated water was +1.5‰ heavier than that of the lake water. These results suggested that the ascending water enriched with these salts and light δ<18>O (up to -1.5‰ than that of the lake water) is thought to be the main gas hydrate-forming fluid. The second type is as follows: the concentrations of Na, K, Mg and Cl<-> did not increase with depth whereas those of SO<2-><,4> and Ca decreased with depth by the chemical reactions such as sulfate reduction and carbonate formation. The δ<18>O value of the hydrated water was up to + 1.4‰ heavier than that of the sea water. These results suggested the two distinct possibilities for the main gas hydrate- forming fluids, i. e., (i) sea water was the main gas hydrate-forming fluid since only free gas was expulsed without any ascending fluid at the gas seepage structures, and (ii) not only free gas was expulsed but also the water with light δ<18>O (up to - 1.9‰ than that of the sea water) was discharged and it was the main gas hydrate-forming fluid.
Okhotsk Sea Lake Baikal gas hydrate sediment pore water ions stable isotopic composition
Hirotsugu Minami Tamara emskaya Young Jin Hitoshi Shoji Hirotoshi Sakagami Akihiro Hachikubo Alexey Krylov Nobuo Takahashi Leonid Mazurenko Tatiana Matveeva Anatoly Obzhirov Oleg Khlystov
Kitami Institute of Technology, Japan Limnological Institute, Siberian Branch of the Russian Academy of Sciences, Russia Korea Ocean Research and Development Institute, Korea All-Russia Research Institute for Geology and Mineral Resources of the Ocean, Russia V. I. Ilichev Pacific Oceanological Institute of the Far Eastern Branch of Russian Academy of Scien