Isotopic difference between hydrate-bound and sediment gases retrieved at Lake Baikal

The objective of our study is to clarify the formation process of natural gas hydrates in the sublacustrine sediments of Lake Baikal, and we are now compiling data on stable isotope ratios of hydrocarbons in hydrate-bound and sediment gases, which have been collected in the framework of collaboration between Russia, Belgium and Japan for 15 years. The results obtained in the VER19-03 cruise at the southern and central Baikal basins were summarized in this report. Comparison of stable isotope ratios of hydrate-bound and sediment gases at the same core and depth was performed. The methane carbon isotope ratio of the hydrate-bound gas was 1 to 2 smaller than the sediment gas. The methane hydrogen isotope ratio was similarly about 5 smaller. These results can be explained generally by the isotopic fractionation of methane. In contrast, some sediment cores from pockmarks in the center of the mud volcano showed small or reversed differences in methane carbon isotope ratios, as well as small differences in methane hydrogen isotope ratios, suggesting that dissociation of gas hydrate is occurring at that point.