Molecular and isotopic characteristics of gas hydrate-bound hydrocarbons in southern and central Lake Baikal

摘要

We investigated the molecular composition (methane, ethane, and propane) and stable isotope composition (methane and ethane) of hydrate-bound gas in sediments of Lake Baikal. Hydrate-bearing sediment cores were retrieved from eight gas seep sites, located in the southern and central Baikal basins. Empirical classification of the methane stable isotopes (delta C-13 and delta D) for all the seep sites indicated the dominant microbial origin of methane via methyl-type fermentation; however, a mixture of thermogenic and microbial gases resulted in relatively high methane delta C-13 signatures at two sites where ethane delta C-13 indicated a typical thermogenic origin. At one of the sites in the southern Baikal basin, we found gas hydrates of enclathrated microbial ethane in which C-13 and deuterium were both highly depleted (mean delta C-13 and delta D of -61.6aEuro degrees V-PDB and -285.4aEuro degrees V-SMOW, respectively). To the best of our knowledge, this is the first report of C-2 delta C-13-delta D classification for hydrate-bound gas in either freshwater or marine environments.