Model of formation of double structure gas hydrates in Lake Baikal based on isotopic data

We measured the isotopic compositions of methane (C1) and ethane (C2) of hydrate‐bound gas and of dissolved gas in pore water retrieved from bottom sediments in Lake Baikal. Both structure I (sI:3%C2) and II (sII:14%C2) gas hydrates are observed in the same sediment cores in Kukuy K‐2 mud volcano. We found that C2δD of sI gas hydrate is larger than that of sII, whereas C1δ13C, C1δD and C2δ13C values are practically the same in both hydrate structures. δ13C of C1 and C2 of hydrate‐bound gas are several permil smaller than those in pore water, showing that the current pore water is not the source of gas hydrates. These findings lead to a new model where the sII gas hydrates were formed prior to the sI hydrates.