We report the field observation of hydrate deposits of different crystal structures in the same cores of a mud volcano in the Kukuy Canyon. We link those deposits to chemical fractionation during gas hydrate crystallization. Gas composition and crystallographic analyses of hydrate samples reveal involvement of two distinct gas source types in gas hydrate formation at present or in the past: microbial (methane) and thermogenic (methane and ethane) gas types. The clathrate structure II, observed for the first time in fresh water sediments, is believed to be formed by higher mixing of thermogenic gas.

This database of Phanerozoic occurrences and isotopic characteristics of metastable cold-water calcium carbonate hexahydrate (ikaite; CaCO₃⚫6H₂O) and their associated carbonate pseudomorphs (glendonites) has been compiled from academic publications, explanatory notes, and reports. Our database including more than 700 occurrences reveals that glendonites characterize cold-water environments, although their distribution is highly irregular in space and time. A significant body of evidence suggests that glendonite occurrences are restricted mainly to cold-water settings; however they do not occur during every glaciation or cooling event of the Phanerozoic. While Quaternary glendonites and ikaites have been described from all major ocean basins, older occurrences have a patchy distribution, which may suggest poor preservation potential of both carbonate concretions and older sediments. The data file described in this paper is available on Zenodo at https://doi.org/10.5281/zenodo.4386335 (Rogov et al., 2020).

Knowledge of cage occupancies and hydration numbers (n) of naturally occurring gas hydrate in a local environment is important for the improvement in global estimates of hydrate‐bound natural gas. We report on local differences in cage occupancies and hydration number of gas hydrates from Lake Baikal. Natural gas hydrates of both structures I and II (sI and sII) and ranging in composition from pure CH₄ to mixed gas hydrate containing up to 15% C₂H₆ are compared. The average hydration numbers are n = 6.1 for the sI CH₄ hydrates recovered from the Malenky and Bolshoy mud volcanoes, n = 6.2 for the sI hydrates, containing 3–4% C₂H₆ recovered from the K‐2 mud volcano, and n = 6.9 for the sII hydrate containing about 15% C₂H₆ recovered from the K‐2 mud volcano. The differences in hydration number are due to the differences in the small cage occupancy of CH₄ among the samples studied.