Do CO2 hydrates exist in nature? If so, how much carbon could they contain?
By far most naturally occurring hydrates are methane hydrates. However, Dillon documents hydration of other gases including carbon dioxide in some places such as the Gulf of Mexico:
Many gas hydrates are stable in the deep ocean conditions, but methane hydrate is by far the dominant type, making up >99% of gas hydrate in the ocean floor. The methane is almost entirely derived from microbial methanogenesis, predominantly through the process of carbon dioxide reduction. In some areas, such as the Gulf of Mexico, gas hydrates are also created by other thermogenically formed hydrocarbon gases and other clathrate-forming gases such as hydrogen sulfide and carbon dioxide. Such gases escape from sediments at depth, rise along faults, and form gas hydrate at or just below the seafloor, but on a worldwide basis these are of minor volumetric importance compared to methane hydrate.
There is no evidence from this that the carbon dioxide is in a purely binary hydrate, and the point is made that methane hydrates are by far more common. But yes, to at least a small degree carbon dioxide is hydrated in nature.
One factor behind the limited occurrence of naturally occurring hydrates of carbon dioxide (and other common gases besides methane) is the solubility of the gas in liquid water. Gas hydrates exist in equilibrium with solutions of the gas in liquid water, so more soluble gases require higher concentrations to form the hydrate (which may be thought of as a precipitate). This table reveals that on a mass basis, carbon dioxide is over 70 times as soluble as methane in water under ambient conditions; even accounting for the heavier molecular weight of carbon dioxide this is still a molar ratio of over 25 to 1. For hydrogen sulfide, also specifically mentioned in Ref. , the corresponding molar ratio is about 80 to 1 over methane. Two-carbon hydrocarbons such as ethane can also form hydrates and their solubility is closer to that of methane (allowing hydrate formation at lower concentrations than carbon dioxide or hydrogen sulfide), but these are less abundant gases in nature.
1. W. P. Dillon, in Robert A. Meyers, Encyclopedia of Physical Science and Technology, Third Edition (2001).