Experimental study on isotopic fractionation in water during gas hydrate formation

Isotopic fractionation of oxygen and hydrogen in water caused by gas hydrate formation was investigated experimentally. Two different gas hydrates in structures, Structure I hydrate and Structure II hydrate, were formed with methane and krypton gases in a NaCl solution. Isotopic fractionation during gas hydrate formation was observed by measuring oxygen and hydrogen isotopic compositions in the solutions sampled before and after gas hydrate formation. Heavy isotopes of oxygen and hydrogen in water were depleted in the solution resulting from that those isotopes were concentrated in the gas hydrate. The isotopic fractionation was larger as increasing amounts of gas hydrates that were calculated from both the decrease of gas pressure and increase of NaCl concentration in solution. The isotopic fractionation factors of oxygen and hydrogen in water between gas hydrate and liquid water were determined to be 1.0023-1.0032 and 1.014-1.022, respectively. The significant difference of the gas hydrate structures was not observed beyond the analytical errors. These factors are similar to those between ice and liquid water.

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