Carbon isotopic fractionation of carbon dioxide between gas and hydrate phases

Abstract

During the formation of gas hydrate crystals, stable isotope fractionation of the guest molecules occurs. For methane and ethane, hydrogen stable isotope fractionation has been reported in the range of a few per mil, but for carbon stable isotope fractionation, there is no difference in ¹³C, or the difference is below the detection limit. As for carbon dioxide in natural gas hydrates, little is known about stable isotope fractionation of carbon dioxide. In this study, we report the stable isotope fractionation during formation of synthetic carbon dioxide hydrates under various temperature conditions. Fine ice powder and pure carbon dioxide gas were introduced into a pressure cell and adjusted to be above the equilibrium pressure of the hydrate and below the liquefaction pressure of the carbon dioxide at each temperature. After the formation of carbon dioxide hydrate, the residual gas was collected and the hydrate was recovered under the liquid nitrogen temperature to obtain the hydrate-bound gas. The carbon stable isotope ratios of carbon dioxide in the hydrate and residual gases were determined using a stable isotope mass spectrometer. Over a wide range from 243 K to 278 K, the ¹³C of residual gas was always larger than the hydrate phase, indicating that the carbon dioxide hydrate preferentially enclathrated lighter molecules. This trend is consistent with the results of a previous study. These results suggest that the equilibrium pressure of ¹³CO₂ hydrate is slightly higher than that of ¹²CO₂ hydrate.