Carbon Isotope Fractionation during the Formation of CO2 Hydrate and Equilibrium Pressures of 12CO2 and 13CO2 Hydrates

Knowledge of carbon isotope fractionation is needed in order to discuss the formation and dissociation of naturally occurring CO2 hydrates. We investigated carbon isotope fractionation during CO2 hydrate formation and measured the three-phase equilibria of (CO2)-C-12-H2O and (CO2)-C-13-H2O systems. From a crystal structure viewpoint, the difference in the Raman spectra of hydrate-bound (CO2)-C-12 and (CO2)-C-13 was revealed, although their unit cell size was similar. The delta C-13 of hydrate-bound CO2 was lower than that of the residual CO2 (1.0-1.5 parts per thousand) in a formation temperature ranging between 226 K and 278 K. The results show that the small difference between equilibrium pressures of similar to 0.01 MPa in (CO2)-C-12 and (CO2)-C-13 hydrates causes carbon isotope fractionation of similar to 1 parts per thousand. However, the difference between equilibrium pressures in the (CO2)-C-12-H2O and (CO2)-C-13-H2O systems was smaller than the standard uncertainties of measurement; more accurate pressure measurement is required for quantitative discussion.

Automated summary

Carbon isotope fractionation was observed during the formation of CO2 hydrates, with δ13C of hydrate-bound CO2 lower than that of residual CO2. The difference in equilibrium pressures between (CO2)-C-12 and (CO2)-C-13 hydrates caused approximately 1 parts per thousand carbon isotope fractionation. More accurate pressure measurements are needed for quantitative discussions in (CO2)-C-12-H2O and (CO2)-C-13-H2O systems.