| Summary: | Knowledge of carbon isotope fractionation is needed in order to discuss the formation and dissociation of naturally occurring CO<sub>2</sub> hydrates. We investigated carbon isotope fractionation during CO<sub>2</sub> hydrate formation and measured the three-phase equilibria of <sup>12</sup>CO<sub>2</sub>–H<sub>2</sub>O and <sup>13</sup>CO<sub>2</sub>–H<sub>2</sub>O systems. From a crystal structure viewpoint, the difference in the Raman spectra of hydrate-bound <sup>12</sup>CO<sub>2</sub> and <sup>13</sup>CO<sub>2</sub> was revealed, although their unit cell size was similar. The δ<sup>13</sup>C of hydrate-bound CO<sub>2</sub> was lower than that of the residual CO<sub>2</sub> (1.0–1.5‰) in a formation temperature ranging between 226 K and 278 K. The results show that the small difference between equilibrium pressures of ~0.01 MPa in <sup>12</sup>CO<sub>2</sub> and <sup>13</sup>CO<sub>2</sub> hydrates causes carbon isotope fractionation of ~1‰. However, the difference between equilibrium pressures in the <sup>12</sup>CO<sub>2</sub>–H<sub>2</sub>O and <sup>13</sup>CO<sub>2</sub>–H<sub>2</sub>O systems was smaller than the standard uncertainties of measurement; more accurate pressure measurement is required for quantitative discussion.
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