Abstract
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 12CO2–H2O and 13CO2–H2O systems. From a crystal structure viewpoint, the difference in the Raman spectra of hydrate-bound 12CO2 and 13CO2 was revealed, although their unit cell size was similar. The δ13C of hydrate-bound CO2 was lower than that of the residual CO2 (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 12CO2 and 13CO2 hydrates causes carbon isotope fractionation of ~1‰. However, the difference between equilibrium pressures in the 12CO2–H2O and 13CO2–H2O systems was smaller than the standard uncertainties of measurement; more accurate pressure measurement is required for quantitative discussion.
Highlights
Gas hydrates are crystalline clathrate compounds that have guest gas molecules encapsulated in hydrogen-bonded water cages and can be thermodynamically stable at high pressures and low temperatures
Two distinct peaks corresponding to the Fermi dyad of CO2 in the hydrate cages were observed at 1278.2 cm−1 and 1381.9 cm−1 for 12CO2 hydrates, corresponding to [19,20,21]
From the Raman spectra of 12CO2 and 13CO2 hydrates, definite differences were found in the Raman shift of Fermi dyad of 12CO2 and 13CO2 encapsulated in hydrate cages, their unit cell size was similar
Summary
Gas hydrates are crystalline clathrate compounds that have guest gas molecules encapsulated in hydrogen-bonded water cages and can be thermodynamically stable at high pressures and low temperatures. Gas hydrates with encapsulated natural gases exist in sub-marine/sublacustrine sediments and below the permafrost on Earth. Because the temperature of existing natural gas hydrates at sea/lake bottom sediments is above 273 K, information about carbon isotope fractionation above the freezing point of water is needed to discuss the formation and dissociation of naturally occurring CO2 hydrates. Because guest molecules, which have lower equilibrium pressure, are preferentially encapsulated in the gas hydrate cages, the difference in equilibrium pressures of CH3D and CH4 hydrates can explain the hydrogen isotope fractionation in methane during methane hydrate formation [18]. Comparing the phase equilibrium p–T conditions of each gas hydrate encapsulating isotopologues can explain the trend of isotopic fractionation of guest gases during gas hydrate formation. We investigated carbon isotope fractionation between hydrate-bound gas and residual gas in a pressure cell in the temperature range of 226 K to 278 K
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
