Achievement of carbon isotope equilibrium in the system HCO3- (solution) CO2 (gas)

The magnitude of stable carbon isotope fractionation between dissolved bicarbonate and gaseous carbon dioxide as a function of isotope exchange time was measured in the temperature range 7 to 70 degrees C at an initial partial pressure of CO2 of 26.7 kPa and from 7 to 60 degrees C at a total initial pressure P(CO2 + H2O vapour) of 28 kPa. Isotope equilibrium was also examined at a temperature of 53.5 degrees C and at an initial pressure of carbon dioxide that varied from 13.3 to 118.7 kPa. The permil fractionation, epsilon(13)C, as a function of exchange time, t, is given by the formula epsilon(13)C = epsilon(infinity)(13)C(1-e(-t/tau)) where the carbon fractionation at isotopic equilibrium, epsilon(infinity)(13)C, and relaxation time, tau, are calculated from the experimental data. It has been found that the relaxation time in a given experimental apparatus depends on two variables: (1) temperature and (2) the initial pressure. The epsilon(infinity)(13)C value as a function of temperature is well described by the equation: epsilon(infinity)(13)C = -(0.0954 +/- 0.0027) T[degrees C] + (10.41 +/- 0.12). The relaxation time as a function of absolute temperature at a total initial pressure of 28 kPa is given by the equation: tau[hours] = (350 +/- 35) T-1/2-(18 +/- 2), and as a function of the total initial pressure, P (in kPa), at temperature 53.5 degrees C is well represented by the following dependence: tau[hours] = (0.03054 +/- 0.00097)P. (C) 1997 Elsevier Science B.V.