Experimental studies of oxygen isotope fractionation in the carbonic acid system at 15°, 25°, and 40°C

In light of recent studies that show oxygen isotope fractionation in carbonate minerals to be a function of HCO3− and CO32− concentrations, the oxygen isotope fractionation and exchange between water and components of the carbonic acid system (HCO3−, CO32−, and CO2(aq)) were investigated at 15°, 25°, and 40°C. To investigate oxygen isotope exchange between HCO3−, CO32−, and H2O, NaHCO3 solutions were prepared and the pH was adjusted over a range of 2 to 12 by the addition of small amounts of HCl or NaOH. After thermal, chemical, and isotopic equilibrium was attained, BaCl2 was added to the NaHCO3 solutions. This resulted in immediate BaCO3 precipitation; thus, recording the isotopic composition of the dissolved inorganic carbon (DIC). Data from experiments at 15°, 25°, and 40°C (1 atm) show that the oxygen isotope fractionation between HCO3− and H2O as a function of temperature is governed by the equation: 1000⁢ ln⁡αHCO3−−H2O=2.59±0.00⁢ (106T−2)+1.89±0.04 where α is the fractionation factor and T is in kelvin. The temperature dependence of oxygen isotope fractionation between CO32− and H2O is 1000⁢ lnαCO32−−H2O=2.39±0.04⁢ (106T−2)−2.70±0.46 The oxygen isotope fractionation between CO2(aq) and H2O was investigated by acid stripping CO2(aq) from low pH solutions; these data yield the following equation: 1000⁢ ln⁡αCO2(aq)−H2O=2.52±0.03⁢ (106T−2)+12.12±0.331000⁢ ln⁡αCO2(aq)−H2O=2.52±0.03⁢ (106T−2)+12.12±0.33These results show that pH can have a significant effect on the δ18O of the DIC, which can vary by as much as 17‰ at a given temperature.