Experimental studies of oxygen isotope fractionation in the carbonic acid system at 15, 25, and 40C
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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 40C. 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 40C (1 atm) show that the oxygen isotope fractionation between HCO3- and H2O as a function of temperature is governed by the equation: 1000 lnHCO3--H2O = 2.59 0.00 (106T-2) + 1.89 0.04 where is the fractionation factor and T is in kelvin. The temperature dependence of oxygen isotope fractionation between CO32- and H2O is 1000 lnCO32--H2O=2.39 0.04 (102T-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 lnCO2(aq)-H2O = 2.52 0.03 (106T-2 + 12.12 0.33 These results show that pH can have a significant effect on the 18O of the DIC, which can vary by as much as 17 at a given temperature. Copyright 2005 Elsevier Ltd.
