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 degrees, 25 degrees, and 40 degrees 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 degrees, 25 degrees, and 40 degrees C (1 atm) show that the oxygen isotope fractionation between HCO3- and H2O as a function of temperature is governed by the equation: 1000 1n alpha(HCO3-H2O) = 2.59 +/- 0.00 (10(6)T(-2)) + 1.89 +/- 0.04 where alpha is the fractionation factor and T is in kelvin. The temperature dependence of oxygen isotope fractionation between CO32- and H2O is 1000 1n alpha(CO32-H2O) = 2.39 +/- 0.04 (10(6)T(-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 1n alpha(CO2(aq)-H2O) = 2.52 +/- 0.03 (10(6)T(-2)) + 12.12 +/- 0.33 These results show that pH can have a significant effect on the delta O-18 of the DIC, which can vary by as much as 17 parts per thousand at a given temperature. Copyright (c) 2005 Elsevier Ltd.