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

Abstract

In light of recent studies that show oxygen isotope fractionation in carbonate minerals to be a function of HCO 3 − and CO 3 2− concentrations, the oxygen isotope fractionation and exchange between water and components of the carbonic acid system (HCO 3 −, CO 3 2−, and CO 2(aq)) were investigated at 15°, 25°, and 40°C. To investigate oxygen isotope exchange between HCO 3 −, CO 3 2−, and H 2O, NaHCO 3 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, BaCl 2 was added to the NaHCO 3 solutions. This resulted in immediate BaCO 3 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 HCO 3 − and H 2O as a function of temperature is governed by the equation: 1000 ⁢ ln ⁡ α HCO 3 − − H 2 O = 2.59 ± 0.00 ⁢ ( 10 6 T − 2 ) + 1.89 ± 0.04 where α is the fractionation factor and T is in kelvin. The temperature dependence of oxygen isotope fractionation between CO 3 2− and H 2O is 1000 ⁢ ln α CO 3 2 − − H 2 O = 2.39 ± 0.04 ⁢ ( 10 6 T − 2 ) − 2.70 ± 0.46 The oxygen isotope fractionation between CO 2(aq) and H 2O was investigated by acid stripping CO 2(aq) from low pH solutions; these data yield the following equation: 1000 ⁢ ln ⁡ α CO 2(aq) − H 2 O = 2.52 ± 0.03 ⁢ ( 10 6 T − 2 ) + 12.12 ± 0.33 These results show that pH can have a significant effect on the δ 18O of the DIC, which can vary by as much as 17‰ at a given temperature.