EXPERIMENTAL STUDIES OF STABLE OXYGEN ISOTOPE FRACTIONATION IN THE SYSTEM CO2-H2O AT 15, 25, AND 40°C

Recent studies have shown that the use of stable oxygen isotope data as a proxy for paleotemperature may be complicated by changes in seawater pH. The d¹⁸O of calcite (CaCO₃) may be affected by the relative proportions of CO₂(aq), HCO₃^-, and CO₃^2- in seawater, and pH controls the speciation of the CO₂-H₂O system components. Thus, it is necessary to resolve the individual oxygen isotope fractionation factors for each of these components as a function of temperature. Stable oxygen isotope fractionation and rates of exchange between CO₂(aq), HCO₃^-, CO₃^2-, and H₂O were investigated experimentally by adding BaCl₂ to 15 mM NaHCO₃ solutions at 15, 25, and 40^oC in a closed system. The quantitatively precipitated BaCO₃ instantaneously records the isotopic composition of the CO₂ species in solution. We used NBS-calibrated pH buffers and the PITZER model to calculate activity coefficients.  Either HCl or NaOH were added to some of the NaHCO₃ solutions to adjust the pH and the distribution of CO₂ species. Time-series experiments were run to assess the kinetics of oxygen isotope exchange in the CO₂-H₂O system. No correction was made for the difference in acid fractionation factors between CaCO₃ and BaCO₃, which is apparently small (Kim and O'Neil, 1997, GCA 61:3461-3475). Our results yield equilibration times between dissolved inorganic carbon species and water at 15°, 25°, and 40°C of approximately 48, 12, and 2 h respectively. Fractionation is greatest in CO₂ (aq), intermediate in HCO₃^-, and least in CO₃^2-, which agrees with trends found in previous studies (Usdowski et al., 1991, Z. Phys. Chem 170:237-249). A pH change from 8.35 to 6.84 at 25^oC can result in a 1.8^o/_oo difference in the isotopic composition of the precipitated BaCO₃. Preliminary results yield the following fractionation relationship for HCO₃^-:

 1000lna(HCO₃-H₂O)  =(3.02 ± .13) (10⁶/T²) – (2.92 ± 1.44).

Our 1000lna(HCO₃-H₂O) values are about 1^o/_oo higher than those of Halas and Wolacewicz (1982, J. Chem. Phys. 76:5470-5472).  Additional experiments are in progress to establish similar relationships for CO₂(aq.) and CO₃^2- as well as reduce the error associated with the HCO₃^- equation.