Influence of dissolved ions on determination of oxygen isotope composition of aqueous solutions using the CO₂-H₂O equilibration method

RATIONALE Stable isotope compositions of natural waters, such as seawater, glaciers and basinal brines, can provide valuable information about Earth's hydrological cycle and its evolutionary history. However, a high concentration of dissolved ions in some natural waters hinders an accurate analysis of their oxygen isotope composition. A laboratory study was carried out in order to provide guidelines on how to resolve this analytical difficulty. METHODS CO ₂ gas was equilibrated with saline aqueous solutions of various chemical compositions at 25 °C. Subsequently, the oxygen isotope composition of the CO₂ was determined at different equilibration times using a dual-inlet isotope ratio mass spectrometer in order to evaluate the oxygen isotope salt effect and the rate of oxygen isotope exchange between CO ₂ and the saline solution. RESULTS Using the experimentally determined oxygen isotope salt effects of aqueous chloride and sulfate solutions, an empirical method for the prediction of the oxygen isotope salt effect of a 1.0 molal chloride or sulfate solution was proposed. The rates of oxygen isotope exchange between CO₂ and saline solutions were also examined. Our experimental data indicates that the sequence of the oxygen isotope exchange time is as: MgSO₄ > CaCl₂ ≈ Na ₂SO₄ > NaCl > MgCl₂ > KCl > H ₂O. CONCLUSIONS The isotope salt effect and the kinetics of isotope exchange must be taken into account when the oxygen isotope composition of a saline aqueous solution is determined using the CO₂-H₂O equilibration method. Our experimental data and the proposed prediction method provide essential guidelines for the accurate δ¹⁸O analysis of saline aqueous solutions.