Oxygen isotope fractionation and equilibration kinetics between CO 2 and H 2O as a function of salinity of aqueous solutions

Abstract

Oxygen isotope fractionation and equilibration kinetics between CO 2 and H 2O have been investigated at 313 K for salinities ( S) ranging from 0 to 250 g L − 1 . In this range of salinity, times needed to reach oxygen isotope equilibrium between CO 2 and H 2O increase from 4 h to 12 h. Isotopic exchanges are comparable for KCl and NaCl-like (sea salt) solutions and are described by first-order kinetic reactions with ln( k) = − 8.1485(± 0.0057) − 0.00474(± 3.87 × 10 − 5 ) S. The oxygen isotope fractionation factor between CO 2 and H 2O increases with salinity for both sea salt and KCl solutions with concentrations ranging from 0 to 250 g L − 1 according to the following equation: 1000 ln( α CO2–H2O)sea salt = 37.02(± 5 × 10 − 3 ) + 3.96 × 10 − 3 (± 1.1 × 10 − 4 ) S − 6.38 × 10 − 6 (± 4.5 × 10 − 7 ) S 2 ( R 2 = 0.998). The oxygen isotope analysis of seawater samples with a salinity of 35 g L − 1 requires minor corrections of − 0.15‰ (V–SMOW). However, oxygen isotope ratios are overestimated by 0.4‰ to 0.6‰ in the case of highly saline natural waters (100 < S < 250 g L − 1 ). Corrections of the oxygen isotope ratios due to changes in the salinity-dependent fractionation factors between CO 2 and H 2O must be taken into account during the study of waters sampled from salt marshes, hypersaline lakes and lagoons, or hydrothermal brines.