D/H equilibrium fractionation between H 2O and H 2 as a function of the salinity of aqueous solutions

Abstract

Hydrogen isotope fractionation between H 2O and H 2 has been investigated at 313 K for sodium chloride solutions with salinities (S) ranging from 0 to 265 g.L − 1 . In the presence of a Pt catalyst, time needed to reach hydrogen isotope equilibrium between H 2O and H 2 is close to 3 h (t 1/2 ≈ 30 min), independently on the salinity of the aqueous solution. Hydrogen isotope fractionation between H 2O and H 2 increases with increasing molality (m) for a NaCl-like (sea salt) solution according to the following linear function: α H 2O -H 2 (sea salt) = 3.387(± 8.4 × 10 − 5 ) + 3.4 × 10 − 3 (± 5 × 10 − 5 )m (R 2 = 0.997). The hydrogen isotope analysis of seawater samples with a salinity of 35 g.L − 1 requires minor corrections of − 2 ± 0.5‰ (V-SMOW) whilst δD values are overestimated by 5 ± 0.5‰ to 10 ± 0.5‰ in the case of highly saline natural waters (100 < S < 265 g.L − 1 ). In combination to previously published salinity-dependent fractionation factors between CO 2 and H 2O ( Lécuyer et al., 2009), corrections of both hydrogen and oxygen isotope ratios must be taken into account during the analysis by equilibration techniques of waters sampled from salt marshes, hypersaline lakes and lagoons, or hydrothermal brines.