D/H equilibrium fractionation between H2O and H2 as a function of the salinity of aqueous solutions

Hydrogen isotope fractionation between H2O and H2 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 H2O and H2 is close to 3 h (t1/2 ≈ 30 min), independently on the salinity of the aqueous solution. Hydrogen isotope fractionation between H2O and H2 increases with increasing molality (m) for a NaCl-like (sea salt) solution according to the following linear function: αH2O-H2(sea salt) = 3.387(± 8.4 × 10− 5) + 3.4 × 10− 3(± 5 × 10− 5)m (R2 = 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 CO2 and H2O (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.

► Hydrogen isotope fractionation between H2O and H2 increases with salinity.
► A minimal time of 3 h is needed to reach isotopic equilibrium at 313 K.
► Corrections must be applied to δD values of waters with S ≥ 35 g.L− 1.
► Isotopic offsets reach 10‰ for hypersaline solutions.