The effect of salinity on water ¹⁷O/¹⁶O ratios in brines

Closed-basin lakes respond strongly to climate related changes in their water balance. Lake water isotopic composition can thus serve as a sensitive indicator of paleo-hydrological conditions. Such lakes are often highly saline, so that the effect of salinity on the isotopic composition of lake water has to be accounted for in analyzing lake water evaporation and lake sediment isotopic records. Whereas the effect of salinity on δ¹⁸O has been well characterized, this has not been done for the emerging proxy ¹⁷O_excess [Oexcess17=10⁶[ln⁡(10⁻³δ¹⁷O+1)−0.528ln⁡(10⁻³δ¹⁸O+1)]. To this end, we measured for the first time δ¹⁷O and δ¹⁸O of water in NaCl, CaCl₂, and MgCl₂ solutions of a wide range of concentrations (1 to 4 molal), using CO₂-H₂O isotope exchange at 25 °C. The results show that although both δ¹⁸O and δ¹⁷O are sensitive to salinity, ¹⁷O_excess is independent of salinity. Therefore, when analyzing lake water composition, the effect of salinity on δ¹⁸O and δ¹⁷O has to be accounted for, whereas ¹⁷O_excess remains unchanged. These findings have important implications when studying evaporation in hypersaline lakes and for reconstructions of past water balance in saline closed-basin lakes.