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

Abstract

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 17Oexcess [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, 17Oexcess is independent of salinity. Therefore, when analyzing lake water composition, the effect of salinity on δ18O and δ17O has to be accounted for, whereas 17Oexcess 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.