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 delta O-18 has been well characterized, this has not been done for the emerging proxy O-17(excess) [O-17(excess) = 106[ln(10(-3)delta O-17 + 1) - 0.528 ln(10(-3)delta O-18 + 1)]. To this end, we measured for the first time delta O-17 and delta O-18 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 delta O-18 and delta O-17 are sensitive to salinity, O-17 excess is independent of salinity. Therefore, when analyzing lake water composition, the effect of salinity on delta O-18 and delta O-17 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. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

Closed-basin lakes show strong response to climate-related changes in water balance, making lake water isotopic composition a sensitive indicator of paleo-hydrological conditions. The effect of salinity on δO-18 has been well studied, but not for the emerging proxy O-17(excess). This study finds that while both δO-18 and δO-17 are sensitive to salinity, O-17(excess) remains unaffected.