Comparison of δ⁸¹Br and δ³⁷Cl composition of volatiles, salt precipitates, and associated water in terrestrial evaporative saline lake systems

This study provides the first characterization of the variability of bromine and chlorine stable isotopic composition in evaporites, associated residual brines, and shoreline gases in terrestrial evaporative saline lakes. The lakes investigated here are groundwater discharge locations, and include both potash-rich alkaline lakes and sodic-dominated neutral pH lakes at a variety of salinities and evaporative stages. The chlorine and bromine isotope systems behave consistently different during evaporative salt precipitation, with ³⁷Cl more enriched in the salt than in the fluid, but ⁸¹Br more enriched in the fluid compared with the precipitated salt. The ⁸¹Br concentration of shore off-gassing was even smaller than mineral precipitate compositions. The trends observed for bromine isotopes are surprising compared with published laboratory studies, indicating that a process besides inorganic mineral precipitation is affecting δ⁸¹Br. Additional processes explored include diffusion, salt deflation, microbial and photolytic conversion to the gas phase, and oxidative bromination of organic matter. Dedicated to Professor Peter Fritz on the occasion of his 80th birthday.