δ81BR AND δ37CL COMPOSITION OF SALT PRECIPITATES AND ASSOCIATED WATER IN SODIC-NEUTRAL PH AND POTASH-ALKALINE SALINE LAKE SYSTEMS

As relatively conservative ions, chemical ratios using chloride and bromide have long been used to identify the origin and evolution of various waters and brines. Despite widespread acceptance for such methods, chloride and bromide concentrations are affected by, and change as a result of, a variety of processes. While the stable isotopic compositions of bromine (⁸¹Br/⁷⁹Br) and chlorine (³⁷Cl/³⁵Cl) provide one avenue to better understand these processes, relatively little is known about the behavior of bromine isotopes in natural systems. The goal of this study is to characterize the chlorine and bromine isotopic compositions of evaporites and source waters in two different terrestrial evaporative settings.

Salt and evaporative lake water samples were collected from several potash-alkaline lakes in western Nebraska in August of 2012, 2013, and 2014 and sodic-neutral pH saline lakes in central Kansas in November of 2014. Stable chlorine and bromine isotopic compositions were determined at the Environmental Isotope Laboratory at the University of Waterloo, on a Micromass Isoprime Continuous Flow Isotope Ratio Mass Spectrometer (CF-IRMS) and a Thermo-Scientific MAT 253 CF-IRMS, respectively, with internal precisions of ±0.03‰. Chlorine and bromine isotopic data are reported relative to standard mean ocean chloride and bromide, respectively (SMOC, SMOB).

The isotopic behavior between water samples and salt samples was similar for both sodic and potash lakes. From the lake water to the salt samples collected at the same time, values for δ³⁷Cl increased anywhere from 0.04 to 0.70 ‰, while δ⁸¹Br values decreased between 0.4 to 1.2 ‰. The opposite behavior for chlorine and bromine isotopic systems is surprising, particularly as the systems studied exhibited this same behavior across a range of evaporative stages and cation/anion dominant fluids. It is suggested a process beyond simple evaporation is affecting the bromine system after comparing results with recently published empirically-derived fractionation factors for chlorine and bromine isotopes during evaporation (Eggenkamp et al., 2016). However, additional study is needed to identify this process.

Reference: Eggenkamp et al., 2016, Chem. Geol. 433: 46-56.