δ37Cl and δ81Br fractionation during evaporation in the Alkaline Lakes Region, Sand Hills, Nebraska
Abstract
Recent improvements in the precision, accuracy, and detection limits of stable bromine isotope analytical techniques have enabled investigation of the natural distribution of stable bromine isotopes (81Br/79Br). However, the extent to which fractionation processes influence these isotopes are not well understood. In contrast, isotopic fractionation for the stable isotopes of another halide, chlorine (37Cl/35Cl), have been studied in greater detail. It is understood that the heavier isotope 37Cl preferentially incorporates into the precipitate during evaporation. However, unlike chloride, bromide is generally not incorporated in precipitates until the final stages of evaporite precipitation, thus bromine isotopes should not significantly fractionate until the late evaporative stages. The objective of this investigation is to observe and identify the effects of evaporation and evaporite mineral precipitation on dissolved stable bromine isotopes in a field setting. This is accomplished by comparing bromine isotopic results with chlorine isotopic results from samples collected throughout an annual evaporation cycle from several lakes located in the Alkaline Lakes Region of the western Nebraska Sand Hills. Lakes in the study area are compositionally diverse, with dissolved solids ranging from 0.2 g L-1 to 100 g L-1. Seasonally, groundwater and direct precipitation inputs exceed evaporation in the fall, winter, and spring, but the situation reverses in summer. Samples for this study were originally collected between August 1992 and October 1993, and were selected for chlorine and bromine isotope analysis from five lakes within the archived samples using the following criteria: (1) multiple seasonal samples from a single lake, (2) sufficient sample volume, and chloride and bromide mass, (3) lack of observable precipitate within the sample, and (4) no acid added for sample preservation. Stable chlorine and bromine isotopic ratios were analyzed at the University of Waterloo Environmental Isotope Laboratory by Isotope Ratio Mass Spectrometry (IRMS). δ37Cl values were between -1.06 % and 0.14 %, relative to SMOC, and δ81Br values were between -0.76 % and 1.39 %, relative to SMOB. TDS values in the same samples varied between 18.60 g L-1 to 384.75 g L-1. In a single lake, δ37Cl and δ81Br values varied by as much as 0.97 % and 0.80 % respectively, and as little as 0.36 % and 0.03 %, respectively. Generally over the course of a single summer, lake waters became depleted in 81Br and 37Cl. As the precipitates observed were dominated by bicarbonate and sulfate minerals, evaporite precipitation should only contribute a small amount to the observed isotopic fractionation. Thus, another process related to low-precipitation/high-evaporation must result in the substantial observed variations in δ81Br and δ37Cl values. Planned investigations at these lakes should provide additional insights into the processes fractionating halide isotopes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMEP41B0621S
- Keywords:
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- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 1818 HYDROLOGY / Evapotranspiration;
- 1830 HYDROLOGY / Groundwater/surface water interaction;
- 1845 HYDROLOGY / Limnology