SULFUR ISOTOPE DYNAMICS IN A HIGH-ELEVATION CATCHMENT, WEST GLACIER LAKE, WYOMING

Stable isotopes of S are used in conjunction with dissolved SO42- concentrations to evaluate the utility of delta(34)S ratios in tracing contributions of bedrock-derived S to SO42- in runoff. Water samples were collected over the annual hydrograph from two tributaries in the West Glacier Lake, Wyoming, catchment. Concentrations of SO42- ranged from 12.6 to 43.0 mu eq L(-1); delta(34)S ratios ranged from -1.8 parts per thousand to +4.9 parts per thousand. The delta(34)S value of atmospherically derived SO42- is about +5.6 parts per thousand; four samples of pyrite from the bedrock had delta(34)S ratios that ranged from +0.7 to +4.1 parts per thousand. Concentrations of SO42- were inversely related to delta(34)S and discharge. The data for the tributary with the higher SO42- concentrations were reasonably consistent with mixing between atmospheric S and S from a bedrock source with a delta(34)S ratio of about -4.5 parts per thousand, The difference from the measured bedrock values presumably indicates that S isotopes in the bedrock pyrite are heterogeneously distributed. The data from the tributary with lower SO42- concentrations did not follow a two-component mixing line. Deviation from a two-component mixing line is most likely caused by preferential elution of SO42- from the snowpack during the early stages of snowmelt, although microbially mediated fractionation of S isotopes in the soil zone also may cause the deviation from the mixing line. Sulfur isotopes are useful in identifying whether or not there is a substantial contribution of bedrock S to runoff, but quantifying that contribution is problematic.