Sulfur isotope dynamics in a high-elevation catchment, West Glacier lake, Wyoming

Abstract

Stable isotopes of S are used in conjunction with dissolved SO 4 2- concentrations to evaluate the utility of δ 34S ratios in tracing contributions of bedrock-derived S to SO 4 2- in runoff. Water samples were collected over the annual hydrograph from two tributaries in the West Glacier Lake, Wyoming, catchment. Concentrations of SO 4 2- ranged from 12.6 to 43.0 μeq L-1; δ 34S ratios ranged from -1.8‰ to +4.9‰. The δ 34S value of atmospherically derived SO 4 2- is about +5.6‰; four samples of pyrite from the bedrock had δ 34S ratios that ranged from +0.7 to +4.1‰. Concentrations of SO4 2- were inversely related to δ 34S and discharge. The data for the tributary with the higher SO 4 2- concentrations were reasonably consistent with mixing between atmospheric S and S from a bedrock source with a δ 34S ratio of about -4.5‰. 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 SO 4 2- concentrations did not follow a two-component mixing line. Deviation from a two-component mixing line is most likely caused by preferential elution of SO 4 2- 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.