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

Stable isotopes of S are used in conjunction with dissolved SO ₄ ^2? concentrations to evaluate the utility ofδ 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‰ Theδ ³⁴S value of atmospherically derived SO ₄ ^2? is about +5.6%c.; four samples of pyrite from the bedrock hadδ ³⁴S ratios that ranged from +0.7 to +4.1‰ Concentrations of SO ₄ ^2? were inversely related toδ 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 aδ 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.