The sulphur isotope and hydrochemical characteristics of Skeidarársandur, Iceland: Identification of solute sources and implications for weathering processes

The hydrochemistry of sulphur in the active outwash plain of Skeidarársandur, SE Iceland shows considerable variety reflecting differences in sources and processes, and is investigated in this article using δ³⁴S values of dissolved sulphate and igneous sulphide minerals, δ¹⁸O_-H2O, and ionic concentrations in waters. Seawater-derived sulphate is a notable component only in the lower sandur (δ³⁴S_-SO4 = +6.8‰ VCDT). δ³⁴S_-SO4 in the Skeidará glacial melt river range from +3.4 to +8.8‰ δ³⁴S. The higher values occur during a flood event and reflect geothermal sulphate contributions from the Grímsvötn caldera. 'Kettle-hole lakes' formed since the November 1996 jökulhlaup form important ecological niches and show evidence of coupled sulphide oxidation and carbonate and possibly silicate dissolution and have the lowest δ³⁴S_-SO4 values (average -0.8‰) attributed to higher proportions of sulphide oxidation-derived SO₄^2- and/or bacterially mediated sulphide oxidation. The hydrochemistry of eastern sandur groundwater is influenced by a hydrothermal source of reduced sulphur from the Grímsvötn caldera or a localized area of geothermal springs, and contrasts with the hydrochemistry of the majority of groundwaters and the Sulá and Gígjukvísl rivers in western and central Skeidarársandur. These have lower [Ca²⁺ + Mg²⁺], [Na⁺ + K⁺] and SO₄ ^2- concentrations and δ³⁴S values consistent with sulphate derived from the oxidation of igneous sulphides. In comparison to catchments dominated by atmospherically transported sulphate, catchments such as this where the sulphur system is dominated by internal sources (rock-derived and geothermal) will not be sensitive to the temporal decreasing trends in contributions from long-range airborne sulphate resulting from reductions of sulphur emissions in North America and Europe.