Multiple sulphur and oxygen isotopes reveal microbial sulphur cycling in spring waters in the Lower Engadin, Switzerland

Abstract

Highly mineralized springs in the Scuol-Tarasp area of the Lower Engadin and in the Albula Valley near Alvaneu, Switzerland, display distinct differences with respect to the source and fate of their dissolved sulphur species. High sulphate concentrations and positive sulphur (δ³⁴S) and oxygen (δ¹⁸O) isotopic compositions argue for the subsurface dissolution of Mesozoic evaporitic sulphate. In contrast, low sulphate concentrations and less positive or even negative δ³⁴S and δ¹⁸O values indicate a substantial contribution of sulphate sulphur from the oxidation of sulphides in the crystalline basement rocks or the Jurassic sedimentary cover rocks. Furthermore, multiple sulphur (δ³⁴S, Δ³³S) isotopes support the identification of microbial sulphate reduction and sulphide oxidation in the subsurface, the latter is also evident through the presence of thick aggregates of sulphide-oxidizing Thiothrix bacteria.

Keywords:

• hydrogen-2
• isotope geochemistry
• microbial sulphur cycling
• oxygen-18
• spring water
• sulphate reduction
• sulphur-33
• sulphur-34
• sulphur-36

Acknowledgements

Samples for this study were collected during student field courses jointly held between the universities of Münster, Tübingen, Zürich, Basel, and Vienna in 2009, 2011, and 2012. Stimulating discussions with R. Schönberg, M. Obst, M. Lehmann, S. Krämer, and T. Eglinton are gratefully acknowledged. We thank the students from these universities for their help during sampling and initial measurements in the field and A. Reschka for ICP-OES and IC measurements in Münster. Maps in figures 1 and 2 are reproduced with permission of swisstopo (BA15039). The Swiss Bundesamt für Umwelt BAFU and the Nationale Grundwasserbeobachtung NAQUA are gratefully acknowledged for providing isotope data of river water and precipitation.

Disclosure statement

No potential conflict of interest was reported by the authors.