Sulphur isotope composition of dissolved sulphate in the Cambrian-Vendian aquifer system in the northern part of the Baltic Artesian Basin

The groundwater in the Cambrian-Vendian aquifer system with its delta O-18(water) values of about -22 parts per thousand (VSMOW) and a low radiocarbon concentration is of glacial origin from the Last Ice Age. Earlier surveys have highlighted a negative co-variance of sulphate and bicarbonate content in the groundwater of the Cambrian-Vendian aquifer system, whereas the most depleted dissolved inorganic carbon delta C-13 values have been measured mainly in groundwater samples with the lowest sulphate concentrations. In this paper we studied the origin of sulphate and the factors controlling the sulphur and carbon isotope geochemistry in the aquifer system. Direct sources of sulphate were not found, but relying upon delta O-18(SO4) measurements we suggest that the sulphate originates from oxidation of sulphide minerals whereas the delta S-34 of the dissolved SO42- in the groundwater is more enriched than the delta S-34 of the surrounding rocks. We show that bacterial activity may have caused the enrichment of delta S-34 of sulphate. (C) 2014 Elsevier B.V. All rights reserved.