Combined sulfur K-edge XANES spectroscopy and stable isotope analyses of fulvic acids and groundwater sulfate identify sulfur cycling in a karstic catchment area

Chemical and isotope analyses on groundwater sulfate, atmospheric deposition sulfate and fulvic acids (FAs) associated sulfur were used to determine the S cycling in a karstic catchment area of the Franconian Alb, Southern Germany. Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy provided information on the oxidation state and the mechanism of the incorporation of sulfur in FAs. During base flow δ34S values of groundwater sulfate were slightly depleted to those of recent atmospheric sulfate deposition with mean amount-weighted δ34S values of around + 3‰. The δ18O values of groundwater sulfate shifted to lower values compared to those of atmospheric deposition and indicated steadiness from base flow to peak flow. The reduced sulfur species (S− 1/thiol; S0/thiophene, disulfide, S+ 2/sulfoxide) of soil FAs averaged around 49% of the total sulfur and δ34S value in FAs was found to be 0.5‰. The formation of polysulfides and thiols in FAs in concert with a decreasing isotope value of δ34S in FAs with respect to those of atmospheric deposition sulfate suggests oxidation of H2S, enriched in the 32S isotope, with organic material. The depletion of δ18O–SO42− by several per mil in groundwater sulfate with respect to those of atmospheric deposition is, therefore, consistent with the hypothesis that SO42− has been cycled through the organic S pool as well as that groundwater sulfate is formed by oxidation of H2S with organic matter in the mineral soil of the catchment area.