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-SO₄^2- by several per mil in groundwater sulfate with respect to those of atmospheric deposition is, therefore, consistent with the hypothesis that SO₄^2- 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.