Influence of upwelling Zechstein sulphate on the concentration and isotope signature of sedimentary sulphides in a fluvioglacial sand aquifer

Very low concentrations of total S, mainly sedimentary sulphides, were quantitatively extracted from Quaternary sands of the Elbe Basin, using HNO₃, Br₂ and HCl, to distinguish 3 aquifer zones:

• •an upper aerobic section, containing low concentrations (only a few ppm) of non-sulphidic S compounds,
• •the central and lower part of the aquifer, dominated by ³⁴S-depleted sedimentary Fe sulphides, formed by reduction of infiltrating SO₄, derived from groundwater recharge, and
• •the lowest 5–10 m of the aquifer, containing high concentrations of ³⁴S-enriched sulphides.

The latter originated from dissolved Zechstein SO₄, which was reduced during upwelling through the organic-rich Tertiary aquiclude. H₂S and HS⁻ reacted and precipitated with Fe and other metal ions shortly after migration into the C_org-poor Quaternary aquifer. The sulphides yield valuable information concerning the ascent of confined saline solutions from isolated Zechstein evaporites inside the “Mühlberger Graben”, which is covered by Cenozoic sediments and whose extension and boundaries are therefore not well defined. Only a few locations, close to faults and geological windows, show deep-water admixture sufficiently strong to cause visible changes in hydrochemistry and isotopic ratios of SO₄ and DIC directly above the base of the Quaternary. Sulphides showing different origins may possibly be used in other areas to provide information concerning underlying geology and hydrodynamics.