Sulphur, oxygen and strontium isotope compositions of Middle Miocene (Badenian) calcium sulphates from the Carpathian Foredeep, Poland: palaeoenvironmental implications

Sulphur, oxygen and strontium isotope compositions have been measured in sulphate (gypsum and anhydrite) samples from the Badenian evaporite complex in the Carpathian Foredeep Basin (Poland) to determine the origin of brines from which these sulphates were formed. Studied samples display the delta values from +22.68 to +24.91 parts per thousand CDT for sulphur(ten samples) and from +12.26 to +13.63 parts per thousand SMOW for oxygen (ten samples), and Sr-87/Sr-86 ratios from 0.708915 to 0.716329 (six samples). Most samples show isotopic values (both delta S-34 and Sr-87/Sr-86) higher than contemporaneous (Badenian) seawater, and thus suggest that these sulphates were formed (i) from brines with a significant component of non-marine waters, (ii) in a restricted system, where an important role was played by meteoric water inputs as well as by bacterial sulphate reduction. The results of this study show clear differences in the isotopic signatures between sedimentary (gypsum) and diagenetic (anhydrite) lithofacies. While sedimentary gypsum displays sulphur, oxygen and strontium isotope ratios close to contemporaneous (Badenian) seawater, diagenetic anhydrite values are largely elevated. The higher isotopic values (delta S-34, delta O-18 and Sr-87/Sr-86) for anhydrite when compared to gypsum are interpreted as reflecting different hydrological provenances of sulphate in the Badenian basin. Gypsum was formed from brines marine in origin that were subject to an important inflow of continental waters and a local bacterial sulphate reduction in a restricted, and therefore sulphate-limited basin, which is consistent with earlier interpretations based on sedimentological and geochemical studies. Our study shows that brines from which anhydrite was formed had a highly-radiogenic non-marine (riverine, groundwater) strontium component, which is a new contribution to the knowledge on the Badenian sulphate formation. The results reflect a lateral compositional evolution of parent waters during sulphate deposition and diagenesis in the Carpathian Foredeep Basin.