Sulfur isotopes in anhydrites from the Upper Devonian Prypiac’ and Dnipro-Donets Basins (Belarus and Ukraine)

Abstract

Upper Devonian evaporite rocks worldwide record a marked sulfur isotope excursion to heavier δ34S values. New δ34S data from anhydrite laminae in halite, anhydrite in water-insoluble residue in halite and sulfate from massive sulfate units in evaporite sequences of the Prypiac’ and Dnipro-Donets Basins of Belarus and Ukraine are presented. The mean δ34Ssulfute is 27.6‰±3.0‰ (n=60) with a range from 21.9‰ to 35.6‰ The mean values for Frasnian and Famennian evaporites are similar: 27.5‰±3.8‰ (n=30) and 27.7‰±1.9‰ (n=30), respectively. The rocks formed in a restricted basin but its parent waters had a marine source. Bacterial sulfate reduction was the dominant process controlling δ34S of Upper Devonian anhydrite and pyrite although also disproportionation processes and secular variation played an important role. True secular isotope variation is indicated between stratigraphic units, and a range of c. 24–29‰ is assumed to be representative for oceanic sulfate of this time. New data from the Lower Eifelian suggest that the value of c. 15‰ was representative for oceanic sulfate of Eifelian time, and this indicates the increase of δ34S values of oceanic sulfate by 12‰ during ca. 11 Ma. Subsequently, δ34S values show high plateau of 27‰ during ca. 12 Ma (Early Frasnian to Late Famennian) with some oscillation reflected in the mean values for particular evaporite units.