Multiple sulfur isotopes signature of Thermochemical Sulfate Reduction (TSR): Insights from Alpine Triassic evaporites

The sulfur cycle is driven by redox processes, among which sulfate reduction of primary importance. Sulfate reduced to sulfide either abiotically Thermochemical Reduction (TSR) or biotically Microbial (MSR). Although these two processes occur at different temperature regimes (>100 °C and <80 °C, respectively), they generate similar by-products (e.g., sulfides, elemental sulfur). 34S/32S ratio often used as the sole criterion identify origin compounds, but overlaps prevent unambiguous conclusions. Contrary MSR, multiple isotopic signatures (?33S, ?34S, ?36S) natural TSR remains uncharacterized. Here, we performed isotopes analyses sulfates, from six sites in Alpine Triassic evaporites formation better constrain TSR. Unlike can induce slight negative deviations (?33S down ?0.08‰) relative initial ?33S value, significantly discriminates between processes. Isotopic equilibria anhydrite sulfides (pyrite chalcopyrite) were verified according their mass-fractionation exponents (?33/34=0.5140 0.5170, respectively). Using sulfate-elemental (?34SSO42?-S8) sulfate-sulfide (?34SSO42?-S2?) fractionation pairs respective factors (?34) for samples that fulfilled criteria equilibrium, determined precipitation temperatures be 194 ± 14 293–488 respectively. Interestingly, obtained corresponds exactly solid-liquid phase transition native sulfur. vs. ?34S ?36S diagrams, are able fully explain disequilibrium mixing with organic aqueous fluid. Mixing curves allow determination proportions sulfur, latter being formed recombination polysulfides during cooling. It appears sulfides' best explained a 33% contribution (i.e., signatures), consistent proportion dissolved previously measured fluid inclusions this >200 °C. Finally, no mass independent (S-MIF) observed evaporitic formation, signature generated both equilibrium mixing. This implies does not S-MIFs. Our results thus provide TSR, may reliably process variable geological settings.