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

Abstract

The sulfur cycle is driven by redox processes, among which sulfate reduction is of primary importance. Sulfate is reduced to sulfide either abiotically by Thermochemical Sulfate Reduction (TSR) or biotically by Microbial Sulfate Reduction (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). The 34S/32S ratio is often used as the sole criterion to identify the origin of reduced sulfur compounds, but overlaps prevent unambiguous conclusions. Contrary to MSR, the multiple sulfur isotopic signatures (δ33S, δ34S, δ36S) of natural TSR remains uncharacterized. Here, we performed multiple sulfur isotopes analyses of sulfates, sulfides, and elemental sulfur from six sites in the Alpine Triassic evaporites formation to better constrain the isotopic signatures of TSR. Unlike MSR, TSR can induce slight negative deviations (Δ33S down to −0.08‰) relative to the initial sulfate Δ33S value, which significantly discriminates between these two processes. Isotopic equilibria between anhydrite and either elemental sulfur or sulfides (pyrite or chalcopyrite) were verified according to their mass-fractionation exponents (θ33/34=0.5140 and 0.5170, respectively). Using sulfate-elemental sulfur (Δ34SSO42−-S8) or sulfate-sulfide (Δ34SSO42−-S2−) fractionation pairs and respective fractionation factors (α34) for samples that fulfilled the criteria of isotopic equilibrium, we determined the precipitation temperatures of elemental sulfur and sulfides (pyrite or chalcopyrite) to be 194 ± 14°C and 293–488°C, respectively. Interestingly, the obtained temperature of elemental sulfur precipitation corresponds exactly to the solid-liquid phase transition of native sulfur. Using Δ33S vs. δ34S and Δ33S vs. Δ36S diagrams, we are able to fully explain the isotopic signatures of disequilibrium sulfides by the mixing of sulfate with either elemental or organic sulfur in the aqueous fluid. Mixing curves allow the determination of the relative proportions of sulfate and organic and elemental sulfur, the latter being formed by the recombination of polysulfides during cooling. It appears that the sulfides' signatures are best explained by a 33% contribution of polysulfides (i.e., elemental sulfur signatures), consistent with the relative proportion of dissolved polysulfides previously measured in fluid inclusions from this formation at >200°C. Finally, no sulfur mass independent fractionation (S-MIF) is observed in this evaporitic formation, consistent with the TSR signature generated both at equilibrium and by mixing. This implies that TSR does not generate S-MIFs. Our results thus provide multiple sulfur isotopes signatures of TSR, which may be used to reliably identify this process in variable geological settings.