Volatile earliest Triassic sulfur cycle: A consequence of persistent low seawater sulfate concentrations and a high sulfur cycle turnover rate?

Abstract Marine biodiversity decreases and ecosystem destruction during the end-Permian mass extinction (EPME) have been linked to widespread marine euxinic conditions. Changes in the biogeochemical sulfur cycle, microbial sulfate reduction (MSR), and marine dissolved sulfate concentrations during the Permian-Triassic transition can provide insights into the role of ocean chemistry change in the largest mass extinction in Earth history. In this study, we constrain marine dissolved sulfate concentrations using the MSR-trend method of Algeo et al. [Algeo, T.J., Luo, G.M., Song, H.Y., Lyons, T.W., Canfield, D.E., 2015. Reconstruction of secular variation in seawater sulfate concentrations. Biogeosciences 12, 2131–2151] on sulfur (S) isotope records from Iran (the Kuh-e-Ali Bashi and Zal sections) and Hungary (the Balvany North and Balvany East sections). This empirically derived transfer function is based on the S isotope fractionation between sulfate and sulfide associated with MSR in natural aquatic environments. This fractionation is proxied by the difference in S isotope compositions between chromium-reducible sulfur (CRS) and carbonate-associated sulfate (CAS), i.e., Δ 34 S CAS-CRS . We show that, despite region-specific redox conditions, Δ 34 S CAS-CRS exhibits a nearly invariant value of 15–16‰ in both study sections. By comparing our record with a Δ 34 S sulfate-sulfide density distribution for modern marine sediments, we deduce that porewater Rayleigh distillation, carbonate diagenesis, and other effects are unlikely to have appreciably altered the S isotope offset between CRS and CAS in the study sections. In addition, differences in sedimentary regimes and organic carbon (OC) fluxes between the Iranian and Hungarian sections exclude major influence of the electron donor on MSR-S isotope fractionation and point to a more universal control, i.e., contemporaneous seawater sulfate concentration. The MSR-trend transfer function yielded estimates of seawater sulfate of 0.6–2.8 mM for the latest Permian to earliest Triassic, suggesting a balanced oceanic S-cycle with equal S inputs and outputs and no major changes in sulfate concentrations during this interval. However, a secular trend toward heavier δ 34 S CAS (by > 5‰) in the earliest Triassic can be explained only by increasing the turnover rate of the S-cycle (by ca. one order of magnitude) and a concomitant change in terrestrial S sources in a box model experiment. Exposure of evaporite deposits having a high δ 34 S may account for the source change, with a possible role for the Siberian Traps volcanism by magmatic remobilization of Cambrian rock salt. A high sulfur cycle turnover rate would have left the ocean system vulnerable to development of widespread euxinic conditions, posing a sustained threat to marine life during the Early Triassic.