Morgan Reed Raven, David A Fike, Alexander S. Bradley, Maya L Gomes, Jeremy Owens, Jeremy D Owens, Samuel L Webb

Sulfur (S) isotope compositions of pyrites in the sedimentary record have played an important part in our understanding of biogeochemical cycling in the geologic record. However, the kinetics of pyritization are complex and depend strongly on the reactivity and mineralogy of available iron. As a second major sink for sulfide in anoxic sediments, organic matter (OM) provides essential context for reconstructing the distribution and isotopic composition of environmental sulfide. To first order, roughly parallel pyrite and OM δ34S profiles reflect changes in sulfide, while independent patterns require alternative explanations, including changes in iron availability or OM characteristics. We apply this framework to Ocean Anoxic Event 2 (OAE-2, 94 Mya), a period of enhanced reduced C and S burial (in OM and pyrite) that was associated with expanded marine anoxia. We present paired S-isotope records for pyrite and OM along with profiles of OM S:C ratio and S redox speciation from four well-characterized lithologic sections (Pont d’Issole, Cismon, Tarfaya Basin, and Demerara Rise) to reconstruct both local redox structure and global mechanisms impacting the C, S and Fe cycles around OAE-2.

OM sulfurization appears to be a major control on OM preservation at all four sites. Similar to modern anoxic environments, there is a positive correlation between OM S:C ratios and OM concentrations for sites with more reducing conditions, implying a link between OM sulfurization and burial. At consistently anoxic sites like Tarfaya Basin and Demerara Rise, strongly sulfurized OM with a consistent S redox speciation and S-isotope composition most likely formed rapidly in sinking particles before, during, and after OAE-2. Particle-hosted OM sulfurization may therefore have been a central mechanism facilitating the massive burial of OM in anoxic environments during this and other periods of enhanced global carbon burial. At the same time, a nearly 25‰ negative shift in the δ34S values of pyrite – but not OM – occurs at multiple, globally distributed sites near the onset of OAE-2, indicating slower pyritization reactions that likely reflect changes in iron delivery due to expanding regional or global anoxia. The combination of pyrite and organic S isotopes thus provides novel constraints on the interwoven cycles of carbon, iron, and sulfur across a major carbon cycle perturbation.