Expansion of Reducing Marine Environments During the Ireviken Biogeochemical Event: Evidence From the Altajme Core, Gotland, Sweden

New δ34Spy (pyrite) and δ34SCAS (carbonate‐associated sulfate) across the Llandovery‐Wenlock boundary (∼432 Ma) provide evidence for the expansion of reduced marine environments during the Ireviken Biogeochemical Event. This event consists of a major positive carbon isotope excursion, increased biotic turnover, and other major perturbations and changes within biogeochemical cycles. This interval of time has been hypothesized to coincide with an expansion of reducing marine environments that caused increased organic carbon burial and led to the Ireviken positive carbon isotope excursion (ICIE). Previous high‐resolution carbon isotope work in the Altajme core from Gotland, Sweden provides the highest resolution record of the ICIE yet documented and provides an ideal expanded stratigraphic section to study this event. Local expansion of reduced marine environments within the deeper shelf setting of the Altajme core is indicated by a positive shift in δ34Spy values and increase in pyrite sulfur concentrations at the onset of the ICIE. These data are indicative of increased microbial sulfate reduction within this portion of the Baltic Basin. Combined with new δ34SCAS data from this core, as well as additional data from distant basins, the new data presented here suggest a global expansion of reduced environments led to an increase in organic carbon burial and the ICIE.