SEDIMENTARY GEOCHEMICAL INSIGHTS INTO MIDDLE–LATE ORDOVICIAN SEAS OF LAURENTIA: IMPLICATIONS FOR MARINE REDOX STATE, WEATHERING, AND BIODIVERSIFICATION (Invited Presentation)

Major phases of the Great Ordovician Biodiversification Event (GOBE) occurred during the Middle Ordovician (Dapingian–Darriwilian), and roughly coincided with sea surface temperatures that cooled into modern equatorial ranges. The unprecedented change in biodiversity of marine life has also been previously linked to an L-chondrite asteroid breakup event and enhanced nutrient supply via weathering of uplifted Taconic highlands. Ocean oxygenation events, associated with major perturbations of the carbon and sulfur cycles, have also been linked to large diversification events of marine life, from the Ediacaran to the late Cambrian in which pulses of oxygen progressively ventilated marine environments. Previous sulfur and carbon isotope studies of Early to Middle Ordovician sequences in Newfoundland and Argentina have highlighted evidence for persistent widespread euxinic (anoxic, sulfidic) deep marine waters that were intermittently oxidized, and recent sulfur and carbon studies Darriwilian–Sandbian carbonates from Nevada, Oklahoma, and the Appalachian Basin have documented antithetical isotope trends that suggest dynamic changes in the marine redox state during this time. Here we present carbonate-associated sulfate (δ³⁴S_CAS) paired with carbonate δ¹³C_carb and δ¹³C_org from an expanded Middle to Upper Ordovician sequence from the southern Appalachian Basin. Data from an expanded sequence at Evans Ferry, TN show several intervals of decoupled trends in δ³⁴S_CAS from δ¹³C_carb and δ¹³C_org values that alternate with intervals of seemingly co-varied trends. Intervals of antithetical isotope records continue throughout the Sandbian. And are best explained by pulses of oceanic ventilation forcing the chemocline into deeper waters and below sediment-water interface thus expanding habitable environments for marine organisms. Thus far only stable isotopic data sets from Laurentia have been generated to assess paleoredox conditions through this interval of the Ordovician but future work is needed from other paleocontinents using both local and global redox proxies within shale- and other carbonate-dominated sequences. A multiproxy approach is necessary to establish a complete framework for understanding the complex evolution of marine redox and its impact on biodiversity in the Ordovician.