SULFUR ISOTOPE EVIDENCE FOR LATE ORDOVICIAN OCEAN OXYGENATION: IMPLICATIONS FOR THE DRIVERS OF THE HIRNANTIAN EXTINCTION
To further constrain the changes to the global sulfur cycle over the late Ordovician, we present paired carbon and sulfur isotope data from both carbonate-associated sulfate (δ34SCAS) and pyrite (δ34Spyrite) from successions from the western Laurentia and Baltica. δ34SCAS data from both successions show little variation during the Hirnantian and are consistent with other recently published Hirnantian δ34SCAS data from other locations, confirming that this trend is global. The lack of a positive sulfate-S isotope excursion parallel to the HICE suggests a driver other than organic carbon burial for the carbon isotope excursion and excludes the expansion of euxinia as a driver of the extinction events. Additionally, our section from Laurentia, which contains a substantial portion of the preceding Katian Stage, contains a large, negative δ34SCAS shift of 15‰ that continues into the earliest Hirnantian. Geochemical box modeling for this drop points to an increase in the weathering flux of sulfur to the oceans, a decrease in the global rate of pyrite burial, or a combination of both. The former would require a lowering of sea level and exposure and erosion of the shelf, evidence for which is lacking. The latter driver, which is our preference, would have likely been induced by ocean oxygenation—an expected product of global cooling that increased dissolved oxygen solubility and enhanced deep ocean circulation.