SULFUR ISOTOPE EVIDENCE FOR LATE ORDOVICIAN OCEAN OXYGENATION: IMPLICATIONS FOR THE DRIVERS OF THE HIRNANTIAN EXTINCTION

The Hirnantian Stage of the Ordovician (445 to 443 Ma) contains several important paleoenvironmental and biological events, including large-scale continental glaciation; large perturbations to the global carbon cycle; and the second largest extinction interval in Earth history. Recent models for Late Ordovician extinction have invoked the spread of euxinia (anoxic, H₂S-containing marine waters) as a kill mechanism. One of the main lines of evidence used to argue for enhanced marine euxinia is a positive sulfur isotope excursion found in sedimentary pyrite that parallels the well known Hirnantian positive carbon isotope excursion or HICE. These pyrite-S and organic-C excursions are interpreted to track changes in the isotopic compositions of the marine sulfate and dissolved inorganic carbon pools that reflect enhanced burial rates of both pyrite and organic carbon during an expansion of euxinia within the oceans.

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 (δ³⁴S_CAS) and pyrite (δ³⁴S_pyrite) from successions from the western Laurentia and Baltica. δ³⁴S_CAS data from both successions show little variation during the Hirnantian and are consistent with other recently published Hirnantian δ³⁴S_CAS 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 δ³⁴S_CAS 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.