EVIDENCE OF OCEANIC EUXINIA ASSOCIATED WITH THE LATE SILURIAN LAU/KOZLOWSKII EXTINCTION FROM LAURENTIAN CARBONATES OF TENNESSEE AND NEVADA

The Silurian period was fraught with environmental upheaval as the ocean-climate system repeatedly oscillated between cool and warm conditions. The transitions between these climate states are often marked by extinctions, faunal turnovers, and major changes to the global carbon cycle. The most notable of these is the mid-Ludfordian Lau Event which includes one of the largest positive carbon isotope excursions in the Phanerozoic, with δ¹³C values > +8‰ at some localities, which coincides with the Lau/Kozlowskii extinction (LKE), the most severe and widespread documented biotic event in the Silurian. This LKE encompasses multiple phases of extinction in nektobenthic fauna, such as brachiopods, fish, and conodonts, and later planktic fauna, such as graptolites and acritarchs. In prior work, the causes of the extinction event and its link to the carbon isotope excursion have not been well constrained with hypotheses ranging from enhanced organic carbon burial and anoxia to increased weathering from eustatic sea level changes. Paired carbon and sulfur isotopic analyses, along with novel I/Ca ratios, have been generated from multiple sections on the western (Nevada) and southern (Tennessee) margins of Laurentia. Local redox conditions for these study areas are investigated using the I/Ca redox proxy. Low I/Ca ratios indicate intervals of low dissolved O₂ concentrations or anoxic waters nearby the carbonate platforms. The δ¹³C_carb, δ¹³C_org, and δ³⁴S_CAS records from these regions lend evidence in support of global increases in organic carbon and pyrite burial due to expansion of euxinia in late Silurian oceans. Supporting the global record is another local proxy, δ³⁴S_pyr, which also documents positive excursions suggesting a response to the global burial of organic carbon and pyrite. The significant burial of reduced compounds, organic carbon and pyrite, had a significant impact on the short- and long-term carbon, sulfur and oxygen cycles. These significant paleoredox perturbations had a devastating effect on the marine biosphere as a critical mechanism that drove the known LKE.