GEOCHEMICAL EVIDENCE OF REDOX CHANGES ASSOCIATED WITH THE LATE SILURIAN LAU/KOZLOWSKII EXTINCTION FROM CARBONATE FACIES OF LAURENTIA AND BALTICA

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 transitions is the mid-Ludfordian Lau Event which coincides with the Lau/Kozlowskii extinction (LKE), the most severe and widespread biotic event in the Silurian, and the Lau positive carbon isotope excursion (CIE), one of the largest CIEs in the Phanerozoic. The LKE encompasses multiple phases of extinction that begin prior to the onset of the CIE, first in nektobenthic fauna and later planktic fauna. In prior work, the causes of the extinction event and its link to the CIE 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 carbonate sections along Laurentian and Baltic paleo-margins. The 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 records of redox change are another local proxy, δ³⁴S_pyr records, which also show positive excursions supporting evidence for the increases in global burial fluxes of organic carbon and pyrite. The increased burial of organic carbon and pyrite, had a significant impact on the long-term carbon, sulfur, and oxygen cycles. Furthermore, these dramatic changes in the late Silurian redox landscape had a devastating effect on the marine biosphere and thus were a critical driving mechanism of the LKE.