NEW LATE SILURIAN (LUDFORDIAN) δ¹³C AND δ³⁴S ANALYSES FROM WESTERN TENNESSEE: AN OUTER RAMP PERSPECTIVE ON THE LAU CIE AND EXTINCTION EVENT

The Lau/Kolzlowskii extinction (LKE) was the last in a series of moderate extinction events that occurred during the Silurian. This event is defined by a global reduction in conodont and graptolite faunal diversity, among many other marine taxa. Records of this event may be seen in localities from Scandinavia, Eastern Europe, North America, and Australia. Coinciding with the LKE, was a major positive carbon isotope excursion (CIE), the Lau CIE. The causes and consequences of the Lau CIE and exact relationship to the extinction event are not well understood. Most investigations invoke changes in the global carbon cycle, oceanic redox conditions and/or eustatic sea level as a driver for the CIE. Here we report new paired carbon (δ13Ccarb, δ13Corg) and sulfur (δ34SCAS : carbonate-associated sulfate) isotope data from the Ludfordian Brownsport Formation, Tennessee, which has been previously studied in shallow-carbonate ramp settings from nearby outcrops. Our new data from the Bath Springs section will be used to assess completeness of shallower sequences, local redox changes within this carbonate ramp setting, and global marine redox changes. The alternating mudstone/wackestone and shale lithofacies along with trilobite-nautiloid biofacies indicate a deeper-water setting of the Bath Springs section. Data from this locality will help to further test the hypotheses put forward for the Lau CIE, with coincident positive δ13Ccarb and δ34SCAS excursions supporting global increases in the burial flux of organic matter and pyrite. Widespread anoxic to euxinic (no oxygen + sulfide) conditions would be necessary to induce these changes in burial fluxes of C and S. Furthermore, this hypothesis provides a critical link of the Lau CIE to the extinction events via spread of reducing conditions within marine environments globally. Conversely, if no major positive shift in δ34SCAS is documented in concert with the Lau CIE this may support a carbonate weathering hypothesis rather than an oceanographic/marine redox hypothesis.