SULFUR AND NITROGEN ISOTOPE CHEMOSTRATIGRAPHY OF THE LATE ORDOVICIAN

The Ordovician period (485-444 Mya) was characterized by major climatic, geologic, and evolutionary changes. Beginning with a prolonged “hot-house” climate in the Early Ordovician, the climate began to cool, resulting in the “ice-house” conditions of the Late Ordovician and Hirnantian glaciation, despite estimated atmospheric CO₂ levels that were 8-16 times higher than modern post-industrial concentrations, with predictions of at least 1500 ppm during glaciation, returning to a warm, stable climate in the Silurian. The short-lived glaciation has been linked to several factors including climatic cooling, increased organic carbon burial, and cessation of volcanism as a result of the Taconic orogeny, in which a volcanic island arc collided with Laurentia due to active subduction of a remnant oceanic basin, resulting in reduced volcanic arc outgassing and enhanced silicate weathering of the newly uplifted terrane. It is also considered a major cause of the first of the ‘Big Five’ Phanerozoic mass extinctions in the end-Ordovician, resulting in the loss of 60-70% of marine species, many of which first appeared during the Great Ordovician Biodiversification Event (GOBE) in the Early to Middle Ordovician. Samples were collected from the IMI Pendleton Quarry in Indiana composed of interbedded fossiliferous limestones and shales, suggesting shallow marine facies, which is consistent with the view that Indiana, positioned ~20° south of the equator on the continent Laurentia, was part of a vast epeiric sea covering the central areas of the continent. Through the Ordovician, this epeiric sea experienced changing sea levels, causing a series of flooding events and a shift in lithology. Both high-resolution and long-term variations in δ³⁴S_pyrite and bulk elemental compositions of carbon and sulfur show coherent responses with these lithological changes. In this study, isotopic analyses of carbonate-associated sulfate (CAS) and redox proxies will be used to help unravel the climatic complexities of this time, providing clues into paleoclimatic, paleoceanographic, and redox cycle changes through the Late Ordovician, Hirnantian glaciation, and end Ordovician mass extinction.