CHEMOSTRATIGRAPHIC INVESTIGATION OF AN END-ORDOVICIAN SHALLOW MARINE SEQUENCE, INDIANA, USA

The Ordovician period was a pivotal era characterized by significant climatic, geologic, and evolutionary transformations. The transition from an Early Ordovician "hot-house" climate to a Late Ordovician "ice-house" condition, marked by glaciation and cooling, led to major environmental changes, increased biodiversity (GOBE), and a mass extinction event. This mass extinction, one of the "big five" in Earth's history, resulted in the loss of up to 50% of genera and a staggering 85% of marine species. In addition, the first appearance and colonization of land plants during the Ordovician may have had extensive consequences on terrestrial and marine ecosystems including fluctuations in atmospheric CO₂ and O₂ concentrations and periods of increased organic carbon burial rates, and subsequent anoxia, evident in two Late Ordovician global positive carbon isotope excursions (GICE and HICE). The evolution of land plants, particularly their ability to reduce CO₂ levels through enhanced calcium-magnesium silicate weathering, including extensive shallow-water phosphate deposits from the Late Ordovician, has been proposed as an important contributor for the decrease in atmospheric CO₂ and subsequent cooling during the Ordovician. Our study focuses on end Ordovician samples collected from the IMI Pendleton Quarry in Indiana, comprised of interbedded dolostones, limestones, and shales, indicative of shallow marine facies. Indiana, positioned ~20° south of the equator on the continent Laurentia, was part of a vast epeiric sea during the Ordovician, experiencing fluctuating sea levels and lithological shifts. By analyzing these samples, we aim to unravel the climatic complexities of the end Ordovician, with a specific focus on the Hirnantian glaciation and the associated mass extinction event. We will investigate paleoclimatic, paleoceanographic, and redox cycle changes that occurred during this time, shedding light on the factors contributing to global cooling and marine extinctions.