PhD, University of Cincinnati, 2024, Arts and Sciences: Geology

The Great Oxygenation Event is the period, between 2.3 and 2.4 billion years ago, when oxygen began to accumulate in Earth's atmosphere. It was arguably the most consequential change in the history of our planet and the life that inhabits it. Knowledge of the events that led to this change is of great scientific interest because it will help us understand how Earth became a planet that can support complex multicellular organisms, including humans, and determine what to look for when planning missions to seek evidence of life beyond Earth, such as NASA's current Mars 2020 mission. This dissertation presents studies of fossilized microorganisms (microfossils) and microbially-influenced carbonate (carbonate microbialites) that formed shortly before the Great Oxygenation Event. Three-dimensional reconstructions along with photomicrography reveal the morphologies and habits of the preserved microbial communities to provide a better understanding of the types of microorganisms that existed just before the Great Oxygenation Event. The results suggest that microorganisms may have evolved to take advantage of transient microoxic-sulfidic interfaces 200 million years before the Great Oxygenation Event. Petrographic examinations and Raman spectroscopy provide insight into the geological processes that led to the preservation of these microfossils and microbialites. Understanding the preservation and detection of life's signatures, or biosignatures, on Earth is both necessary to the study of life on Earth and foundational to planning missions to seek biosignatures elsewhere in the solar system. The results of this work include the first reported instance of early diagenetic silicification that occurred via silica spherule nucleation directly on organic matter in a deep-water environment. To support the interpretation of DUV Raman data collected on Mars, this dissertation presents a comparison of Raman data collected from terrestrial biogenic microbialites with a Mars 2020 analo (open full item for complete abstract)

Committee: Andrew Czaja Ph.D. (Committee Chair); Carlton Brett Ph.D. (Committee Member); Susannah Porter Ph.D. (Committee Member); Annette Rowe Ph.D. (Committee Member); Joshua Miller Ph.D. (Committee Member) Subjects: Paleontology

Doctor of Philosophy, The Ohio State University, 2014, Geological Sciences

This chemostratigraphic study uses new carbon (d13C) and sulfur (d34S) isotope data measured from Lower–Middle Ordovician carbonate rocks from the Great Basin region, USA. The Pogonip Group was sampled at meter-scale from Shingle Pass (east-central Nevada) and the Ibex area (western Utah) to integrate the stable isotope stratigraphy with a well-studied conodont biostratigraphic framework. The Pogonip Group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the Late Cambrian to Middle Ordovician. The d13C trend has four distinct characteristics recognized in both Great Basin sections: 1) a drop in d13C from +1‰ at the base of the Ordovician (Tremadocian) to -0.7‰, 2) a 1–2‰ positive d13C shift during the late Tremadocian, 3) a gradual d13C increase from -2‰ to ca. 0‰ during the end of the Early Ordovician (Floian), and 4) a steady d13C decrease from 0‰ to -4 to -5‰ during Middle Ordovician (Dapingian–Darriwilian). The d34S trend measured from carbonate-associated sulfate (CAS) at Shingle Pass has an overall decrease from +35‰ during the Tremadocian to +20 to +25‰ during the Floian. A 15‰ negative excursion is present near the Dapingian-Darriwilian boundary before d34S values increase up to +35‰ at the top of the section. Corresponding d34S measured from sedimentary pyrite shows an overall similar drop in the Lower Ordovician but pyrite d34S values are more variable. During the early Tremadocian Stage pyrite d34S varies between 0 to +20‰ but makes a major drop of 20‰ during the late Tremadocian to values between -10–0‰ throughout the Floian and Dapingian Stages. Pyrite d34S increases gradually near the Dapingian-Darriwilian boundary to values between +10 to +20‰ at the top of the section. The Lower–Middle Ordovician d13C and d34S trends reported here from the Great Basin are not consistent with a causal mechanism involving sea level change and subsequent migration of isoto (open full item for complete abstract)

Committee: Matthew Saltzman (Advisor); William Ausich (Committee Member); Stig Bergström (Committee Member); Lawrence Krissek (Committee Member) Subjects: Geochemistry; Geology