Doctoral Dissertations

Orcid ID

Date of Award


Degree Type


Degree Name

Doctor of Philosophy



Major Professor

Linda C. Kah

Committee Members

Christopher M. Fedo, Jill M. Mikucki, Andrew D. Steen, Colin D. Sumrall


The Ordovician is a critical period of Earth evolution where we observe a dramatic increase in the diversity of marine organisms called the Great Ordovician Biodiversification Event (GOBE). Major pulses in biodiversity of marine phytoplankton are identified in the Middle Ordovician during the GOBE suggesting dramatic changes in nutrient budgets of the shallow oceans. Additionally, carbon isotope evidence suggesting changeover from a greenhouse climate state to an icehouse climate state is also identified in the Middle Ordovician. Disruption of the Early Ordovician stratified ocean with invigorated ocean circulation associated with global cooling through the Middle Ordovician would lead to increased ventilation and upwelling of nutrient rich anoxic deep-waters potentially driving the biodiversity trends observed during the GOBE. Evidence of such a mixing event in the Middle Ordovician may recorded as the globally recognized Middle Darriwilian Isotope Carbon Excursion (MDICE).In this study, I explore the potential for disruption of ocean stratification driven by global cooling in the Middle Ordovician by investigating the isotopic compositions of carbon, sulfur, strontium, and neodymium from the Precordillera of Argentina, which contains distinct marine facies representing coeval, yet distinct, water depths. In Chapter 1, I evaluate the carbon isotope composition of marine carbonate and organic carbon. Paired C-isotope profiles show decoupling of carbon isotope compositions between shallow- and deep-water environments and confirms the presence of a strong chemocline in the Darriwilian. In Chapter 2, I characterize potential changes in redox conditions in the Precordillera through the sulfur isotope record of carbonate associated sulfate (CAS) and pyrite. Variation in the behavior of marine sulfur between shallow, more oxygenated surface waters and between deeper-water environments, below the marine chemocline is consistent with increased oxygenation of the deep-ocean in the Middle Ordovician. In Chapter 3, I determined the isotopic compositions of neodymium and strontium to explore the potential for physical stratification of the Middle Ordovician water column. Neodymium and strontium isotope compositions show decoupling between shallow- and deep-water environments suggesting physical stratification of the water column in addition to dynamic stratification indicated in the biogeochemical records of carbon and sulfur.

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