Hello and thanks for visiting. I’m a recent PhD graduate from the School of Earth Sciences at Ohio State University where I worked with Dr. Matt Saltzman to study drivers of change in the Early Paleozoic global carbon cycle using inorganic isotope geochemistry and carbonate stratigraphy.

Email: conwell.30@osu.edu
Link to combined Resume & CV [May 2023]

I published some of my dissertation work—link here. Please send me an email if you don’t have access and would like a PDF copy.

Summary of dissertation research to understand the evolution of Earth’s carbon cycle
Although we’ve directly observed Earth’s changing climate over the last ~200 years, many questions remain about the mechanics of the Earth’s global carbon cycle (the sum of processes that exchange carbon between the atmosphere, land, oceans, and solid earth) and its connection to climate, life, and other Earth systems. To try to understand this complex system, we look to the multimillion-year record of Earth’s surface environments preserved in ancient sediments. Earth’s paleotemperature record spanning the last 500 million years reveals massive temperature fluctuations as large as 20°C (compared to the ~1°C warming of the last 200 years). What changes in the behavior of Earth’s global carbon cycle could have produced these climate fluctuations? Also, what processes act to alter chemical signals recorded in sedimentary minerals, and how does this alteration affect our interpretations of the evolution of Earth’s carbon cycle?

My dissertation research focuses on a global climatic transition during the Ordovician Period of Earth’s history (~465 million years ago) when Earth’s climate cooled dramatically over ~40 million years, leading to the first major glaciation and mass extinction event of the Phanerozoic. Within this time interval, I investigated:

1. The role of mafic (Ca,Mg-silicate) weathering in driving the Ordovician climate transition. I studied bulk carbonate isotope records of strontium (⁸⁷Sr/⁸⁶Sr) and neodymium (ε_Nd(t)) from the tropical Middle–Late Ordovician (~470–450 Ma) carbonate succession in the Antelope Range, central Nevada. These data are paired to published records of oxygen isotope (δ¹⁸O) paleotemperature measurements from the same locality. I also adapted a published carbon-strontium mass balance model to further test the hypothesis that enhanced mafic weathering led to Ordovician cooling. This work marks one of the first demonstrations of a global climatic response to enhanced mafic weathering in the geologic record, supporting the hypothesis that atmospheric CO₂ drawdown by the silicate–carbonate system can act as a primary driver of multimillion-year climate. This work is published in Geology.
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2. Factors affecting seawater ⁸⁷Sr/⁸⁶Sr preservation in conodont apatite. Conodont fossils are a crucial archive for high-resolution studies of Paleozoic seawater ⁸⁷Sr/⁸⁶Sr, but our knowledge of Sr systematics in conodont–porewater systems, and thus our ability to screen samples for diagenetic alteration, is underdeveloped. This work investigates ⁸⁷Sr/⁸⁶Sr in exceptionally well-preserved conodonts from two classical Ordovician carbonate sections in central Sweden (Fjäcka and Kärdgarde), as well as broader trends using a Paleozoic compilation of published conodont ⁸⁷Sr/⁸⁶Sr measurements, to assess relationships between host-rock lithology, sedimentation rate, thermal history (Conodont Alteration Index), and the degree of ⁸⁷Sr/⁸⁶Sr alteration from seawater values. This manuscript is in preparation for journal publication.
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3. The influence of early marine diagenesis on the carbon isotope (δ¹³C) record of shallow marine sediments. Recent investigations of modern carbonate platforms have led researchers to suggest that large fluctuations in Earth’s seawater δ¹³C record, traditionally interpreted as disturbances in Earth’s carbon cycle, are actually the result of processes that alter carbonate minerals after their formation. This work uses Ca isotope (δ^44/40Ca) measurements of bulk carbonate rock to constrain water–rock interactions during shallow marine burial, probing for a relationship between regimes of sediment recrystallization and paired δ¹³C measurements from published literature. Our results indicate that a globally-resolved event in the Ordovician δ¹³C record (the Middle Darriwilian Carbon Isotope Excursion) is not solely a product of early marine diagenesis, supporting the view that the C isotope record of shallow marine carbonates can record information about Earth’s carbon cycle. This manuscript is in preparation for journal publication.

 

I prepared all samples in our class 100 clean lab at Ohio State, and performed isotope measurements using our ThermoFisher Triton Plus TIMS (P.I. Elizabeth M. Griffith; see this page for more info on facilities).