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Topic

Climate Variability and the Oxygenation of the Atmosphere

School of Earth and Ocean Sciences

Date & location

• Wednesday, September 10, 2025
• 1:30 P.M.
• Clearihue Building, Room A326

Examining Committee

Supervisory Committee

• Dr. Colin Goldblatt, School of Earth and Ocean Sciences, University of Victoria (Supervisor)
• Dr. Anne-Sofie Ahm, School of Earth and Ocean Sciences, UVic (Co-Supervisor)
• Dr. Jay Cullen, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Shannon Fargey, Department of Geography, UVic (Outside Member)

External Examiner

• Dr. Timothy Lyons, Department of Earth and Planetary Sciences, University of California, Riverside

Chair of Oral Examination

• Dr. Randy Scharien, Department of Geography, UVic

Abstract

The oxygenation of the atmosphere was perhaps the most important transition in Earth’s history. This transition, known as the Great Oxidation Event (GOE), occurred approximately 2.5 billion years ago, leading to the oxidation of the Earth’s surface environment. The GOE occurred around the same time as the global snowball Earth Huronian glaciations, suggesting that the oxygenation of the atmosphere was accompanied by a period of extreme climate variability. Snowball Earth climates were likely terminated by the accumulation of greenhouse gases in the atmosphere, leading to hot moist greenhouse climates with mean surface temperatures of up to 350K. In this thesis, I investigate how climate variability associated with global glaciations affected the oxygenation of the atmosphere. I develop this investigation in five chapters.

In the first chapter, I use a one-dimensional photochemical model to investigate how climate variability affected the concentration of oxygen across the GOE. I find that climate variability affects the concentration of oxygen through changes in the concentration of oxidizing radicals in the atmosphere. Cold snowball Earth climates lead to less oxygen in the atmosphere and hot moist greenhouse climates lead to more oxygen after the GOE.

In the second chapter, I use transient photochemical simulations of the GOE to investigate whether climate variability can help explain the Mass Independent fractionation of sulfur isotopes (MIF-S) geological record across the GOE. The intermittent nature of this record has been interpreted as evidence for oxygen oscillations across the GOE. I find that climate variability can lead to oxygen oscillations across the GOE, helping explain the MIF-S rock record.

In the third chapter, I document the development of Chempath: a new open-source pathway analysis program for photochemical models. This algorithm constructs the reaction chains (pathways) that produce or destroy a species of interest in a reaction system and quantifies the importance of each pathway to explain the concentration change of the species of interest. Chempath can be a useful tool to understand the results of complex photochemical models.

In chapter four, I apply Chempath to transient simulations of the GOE to investigate the chemical mechanisms that produce and destroy oxygen, ozone, and methane across the oxygenation of the atmosphere. I find that the GOE is the result of a positive feedback loop for oxygen accumulation in the atmosphere triggered by an increase in oxidizing radicals. As oxygen starts to increase during the GOE, the production of oxidizing radicals increases. These radicals react with and deplete reduced species like methane, allowing oxygen to increase to higher levels and produce more oxidizing radicals, forming a positive feedback loop for oxygen accumulation. This feedback is stabilized by the formation of the ozone layer, decreasing the rate of production of oxidizing radicals through water vapor photolysis.

In chapter five I measure iodine and cerium anomalies in carbonates deposited before, during, and after the GOE to investigate shallow water redox conditions across the oxygenation of the atmosphere. I find that these redox proxies do not agree and carbonates can preserve either iodine without negative cerium anomalies or negative cerium anomalies without iodine. These results emphasize the importance of using multiple proxies to study past redox conditions.

The GOE was probably the most significant transition in Earth’s history, fundamentally changing biogeochemical cycles and likely contributing to the development of complex life. Investigating this transition has important implications for understanding the evolution of life and the environment on Earth, and for investigating potential signs of life on other planets. My research improved our understanding of this transition showing that climate variability exerts a strong control on atmospheric oxidation processes and by showing the chemical mechanisms responsible for the accumulation of oxygen during this transition.