What The rise in atmospheric oxygen 2.3 billion years ago dramatically altered the redox state of Earth's atmosphere, permanently changed all major biogeochemical cycles, and set the stage for the evolution of multicellular life. Despite the ever-lasting effects on Earth's surface processes, atmospheric oxygen initially rose to only ~1% of present atmospheric levels. Remarkably, an additional ~1.8 billion years passed before oxygen concentrations reached levels comparable to the present. Throughout this period, substantial uncertainty exists as to whether oxygenation was slow and continuous, or fast and step-wise. My research focuses on extracting a robust and more detailed history of our planet's evolving redox state from the ancient sedimentary record. Why The chemistry of carbonate rocks is an excellent, if temperamental, archive of the chemical composition of ancient seawater. However, trace element proxies in carbonate rocks are difficult to interpret because the geochemical cycles of these elements are not fully understood over a range of timescales including the modern. In addition, we lack fundamental knowledge of how trace elements are affected by the post-depositional diagenetic processes that are common to all ancient carbonate sediments. Despite these open questions, the use of trace element proxies is a rapidly growing pursuit in geochemistry, and understanding how these proxies are preserved in ancient sediments is an important avenue of ongoing research. How With no direct way of measuring oxygen concentrations in deep geological time, key constraints are inferred from proxies such as the stable isotopic composition and concentration of trace elements in ancient carbonate rocks. Measurements of ratios of iodine to calcium (I/Ca) in carbonate rocks are an example of a promising, but not yet fully tested, technique that may greatly improve our ability to investigate the early history of oxygen on Earth. However, like most geochemical proxies the I/Ca ratio is sensitive to diagenesis, which can erase the primary redox signal. To overcome this problem, I will combine proxies for carbonate diagenesis and alteration (Ca and Mg isotopes) with numerical modeling and measurements of I/Ca ratios in ancient carbonate samples. SSR The broader impacts of this project are to improve our understanding of the evolution of climate on Earth and beyond. First, as the development of an oxygenated ocean is a necessary precursor for multicellular life, understanding the rate and extent of oxygenation on Earth offers a critical perspective on the rarity and role of life in our universe. Secondly, observations of environmental changes throughout Earth history are the only baseline for understanding variations in the natural system that do not occur on human time scales. As we face a rapidly approaching warmer world, that may be drastically different from the one where civilization began, these ancient observational baselines become a critical component to predicting and managing future climate change.