Tracing Earth’s Oxygen Cycle through High-Precision Multiple-Isotope Analyses

Abstract

The oxygen cycle is fundamental to the past, present and future of Earth’s climate. This thesis focuses on three central components of the oxygen cycle: seawater sulfate (Chapter 1), molecular oxygen (Chapter 2), and odd-oxygen (Chapter 3). Together, these components encompass oxygen cycling in all regions of the fluid Earth (solid-fluid Earth interface, ocean, and atmosphere), and on timescales from geologic to transient. The study of each is enabled by emerging, high-precision multiple-isotopologue analyses. Chapter 1: Seawater sulfate is a major carrier of oxidizing capacity, and its budget is therefore a critical control on Earth’s redox state. Here, the influence of submarine hydrothermal systems on seawater sulfate is investigated through triple-oxygen-isotope analyses. These analyses suggest that the seawater sulfate budget may contain a significant, and previously overlooked, hydrothermal contribution. Chapter 2: The flux of O2 generated by gross primary production is critical to Earth’s oxygen and carbon cycles, but accurate estimation of this flux remains challenging. Here, triple-oxygen-isotope analyses of O2 are used to characterize the uncertainty in current estimates. A new method for estimating gross productivity, based on the fractionation of multiply-substituted O2 isotopologues (17O18O or 18O18O), is then proposed. This may reduce current uncertainties in gross primary production by a factor of 4. Chapter 3: Odd-oxygen (i.e. atomic oxygen, O, and ozone, O3) is a central control on atmospheric oxidising capacity and therefore on climate. Reconstruction of recent changes in atmospheric odd-oxygen is inhibited by the short lifetime of each species. Here, experiments are used to identify signals of odd-oxygen photochemistry in the relative abundances of 18O18O and 17O18O. The experiments are used to propose that atmospheric 18O18O and 17O18O may provide unique constraints on past and present O3 cycling.