Oxygen isotope constraints on the composition of seawater through time

Abstract

The chemical components of seawater are broadly set by weathering processes and exchange with the atmosphere. This means that the composition of the ocean reflects the contemporaneous climate state and tectonic setting. The deposition of minerals formed within the ocean represent the major long-term export of these chemical components and are thus a record of seawater chemistry through time. The chapters that follow examine the chemical sediment record across Earth history with a broad goal of constraining the evolution of the ocean-atmosphere system. We specifically focus on oxygen-bearing species that are robust to secondary alteration and thus have a greater chance of retaining primary information: sulfate (SO4), zircon (ZrSiO4), and phosphate (PO4).

First, we explore the environmental information contained in sulfate minerals. We determine that the triple oxygen isotope composition of sulfate-bearing evaporite deposits is broadly representative of the marine sulfate reservoir, with some isotopic resetting due to processes that occur in the evaporite basin (Chapter 1, Appendix A). We use this framework for understanding evaporite basins to examine the oxygen isotope composition of the evaporite record over the last ~1,000 Ma (Appendix E). We identify a step-change in the triple oxygen isotope composition of evaporite basins at ~400 Ma, which we posit is the result of the evolution of root systems changing the locus of pyrite oxidation. The evaporite record does not extend back into the Archean, however, when sulfate concentrations were low. In fact, Archean sulfate was likely formed via distinct oxidation pathways compared to the evaporite record and can be a proxy for contemporaneous seawater. Accordingly, we use Archean barite (Chapter 2, Appendix B) and then the detrital zircon record (Chapter 3, Appendix C) to gain insight into the oxygen isotope composition of contemporaneous seawater. The most parsimonious explanation for these records is an early ocean with an oxygen isotope composition close to the modern. Finally, we explore the alteration history of carbonate fluorapatite, a carbonate and phosphate-bearing mineral, from the Permian Phosphoria Rock Complex (Chapter 4, Appendix D). Though this unit has been subject to significant diagenetic alteration, our multi-proxy approach is a promising framework for the future analysis of less-altered phosphorite units. Overall, these studies span geologic time (~4,400 Ma to 5.6 Ma) and use multiple proxies and analytical techniques to better understand the history of Earth's ocean-atmosphere system.