Collaborative Research: Tracking Middle-late Paleozoic Global-ocean Redox Conditions Using U Isotopes in Marine Carbonates

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The goal of the funded proposal is to determine the relative amounts of oxygen dissolved in ancient oceans during a unique geologic time interval (300 to 420 million years ago) when three major mass extinctions occurred and the Earth was significantly warmer than today. Understanding the history of oxygen levels in ancient oceans is important because oxygen concentrations influence all marine animal life, control the formation and preservation of petroleum and many economic mineral deposits, and affect important biochemical reactions in the oceans and atmosphere. This project utilizes the relatively new geochemical tool of uranium isotopes to develop the most detailed history of changing ocean oxygen concentrations to date. The funding supports the training of the next generation of Earth scientists, will provide hands-on field and laboratory experience for first-generation college students, and will support multiple primary school science teachers to conduct real-world science to gain confidence and provide new ideas for their own classrooms. The project goal is to develop a global-ocean redox curve using U isotopes from Devonian-Mississippian marine carbonates. The ~100 million year study interval, which bridges the transition between poorly oxygenated Precambrian-Early Paleozoic oceans and well-oxygenated Late Paleozoic-Recent oceans, is important because of: 1) a doubling of atmospheric pO2 levels, 2) a transition from greenhouse to icehouse climate and intensifying ocean circulation, 3) the occurrence of three mass extinctions, two of which are attributed to marine anoxia, and 4) multiple large positive 13C excursions. The U-isotope composition of marine carbonates has the potential to record mean global-ocean redox conditions owing to the long residence time (~500 thousand years) of U in seawater. Samples will be collected from biostratigraphically well-dated successions in Nevada and Utah at spacings ranging from ~100 thousand years/sample to 20 thousand years/sample through specific event intervals. To assess samples for local redox influences, we will analyze redox-sensitive metals, Fe-speciation, and Corg:P ratios. To verify the global nature of the composite curve, we will sample and compare to selected shorter stratigraphic intervals on other continents.