Critical new perspectives on molybdenum cycling under modern and experimental euxinic conditions: Tuning the paleoredox proxy

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The story of ancient ocean chemistry is the story of our origins. The geologic record reveals the presence of animals over only the last 10% of Earth?s 4.6-billion-year history and the origins of modern humans only a flicker of time ago. Most researchers link the appearance of the first animals to a rise in atmospheric oxygen to a level high enough to support complex animal metabolisms. The first 90% of Earth history, however, is a story of profound change in the oxygen state of the atmosphere and ocean, with complete absence of oxygen over the first half of Earth history and an oxygen-free deep ocean for most of the next half. Oxygen abounds today, and the ocean teems with animals, yet the oxygen-lean landscape of the early ocean was the cradle of our ancestors. The best view of these early organisms and their surroundings comes from fossils and the chemical properties of sediments deposited within the ancient ocean, and no element in the periodic table has played a greater role than molybdenum (Mo) as a window to the past.Molybdenum, a nutrient essential in the biological cycling of nitrogen, must have been a determining factor in the paths and rates that led ultimately to the early evolution of marine animals, particularly since its concentration in seawater varies dramatically with the availability of oxygen. This and other life-sustaining metals can be scrubbed out of seawater in the presence of hydrogen sulfide, which probably spread through sizeable portions of the ancient ocean. Recent research by this group and a few others has come a long way in providing a roadmap to the chemistry of ancient seawater based on the distributions and isotopic properties of Mo in ancient sediments, but key questions remain. Our understanding of the co-evolution of early life and ocean chemistry can only be as strong as the knowledge of how Mo is cycled now and in the past?particularly under the oxygen-poor conditions that dominated the early ocean. This study seeks to use novel, cutting-edge analytical methods in the lab and in the field to fill essential gaps in our grasp of Mo biogeochemistry. Among the key issues and questions are the mechanisms of Mo uptake across diverse depositional settings, particularly those with abundant hydrogen sulfide in the seawater. What specifically are the relationships to organic matter, the remains of organisms, and are the isotopes of Mo fractionated during interactions with organic substrates? Isotopes are atoms of the same element that differ in their masses and specific reaction behaviors and so can provide unique information about chemical pathways and the controlling environmental factors, such as oxygen availability. What is the full range of potential Mo hosts in settings rich in hydrogen sulfide, and how does dissolved Mo speciate and fractionate isotopically under those conditions? Although these chemical questions are quite specific, the implications are broad and speak to the ability to fingerprint conditions in the ocean that either fostered or challenged the origins and diversification of early life. The impact of this study will extend widely. A far greater ability to unravel the history of life is an expected outcome, and investigators will develop and refine novel analytical methods with broader relevance to many other trace metals and their biogeochemical cycles, including those in the modern ocean. Thanks to their hard-earned experience they are now able to demonstrate the great utility of particle accelerators in geobiological research. They aim to smooth the path for others traveling the same route by providing a virtual experience (website and short course) that captures their analytical maturation from beginners to experts, spanning from the particulars of our experience to more general logistical and scientific details (a sort of ?Synchrotron for Dummies?). They plan a high level of undergraduate involvement that mirrors UCR?s status as one of the most culturally diverse campuses in America. Finally, in an effort to reach across international boundaries, investigators have planned monthly group meetings with their colleagues in Beijing designed to emphasize student presentations, to enhance the flow of ideas in both directions, and to foster additional collaborative exploration of the early ocean and its co-evolving life. Although the methods are diverse and demanding, their motivations distill down to a single simple question: where did we come from?