Molybdenum Isotopes in the Geologic Record: Paleoredox Proxies?

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ABSTRACTMolybdenum Isotope in the Geologic Record: Paleoredox Proxies?This project aims to investigate natural variations in the isotopic composition of the element molybdenum (Mo). Mo is an element that is sensitive to the presence of oxygen.As the abundance of oxygen in the environment varies, so does the chemical behavior ofMo. We have made a preliminary study and found that Mo that has accumulated from seawater under oxygen-rich conditions is lighter than Mo that accumulates in sediments under oxygen-absent conditions in the presence of sulfur. Of the two types of accumulation, it is the Mo that accumulates under oxygen-absent conditions that is similar to Mo in seawater. We therefore believe that changes in the relative proportion of light Mo extracted from the oceans under oxygen-rich conditions will be reflected in the isotopic composition of Mo in seawater and in sediments deposited under oxygen-absent conditions. The more of the ocean that is oxygen-rich, the more light Mo will be extracted from seawater and the heavier the Mo remaining in seawater will become.The key to this hypothesis is the extraction of light Mo from seawater under oxygen-rich conditions. It is therefore the purpose of this project to investigate this process both in the laboratory and in natural samples in order to understand the mechanisms involved and to quantify their effects.If we are correct, then the isotopic composition of Mo in sediments deposited under oxygen-absent waters could be used to track the relative proportions of oxygen-rich and oxygen-absent ocean water through time. An understanding of the evolution of our present-day oxygen-rich ocean is one of the keys to understanding the evolution of theEarth 's environment, and of life itself, from primordial times to the present.