Collaborative Research: Transition Metal Isotope Fractionation During Adsorption to Authigenic Oxides

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ABSTRACTOCE-0526618OCE-0526495Isotope fractionation during adsorption to authigenic oxide minerals is emerging as an important process for paleoproxy applications of many metals. Adsorptive isotope effects have been demonstrated for Mo and Fe, are strongly suspected for Tl, and are at least likely for Cr and Cd. Such fractionation may affect the ocean isotope budgets of these elements and also have important implications for reconstruction of metal isotope paleorecords from isotopic measurements in authigenic oxides such as marine ferromanganese crusts. To realize this potential, it is essential to quantitatively understand the controls on isotopic fractionation and focus on the experiments and theoretical calculations of the natural samples, which have not been the focus of the community's most current work. In this study, researchers at Arizona State University and Princeton University will carry out a laboratory investigation and quantum chemical modeling to attain the following objectives: (1) quantify the magnitude of isotope fractionation of Cr, Fe, Mo, Cd and Tl during adsorption onto various manganese and iron oxides; (2) assess the sensitivity of these effects to intensive variables such as temperature, pH and salinity; and (3) elucidate the reaction mechanisms causing observed fractionations, including definitive determination of whether they arise from equilibrium isotope exchange or kinetics effects. This information is needed to assess whether these new isotopic tools can be used to understand marine geochemical cycles, reconstruct the chemical evolution of the marine environment or determine the ocean redox history. In terms of broader impacts, the project will support not only the early career development of the Co-PI, but also the training of a graduate student, an undergraduate student, and a post-doc in state-of-the-art analytical geochemistry and theoretical chemistry. It will also enhance the infrastructure for research and education by supporting a partnership between geologists, geochemists, and chemists at four different academic institutions, including two outside the United States, who will integrate experiments, state-of-the-art analytical geochemistry and modern theoretical chemistry.