This work tests the application of Uranium (U) isotopes preserved in carbonate sediments as a paleo-redox proxy. Significant variability exists in U isotope composition, due largely to isotope fractionation during redox transformations of U in solution. This suggests that the U isotope composition of seawater may be determined by the redox state of the global ocean. This research will involve laboratory experiments to determine whether U isotopes fractionate during abiotic precipitation of calcite and aragonite, which will serve as a baseline against which to compare any biogenic fractionation. A field component will investigate fractionation of U isotopes in natural carbonates, both biogenic and abiotic. This will include study of how low temperature alteration during deposition and burial might affect the preservation of the primary U isotope signal. Funding provides research opportunities for undergraduates from the Arizona State University Honors College, and supports a PhD thesis.