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.
The overarching goal of this project was to assess whether variations in the isotope composition of uranium (U) in ancient ocean carbonate rocks can be used to track changes on the amount of O2 dissolved in the oceans through time. O2 is critical to support marine animals and many other organisms and so our understanding of life's history in the oceans would be improved with better ways to track this parameter. For this method to work, we need to be able to relate the U isotope composition in carbonate rocks to the U isotope composition of seawater. Are the compositions in rocks faithful recorders of those in seawater? In order to assess this possibility, we needed to assess the extent to U isotopes vary during: Precipitation of carbonate minerals (aragonite and calcite), which incorporate U; Interaction of U in shallow sediments and pore waters with H2S-bearing fluids; Interaction of U with fluids after burial. The major results of this project (published and in preparation) assess all three aspects of this goal: Chen et al (in prep) assess U isotopic variation during coprecipitation with both calcite and aragonite. We find that contrary to prior expectations, U in aragonite demonstrates a small variation, while no measurable fractionation occurs during incorporation in calcite. Romaniello et al. (2013) assessed U isotope variations in natural primary carbonate precipitates and in shallow cores from the Bahamas. The major finding of this study was that although U isotopes in natural primary precipitates are indistinguishable from seawater, bulk carbonate sediments from shallow cores demonstrate a significant offset from seawater, due to the incorporation of isotopically heavy U(IV) from shallow H2S-rich pore waters. Finally, Romaniello et al (in prep) addressed what happens during burial by studying U isotope variations in ODP drill cores from the Bahamas dating back ~800 ka. They found similar results to those observed in the shallow cores, suggesting that variations likely occur during the first meter of sedimentation. In addtion, the project supported final interpretation, preparation, and publicaton of Brennecka et al (2011) which reportd variations in U isotopes in ancient rocks that we interpreted as reflecting changes in ocean O2 associated with the greatest mass extinction in Earth history. These findings helped motivate the rest of the project activities. Our findings have important implications for the interpretation of U isotopes in ancient marine carbonate rocks. Contrary to the previously held assumption that marine carbonates directly record seawater U isotopes, our results demonstrate that a small correction may be required. This correction would be significant because it is similar in magnitude to the size of shifts we expect to see in the geologic record. Further work is needed to determine how and when this correction is needed. The award supported several related secondary projects that provided further baseline information about the U isotope system, improved analytical approaches, and and suggested a new application of U isotope variations as an indicator of biology. The award contributed substantially to the training of two Ph.D. students and two undergraduates in field and laboratory methods, as well as in mass spectrometric analyses, enhancing the skilled scientific workforce.
