Redox changes in the global ocean have played a major role in the evolution of life. In recent years, new proxies have been developed to track these changes through Earth history and to understand the underlying causes. However, most of these proxies are tied to black shale deposits. So far no proxy exists that reliably tracks redox changes of the global ocean in carbonate rocks. A carbonate-based proxy would allow access to a more complete geologic record and a wider range of depositional environments than proxies based on analyses of black shales. One potential proxy is the uranium isotope system. Uranium is an abundant trace element in carbonate sediments and carbonate rocks. Recent work has revealed significant variations in the natural isotope composition of uranium isotopes, driven by isotope fractionation. Much of this fractionation is driven by redox transformations of uranium in solution, suggesting that the uranium isotope composition of seawater may vary with redox conditions. Because the residence time of uranium in the oceans far exceeds ocean mixing times, this isotope system could provide a proxy that integrates global average ocean conditions. Limited preliminary evidence suggests that biogenic carbonates record the uranium isotopic composition of seawater. As a consequence, changes in the uranium isotopic composition of carbonates through time could reflect changes in the redox conditions of the global oceans. However, before this new proxy can be used, it needs to be established that carbonates indeed faithfully record the seawater composition without fractionation. We propose to address the question of whether modern carbonates can capture the uranium isotopic composition of seawater without fractionation through a combination of laboratory and field studies. We will perform carefully controlled experiments to determine whether uranium isotopes fractionate during abiotic precipitation under a range of different environmental conditions (e.g., temperature, carbonate ion concentration, and pH). In addition, we will also sample modern biotic and abiotic carbonates from the Bahamas to assess the uranium isotope fractionation in natural samples. We will target a range of natural samples (e.g., ooids, aragonite mud, stromatolites, biogenic carbonates, etc.) to obtain a variety of carbonate materials (i.e., different mineralogies and taxonomic affinities). The broader impacts will focus on education and training. In education, we will leverage the connections of the investigators to the Honors College at ASU by offering a 1 credit Honors freshman seminar in Earth system science, designed to expose students to the insights that isotope geoscience brings to understanding of climate and biogeochemical cycles. We will convert the notes from the class into an accessible book or other publication at the introductory undergraduate level. In training, one graduate student and at least three undergraduate students will participate in the study. These students will be actively engaged in data collection, laboratory and field work, and data analysis exposing them to the process of interdisciplinary scientific research.
|Effective start/end date||6/1/10 → 5/31/14|
- National Science Foundation (NSF): $368,544.00