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Trace metal signatures in continental sedimentary systems: Isotope constraints, biogeochemistry, and potential as a paleoclimatic tracer

Intellectual merit. The proposed work will address hypotheses regarding the<br/>biogeochemical interactions that lead to the removal of molybdenum, uranium, and<br/>vanadium from aquatic systems and the fractionation of Mo isotopes that occurs during<br/>organic matter decomposition. Our study site, Lake Tanganyika, is an ancient rift lake that<br/>has an oxygenated upper water column and sulfidic deep waters. The existence of<br/>sedimentary deposits underlying both these waters offers an exciting and timely opportunity<br/>to quantify changes in metal solubility and to test predictions regarding isotope fractionation<br/>associated with metal precipitation. Our work is guided by two hypotheses that predict a<br/>close link between metal accumulation rate and carbon cycling. These hypotheses predict<br/>different behavior for Mo isotopes depending on whether or not the overlying water has<br/>sulfide present. (1) In regions where the overlying water column is absent of sulfide, we<br/>hypothesize that trace element accumulation in the sediments and Mo isotope compositions<br/>are closely coupled to the cycling of organic carbon. (2) Where sediments are bathed with<br/>sulfide-rich water, we hypothesize that trace element enrichments remain sensitive to the<br/>organic carbon cycling but Mo isotope values are invariant. Although we use these<br/>hypotheses as a guide for our research, we do expect that the targeted elements will exhibit a<br/>range of sensitivities to organic carbon cycling and water column sulfide concentrations.<br/>Broader impacts I: Scientific value. The targeted elements already offer potential as<br/>biogeochemical tracers, and this proposal represents an exciting opportunity to address<br/>important issues regarding their environmental chemistry. Molybdenum isotopes also<br/>retain potential as a tracer for the availability of oxygen and sulfide through geologic time,<br/>and this proposal will allow us to examine how the presence of water column sulfide<br/>influences Mo isotope behavior. Finally, the African Great Lakes harbor rich records of<br/>climate history extending back to the Miocene. What we offer here is an opportunity to<br/>calibrate a suite of sedimentary tracers that could ultimately provide powerful insights on<br/>biogeochemical process, not only in Tanganyika, but in other sedimentary systems as well.<br/>Broader impacts II: Human impacts and opportunities for education. Lake<br/>Tanganyika is a valuable resource to the East African people. The lake contains a<br/>significant fraction of the Earth's fresh water and supplies the economies of several nations<br/>with a valuable fisheries resource. In a broad sense, this proposal offers tools for<br/>addressing questions related to how the lake's carbon dynamics have responded to past<br/>changes in climate. As part of this proposal, McManus will be participating in the Nyanza<br/>project during the summer of 2006. The Nyanza Project is an undergraduate/graduate field<br/>research program that offers opportunities to study paleoclimatology and geology in a<br/>continental rift cradling one of the world's oldest lakes. As a participant in the program<br/>McManus will be engaged in training both U.S. and African students, and we plan on<br/>having one African student and two U.S. students working on our field research. We will<br/>also support an OSU undergraduate during our second year of funding. Undergraduate<br/>students have contributed significantly to the PI's research program resulting in abstracts<br/>delivered at major conferences, university-wide student research programs, and coauthorship<br/>on two manuscripts. Our college is beginning to engage students from our university's<br/>honors college in research and we are planning on initiating an REU program in the near<br/>term. We anticipate having a student from one of these programs participating in the<br/>proposed research as well. Finally, this proposal will provide support for a postdoctoral<br/>research associate (Chris Siebert).

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