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

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Intellectual merit. The proposed work will address hypotheses regarding thebiogeochemical interactions that lead to the removal of molybdenum, uranium, andvanadium from aquatic systems and the fractionation of Mo isotopes that occurs duringorganic matter decomposition. Our study site, Lake Tanganyika, is an ancient rift lake thathas an oxygenated upper water column and sulfidic deep waters. The existence ofsedimentary deposits underlying both these waters offers an exciting and timely opportunityto quantify changes in metal solubility and to test predictions regarding isotope fractionationassociated with metal precipitation. Our work is guided by two hypotheses that predict aclose link between metal accumulation rate and carbon cycling. These hypotheses predictdifferent behavior for Mo isotopes depending on whether or not the overlying water hassulfide present. (1) In regions where the overlying water column is absent of sulfide, wehypothesize that trace element accumulation in the sediments and Mo isotope compositionsare closely coupled to the cycling of organic carbon. (2) Where sediments are bathed withsulfide-rich water, we hypothesize that trace element enrichments remain sensitive to theorganic carbon cycling but Mo isotope values are invariant. Although we use thesehypotheses as a guide for our research, we do expect that the targeted elements will exhibit arange of sensitivities to organic carbon cycling and water column sulfide concentrations.Broader impacts I: Scientific value. The targeted elements already offer potential asbiogeochemical tracers, and this proposal represents an exciting opportunity to addressimportant issues regarding their environmental chemistry. Molybdenum isotopes alsoretain potential as a tracer for the availability of oxygen and sulfide through geologic time,and this proposal will allow us to examine how the presence of water column sulfideinfluences Mo isotope behavior. Finally, the African Great Lakes harbor rich records ofclimate history extending back to the Miocene. What we offer here is an opportunity tocalibrate a suite of sedimentary tracers that could ultimately provide powerful insights onbiogeochemical process, not only in Tanganyika, but in other sedimentary systems as well.Broader impacts II: Human impacts and opportunities for education. LakeTanganyika is a valuable resource to the East African people. The lake contains asignificant fraction of the Earth's fresh water and supplies the economies of several nationswith a valuable fisheries resource. In a broad sense, this proposal offers tools foraddressing questions related to how the lake's carbon dynamics have responded to pastchanges in climate. As part of this proposal, McManus will be participating in the Nyanzaproject during the summer of 2006. The Nyanza Project is an undergraduate/graduate fieldresearch program that offers opportunities to study paleoclimatology and geology in acontinental rift cradling one of the world's oldest lakes. As a participant in the programMcManus will be engaged in training both U.S. and African students, and we plan onhaving one African student and two U.S. students working on our field research. We willalso support an OSU undergraduate during our second year of funding. Undergraduatestudents have contributed significantly to the PI's research program resulting in abstractsdelivered at major conferences, university-wide student research programs, and coauthorshipon two manuscripts. Our college is beginning to engage students from our university'shonors college in research and we are planning on initiating an REU program in the nearterm. We anticipate having a student from one of these programs participating in theproposed research as well. Finally, this proposal will provide support for a postdoctoralresearch associate (Chris Siebert).