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

Oregon State University, Corvallis OR

Investigators

Abstract

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