Radiogenic Isotope Systematics of Granitoid Weathering: the Influence of Climate and Glacial History.

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The proposed research will utilize Pb and Sr isotope systematics to investigate weathering rates and mechanisms in areas of granitoid bedrock. Waters, soils and bedrock will be investigated from drainages with variable glacial histories in arctic, temperate and tropical regions. Pb and Sr are well suited for this study because each has highly variable isotope ratios in granitic minerals, the two elements are concentrated into different minerals, and they have complementary aquatic behavior. Pb is relatively immobile in the hydrocycle (adsorbed by O-donor surface groups), whereas Sr is mobile in the hydrocycle (remains in solution). Pb and Sr isotopes studied in conjunction with other trace metals, major cations and anions can be used to look through secondary geochemical and biochemical processes and directly determine the relative rates of weathering of the various bedrock minerals. This study will have applications to estimations of modern weathering rates and mechanism in areas of variable climate and glacial history. We will also investigate the effects of global climate change of weathering by studying the fluvial and lacustrine record of changes in the isotopic composition of Pb (in Fe-Mn oxide coatings on sediment particles) and Sr (in calcareous shells) released by weathering back through the stratigraphic record of glacial and interglacial cycles.