This research has two primary goals: 1. to compare rigorously eight different techniques used for determining the exposure-ages of geomorphic surfaces; 2. to understand the timing of alluvial fan aggradation on the bajada below the eastern Sierra Nevada. During the first year of the project, cores will be collected from boulders on a chronologic sequence of five geomorphic surfaces for which exposure-ages are constrained by previous geologic investigation. For each boulder and surface, model exposure-ages will be calculated from the measured accumulation of different cosmogenic isotopes produced in situ including 3He, 10Be, 36Cl, and in some cases 26Al and 14C. After comparing the ages obtained from isotopic methods with existing ages estimates, varnish cation-ratio and radiocarbon ages, and 10Be accumulation in soils, the most appropriate system(s) for determining the temporal and spatial distribution of alluvial fan aggradation will be chosen, also, there will be an intensive study of two drainages in the eastern Sierra Nevada, one glaciated, the other unglaciated. To determine if glaciation and fan aggradation were synchronous, the abundance of cosmogenic isotopes and calculate model exposure-ages for samples taken from boulders on fan surfaces and glacial moraines will be obtained. Age-data gathered in the second year of this study will provide a direct and important link between the marine isotope and terrestrial records of climate change. Because of the inter-disciplinary project team and because this investigation is part of a larger, coordinated University of Washington effort to understand the genesis, chronology, and response to climate change of Sierra Nevada alluvial fans, this project represents a unique opportunity for advancement in the fields of geomorphology and Quaternary geochronology.
