HR: 09:30h
AN: T31E-07    [Abstracts]
TI: The Isotopic Rain Shadow and Elevation History of the Sierra Nevada Mountains, California
AU: * Koch, P L
EM: pkoch@pmc.ucsc.edu
AF: Department of Earth Sciences, University of California, Santa Cruz, CA 95064 United States
AB: Research on the elevation history of the Sierran Nevada Mountains has yielded conflicting results. Some studies argue for substantial uplift within the last 3 to 5 million years (Ma); others propose that high elevations may have existed since the Cretaceous. Because heavier isotopes of oxygen are concentrated in rain and snow as they form from clouds, the rain shadow generated by the Sierra Nevada creates a strong isotopic gradient, with ¹⁸O-depleted precipitation on the eastern, leeward side of the range. Thus, stable oxygen isotope ratios can be used to monitor the development of the Sierran rain shadow as a proxy for high mountains blocking the flow of moisture from the Pacific Ocean. Prior work using this approach has focused exclusively on samples from the eastern side of the Sierras. When looking solely at the leeside of a mountain range, however, it is difficult to preclude alternate explanation for shifts in the isotopic composition of local precipitation (e.g., climate change, changes in moisture sources). We address this problem by analyzing the oxygen isotope composition of tooth enamel bioapatite from contemporaneous mammalian fossils on either side of the present Sierran range. Mammals ingest meteoric water, incorporating its oxygen into their tooth enamel, so isotopic analysis of enamel bioapatite can be used to monitor the isotopic composition of surface water. By sampling on both sides of the range, differences in oxygen isotope composition induced by a rain shadow can be isolated from regional and global factors like climate fluctuations. Our results indicate that the Sierras have generated a strong rain shadow for at least 18 Ma. Unfortunately, resolution for locality ages is not fine enough to differentiate between glacial and interglacial climate intervals during the last 2 Ma. As a consequence, we cannot determine whether the Sierras experienced a Late Cenozoic pulse of uplift, but we can conclude that very substantial elevation was present across the range prior to the Late Cenozoic.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1055 Organic and biogenic geochemistry
DE: 4914 Continental climate records
DE: 8177 Tectonics and climatic interactions
SC: Tectonophysics [T]
MN: Fall Meeting 2005