HR: 10:35h
AN: PP32A-02    [Abstracts]
TI: Glacial Chronologies Spanning the Past >100 Ky Along the Eastern Sierra Nevada From ¹⁰Be Surface Exposure Dating
AU: * Rood, D H
EM: rood5@llnl.gov
AF: Lawrence Livermore National Laboratory, Center for Accelerator Mass Spectrometry, MS L-397, 7000 East Avenue, Livermore, CA 94550,
AU: Burbank, D W
EM: burbank@crustal.ucsb.edu
AF: Department of Earth Science, University of California, Santa Barbara, CA 93106,
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: Lawrence Livermore National Laboratory, Center for Accelerator Mass Spectrometry, MS L-397, 7000 East Avenue, Livermore, CA 94550,
AB: A deeper understanding of the timing and extent of global paleoclimate variations includes knowledge of the age, duration, and extent of alpine glaciations around the world. Despite numerous past studies, the 10 ky-200 ky time span has been poorly explored due either to the paucity of dating techniques with sufficient resolution in this interval or to the absence of datable materials. Terrestrial cosmogenic nuclide (TCN) surface exposure dating allows more accurate, high-resolution chronologies in the 10²-10⁵ yr temporal window to be developed for geomorphic features that serve as records for past alpine glaciations. Answers to fundamental global climate questions have remained elusive at time scales where TCN surface exposure dating methods have proven an effective tool. Our study focuses on field sites where glacial landforms are well preserved for the past >100 ky. Along the eastern escarpment of the Sierra Nevada, the use of glacial deposits of different ages with high-resolution TCN dates permit a robust discrimination of the climate history in the region. Previous paleoclimate studies using TCN surface exposure dating methods along the eastern Sierra Nevada were generally constructed for glacial moraines using ³⁶Cl. We construct a record of moraines and outwash terrace ages using ¹⁰Be data from quartz, which can be directly compared to existing ³⁶Cl chronologies and other ¹⁰Be data. Our ¹⁰Be TCN ages on glacial landforms between Sonora Pass and Mono Basin (including deposits in the West Walker River, Buckeye Creek, Robinson Creek, Green Creek, Virginia Creek, and Mill Creek catchments) illustrate the viability of our approach. We produced >100 ¹⁰Be ages for boulder samples and depth profiles from 8 glacial moraines and 4 outwash terraces spanning MIS 2 through 6. Our data for the Last Glacial Maximum (MIS 2) are remarkably tightly clustered and support a central age with a small error (~3-6%; e.g. 17.2 ± 0.5 ka and 19.5 ± 1.2 ka). A similar clustering even holds for an outwash surface associated with the penultimate glaciation (MIS 6) for which the error is ~7% (136 ± 10 ka). We believe that with judicious selection of surface boulders and depth profiles to sample, it is possible to consistently generate geomorphic ages with <10% errors: this sets an important lower limit on our ability to delineate differences in the ages of glacial deposits (e.g. moraines of different ages and associated outwash surfaces). In short, our results generate within the study area a paleoclimate record spanning the past >100 ky using ¹⁰Be that allows comparison to LGM and older glacial chronologies globally.
DE: 0720 Glaciers
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 1621 Cryospheric change (0776)
DE: 1637 Regional climate change
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting