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2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 10
Presentation Time: 9:00 AM-6:00 PM

IN SITU COSMOGENIC 10Be ESTIMATES OF DEGLACIATION TIMING AND GLACIAL EROSION EFFICIENCY, WESTERN GREENLAND


CORBETT, Lee B., Department of Geology and Rubenstein School of Environment and Natural Resources, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, BIERMAN, Paul, Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, GRALY, Joseph A., Department of Geology and Geophysics, University of Wyoming, 1000 University Ave, Laramie, WY 82071, NEUMANN, Thomas A., NASA Goddard Space Flight Center, Cryospheric Sciences Branch, Code 614.1, 8800 Greenbelt Road, Greenbelt, MD 20770, ROOD, Dylan H., Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, MS L-397, 7000 East Avenue, Livermore, CA 94550-9234 and FINKEL, Robert C., Department of Earth and Planetary Science, University of California, Berkeley, 371 McCone Hall, Berkeley, CA 94720, Ashley.Corbett@uvm.edu

To investigate the retreat of the Greenland Ice Sheet during interglacial periods and the erosivity of the Greenland Ice Sheet during glacial periods, we measured in situ cosmogenic 10Be in 12 samples from Western Greenland.

Boulder/bedrock pairs from three locations near Upernavik (72°N) were sampled in a transect stretching from the sea to the present-day ice margin. Two low-elevation (~ 25 m a.s.l.) erratic boulders located 35 km apart have exposure ages of 11.1 and 12.5 ka, providing an estimated deglaciation age for central-western Greenland. Adjacent bedrock samples are discordant, having modeled exposure ages of 13.3 and 16.7 ka, respectively. It appears that low-elevation ice was not erosive enough to completely remove rock containing 10Be accumulated during previous interglacial periods. One high-elevation (~1000 m a.s.l.) erratic boulder (43 ka) and a corresponding bedrock sample (80 ka) indicate that high-elevation ice during the latest Pleistocene glaciation was even less erosive and failed to remove cosmogenic 10Be inherited from previous periods of exposure. One possible scenario is that warm-based, erosive ice existed in the valleys, while cold-based, less-erosive ice existed in the highlands. It is also possible that the ice grew thinner after 40 ka BP, exposing the high-elevation boulder we sampled. Analytic uncertainties of exposure ages are 2 – 4%.

To determine the amount of inherited 10Be in clasts carried by the Greenland Ice Sheet, icebound rocks were removed from the glacier margin near Kangerlussuaq (67°N), Ilulissat (69°N), and Upernavik (72°N). These clasts were sourced up-ice of the present-day ice margin. So far, two clasts from each site (n = 6) have been analyzed for cosmogenic 10Be; only one (from Kangerlussuaq) yielded a 10Be concentration robustly above blank level, indicative of about 1 ka of surface exposure. We hypothesize that the ice sheet retreated or thinned enough during a previous interglacial period to expose this clast or its source outcrop to cosmic radiation, suggesting that the ice sheet was at some point smaller than it is at present.

Session No. 244--Booth# 51

Geomorphology (Posters)
Wednesday, 21 October 2009: 9:00 AM-6:00 PM
Hall A (Oregon Convention Center)
Geological Society of America Abstracts with Programs. Vol. 41, No. 7, p.621
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