Paper No. 16-6
Presentation Time: 3:10 PM
COSMOGENIC 26AL AND 10BE MEASUREMENTS FROM CAMP CENTURY BASAL SEDIMENT INDICATE GREENLAND ICE SHEET ABSENCE IN THE LAST MILLION YEARS
CHRIST, Andrew J.1, BIERMAN, Paul R.2, CORBETT, Lee B.2, SCHAEFER, Joerg M.3, HIDY, Alan J.4, CAFFEE, Marc W.5 and DAHL-JENSEN, Dorthe6, (1)Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave., Burlington, VT 05405, (2)Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, (3)Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, (4)Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, (5)Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN 47907, (6)Neils Bohr Institute, University of Copenhagen, Tagensvej 16, Copenhagen, 2200, Denmark
The Greenland Ice Sheet is melting in response to a rapidly warming climate. Because Greenland contains ~7 m of sea level rise equivalent, it is imperative to understand its sensitivity to past climate, especially during periods when the ice sheet was smaller or even absent. The Camp Century ice core, collected in 1966 and the first to be drilled to the bed of the Greenland Ice Sheet, revolutionized our understanding of global paleoclimate over the last 125 kyr. However, basal sediment from the ice core was not fully explored and then lost for decades – until it was re-discovered in a Danish freezer two years ago. These unique samples of the sub-glacial environment are now being investigated using modern, state-of-the-art analyses including cosmogenic radionuclide analyses.
We wet sieved samples from the upper and lower parts of 4 m of basal sediment into multiple grain size fractions. We measured in situ cosmogenic 10Be and 26Al concentrations in quartz from the 250-500 and 500-850 um fractions to evaluate the exposure and burial history of sediment below the Greenland Ice Sheet. All samples yielded 10Be concentrations on the order of 104 atoms/g. In the upper basal sediment, 10Be concentrations decrease with finer grain size, from 7.7 ± 0.1 x104 atoms/g (500-850 µm) to 6.6 ± 0.2 x104 (250-500 µm). In the lower basal sediment, 10Be concentrations show less variability with grain size: 1.6 ± 0.1 x104 atoms/g (500-850 µm) and 1.8 ± 0.1 x104 (250-500 µm). 26Al/10Be ratios range between 4.4 and 5.1, indicating a complex exposure and burial history. Such ratios require 20 to 30 ka of exposure and 600 to 900 ka of burial. 10Be concentrations and 26Al/10Be ratios from the basal sediment of the Camp Century core suggest the absence of ice cover in northwest Greenland within the last 1 Ma. These data agree with a growing body of evidence that indicates instability and large-scale absence of the Greenland Ice Sheet during periods of the Late Pleistocene.
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