GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 51-3
Presentation Time: 2:05 PM

ANALYSIS OF THREE COSMOGENIC ISOTOPES IN SUBGLACIAL COBBLES HELPS UNRAVEL GREENLAND’S EXPOSURE AND EROSION HISTORY


CORBETT, Lee B.1, BIERMAN, Paul R.1, NEUMANN, Thomas A.2, GRALY, Joseph A.3, SHAKUN, Jeremy D.4, CAFFEE, Marc W.5, DUNAI, Tibor6 and ZIMMERMAN, Susan H.7, (1)Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, (2)NASA Goddard Space Flight Center, Cryospheric Sciences Branch, Code 614.1, 8800 Greenbelt Road, Greenbelt, MD 20770, (3)Earth Sciences, IUPUI, 723 West Michigan Street, SL 118, Indianapolis, IN 46202, (4)Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA 02467, (5)Department of Physics, Purdue University, West Lafayette, IN 47906, (6)Institute of Geology and Mineralogy, University of Cologne, Albertus-Magnus-Platz, Cologne, 50923, Germany, (7)Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550, Ashley.Corbett@uvm.edu

The size and erosion history of the Greenland Ice Sheet over the Pleistocene remain poorly constrained despite extensive research employing a variety of different chemical, physical, and isotopic techniques. Here, we report results of a new approach, cosmogenic nuclide analysis of subglacial and englacial cobbles, which allows us to infer the behavior of the ice sheet over time and space.

We collected 86 cobbles from glacial ice at the current margin and from proximal outwash channels at three locations in western Greenland: Kangerlussuaq (67°N), Ilulissat (69°N), and Upernavik (72°N). We measured the concentration of in situ 10Be in all cobbles (n = 86) and 26Al and 14C in a subset (n= 14, those with the highest 10Be). Most cobbles contain very low concentrations of 10Be (median 1.0 x 103 atoms g-1), although the population is highly skewed; several cobbles have ~104 and one cobble has ~105 atoms g-1 of 10Be. Ratios of 26Al/10Be, which reflect exposure/burial histories, range from 5.0 ± 1.1 to 8.4 ± 1.2. Nine cobbles have 14C above detection limits, and the 14C concentrations form a significant linear relationship with the 10Be and 26Al concentrations.

Overall, the low concentration of 10Be in most subglacial cobbles suggests that their source areas under the Greenland Ice Sheet have been deeply eroded since the ice sheet was established ~2.5 Ma. The source of the low but measurable 10Be is likely from muon-induced production in deep bedrock prior to ice sheet growth; the rocks we analyzed were then exhumed, plucked, and transported to the margin with little or no surface exposure. The few cobbles containing higher concentrations of 10Be were re-exposed during at least one warm period when the ice sheet was smaller than at present. The range of 26Al/10Be ratios, with most showing no extended burial, but several indicating burial of at least hundreds of ky, indicates a variety of exposure/burial histories. The presence of measurable 14C (10 cobbles) and the positive 14C/10Be covariance (8 of 10 cobbles) suggest near-surface exposure after the Last Glacial Maximum, presumably during the Holocene Climate Optimum. The 14C/10Be ratio of ~6 that we measure is more consistent with muon-induced production under ice or soil cover than surface exposure (for which 14C/10Be is ~3).