Collaborative Research: Detrital Cosmochronology of the Greenland Ice Sheet

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ABSTRACTBiermanOPP-0713956FinkelOPP-0713938 Understanding Greenland Ice Sheet (GIS) history to determine when the ice sheet was smaller than today, is critical to understand increased atmospheric CO2 levels and warming climate. The Principal Investigators will associate the previously diminished GIS with other climate and environmental parameters and provide a foundation for predicting its future behavior. The measurement of in-situ-produced cosmogenic nuclides in samples collected from below the ice sheet has the potential to date past episodes of deglaciation through the analysis of isotopic ratios known as burial dating. When the ice cover diminishes, underlying rock and sediment are exposed to cosmic radiation, and radionuclides with differing half-lives (10Be, 26Al, 36Cl and 14C) are produced. When ice returns, exposed surfaces are buried and cosmic rays no longer reach the once-exposed surfaces. The inventory of radionuclides is unsupported by production and begins to diminish by radio-decay; isotopic ratios change predictably because each isotope has a different half-life. Preliminary multi-isotope cosmogenic analysis of rock collected from the bottom of the GISP2 borehole suggests that the summit area was deglaciated about 0.5 ky raising the specter that when climate warms, the ice sheet can disintegrate completely and perhaps not reform. The Principal Investigators will investigate an alternative approach to deep ice coring by studying the products of subglacial erosion and identify when Greenland was ice-free or partially ice-free. The results have the potential to tell us how the GIS responded to intervals of major climate warming over the past several million years.Intellectual Merit: This study will rely on sub-glacial erosion to sample previously-exposed rock surfaces and sediment. It will provide previously unavailable information about the GIS by using isotope ratio analysis to identify past times when the rock and sediment beneath the GIS were exposed to cosmic radiation. Ice flow and englacial drainages deliver sediments to the ice margin where they will collect and analyze clasts directly from outcropping ice. The Principal Investigators will analyze populations of burial ages to determine modes of initial exposure time from which they will infer times of major ice retreat in the past. To interpret isotopic data in glaciological context, they will use existing ice flow and thermal models to infer basal conditions and clast transit histories. Isotopic data will indicate glacial erosion efficiency. Because the penetration depth of most cosmic radiation is only several meters, large numbers of clasts containing no cosmogenic nuclides would indicate efficient sub-ice erosion whereas many clasts with significant burial ages would indicate long subglacial residence times and low rates of bed erosion and sediment transport.Broader Impact: This methodology should have wide application in other areas currently covered by ice. Model ages for GIS shrinkage will provide information for understanding a major driver of sea-level change and have important paleoclimaticimplications. Isotopic data will guide future ice coring efforts, particularly those intended to sample sub-ice rock. Two graduate students will be supported.