EGU2020-12243
https://doi.org/10.5194/egusphere-egu2020-12243
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Camp Century ice core basal sediments record the absence of the Greenland Ice Sheet within the last million years

Andrew Christ1,2, Paul Bierman1, Dorthe Dahl-Jensen3, Jørgen Steffensen3, Dorothy Peteet4, Elizabeth Thomas5, Owen Cowling5, Eric Steig6, Lee Corbett1, Joerg Schaefer4, Alan Hidy7, Marc Caffee8,9, Tammy Rittenour10, Jean-Louis Tison11, Pierre-Henri Blard12, Marie Protin12, and John Southon13
Andrew Christ et al.
  • 1Department of Geology, University of Vermont, Burlington, USA
  • 2Gund Institute for Environment, University of Vermont, Burlington, USA
  • 3Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
  • 4Lamont-Doherty Earth Observatory, Columbia University, New York, USA
  • 5Department of Geology, University at Buffalo, Buffalo, USA
  • 6Department of Earth and Space Sciences, University of Washington, Seattle, USA
  • 7Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, USA
  • 8Department of Physics and Astronomy, Purdue University, West Lafayette, USA
  • 9Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, USA
  • 10Department of Geosciences, Utah State University, Logan, USA
  • 11Laboratoire de Glaciologie, Université Libre de Bruxelles, Brussels, Belgium
  • 12CRPG, CNRS-Université de Lorraine, Vandoeuvre-lès-Nancy, France
  • 13Department of Earth System Science, University of California Irvine, Irvine, USA

The Greenland Ice Sheet (GrIS) is melting in response to a rapidly warming climate.  It is imperative to understand GrIS sensitivity to past climate, especially during periods when the ice sheet was smaller than present or possibly absent. The Camp Century ice core from NW Greenland, collected in 1966 and the first ice core to be drilled to the bed of the GrIS, revolutionized our understanding of global paleoclimate since 125 ka. However, basal sediment from the ice core was not fully explored and then sat in storage for decades – until it was re-discovered two years ago. We are now investigating these unique samples from the sub-glacial environment using modern analyses. 

Here, we present initial results from two samples, the upper and lower portions of >4 m of basal sediment. We applied an array of geochemical analyses to characterize paleoenvironment (lipid biomarkers, δ13C, δ15N), to infer past climatic conditions (δ18O, δD) from frozen pore water, and to determine the exposure and burial history of the sediments below the ice sheet (optically stimulated luminescence [OSL], cosmogenic 10Be, 26Al, and 21Ne). 

The sub-glacial sediment consists of poorly sorted, reddish-brown, quartz-rich diamict, with paleo-permafrost features in some layers. This material contains woody macrofossils, fungal sclerotia (Cenococcum geophilum), and mosses (Tomenthypnum nitens, Polytrichum juniperinum) that yield a 14C age >55 ka. Woody tissue from the upper and lower samples yield stable δ13C ratios of -26.7±0.1‰ and -29.6±0.1‰ and δ15N ratios of 2.4±0.8‰ and -2.3±0.8‰. Leaf wax (n-alkanoic acid) distributions are similar to modern Arctic shrubs. Frozen pore water yielded δ18O ratios of -23.06±0.08‰ and -21.49±0.08‰, enriched relative to all overlying ice (<-27‰). Deuterium-excess values are 4.3±0.8 ‰ and 13.4±0.4 ‰, respectively.  These stable isotope measurements of pore water suggest snowfall precipitation at temperatures similar to today if the site were ice-free. OSL measurements from the lower sediment suggest a minimum depositional age >600 ka. In situ 10Be concentrations in quartz decrease with depth from 7.7±0.1 x104 atoms/g (500-850 µm) and  6.6±0.2 x104 (250-500 µm) in the upper sediment to 1.6±0.1 x104 atoms/g (500-850 µm) and 1.8±0.1 x104 (250-500 µm) in the lower sediment. The 26Al/10Be ratio also decreases with depth. In the upper sediment, 26Al/10Be ratios range between 4.2 and 4.9 indicating > 900 ka of burial. In the lower sediment, 26Al/10Be ratios range from 1.4 to 2.0 indicating >2 Ma of burial. Measured 21Ne/10Be ratios in quartz exceed 1000, which could indicate long-term burial and/or the presence of nucleogenic 21Ne.

These results demonstrate that Camp Century basal sediment was exposed under ice-free conditions that supported vegetation similar to today. Cosmogenic data indicate the deeper sediment has been buried for most of the Pleistocene and the OSL date rules out surface exposure of the deeper material at MIS 11. Cosmogenic analysis indicates that the upper sample experienced less burial or more recent re-exposure. These data are consistent with a growing body of evidence indicating a dynamic Pleistocene GrIS, even under a pre-industrial climate system in which atmospheric CO2 concentrations did not exceed ~300 ppm.

How to cite: Christ, A., Bierman, P., Dahl-Jensen, D., Steffensen, J., Peteet, D., Thomas, E., Cowling, O., Steig, E., Corbett, L., Schaefer, J., Hidy, A., Caffee, M., Rittenour, T., Tison, J.-L., Blard, P.-H., Protin, M., and Southon, J.: Camp Century ice core basal sediments record the absence of the Greenland Ice Sheet within the last million years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12243, https://doi.org/10.5194/egusphere-egu2020-12243, 2020

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