GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 218-8
Presentation Time: 3:40 PM


CHRIST, Andrew J.1, BIERMAN, Paul R.2, KNUTZ, Paul C.3, CORBETT, Lee B.2, FOSDICK, Julie C.4, THOMAS, Elizabeth K.5, COWLING, Owen5, HIDY, Alan J.6 and CAFFEE, Marc W.7, (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)Geological Survey of Denmark and Greenland, Ă˜ster Voldgade 10, Copenhagen, 1350, Denmark, (4)Geosciences, University of Connecticut, 354 Mansfield Road, U-1045, Storrs, CT 06269, (5)Department of Geology, University at Buffalo, 126 Cooke Hall, University at Buffalo, North Campus, Buffalo, NY 14260-4130, (6)Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, (7)Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN 47907

Millions of years of glacial erosion by the Greenland Ice Sheet (GrIS) has overrun and transported terrestrial evidence of past changes in climate, landscape, and vegetation to the ocean where it has been deposited as glacial marine diamict. This poorly sorted mixture of grain sizes and lithologies, deposited by or proximal to grounded ice on the continental shelf, provides a direct record of ice sheet and paleoclimate history. However, glacial marine diamict has been historically under-utilized because it can be difficult to recover by drilling, to date, and to interpret. Yet, the diversity and abundance of coarse-grained material in diamict allows for chemical and isotopic analyses commonly applied to terrestrial archives, presenting the possibility for a promising new archive of information.

Here, we present a novel multi-proxy geochemical analysis of Pleistocene diamict cored in Melville Bugt, northwestern Greenland, to resolve ice sheet, landscape, and vegetation history. Extremely low cosmogenic nuclide concentrations (meteoric 10Be and in situ 10Be and 26Al) indicate the diamict is composed of sediment with minimal near-surface exposure. 26Al/10Be ratios in this diamict exceed the observed Greenland surface production ratio, suggesting the sediment is comprised of sub-glacial material sourced more than several meters deep, where nuclide production by muons dominates. (U-Th-Sm)/He ages of detrital apatite grains all pre-date widespread glaciation of Greenland by >150 million years, suggesting this sector of the ice sheet had not yet incised deep fjords required to excavate grains from sufficient depth to have young (U-Th-Sm)/He ages when the diamict was deposited. The diamict contains well-preserved leaf waxes produced by terrestrial plants, demonstrating that there were exposed land surfaces surrounding the ice sheet for at least short periods.

These data indicate that a persistent, dynamic GrIS existed in northwestern Greenland prior to 1.8 Ma. Similar to today, an erosive, marine-terminating ice sheet covered much of Greenland and harbored coastal ice-free areas supporting vegetation. The novel approach we demonstrate could be applied to many sediment cores from formerly glaciated continental margins to reveal new information about past landscape, climate, and vegetation.