GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 121-3
Presentation Time: 9:00 AM-6:30 PM


HALSTED, Christopher T.1, SHAKUN, Jeremy D.2, CORBETT, Lee B.3, BIERMAN, Paul R.4, DAVIS, P. Thompson5, GOEHRING, Brent M.6, KOESTER, Alexandria J.2 and CAFFEE, M.W.7, (1)Department of Geology, University of Vermont, Burlington, VT 05405; Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, VT 05405, (2)Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA 02467, (3)Department of Geology, The University of Vermont, 180 Colchester Ave., Burlington, VT 05405, (4)Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, (5)Department of Natural & Applied Sciences, Bentley University, 175 Forest St, Waltham, MA 02452, (6)Dept. of Earth and Environmental Sciences, Tulane University, 6823 St Charles Ave, New Orleans, LA 70118, (7)Department of Physics, Purdue University, 1396 PHYSICS BLDG, W. Lafayette, IN 47907-1396

A lack of empirical data constraining the thinning history of the Laurentide Ice Sheet (LIS) has resulted in uncertainty about its volume changes and dynamics during retreat – information critical to understanding its contribution to sea level rise, influence on paleoclimate, and response to the changing climate of the deglacial period. To provide insight about ice sheet history and behavior, we collected 150 samples for in-situ 10Be cosmogenic exposure dating from various elevations on numerous mountains in the northeastern United States. By calculating ages of exposure at different elevations (i.e., ice sheet “dipsticks”), we are reconstructing the lowering paleo-ice surface of the southeastern LIS. Using radionuclide pairs (in-situ 10Be and 14C) at critical locations (n=10), we are also investigating erosion that took place during glaciation.

Low-elevation (<1200 m a.s.l.) samples with measured 10Be concentrations (n=89) mostly agree with published deglacial chronologies in the region and suggest rapid ice thinning during retreat. Mountain-top exposure ages located within 150 km of the southeastern LIS terminal moraine indicate that near-margin thinning began early in the deglacial period (~19.5 to 17.5 ka), coincident with the slow initial margin retreat indicated by varve records. Further inland (>400 km north of the terminal moraine), exposure ages collected over ~1000 m elevation spans suggest ice thinning between 14.5 and 13 ka, occurring at about the same time that varve records indicate accelerated ice retreat during the Bølling-Allerød warm period (14.6–12.9 ka). Ages across the inland vertical transects are similar to each other within 1σ internal uncertainties, indicating that ice thinning was rapid, taking place over hundreds of years at most.

High-elevation (>1200 m a.s.l.) samples across the study area (n=30) exhibit isotopic evidence of burial and minimal subglacial erosion. In-situ 14C exposure ages younger than 10Be ages from the same elevations indicate that the older 10Be ages are likely due to incomplete removal of pre-Last Glacial Maximum 10Be by the LIS. This dynamic suggests that high-elevation mountain landscapes in this region were covered by cold-based, non-erosive ice during the last glaciation, consistent with a paleo-ice surface not far above the tops of these mountains.