DETERMINING THE TIMING AND RATE OF SOUTHEASTERN LAURENTIDE ICE SHEET THINNING USING IN-SITU COSMOGENIC 10BE

A lack of empirical data constraining the thinning history of the Laurentide Ice Sheet has resulted in uncertainty about volume changes and dynamics of the ice sheet – information critical to understanding its contribution to sea level rise, influence on paleoclimate, and response to the changing climate of the deglacial period. To address the lack of thinning information we collected 120 samples for in-situ ^`10Be exposure dating from various elevations at numerous mountains in the northeastern United States and southern Quebec, Canada. By calculating ages of exposure at different elevations across this region we will track the lowering paleo-ice surface of the southeastern Laurentide Ice Sheet.

Presently, we have processed 62 samples. Exposure ages from Peekamoose Mt., southern NY, and Mt. Greylock, MA, indicate ice thinning early in the deglacial period (~19.5 – 17.5 and 15.5-14 ka, respectively). Samples from Franconia Notch, NH, Mt. Washington, NH, and Mt. Mansfield, VT, suggest ice thinning from between 14.5 and 13 ka in northern New England. Exposure ages from each of the northern New England mountains are similar within 1σ internal uncertainty, indicating that ice thinning was rapid.

Higher elevation (>1200 m a.s.l.) samples from the northern New England mountains contain high ¹⁰Be concentrations that yield simple exposure ages older than the Last Glacial Maximum, including some older than 50 ka. These samples likely contain ¹⁰Be inherited from previous periods of exposure, indicating a lack of deep glacial erosion on these surfaces. These summit landscapes may thus have been preserved beneath cold-based, non-erosive ice during the last glacial period.

Our exposure ages for the southeastern Laurentide Ice Sheet span much of the last deglaciation, indicating that sections of this ice mass were thinning at almost all times. Early deglacial thinning in southern New York and New England occurred in tandem with the slow initial margin retreat from 19.5 to15 ka, while rapid thinning in northern New England occurred at approximately the same time (14.5-13 ka) as varve records indicate accelerated ice retreat. Together, these data suggest that the Bølling-Allerød warm period was marked by significant ice volume loss from the southeastern Laurentide Ice Sheet.