RAPID THINNING OF THE LAURENTIDE ICE SHEET AT MT. WASHINGTON, NH, DURING THE BØLLING WARMING, CONSTRAINED BY ANALYSIS OF COSMOGENIC 14C AND 10BE

The rate and timing of the lateral retreat of the Laurentide Ice Sheet in northeastern United States is relatively well established; however, its thickness history is largely unknown due to a lack of direct constraints on ice elevation through time, particularly at the end of the last glaciation. Reconstructing the thinning history of the ice sheet is necessary to understand better its response to climate change, freshwater input to the ocean, and contribution to sea level rise.

We generated 20 new cosmogenic ¹⁰Be exposure ages and six new cosmogenic ¹⁴C exposure ages along a vertical transect from 700 to 1900 m a.s.l. on Mt. Washington, New Hampshire, the tallest peak in New England, to constrain the timing and rate of the Laurentide Ice Sheet thinning in this region. ¹⁰Be ages above ~1500 m a.s.l. record exposure histories 2-6 times longer than the regional deglaciation age (~14 ka) derived from the North American varve chronology, suggestive of non-erosive, cold-based ice on the upper reaches of the mountain that did not remove nuclides inherited from prior periods of exposure. The Presidential Upland, a geomorphic surface 200-300 m below the Mt. Washington summit cone, might thus owe its morphology to this non-erosive ice cover. ¹⁴C exposure ages on the Presidential Upland (30.0 ± 17.2 and 24.0 ± 8.4 ka) are older than those both on the summit (16.4 ± 2.9 and 18.2 ± 3.8 ka) as well as halfway down the mountain (11.7 ± 1.7 ka and 9.7 ± 1.1 ka). Below ~1500 m a.s.l., ¹⁰Be exposure ages are indistinguishable down to 700 m a.s.l. centered on ~14.5 ka. Together with ¹⁰Be ages of 13-14 ka on the Littleton-Bethlehem and Androscoggin moraines at ~300 m a.s.l. just north of Mt. Washington (Bromley et al., 2015), these data imply century-scale ice-sheet thinning of over ~1 km during the Bølling Interstadial.