MEASURING 10BE AGES FROM TOP TO BOTTOM IN FRANCONIA NOTCH, NH, TO CONSTRAIN LAURENTIDE ICE SHEET HISTORY

As part of an ongoing effort to constrain the thinning history of the southeastern sector of the Laurentide Ice Sheet (LIS) during its retreat from New England, we obtained nine in situ ¹⁰Be exposure ages for glacially-deposited boulders and polished bedrock at various elevations (623 - 1597 m a.s.l.) in Franconia Notch, N.H. Accurate cosmogenic exposure dating requires that the LIS caused sufficient erosion to remove most ¹⁰Be produced during previous periods of exposure; if this were the case, then ¹⁰Be would begin accumulating in exposed bedrock and boulders when the ice surface lowered below sampled elevations, providing a chronometer of the time elapsed since deglaciation.

Previous attempts to constrain the thinning history of the LIS in the Franconia Notch area have primarily used radiocarbon dating of basal lake sediments to provide minimum-limiting ages of ice retreat. Examples include a near-basal sediment core ages of 11.1-13.4 cal ¹⁴C ka on bulk sediments from Lonesome Lake (832 m a.s.l.) by Spear et al. (1994) and 12.8-12.9 ¹⁴C cal ka on wood from Profile Lake (588 m asl) by Rogers et al. (2010).

Exposure ages along our vertical transect can be divided into two groups. Below 1400 m a.s.l., ages (n=5) are consistent with the timing of regional deglaciation and overlap within uncertainty, suggesting that the ice sheet rapidly thinned in this area during the Bølling/Allerød, 14.6 - 12.7 ka. Ages above 1400 m a.s.l. (n=4), in contrast, are old, ranging from ~26 to 155 ka. These old exposure ages, which predate the Last Glacial Maximum, are consistent with a previous ¹⁰Be age (59.6 ka) obtained from Little Haystack Mountain at the southern end of Franconia Ridge and suggest that at least some and in one case, much ¹⁰Be was inherited from previous exposure due to little if any glacial erosion at these higher elevations. This phenomenon has been observed at high elevations on Katahdin in Maine and Mount Washington in N.H. (Bierman et al., 2015; Koester, 2016; Davis et al., 2017), suggesting that the basal thermal regime of the LIS may have had an elevation threshold, above which ‘warm-based’ erosive ice transitioned into ‘cold-based’ with significantly reduced erosion.