MULTIPLE COSMOGENIC NUCLIDES CONSTRAIN LAURENTIDE ICE SHEET HISTORY AND PROCESS ON MT. MANSFIELD, VERMONT’S HIGHEST PEAK (Invited Presentation)

Understanding glacial history and process on Mt Mansfield, the highest peak in Vermont (1339 m), provides insight into how the Laurentide Ice Sheet shaped the underlying landscape, how rapidly Late Pleistocene ice retreated, and how upland and lowland glacial histories relate. Here, we report in situ cosmogenic ¹⁰Be data for 20 bedrock and boulder surfaces (sample elevations 409-1305 m), as well as cosmogenic ¹⁴C in three of those surfaces, to assess subglacial erosion and exposure history.

Consistent exposure ages that match other chronometers in the region indicate that Mt. Mansfield’s lower elevations (~400-1200 m a.s.l.) were deeply eroded during the last glaciation. A boulder and a bedrock sample collected near one another have indistinguishable ¹⁰Be ages suggesting similar erosion and exposure histories (14.9 ± 0.3 and 15.3 ± 0.3 ka, 1σ internal uncertainties). Another boulder has similar ¹⁰Be and ¹⁴C ages (¹⁰Be 14.1 ± 0.7 ka; ¹⁴C 12.9 ± 1.7 ka; 1σ external uncertainties), suggesting that most nuclides formed prior to the LGM were removed by erosion and that the boulder has a simple exposure history. These lower elevations deglaciated rapidly; ¹⁰Be ages over an 800 m elevation range are indistinguishable, averaging 14.2 ± 0.7 ka (n = 15). Ice thinning occurred within at most about a millennium, possibly coincident with Bølling-Allerød warming.

Conversely, the higher elevations (>1200 m a.s.l.) preserve a more complex geomorphic history. Mt. Mansfield’s summit surfaces contain ¹⁰Be from previous periods of exposure, yielding exposure ages up to 24.4 ka, and indicating that the summit landscape was likely preserved beneath cold-based, weakly-erosive glacial ice. Exposure ages from the shorter-lived isotope, ¹⁴C, are much younger (8.5 and 10.2 ka), suggesting that Mt. Mansfield’s summit was shielded for several thousand years after deglaciation perhaps by snowfields, ice carapaces, and/or till. Our findings suggest that thinning Laurentide ice flowed through the valleys for hundreds of years following deglaciation of the uplands, but that ice or till cover may have persisted on at least several of New England’s highest summits for millennia after the valleys became ice-free.