LACK OF MIS-3 ICE IN NORTHERN NEW ENGLAND, USA, AS CONTRAINED BY MULTIPLE COSMOGENIC NUCLIDES

Long-standing questions in glacial geology are whether New England summits were covered by glacial ice, and if so, when? All ¹⁴C ages on organics buried by till in northern New England are infinite and ambiguous concerning ice cover history. However, conifer wood from the basal part of a lake sediment core near Katahdin dates ~32.5 ka (Anderson et al., 1988) and cave sediments near Mt. Mansfield provide OSL and IRSL ages that preclude Laurentide Ice Sheet (LIS) advance into northern New England from ~68 ka until after ~35 ka (Munroe et al., 2016). Here, we use multiple cosmogenic nuclides with different half-lives to explore the history of glacial erosion, burial without erosion by cold-based ice, and inter-glacial exposure of New England’s highest peaks. Cosmogenic nuclide data from the summits of Katahdin (ME; Bierman et al., 2015), Mt. Washington (NH; Bierman et al., 2015; Koester et al., 2020), and Mt. Mansfield (VT, Corbett et al. 2019) indicate at least two periods of exposure separated by a period of burial during which in situ ¹⁴C produced earlier decayed, whereas long-lived ¹⁰Be and ²⁶Al remained. The old ¹⁰Be and ²⁶Al ages on and near the summits suggest that overriding ice during the Last Glacial Maximum (LGM) was cold-based and weakly erosive. If summit areas had remained exposed throughout the last glacial cycle, and sampled surfaces were not eroded or covered by till, ¹⁰Be and ²⁶Al ages should indicate >100 k.y. of exposure. Instead, ¹⁰Be and ²⁶Al ages from mountain summits are mostly between 20 and 80 ka. We cannot differentiate between local ice caps or the LIS because boulders are the same composition as the bedrock with one exception being an erratic granite boulder that rests on the schist bedrock of Mt. Jefferson (NH) derived from a lower elevation to the north, which suggests LIS transport. Five in situ ¹⁴C samples from Mt. Washington are within the error of saturation, suggesting a total pre-LGM exposure of >21 k.y. Seven in situ ¹⁴C exposure ages from the summits are within 2 sigma of regional deglacial ages, which could be in part the result of post-glacial erosion and in part burial by ice during the LGM that allowed pre-LGM in situ ¹⁴C to decay away. Together with the conifer wood in Maine and cave sediments in Vermont, the in situ ¹⁴C data from summits suggest that northern New England was free of Laurentide ice for much of MIS-3.