CONSTRAINING GLACIAL HISTORY AND PROCESS ON MOUNT MANSFIELD, VERMONT'S HIGHEST PEAK, WITH IN SITU COSMOGENIC 10BE AND 14C

Studying the timing and rate of deglaciation of Mount Mansfield, Vermont’s highest peak (1339 m), provides information about the dynamics of the collapsing Laurentide Ice Sheet during the Latest Pleistocene. To determine the timing of deglaciation of Mount Mansfield’s uplands and infer the efficiency of subglacial erosion, we measured in situ cosmogenic ¹⁰Be in 12 samples collected from bedrock surfaces and three from erratic boulders (sample elevations 1025-1305 m). In one mid-elevation boulder and two summit bedrock surfaces, we also measured in situ cosmogenic ¹⁴C.

Below 1200 m, seven bedrock surfaces and three boulders have ¹⁰Be exposure ages that form a single population (average, 14.2 ± 0.4 ka, 1SD), suggesting rapid lowering of the ice surface during deglaciation, possibly coincident with Bølling/Allerød warming. A boulder and a bedrock sample collected near one another have indistinguishable ages suggesting similar erosion and exposure histories (15.0 ± 0.3 and 14.4 ± 0.3 ka, 1σ internal uncertainties). Another boulder has similar ¹⁰Be and ¹⁴C ages (¹⁰Be 13.8 ± 1.1 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. Mount Mansfield’s mid-elevations were likely glaciated by warm-based, erosive ice, and hence isotopic concentrations record the timing of Latest Pleistocene deglaciation.

The higher elevations of Mount Mansfield (>1200) m are characterized by older ¹⁰Be ages but younger ¹⁴C ages. Five samples (all bedrock) have ¹⁰Be exposure ages ranging from 15.5 to 24.4 ka, forming a population statistically separable from the low-elevation samples. For the two samples in which we measured ¹⁴C and ¹⁰Be, there is a several-fold age difference (¹⁴C 10.2 ± 1.1 ka, ¹⁰Be 18.1 ± 1.5 ka; and ¹⁴C 8.5 ± 0.8 ka, ¹⁰Be 24.4 ± 2.0 ka, 1σ external uncertainties). The two summit ¹⁴C ages likely underestimate the true age of deglaciation of the mountaintop, perhaps due to post-glacial burial by snow/ice. The high ¹⁰Be ages likely represent inheritance of nuclides produced prior to the LGM due to shallow erosion depths, perhaps because Laurentide ice was frozen to the bed. Isotopic data suggest that the summit of Mount Mansfield likely experienced a complex interplay of glacial and periglacial processes.