DETERMINING THE TIMING AND RATE OF SOUTHEASTERN LAURENTIDE ICE SHEET THINNING USING IN-SITU 10BE DIPSTICKS

The deglacial chronology of the southeastern Laurentide Ice Sheet, which retreated through the northeastern United States and southern Quebec, has been well constrained by multiple lines of evidence including minimum-limiting radiocarbon dates, varve chronologies, and ¹⁰Be exposure dating of boulders perched on moraines. By comparison, few data exist to constrain the thinning history of the southeastern Laurentide, resulting in lingering uncertainty about volume changes and dynamics of this ice mass during the deglacial period. To address the lack of thinning information we collected 82 samples for in-situ ¹⁰Be exposure dating from a variety of elevations using the mountains of New England, USA, and southern Quebec. We performed Monte Carlo analyses using the internal uncertainties of exposure ages from each mountain to determine the most-likely timing and rate of ice thinning for that location, a technique known as the ‘dipstick approach’.

Presently, 62 samples have been processed and had ¹⁰Be concentrations measured. Exposure ages from Peekamoose Mt. in southern NY suggest ice thinning early in the deglacial period (~19.5-17.5 ka), prior to the onset of the Heinrich Stadial I cold period. Samples from Franconia Notch, NH, Mt. Washington, NH, and Mt. Mansfield VT, suggest ice thinning from 15-13 ka, roughly coincident with the Bølling-Allerød warm period. Exposure ages from each of the northern New England dipsticks are nearly identical within 1σ internal uncertainty, indicating that the ice thinned rapidly.

Higher elevation (>1200 m a.s.l.) samples from the northern New England mountains appear to reflect a more complex geomorphology history than lower elevations. High ¹⁰Be concentrations in these samples yield simple exposure ages older than the LGM, including some older than 50 ka. These samples likely contain inherited ¹⁰Be from previous periods of exposure, indicating a lack of glacial erosion on these surfaces. Our high-elevation samples with inherited ¹⁰Be support a hypothesis proposed by our project members that these summit landscapes were preserved beneath cold-based, non-erosive ice during the last glacial and deglacial periods.