EXPLORING THE TIMING OF LAURENTIDE ICE SHEET RETREAT AND 10BE/14C DATING MISMATCH IN CENTRAL NEW ENGLAND

Constraining the timing and pace of Laurentide Ice Sheet (LIS) deglaciation at the end of the last glacial maximum provides a useful analogue for understanding how modern ice sheets might respond to anthropogenic climate change. Ice sheet reconstructions are based on empirically determined exposure ages; however, different methods used to constrain retreat of the LIS in the northeastern US provide contrasting age estimates for deglaciation. Cosmogenic ¹⁰Be ages imply retreat from the terminal moraine in Massachusetts sometime between 28 and 25 ka, whereas calibrated organic ¹⁴C ages suggest retreat around 16 ka. However, ages determined by the two methods converge 200 kilometers north, near the Vermont-Massachusetts border. No ¹⁰Be exposure ages have been measured between central Connecticut and northern Massachusetts, leaving a gap in the data. Here, we use 14 new ¹⁰Be exposure ages from 4 sites in Connecticut and Massachusetts to determine timing of LIS deglaciation in central New England and explore the 10,000 year age difference between in-situ cosmogenic ¹⁰Be exposure ages and organic ¹⁴C ages near the LIS terminal moraine.

Our 14 exposure ages range from 12.5 to 22.4 ka and are progressively younger from south to north (two young outliers <10 ka are unlikely to reflect the timing of deglaciation). These ages demonstrate that ice retreated from northern Connecticut by 19.2 ka and northern Massachusetts by 17.4 ka. We infer, based on younger exposure ages, that ice lobes persisted in valleys after uplands became exposed. Additionally, individual river valleys may not have become ice free simultaneously. In Massachusetts, results from ~42.5˚ N suggest ice persisted in the Hudson River Valley until 15.2 ka, 2 ka longer than in the Connecticut River Valley where ice retreated by 17.4 ka.

The difference between local deglacial estimates based on ¹⁰Be and ¹⁴C decreases to the north, farther from the LIS maximum extent. We conclude that incomplete removal of inherited ¹⁰Be from prior periods of exposure, reduced ice sheet occupation duration, and reduced erosivity of the overlying ice mass cause some cosmogenic exposure ages to be too old near the terminal moraine. Organic ¹⁴C ages may be too young due to delayed revegetation of the landscape in the harsh climates that followed the glacial maximum.