10-BERYLLIUM EVIDENCE FOR RAPID LAURENTIDE DEGLACIATION OF THE KATAHDIN AREA, WEST-CENTRAL MAINE, USA, 16 TO 15 KY

Until recently, the deglacial chronology of the Laurentide Ice Sheet in New England was primarily constrained by minimum-limiting ¹⁴C ages on organic material deposited in ponds and bogs after deglaciation. However, an unknown lag time between glaciation and the deposition of the first datable organic material, the uncertainty of ¹⁴C reservoir corrections for marine samples, and the paucity of ¹⁴C datable samples meant that in many places the chronologic framework was insufficient to test competing hypotheses for the style and timing of deglaciation. For example, did large parts of the Laurentide ice sheet stagnate and down waste in a warming climate, or did the ice sheet retreat as an active ice margin? Recent work has produced a numerical chronology for the glacial Lake Hitchcock varve record, which has been used in conjunction with other data to draw glacial retreat isochrones across western New England. Here we expand the application of cosmogenic nuclide exposure dating and recalibrate glacial retreat isochrones across eastern New England.

Katahdin, the highest peak in Maine and part of the second highest mountain range in New England, is flanked on its east side by the Basin Ponds moraine, the age of which was previously not well known. We collected 12 samples from boulders on the moraine and from bedrock and boulders on the upper part of the mountain to estimate the age at which they were exposed by deglaciation of the Laurentide Ice Sheet. Measurements of in situ produced ¹⁰Be indicate that the Katahdin edifice became exposed from under ice by 14.9±2.1 ky (n = 6), an age indistinguishable from the adjacent Basin Ponds moraine complex (16.1±1.2 ky, n = 6). Also, a boulder in the lowlands several km south of the moraine dates to 15.1±1.2 ky, and a boulder deposited at Pineo Ridge, about 140 km SE of Katahdin, dates to 16.4±1.4 ky. These data show that samples spanning about 1.6 km and over 140 km in vertical and horizontal dimensions all have exposure ages that are indistinguishable within one-sigma analytic uncertainties. Together these data suggest that the Laurentide Ice Sheet surface dropped rapidly and the ice sheet margin retreated quickly across Maine about 16.0 to 15.0 ky, perhaps influenced by calving bays in the marine-based ice sheet in the St. Lawrence Lowlands to the north and the Penobscot basin to the south.