INVESTIGATING 10BE/14C CHRONOMETER AGE DIVERGENCE IN SOUTHERN NEW ENGLAND TO ASSESS THE TIMING OF LAURENTIDE ICE SHEET DEGLACIATION

Accurately understanding the timing of past deglaciations is important for studying ice sheet response to climate change. Predictions of how quickly ice sheets will lose mass in the future rely on paleo ice sheet reconstructions due to the short direct observational period of modern ice sheets. Ice-sheet reconstructions are based on empirically-determined ages; however, challenges arise when ages derived from different methods are used to constrain the same deglacial chronology but diverge.

Here, we focus on differences between in-situ cosmogenic ¹⁰Be exposure ages and organic ¹⁴C ages which should constrain the same deglacial events. Each method has its own assumptions: ¹⁰Be assumes complete removal of inherited nuclides from prior periods of exposure, whereas ¹⁴C assumes revegetation began immediately following local deglaciation. Cosmogenic nuclide inheritance causes exposure ages to be too old, whereas harsher climates close to glacial maxima may delay revegetation, causing ¹⁴C ages to be too young.

These complications appear pronounced near the terminal moraine of the Laurentide ice sheet in New England. ¹⁰Be ages imply retreat from the terminal moraine occurred sometime between 28 and 25 ka, while ¹⁴C, particularly from macrofossils, suggest retreat occurred around 16 ka. However, ages determined by the two methods converge 200 kilometers north of the terminal moraine, at the Vermont-Massachusetts border, and remain similar north along the axis of retreat. ¹⁰Be inheritance modelling suggests exposure ages from the terminal moraine contain excess ¹⁰Be equivalent to several-thousand years of surface exposure. Paleoclimate reconstructions show North Atlantic warming only began around 17.5 ka, consistent with young ¹⁴C ages reflecting a substantial lag between deglaciation and organic matter accumulation. Further north, deglaciation age estimates converge due to greater erosivity from longer ice sheet occupation and milder regional climate during the Bølling-Allerød, consistent with rapid revegetation following ice retreat.

Determining where ages converge along the axis of retreat is difficult because there are no ¹⁰Be samples between southern Connecticut and northern Massachusetts. We collected 14 additional ¹⁰Be samples from this region to fill the data gap; they are awaiting measurement.