INACCURACIES IN DEGLACIAL CHRONOMETERS RECORDING LAURENTIDE ICE SHEET RETREAT IN THE NORTHEASTERN UNITED STATES: PRESENCE, MAGNITUDE, AND CONTROLS

Accurately reconstructing when and how quickly ice sheets retreated after the Last Glacial Maximum is critical for understanding their sensitivity to changes in climate. Such reconstructions rely on accurate dates from deglacial chronometers such as organic ¹⁴C, in situ cosmogenic nuclides, and calibrated glacial varve chronologies. Here, we use a compilation of 204 previously published organic ¹⁴C (n=132) and in situ ¹⁰Be (n=72) ages, two calibrated glacial varve chronologies, and 13 new in situ ¹⁰Be ages to quantify differences between dating methods recording Laurentide Ice Sheet retreat in the northeastern United States. We find evidence of spatial and temporal controls on chronometer accuracy, as indicated by agreement (accurate) or disagreement (inaccurate) between ages inferred using different methods. ¹⁰Be exposure ages and ¹⁴C ages from macrofossils purported to constrain ice retreat from the terminal moraines disagree by thousands of years. We infer that ¹⁰Be from pre-glacial exposure (“inherited” ¹⁰Be) adds >4 kyr to some ages, while macrofossil ¹⁴C ages are typically 6 to 8 kyr too young due sustained arctic conditions preventing colonization of the deglaciated landscape by vegetation; these inaccuracies are especially prevalent at the location of the terminal moraine. Age disagreements decrease with distance from the terminal ice margin, due to both less ¹⁰Be inherited from prior exposure, the result of longer and thus deeper glacial erosion, and a reduced lag between ice retreat and revegetation as the climate warmed later in the deglacial period. Ages between methods agree well at locations >200 km from the terminal moraines, indicating that deglacial chronometers are more likely to be accurate farther from the Last Glacial Maximum margin and later in the deglacial period. The evidence of deglacial chronometer inaccuracy near the periphery of the southeastern Laurentide Ice Sheet suggests that insufficient glacial erosion and cold climate conditions during deglaciation may bias ice retreat ages in other ice sheet peripheral areas, particularly when retreat began early following the Last Glacial Maximum and ice occupation times were short.