Geomagnetic Secular Variation Models for Latitude Scaling of Cosmic Ray Flux and Considerations for ¹⁰Be Exposure Dating of Laurentide Ice Sheet Retreat

Published cosmogenic ¹⁰Be exposure ages from the terminal moraine of the Laurentide Ice Sheet (LIS) in northeastern North America have been interpreted to date the start of the retreat of the LIS at the Last Glacial Maximum (LGM) about 25 thousand years ago (ka). In contrast, published ¹⁴C accelerator mass spectrometry (AMS) dates for terrestrial plant macrofossils in LIS basal deglacial clay deposits range back to only ~16 calibrated (cal) ka, more consistent with the timing of glacio-eustatic rise and associated meltwater discharge to the North Atlantic and Gulf of Mexico associated with LGM deglaciation. We apply statistical models of geomagnetic secular variation, including dipole moment, to the latitudinal scaling of cosmic ray flux to see how well the age discrepancy can be addressed. A preferred new scaling, which is essentially time-invariant over the relevant LGM age range, shifts the exposure ages only a few thousand years younger. The age discrepancy may thus stem more from potential local biases toward higher ¹⁰Be concentrations (older apparent ages) at the terminal moraine sites, such as much higher ¹⁰Be production rates at the LIS front, and especially from inheritance. Such biases can be tested by obtaining primary ¹⁰Be calibration sites in the LGM time frame, and by more comprehensive sampling strategies for glaciated terrain to discern inheritance.