Dennis Kent , Luca Lanci, Dorothy Peteet

What we regard as anomalously old 10Be exposure dates reported from the terminal moraine of the Laurentide Ice Sheet (LIS) in northeastern North America, such as recently published for Allamuchy NJ, ostensibly point to the start of deglaciation at 25 thousand calendar years before present (cal. ka). These dates are well within the conventional age span of the Last Glacial Maximum (LGM) and are in stark contrast with published 14C accelerator mass spectrometry (AMS) dates for earliest terrestrial plant macrofossils found in LIS deglacial clay deposits that range back to only ~16 cal. ka, which more plausibly coincide with the known timing of the glacio-eustatic rise and meltwater discharge to the North Atlantic and Gulf of Mexico that mark the demise of the LGM in the marine record. To explore possible explanations for this inconsistency, we first employed a statistical model of the geomagnetic field that includes secular variation with nondipole terms and can be applied globally. The model results in a decrease in the magnetic shielding factor by about 10% at mid-latitudes compared to oft-used geomagnetic scaling schemes. However, the time-integrated axial dipole moment estimated separately suggests little overall change in average shielding since about 20 cal. ka. This seems to leave cosmic ray flux modulated by a time-varying heliomagnetic field linked to sunspot activity as an underestimated factor in widely used 10Be exposure age calculators. If generally biased by about 23% higher compared to modern levels as reported for the past 9.4 cal. ka, the elevated high cosmic ray flux would make 10Be reference production rates proportionately higher, to about 5.5 at/g/y at sea level-high latitude, and reduce exposure ages to about 3/4 of those that have been previously calculated for LGM and younger rocks (to less than 20 cal. ka in the case of Allamuchy). Varying but generally higher solar modulation will require reevaluation of cosmogenic exposure dates in general, as in the case of Allamuchy, that would allow improved synchronization of marine and terrestrial records of glaciation. Other test cases can result in improved GIA deglaciation models and alternative estimates of effects of shielding in ice-flow models.