Variation in glacial erosion near the southern margin of the Laurentide ice sheet, south-central Wisconsin, USA; implications for cosmogenic dating of glacial terrains

We measured the abundance of cosmogenic (super 10) Be and (super 26) Al in 22 samples collected from five striated granite, metarhyolite, and quartzite outcrops in south-central Wisconsin that were covered by the late Wisconsin Laurentide Ice Sheet. In two outcrops, measured nuclide abundances are consistent with the existing radiocarbon chronology of ice retreat. In three outcrops, nuclide abundances were up to eight times higher than predicted by the radiocarbon chronology. At these three sites, several thousand years of ice flow eroded only centimeters to decimeters of rock, allowing a significant quantity of nuclides (10 (super 5) -10 (super 6) atoms of (super 10) Be and (super 26) Al per gram of quartz), produced during prior periods of exposure, to remain. We calculate minimum-limiting glacial erosion rates of 0.01-0.25 mm.yr (super -1) for these rocks. Rock properties, sample location on outcrops, and outcrop proximity to the former ice margin control the magnitude of cosmogenic nuclides inherited from periods of prior exposure. Four of five samples from very hard metarhyolite outcrops with widely spaced joints contain inherited nuclides; two samples carry the equivalent of >150,000 yr of surface exposure, even though they were covered by ice during the last-glacial-maximum advance. Samples highest on the landscape or in plucked areas have less inheritance than those from the lee sides of large hills or lower in the landscape. Three quartzite samples collected approximately 10 km up-ice from the margin contain three to four times the expected nuclide abundance (10 (super 5) to 10 (super 6) atoms per gram of quartz). In contrast, eight other quartzite and granite samples from two outcrops >50 km up-ice from the former margin contain only 10 (super 5) atoms of (super 10) Be per gram of quartz, consistent with late Pleistocene exposure and little, if any, nuclide inheritance. This relationship between glacial erosion and distance from the former terminus is consistent with a marginal zone of minimal subglacial erosion; the ice was either frozen to its bed there, or the ice thickness and duration of ice cover were less near the terminus. These data, together with simple modeling of nuclide production by deeply penetrating muons, suggest that many meters of rock must be removed to reduce inheritance to negligible levels (<1000 yr) in continental terrains with low long-term erosion rates. Our results indicate that cosmogenic dating of exposed bedrock surfaces near former ice margins or in areas where ice was frozen to the bed may be uncertain, and in some cases impossible, because nuclides are inherited from prior periods of cosmic-ray exposure. Unfortunately, striated glacial outcrops that can be used to demonstrate most easily the assumption of "no postglacial erosion" are also the most likely to have undergone little glacial erosion. This finding suggests that cosmogenic nuclide production rates based on glacially striated surfaces may include cosmogenic nuclides inherited from prior exposure.