Mid-Pleistocene cosmogenic minimum-age limits for pre-Wisconsinan glacial surfaces in southwestern Minnesota and southern Baffin island: a multiple nuclide approach

Paired Be-10 and Al-26 analyses (n = 14) indicate that pre-Wisconsinan, gla ciated bedrock surfaces near the northern (Baffin Island) and southern (Min nesota) paleo-margins of the Laurentide Ice Sheet have long and complex his tories of cosmic-ray exposure, including significant periods of partial or complete shielding from cosmic rays. Using the ratio, Al-26/Be-10, we calcu late that striated outcrops of Sioux Quartzite in southwestern Minnesota (s outhern margin) were last overrun by ice at least 500,000 years ago. Weathe red bedrock tors on the once-glaciated uplands of Baffin Island (northern m argin) are eroding no faster than 1.1 m Myr(-1), the equivalent of at least 450,000 years of surface and near-surface exposure. Our data demonstrate t hat exposure ages and erosion rates calculated from single nuclides can und erestimate surface stability dramatically because any intermittent burial, and the resultant lowering of nuclide production rates and nuclide abundanc es, will remain undetected. (C) 1999 Elsevier Science B.V. All rights reser ved.