Glacial erosion near the southern margin of the Laurentide Ice Sheet, south-central Wisconsin, USA: implications for cosmogenic dating of glacial terrains
Patrick M. Colgan1, Paul R. Bierman2, David M. Mickelson3, and Marc Caffee4
1 Department of Geology, Northeastern University, pcolgan@lynx.neu.edu.
2 Department of Geology, University of Vermont, pbierman@zoo.uvm.edu.
3 Department of Geology and Geophysics, University of Wisconsin, davem@geology.wisc.edu.
4 Center for Accelerator Mass Spectrometry, Lawrence Livermore National
Laboratory, caffee1@llnl.gov
We analyzed 22 samples, collected from five striated granite, meta-rhyolite,
and quartzite outcrops in south-central Wisconsin to estimate the timing
of late Pleistocene deglaciation using cosmogenic 10Be and 26Al
methods. In three of the five outcrops, nuclide abundance was up
to eight times higher than expected considering that radiocarbon ages indicate
that the outcrops had been overrun by the Laurentide Ice Sheet during the
last 30,000 years. At these sites, several thousand years of ice
flow probably removed only centimeters to decimeters of rock, allowing
a significant quantity of nuclides (105 to 106 atoms
g-1 10Be and 26Al ) produced during prior
periods of exposure to remain. Minimum limiting glacial erosion rates
are 0.01 to 0.25 mm yr-1 for these extremely strong rocks. At
two other sites, farthest from the former ice margin, erosion rates were
significant and exposure ages probably do reflect the time since deglaciation.
Rock strength, sample location on outcrops, and outcrop proximity to the former ice margin control the magnitiude of inheritance of cosmogenic nuclides from prior periods of exposure. Four of five samples from extremely hard meta-rhyolite outcrops contain inherited nuclides; two samples carry the equivalent of >150,000 years of surface exposure. Samples highest in the landscape or in plucked areas, carry less inheritance than those from the lee sides of landforms or lower in the landscape. Three quartzite samples collected about 10 km from the ice margin, contain three to four times the expected nuclide abundance. In contrast, eight other quartzite and granite samples from two outcrops more than 50 km behind the former ice margin, contain only 105 atoms g-1 10Be, consistent with late Pleistocene exposure and little, if any, nuclide inheritance.
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). 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. Simple age models that assume no inheritance of nuclides have the potential to inflate both exposure age and production rate estimates.