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Challenges in the Use of Cosmogenic Exposure Dating of Moraine Boulders
to Trace the Geographic Extents of Abrupt Climate Changes:
The Younger Dryas Example
Patrick J. Applegate
1
and Richard B. Alley
Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA
Cosmogenic exposure dating has sometimes been used to identify moraines
associated with short-lived climatic events, such as the Younger Dryas (12.9
-
11.7 ka). Here we point out two remaining challenges in using exposure dating to
identify moraines produced by abrupt climate changes. Speci
cally, (1) a com-
monly applied sampling criterion likely yields incorrect exposure dates at some
sites, and (2) geomorphic processes may introduce bias into presently accepted
nuclide production rate estimates. We
fit a geomorphic process model that treats
both moraine degradation and boulder erosion to collections of exposure dates from
two moraines that were deposited within a few thousand years of the Younger
Dryas. Subsampling of the modeled distributions shows that choosing boulders for
exposure dating based on surface freshness yields exposure dates that underesti-
mate the true age of the moraine by up to several thousand years. This conclusion
applies only where boulders do not erode while buried but do erode after exhuma-
tion. Moreover, one of our
fitted data sets is part of the global nuclide production
rate database. Our
fit of the moraine degradation model to this data set suggests that
nuclide production rates at that site are several percent higher than previously
thought. Potential errors associated with sampling strategies and production rate
estimates are large enough to interfere with exposure dating of moraines, especially
when the moraines are associated with abrupt climate changes. We suggest sam-
pling strategies that may help minimize these problems, including a guide for
determining the minimum number of samples that must be collected to answer
particular paleoclimate questions.
1. INTRODUCTION
Cosmogenic exposure dating of moraines is an attractive
method for tracing the geographic extents of former abrupt
climate changes. Glaciers grow and shrink in response to
climate changes [Lowell, 2000; Oerlemans, 2005; Denton et
al., 2005], and they deposit ridges called moraines at their
margins [Gibbons et al., 1984]. Thus, for abrupt climate
changes that propagated over long distances quickly, we
expect to
1
Now at Department of Physical Geography and Quaternary
Geology, Stockholm University, Stockholm, Sweden.
find moraines of about the same age in mountain
ranges within the area affected by the change. The crests of
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