GEOPHYSICAL MONOGRAPH SERIES, VOL. 193, PP. 111-122, 2011
Challenges in the Use of Cosmogenic Exposure Dating of Moraine Boulders to Trace the Geographic Extents of Abrupt Climate Changes: The Younger Dryas Example
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. Specifically, (1) a commonly 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 underestimate 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 exhumation. 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 sampling 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.
Citation: Applegate, P. J., and