INTERPRETING COSMOGENIC EXPOSURE DATES FROM MORAINES USING GEOMORPHIC PROCESS MODELING

The distribution of cosmogenic exposure dates from a moraine contains information about both the age of the moraine and the processes affecting the moraine's morphology. The concentrations of cosmogenic nuclides in boulders on the crest of a moraine can be measured to give an estimate of the moraine's age. However, cosmogenic exposure dates are often biased by geomorphic processes. These processes tend to increase the scatter between exposure dates from individual moraines. In many cases, this excess scatter is accommodated by discarding a subset of the dates and taking an average of the rest. Here, we present an alternative means of interpreting cosmogenic exposure dates, using geomorphic process modeling. This method honors all the dates from a particular moraine, and returns explicit information on the geomorphic processes affecting a given moraine. We have updated an existing model of moraine degradation to predict the production of cosmogenic beryllium-10 beneath moraine surfaces by both nucleons and muons. The updated model returns the expected topographic profile and distribution of cosmogenic exposure dates from a moraine of given age, initial height, initial slope, and topographic diffusivity. We apply the model to the outer Lake Fork moraine in the Uinta Mountains of Utah, where seven beryllium-10 exposure dates have been reported, and where we have recently measured topographic profiles. The exposure dates cluster around 18 ka, with one outlier at 11.5 ka. This observed distribution is consistent with distributions predicted by the model, suggesting that the observed scatter is, indeed, due to moraine degradation. We use Bayesian methods to invert the model against the topographic profiles and cosmogenic exposure dates from the Lake Fork moraine. The inversion yields a probability distribution of moraine age centered on 18 ka, consistent with earlier estimates. The probability distribution of topographic diffusivity returned by the inversion is broad, but also consistent with other estimates. We propose that this interpretive method may be usefully applied to other moraines whose true ages are less clear.