Deciphering boulder mobility and erosion from cosmogenic nuclide exposure dating

Abstract

Large boulders are prominent features in many geomorphic systems and are frequently targeted for cosmogenic exposure dating. Presently, there are little data or theory predicting exposure age, erosion rate, and mobilization frequency of boulders in environments such as channels, talus slopes, or moraines. Here we explore the potential for cosmogenic isotope analysis to constrain the transport and erosion history of boulders. Through a series of numerical experiments, we model the statistical evolution of nuclide concentrations around the surface of boulders. Stable boulders have distinctive radial distributions of surface concentration in comparison to those that are periodically mobile, and this can be used to establish boulder stability. Mean nuclide accumulation rates around the surface of an eroding boulder increase when the radius is smaller than approximately 1.5 e-folding lengths (~1.2 m) of neutron flux intensity, whereupon nuclide accumulation on the underside of the boulder becomes non-negligible (~10%). Model results for cases of no cosmogenic inheritance and uniform erosion indicate the normalized standard deviation of nuclide surface concentration systematically decreases with increasing number of boulder mobilization events. This may be used to constrain the minimum number of times a boulder has moved for up to approximately four events, or distinguish between rarely and frequently mobilized boulders. Using non-dimensional scaling relations between surface concentration statistics, boulder size, and time, we propose methods to estimate the minimum age, frequency of movement, and erosion rate of mobile boulders with application to a range of geomorphic problems.

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