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Deciphering boulder mobility and erosion from cosmogenic nuclide exposure dating

Mackey, Benjamin H. and Lamb, Michael P. (2013) Deciphering boulder mobility and erosion from cosmogenic nuclide exposure dating. Journal of Geophysical Research. Earth Surface, 118 (1). pp. 184-197. ISSN 2169-9003. doi:10.1002/jgrf.20035.

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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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Lamb, Michael P.0000-0002-5701-0504
Additional Information:© 2013 American Geophysical Union. Received 24 July 2012; revised 14 January 2013; accepted 24 January 2013; published 6 March 2013. This work was supported by NSF grants EAR-0922199 and EAR-1204375 to MPL. We thank Ken Farley for helpful discussions. Comments from Joel Johnson, Simon Brocklehurst, Alex Densmore, and two anonymous reviewers improved this manuscript.
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Subject Keywords:Cosmogenic dating; Erosion; Boulder
Issue or Number:1
Record Number:CaltechAUTHORS:20130606-104305806
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Official Citation:Mackey, B. H., and M. P. Lamb (2013), Deciphering boulder mobility and erosion from cosmogenic nuclide exposure dating, J. Geophys. Res. Earth Surf., 118, 184–197, doi:10.1002/jgrf.20035
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:38833
Deposited By: Tony Diaz
Deposited On:07 Jun 2013 19:57
Last Modified:09 Nov 2021 23:40

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