Working with samples from terraces of the Fremont River, we demonstrate that samples amalgamated from 30 clasts represent well the mean concentration.
Depth profiles show the expected shifted exponential concentration profile that we attribute to the sum of uniform mean inheritance and depth-dependent post-depositional nuclide production.
The stochastic nature of burial depth and hence in nuclide production in these clasts during exhumation and fluvial transport, and during post-depositional stirring, results in great variability in clast nuclide concentrations.
We present a method for dealing with the problem of pre-depositional inheritance of cosmogenic nuclides.
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While surface exposure dating using cosmogenic Al would seem to be an ideal dating method, the surfaces are composed of individual clasts, each with its own complex history of exposure and burial.We generate samples by amalgamating many individual clasts in order to average over their widely different exposure histories.Depth profiles of such amalgamated samples allow us to constrain the mean inheritance, to test for the possible importance of stirring, and to estimate the age of the surface.That the depth-dependent parts of the profiles are exponential argues against significant post-depositional displacement of clasts within the deposit.Our technique yields Be age estimates of 60±9, 102±16 and 151±24 ka for the three highest terraces, corresponding to isotope stages 4, 5d and 6, respectively.