Absolute dating of river terraces can yield long-term incision rates.
clarify the role of climate in setting times of aggradation and incisi
on, and establish the rates of pedogenic processes. While surface expo
sure dating using cosmogenic Be-10 and Al-26 would Seem to be an ideal
dating method, the surfaces are composed of individual clasts, each w
ith its own complex history of exposure and burial. The stochastic nat
ure of burial depth and hence in nuclide production in these clasts du
ring exhumation and fluvial transport, and during post-depositional st
irring, results in great variability in clast nuclide concentrations.
We present a method for dealing with the problem of pre-depositional i
nheritance of cosmogenic nuclides. We generate samples by amalgamating
many individual clasts in order to average over their widely differen
t exposure histories. Depth profiles of such amalgamated samples allow
us to constrain the mean inheritance, to test for the possible import
ance of stirring, and to estimate the age of the surface. Working with
samples from terraces of the Fremont River, we demonstrate that sampl
es amalgamated from 30 clasts represent well the mean concentration. D
epth profiles show the expected shifted exponential concentration prof
ile that we attribute to the sum of uniform mean inheritance and depth
-dependent post-depositional nuclide production. That the depth-depend
ent parts of the profiles are exponential argues against significant p
ost-depositional displacement of clasts within the deposit. Our techni
que yields Be-10 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. The mean inheritance is similar from terrace t
o terrace and would correspond to an error of similar to 30-40 ka if n
ot taken into account. The inheritance likely reflects primarily the m
ean exhumation rates in the headwaters, of order 30 m/Ma. (C) 1997 Els
evier Science B.V.