Ma. Seidl et al., COSMOGENIC ISOTOPE ANALYSES APPLIED TO RIVER LONGITUDINAL PROFILE EVOLUTION - PROBLEMS AND INTERPRETATIONS, Earth surface processes and landforms, 22(3), 1997, pp. 195-209
The use of cosmogenic isotopes to determine surface exposure ages has
grown rapidly in recent years. The extent to which cosmogenic nuclides
can distinguish between mechanistic hypotheses of landscape evolution
is an important issue in geomorphology. We present a case study to de
termine whether surface exposure dating techniques can elucidate the r
ole knickpoint propagation plays in longitudinal profile evolution. Co
smogenically produced Be-10, Al-26, Cl-36, H-3 and Ne-21 were measured
in olivines collected from 5.2 Ma basalt flows on Kauai, Hawaii. Seve
ral obstacles had to be overcome prior to the measurement of in situ-p
roduced radionuclides, including removal of meteoric Be-10 from the ol
ivine grains. Discrepancies between the radionuclide and noble gas dat
a may suggest limits for exposure dating. Approximate surface exposure
ages calculated from the nuclide concentrations indicate that large b
oulders may remain in the Hawaiian valley below the knickpoint for hun
dreds of thousands of years. The ages of samples collected above the k
nickpoint are consistent with estimates of erosion based on the preser
vation of palaeosurfaces. Although the exposure ages can neither confi
rm nor reject the knickpoint hypothesis, boulder ages downstream of th
e knickpoint are consistent with a wave of incision passing upvalley.
The long residence time off the coarse material in the valley bottom f
urther suggests that knickpoint propagation beneath a boulder pile is
necessary for incision of the bedrock underlying the boulders to occur
. (C) 1997 by John Wiley & Sons, Ltd.