COSMOGENIC ISOTOPE ANALYSES APPLIED TO RIVER LONGITUDINAL PROFILE EVOLUTION - PROBLEMS AND INTERPRETATIONS

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.