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 determine whether surface exposure dating techniques can elucidatethe role knickpoint propagation plays in longitudinal profile evolution. Cosmogenically produced{sup}10{end}Be, {sup}26{end}Al, {sup}36{end}Cl, {sup}3{end}He, and {sup}21{end}Ne were measured in olivines collected from 5.2 Ma basalt flows on Kauai, Hawaii. Several obstacles had to be overcome prior to the measurement of in situ-produced radionuclides, including removal of meteoric lobe from the olivine grains.Discrepancies between the radionuclide and noble gas data may suggest limits for exposure dating. Approximate surface exposure ages calculated from the nuclide concentrations indicate that large boulders may remain in the Hawaiian valley below the knickpoint for hundreds of thousands of years. The ages of samples collected above the knickpoint are consistent with estimates of erosion based on the preservation of palaeosurfaces. Although the exposure ages can neither confirm norreject the knickpoint hypothesis, boulder ages downstream of the knickpoint are consistent with a wave of incision passing upvalley. The long residence time off the coarse material in the valley bottom further suggests that knickpomt propagation beneath a boulder pile is necessary for incision of the bedrock underlying the boulders to occur. [ABSTRACT FROM AUTHOR]