USING IN-SITU PRODUCED COSMOGENIC ISOTOPES TO ESTIMATE RATES OF LANDSCAPE EVOLUTION - A REVIEW FROM THE GEOMORPHIC PERSPECTIVE

The application of in-situ produced cosmogenic isotopes to problems in geomorphology has increased rapidly over the past decade. At least 57 papers and numerous abstracts have been published since the mid-1980s when the first mass-spectrometric measurements of terrestrially produ ced cosmogenic isotopes were made. Taken at face value, these studies provide quantitative information about rates of landscape evolution an d landform age; however, the significance of calculated erosion rates and exposure ages depends strongly on the models used to interpret iso topic data, the validity of assumptions inherent to these models, and the geologic surroundings in which the samples were collected. This pa per attempts to place cosmogenic isotope studies in a geomorphic conte xt by reviewing fundamentals of the method and evaluating the validity of assumptions under which these data have been interpreted. At prese nt, the establishment of high-precision, cosmogenically based glacial and alluvial chronologies is stymied by the evolution of geomorphic su rfaces, the erosion of rock from sampled boulders, the potential for i sotope inheritance from previous exposure, and the uncertainty of isot opic measurements. Uncertainties in isotope production rates and die o bserved variability of exposure ages on individual geomorphic surfaces limit the confidence with which cosmogenic ages can be correlated rel iably with those obtained by other techniques. Estimation of erosion r ates at single points on the landscape gives useful small-scale inform ation. Extrapolation of these rates over longer time and larger spatia l scales is less sure and most likely biased toward lower erosion rate s by the inadvertent selection of resistant sample sites. However, bec ause erosion rates are so poorly constrained at present, even estimate s to within a factor of 2 may be of significant value to geomorphologi sts and tectonicists.