Using in situ produced cosmogenic isotopes to estimate rates of landscape evolution: A review from the geomorphic perspective

Abstract

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 produced cosmogenic isotopes were made. Taken at face value, these studies provide quantitative information about rates of landscape evolution and landform age; however, the significance of calculated erosion rates and exposure ages depends strongly on the models used to interpret isotopic data, the validity of assumptions inherent to these models, and the geologic surroundings in which the samples were collected. This paper attempts to place cosmogenic isotope studies in a geomorphic context by reviewing fundamentals of the method and evaluating the validity of assumptions under which these data have been interpreted. At present, the establishment of high‐precision, cosmogenically based glacial and alluvial chronologies is stymied by the evolution of geomorphic surfaces, the erosion of rock from sampled boulders, the potential for isotope inheritance from previous exposure, and the uncertainty of isotopic measurements. Uncertainties in isotope production rates and the observed variability of exposure ages on individual geomorphic surfaces limit the confidence with which cosmogenic ages can be correlated reliably with those obtained by other techniques. Estimation of erosion rates at single points on the landscape gives useful small‐scale information. Extrapolation of these rates over longer time and larger spatial scales is less sure and most likely biased toward lower erosion rates by die inadvertent selection of resistant sample sites. However, because erosion rates are so poorly constrained at present, even estimates to within a factor of 2 may be of significant value to geomorphologists and tectonicists.

Erosion in northwest Tibet from in‐situ‐produced cosmogenic 10Be and 26Al in bedrock

“…This is because the study area is located in a low-relief plateau and in fact our results are consistent with that obtained for the interior of the Tibetan plateau (Lal et al, 2003). Previous comparison of erosion rates for bedrock and catchment sediments shows that the differences are within a factor of two Clapp et al (2000) 240 30 29 ± 6 active to five (Bierman, 1994;Heimsath et al, 2001). Clearly the tremendous difference in denudation rate within different time intervals in northwest Tibet is not an artifact of dating material.…”

New constraints on the late Cenozoic incision history of the New River, Virginia

“…where λ is the decay constant of 10 Be (4.6x10 -7 yr -1 ) and N is the measured concentration of the isotope in the sample (Bierman, 1994). Here, P 0 is the scaled spallogenic production rate; muogenic production is ignored, but is generally insignificant in surface samples.…”

“…10 Be production rates are constrained by models (Masarik and Reedy, 1995), using measured neutron and muon fluxes (Dunai, 2000), by direct measurement in artificial targets (Nishiizumi et al, 1996), and by calibration against deposits with independent age constraint (Nishiizumi et al, 1986;Brown et al, 1991;Clark et al, 1995). The production rate at any given location fluctuates due to changes in solar output, geomagnetic field configuration, and atmospheric density (Dunai, 2000;Bierman, 1994). Variations of the production rate over different timescales are not well known, but are becoming better constrained as more calibration sites are utilized and better physical models become available (Gillespie and Bierman, 1995).…”

“…Bierman, 1994). Several methods exist to take advantage of CRNs, including burial dating by differential decay of cosmogenic isotopes and long-term erosion rate determination by CRN concentrations in sediment (Bierman, 1994). The method relevant to this study is cosmogenic exposure dating, which allows determination of the amount of time a particular deposit such as a river terrace has been exposed at the Earth's surface.…”

“…Over the past two decades technological advances in mass spectrometry and nuclear physics have made it possible to measure the concentrations of these CRNs in soil and rock material collected from geomorphic surfaces (e.g. Bierman, 1994). Several methods exist to take advantage of CRNs, including burial dating by differential decay of cosmogenic isotopes and long-term erosion rate determination by CRN concentrations in sediment (Bierman, 1994).…”