Precision of terrestrial exposure ages and erosion rates estimated from analysis of cosmogenic isotopes produced in situ

The concentration of cosmogenic isotopes produced within mineral grains varies with both the exposure age and erosion rate of the rock surface. In principle therefore, exposure age and erosion rate may be determined by analyzing two cosmogenic isotopes from the same sample, provided the erosion rate is constant. It is also possible to find either age or erosion rate from one isotope if the other parameter can be determined independently. Simple mathematical models predict the precision and accuracy of the exposure ages and erosion rates. The results provide insight into the use of cosmogenic isotopes and a framework to optimize dating experiments and predict tractable geologic questions. The precision and accuracy of the exposure ages and erosion rates depend on the precision of the measured isotope concentrations, the half-lives of the isotopes, and the age and erosion rate of the sampled rock surface. They also vary with the analytic strategy. Exposure age estimates from some isotope pairs approach measurement precision for late-Pleistocene surfaces eroding at <1 cm kyr(-1). Uncertainties in erosion rate for the same surfaces may be better than +/-0.2 cm kyr(-1). For older surfaces the upper limits for both age and erosion rate estimates become infinite. Pairing of isotopes with different half-lives may give misleading results unless the erosion rate is constant because they record different erosion histories. Analyzing a single isotope removes this source of error but is appropriate only if either the age or erosion rate can be well determined independently. If the erosion rate is poorly constrained, ages for some realistic situations may be inaccurate by 50% or more even though their precision may approach the measurement precision. Although it is important to understand the limitations of cosmogenic dating, it is equally important not to lose sight of the potential of this powerful tool for quantitative geomorphologic studies.