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 grain s varies with both the exposure age and erosion rate of the rock surfa ce. In principle therefore, exposure age and erosion rate may be deter mined by analyzing two cosmogenic isotopes from the same sample, provi ded the erosion rate is constant. It is also possible to find either a ge or erosion rate from one isotope if the other parameter can be dete rmined independently. Simple mathematical models predict the precision and accuracy of the exposure ages and erosion rates. The results prov ide insight into the use of cosmogenic isotopes and a framework to opt imize 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-live s of the isotopes, and the age and erosion rate of the sampled rock su rface. They also vary with the analytic strategy. Exposure age estimat es from some isotope pairs approach measurement precision for late-Ple istocene surfaces eroding at <1 cm kyr(-1). Uncertainties in erosion r ate for the same surfaces may be better than +/-0.2 cm kyr(-1). For ol der surfaces the upper limits for both age and erosion rate estimates become infinite. Pairing of isotopes with different half-lives may giv e 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 ero sion rate can be well determined independently. If the erosion rate is poorly constrained, ages for some realistic situations may be inaccur ate by 50% or more even though their precision may approach the measur ement precision. Although it is important to understand the limitation s of cosmogenic dating, it is equally important not to lose sight of t he potential of this powerful tool for quantitative geomorphologic stu dies.