EROSION RATES OF ALPINE BEDROCK SUMMIT SURFACES DEDUCED FROM IN-SITU BE-10 AND AL-26

We have measured the concentration of in situ produced cosmogenic Be-1 0 and Al-26 from bare bedrock surfaces on summit flats in four western U.S. mountain ranges. The maximum mean bare-bedrock erosion rate from these alpine environments is 7.6 +/- 3.9 m My(-1). Individual measure ments vary between 2 and 19 m My(-1). These erosion rates are similar to previous cosmogenic radionuclide (CRN) erosion rates measured in ot her environments, except for those from extremely arid regions. This i ndicates that bare bedrock is not weathered into transportable materia l more rapidly in alpine environments than in other environments, even though frost weathering should be intense in these areas. Our CRN-ded uced point measurements of bedrock erosion are slower than typical bas in-averaged denudation rates (similar to 50 m My(-1)). If our measured CRN erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion, then relief in the mountain ranges exam ined here is probably increasing. We develop a model of outcrop erosio n to investigate the magnitude of errors associated with applying the steady-state erosion model to episodically eroding outcrops. Our simul ations show that interpreting measurements with the steady-state erasi on model can yield erosion rates which are either greater or less than the actual long-term mean erosion rate. While errors resulting from e pisodic erosion are potentially greater than both measurement and prod uction rate errors for single samples, the mean value of many steady-s tate erosion rate measurements provides a much better estimate of the long-term erosion rate. (C) 1997 Elsevier Science B.V.