Rates of sediment supply to arroyos from upland erosion determined using in situ produced cosmogenic Be-10 and Al-26

Using Be-10 and Al-26 measured in sediment and bedrock, we quantify rates o f upland erosion and sediment supply to a small basin in northwestern New M exico. This and many other similar basins in the southwestern United States have been affected by cycles of arroyo incision and backfilling several ti mes in the past few millennia. The sediment generation (275 +/- 65 g m(-2) yr(-1)) and bedrock equivalent lowering rates (102 +/- 24 m myr(-1)) we det ermine are sufficient to support at least three arroyo cycles in the past 3 ,000 years, consistent with rates calculated from a physical sediment budge t within the basin and regional rates determined using other techniques. Nu clide concentrations measured in different sediment sources and reservoirs suggest that the arroyo is a good spatial and temporal integrator of sedime nt and associated nuclide concentrations from throughout the basin, that th e basin is in steady-state, and that nuclide concentration is independent o f sediment grain size. Differences between nuclide concentrations measured in sediment sources and reservoirs reflect sediment residence times and ind icate that subcolluvial bedrock weathering on hillslopes supplies more sedi ment to the basin than erosion of exposed bedrock. (C) 2001 University of W ashington.