Estimates of the rate of regolith production using Be-10 and Al-26 from analpine hillslope

The production of regolith is a fundamental geomorphic process because most surface processes transport only unconsolidated material. We use concentra tions of the cosmogenic radionuclides (CRNs) Be-10 and Al-26 in regolith an d bedrock to deduce the rate of production of regolith on an alpine hillslo pe in the Wind River Range, WY. These calculations are based on a theoretic al model which we develop here. This model shows that it is important to co nsider dissolution of regolith in regolith production and in basin-averaged erosion rate studies. Rates of production of regolith are uniform along th e hillslope and the mean rates for the entire hillslope deduced from Be-10 and Al-26 are 14.3 +/- 4.0 and 13.0 +/- 4.0 m Ma(-1), respectively. Rates o f production of regolith deduced from Be-10 concentrations in regolith-mant led bedrock support the rates deduced from regolith concentrations. In the alpine environment examined here, the rate of production of regolith beneat h similar to 90 cm of regolith is nearly twice as fast as the average rate of production of regolith on bare rock surfaces, which Small et al. [Small, E.E., Anderson, R.S., Repka, J.L., Finkel, R., 1997. Erosion rates of alpi ne bedrock summit surfaces deduced from in situ Be-10 and Al-26. Earth and Planetary Science Letters 150, 413-425] previously documented. Rock-mantled with regolith probably weathers more rapidly than bare rock because the wa ter required for frost weathering is limited on bare rock surfaces. Because the hillslope examined here is convex with constant curvature and regolith production and thickness are uniform down the slope, the regolith volume f lux must be proportional to the local slope of the hillside. Therefore, our results are consistent with Gilbert's [Gilbert, G.K., 1909. The convexity of hilltops. Journal of Geology 17, 344-350] steady state hillslope hypothe sis. If tor height and the difference between rates of weathering on bare a nd regolith-mantled rock provide a fair estimate of the age of summit flats , steady-state hillslope conditions have been attained in less than several million years. (C) 1999 Elsevier Science B.V. All rights reserved.