Estimates of the rate of regolith production using [formula omitted] and [formula omitted] from an alpine hillslope

Abstract

The production of regolith is a fundamental geomorphic process because most surface processes transport only unconsolidated material. We use concentrations of the cosmogenic radionuclides (CRNs) 10 Be and 26 Al in regolith and bedrock to deduce the rate of production of regolith on an alpine hillslope in the Wind River Range, WY. These calculations are based on a theoretical model which we develop here. This model shows that it is important to consider dissolution of regolith in regolith production and in basin-averaged erosion rate studies. Rates of production of regolith are uniform along the hillslope and the mean rates for the entire hillslope deduced from 10 Be and 26 Al are 14.3±4.0 and 13.0±4.0 m Ma −1, respectively. Rates of production of regolith deduced from 10 Be concentrations in regolith-mantled bedrock support the rates deduced from regolith concentrations. In the alpine environment examined here, the rate of production of regolith beneath ∼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 alpine bedrock summit surfaces deduced from in situ 10 Be and 26 Al . Earth and Planetary Science Letters 150, 413–425] previously documented. Rock-mantled with regolith probably weathers more rapidly than bare rock because the water 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 flux 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 hypothesis. If tor height and the difference between rates of weathering on bare and 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.