We use cosmogenic 10Be concentrations in amalgamated rock samples from active, ice-cored medial moraines to constrain glacial valley sidewall backwearing rates in the Kichatna Mountains, Alaska Range, Alaska. This dramatic landscape is carved into a small ∼65 Ma granitic pluton about 100 km west of Denali, where kilometer-tall rock walls and ‘cathedral’ spires tower over a radial array of over a dozen valley glaciers. These supraglacial landforms erode primarily by rockfall, but erosion rates are difficult to determine. We use cosmogenic 10Be to measure rockwall backwearing rates on timescales of 103–104 years, with a straightforward sampling strategy that exploits ablation-dominated medial moraines.
A medial moraine and its associated englacial debris serve as a conveyor system, bringing supraglacial rockfall debris from accumulation-zone valley walls to the moraine crest in the ablation zone. We discuss quantitatively several factors that complicate interpretation of cosmogenic concentrations in this material, including the complex scaling of production rates in very steep terrain, the stochastic nature of the rockfall erosion process, the unmixed nature of the moraine sediment, and additional cosmogenic accumulation during transport of the sediment.
We sampled medial moraines on each of three glaciers of different sizes and topographic aspects. All three moraines are sourced in areas with identical rock and similar sidewall relief of ∼1 km. Each sample was amalgamated from 25 to 35 clasts collected over a 1-km longitudinal transect of each moraine. Two of the glaciers yield similar 10Be concentrations (∼1·6–2·2 × 104 at/g) and minimum sidewall slope-normal erosion rates (∼0·5–0·7 mm/yr). The lowest 10Be concentrations (8 × 103 at/g) and the highest erosion rates (1·3 mm/yr) come from the largest glacier in the range with the lowest late-summer snowline. These rates are reasonable in an alpine glacial setting, and are much faster than long-term exhumation rates of the western Alaska Range as determined by thermochronometric studies. Copyright © 2010 John Wiley & Sons, Ltd.