Soil transport on a moraine foreslope

Recent work has demonstrated that glacial moraine erosion and soil mixing occur at rates high enough to influence chemical weathering processes and the determination of depositional ages. In this study, we complement previous moraine crest studies by investigating a 1.8-m-deep soil profile from the foreslope of a terminal, Bull Lake-age moraine (~ 140 ky) in the Wind River Mountains (Wyoming, USA). Techniques applied include measurement of the grain size distribution, major element concentrations, and cosmogenic 10Be concentrations in quartz. Results suggest a 30-cm-thick moving soil layer overriding a previously eroded, mixed soil layer. Mass balance calculation based on the cosmogenic nuclide concentration in the different soil layers of the slope profile as well as a previously investigated crest profile suggests that one-third of the 30-cm-thick layer is coming from the crest area and two-thirds of the material from the eroding moraine slope. Foreslope denudation rates are 0.015 to 0.018 mm y− 1 based on the 10Be concentration of the lowermost samples and assuming a depositional age of 140 ky. The 10Be concentration of the foreslope surface sample suggests a denudation rate of 0.020 mm y− 1 for the sediment source area. Analysis of major elements in a limited number of surface samples reveals that the immobile element Ti is enriched downslope, whereas more mobile elements are generally depleted downslope. The internal structure of a moraine foreslope is complex, and care must be taken in calculating moraine age and denudation rate with depth profiles.