Creeping soil

Soil creep is the most widespread and perhaps the least understood process of erosion on soil-mantled hillslopes. Soil is slowly ‘‘stirred’’ by burrowing creatures, and particles are displaced in wetting-drying cycles. These actions can cause downslope creep by processes analogous to particle diffusion. Other possible transport mechanisms include shear and viscous-like creep, such that precise characterization of the entire process appears to require the tracing of labeled soil grains. Here we use natural quartz grains in a mature soil to determine grain movements from the time elapsed since each grain last visited the ground surface, measured by single-grain optical dating. Downslope flux is calculated from soil production by rock weathering at the soil base, measured with in situ–produced cosmogenic nuclide concentrations (10Be and 26Al). Results show that grains throughout the soil profile repeatedly visit the surface, and give the first quantitative characterization of grain-scale transport processes within creeping soil. These unique field data are interpreted with a Monte Carlo simulation to suggest that soil creep involves independent movements of mineral grains throughout the soil body and that grains are reburied or eroded by overland flow upon reaching the surface.