Soil Particle Transport and Mixing Near a Hillslope Crest: 2. Cosmogenic Nuclide and Optically Stimulated Luminescence Tracers

Abstract

We examine probabilistic elements of how cosmogenic nuclide (e.g., 10Be) concentrations and optically stimulated luminescence (OSL) particle ages are distributed within a soil mantle near a hillslope crest as a consequence of disturbance-driven transport and particle mixing. We use an Eulerian-Lagrangian algorithm in which fluctuating particle motions, representing depth-dependent mixing, are superimposed on a two-dimensional mean motion. The intensity of mixing is characterized by a Péclet number involving the vertical speed of particles entering the soil mantle at the soil-bedrock interface, the mechanically active soil thickness, and a particle diffusivity at the soil surface. With weak mixing, the vertical profile of 10Be concentration reflects the strong influence of the mean motion in which particles spend much of their lives in the higher part of the soil column with higher 10Be production rates. With increasing mixing intensity, the profile becomes linear, then uniform, and the vertically averaged concentration is larger than that expected with one-dimensional motion. With weak mixing, particles possessing a finite OSL age tend to remain near the soil surface; with increasing mixing they become more uniformly distributed with depth. Depth-interval-averaged OSL ages increase linearly with depth and then become uniform with strong mixing. With moderate to strong mixing, the probability distribution of OSL ages is approximately exponential with an average much less than the mean residence time of particles. The formulation is consistent with profiles of 10Be concentrations and interval-averaged OSL particle ages compiled from published data, suggesting moderate to strong mixing in the cases examined.