Creeping soil by optically stimulated luminescence and cosmogenic nuclides
Abstract
Soil creep occurs across hilly landscapes, yet may be the least understood process of erosion on soil-mantled hillslopes. Soil is slowly stirred by burrowing creatures and also as particles are displaced in wetting-drying cycles. Such processes cause creep by particle diffusion. Other possible transport mechanisms include pure shear and viscous creep, such that precise characterization requires tracing 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 rates, measured with cosmogenic nuclide concentrations. Results show that grains throughout the entire soil repeatedly visit the surface, and yield a new quantitative characterization of diffusive processes within creeping soil: (i) Samples from 10 cm have low mean ages (<1000 years) and include grains with near-zero ages, but 35-60 % of the grains have not visited the surface; (ii) Mean age increases with depth and exceeds 10 ka at the greatest depth (85 cm), but the proportion of surface-visiting grains is substantial even near soil base; (iii) Many grains must visit the surface several times: mean ages of the finite-age grains are significantly less than the soil generation time, calculated by dividing soil depth by the soil production rate; (iv) The ratio of age-variance to age mean is large for most samples, suggesting that grain movements are independent and random. Overall, results show that repeated displacements mix the entire soil column as the soil moves downhill. Furthermore, likelihood of displacement seems depth-independent, as net rates of grain movement downwards from the soil surface are similar, calculated by dividing sample depth by mean OSL age. Transport has a downslope component; results imply soil creep by diffusion of throughout the soil mass. We simulated sediment transport with a Monte Carlo model that randomly displaces grains: creep at the study site is by particle diffusion with about 20 % of the grains transported by surface wash. Finally, we compare depth-averaged velocities from the model with those from nuclide determined soil production rates to discuss the differences.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2001
- Bibcode:
- 2001AGUSM...H52A03H
- Keywords:
-
- 1625 Geomorphology and weathering (1824;
- 1886);
- 1815 Erosion and sedimentation;
- 1824 Geomorphology (1625);
- 1886 Weathering (1625)