Tracing hillslope sediment production and transport with in situ and meteoric 10Be

Abstract

We use in situ‐produced and meteoric 10Be, analyzed in soils from 28 pits on four hillcrest‐parallel transects along a 14° hillslope in the Great Smoky Mountains, North Carolina, as tracers of soil production and transport. We rely upon amalgamation both to investigate and smooth spatial variability in 10Be concentrations. Lidar indicates that the hillslope is topographically complex and that soil is moved downslope diffusively until it encounters the ephemeral channel network and is rapidly exported. In situ‐produced 10Be, measured in depth profiles, indicates that over millennial timescales, soils are mixed above the soil‐saprolite boundary. In contrast, meteoric 10Be concentrations increase with depth and are correlated to concurrent increases of dithionite‐extractable Al and pH, observations explained by similar Al and Be mobility in the soil. The concentrations of both meteoric and in situ‐produced 10Be increase downslope proportional to the maximum soil particle path length. The data suggest virtual downslope soil velocities of 1.1–1.7 cm yr−1 in a well‐mixed active transport layer ∼60 cm thick. The thickness of this transport layer is constant downslope and depends on the rooting depth and consequent root wad thickness of downed trees on the slope, both of which reflect depth to the soil/saprolite boundary. Both meteoric and in situ‐produced 10Be suggest that soil production is balanced by surface denudation at rates between 10 and 13 m Myr−1. Soil residence times on the slope range from 21 to 33 kyr based on the meteoric 10Be inventories. Major element geochemical analysis suggests little if any elemental loss during soil transport downslope.