ASPECT-DEPENDENT REGOLITH FLUX REVEALED BY METEORIC 10BE

We use a combination of high resolution, LiDAR-derived digital topography and the cosmogenic radionuclide meteoric ¹⁰Be to measure rates of regolith transport along six hillslopes in three en echelon watersheds, test transport rules, and elucidate the mechanisms acting to transport regolith materials at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Topographic observations reveal a systematic asymmetry at SSHO, where north-facing hillslopes exhibit steeper gradients than south-facing hillslopes. Despite this asymmetry, our meteoric ¹⁰Be results suggest that regolith flux is similar along all six hillslopes, and that the landscape is steadily lowering at ~ 20-30 m/My. We compare our measured regolith fluxes with two frequently invoked transport rules; one that sets regolith flux linearly proportional to topographic slope, and one that sets regolith flux linearly proportional to the product of slope and a probability distribution of disturbance events with regolith depth. These comparisons suggest that downslope regolith transport is depth-dependent at SSHO, and that regolith transport efficiency is consistently higher on south-facing hillslopes than north-facing hillslopes by a factor of two. Our results imply that subtle differences in insolation between north- and south-facing slopes impact the frequency of expansion-and-collapse regolith transport processes (i.e., wetting-drying cycles, freezing-thawing cycles), driving more frequent cycles and thus higher transport efficiencies on south-facing slopes. This interpretation is supported by seasonal δ¹⁸O measurements in soil pore-waters on north- and south-facing slopes at SSHO, as well as heat flux models for the SSHO watershed. We contend that the differences in regolith transport efficiency between north- and south-facing slopes at SSHO have conspired to drive the evolution of asymmetric topography over geologic time.