Hillslope response to differences in aspect-related microclimate

Abstract:

For much of the last century, it has been hypothesized that the downslope flux of regolith is dependent on hillslope gradient, but the functional form of this dependence remains uncertain. The development of new isotopic methods and the increasing availability of high resolution topographic data sets allows for testing of simple transport models in a variety of landscapes. At the Susquehanna Shale Hills Critical Zone Observatory (SSHO), 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 (3 north-facing, 3 south-facing), test commonly invoked transport rules, and elucidate the mechanisms acting to transport regolith materials. 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. Tests of frequently invoked transport rules 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. However, along ridgetops, where regolith thickness is uniformly thin, our results suggest that aspect-related differences in transport efficiency are minimal and ridgetop curvature reflects erosion rate and transport efficiency. Therefore, we contend that hillslopes at SSHO are approaching an equilibrium condition, where ridgetop curvature and hillslope gradient are adjusted to maintain a steady flux of materials out of the watershed.