ASYMMETRIC TOPOGRAPHY REFLECTS VARIABLE TRANSPORT EFFICIENCY ON SOIL-MANTLED HILLSLOPES IN THE CENTRAL APPALACHIANS

Variations in microclimate and their influence on soil moisture and cohesion are commonly invoked to explain topographic asymmetry in a variety of landscapes. Despite these assertions, no direct measures of erosion or transport efficiency have been reported for hillslopes of opposing aspect in a single valley. Here, we present an analysis of 131 meteoric ¹⁰Be measurements from regolith and bedrock to quantify mobile regolith flux and test the utility of different transport rules within the Susquehanna Shale Hills Critical Zone Observatory (SSHO), in central Pennsylvania. Regolith samples were collected from north- and south-facing hillslopes in three en echelon watersheds in and adjacent to the SSHO. Hillslopes are mantled by thin (30-80 cm), clay-rich, unstructured regolith that directly overlies fractured and weathered bedrock on gently-sloping south-facing hillslopes; on steeper, north-facing hillslopes regolith overlies a 1 – 2 m-thick layer of coarse colluvium. Meteoric ¹⁰Be data show that along all six hillslopes, mobile regolith fluxes are similar and increase linearly with distance from ridgecrests. Along ridgelines at SSHO, where mobile regolith thickness is uniformly thin, flux is linearly proportional to local gradient. At lower positions on the hillslopes, where mobile regolith thicknesses are greatest, regolith fluxes depend on both local gradient and the depth of mobile regolith. Our data imply that in order for mobile regolith flux on shallow, south-facing hillslopes to keep pace with fluxes on steep, north-facing hillslopes, transport efficiencies must be greater on south-facing hillslopes by nearly a factor of two. Our results provide systematic evidence that the critical zone responds to aspect-related microclimate differences by modulating transport efficiency. We suggest that the observed topographic asymmetry in these watersheds has evolved as a consequence of sustained differences in the efficiency of regolith transport over geologic time.