Aspect-dependent variations in regolith creep revealed by meteoric ¹⁰Be

Although variations in insolation and emergent feedbacks among soil moisture, vegetation, and soil cohesion are commonly invoked to explain topographic asymmetry that depends on aspect, few studies have directly quantifi ed the effi ciency of regolith transport along hillslopes of opposing aspect. We utilize meteoric ¹⁰Be concentrations in regolith (n = 74) to determine mass fl ux along equatorial-facing and polar-facing hillslopes in three forested upland watersheds in and adjacent to the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania (USA). In combination with regolith depth measurements and high-resolution topography, these fl uxes allow us to evaluate transport rate laws and the effi ciency of regolith creep. Concentrations of meteoric ¹⁰Be in regolith along six separate transects imply that regolith fl ux is similar along all hillslopes, despite differences in topographic gradient and regolith thickness. Comparison of fl ux with regolith depth and topographic gradient reveals that transport depends on regolith depth, and that regolith creep is twice as effi cient along low-gradient, south-facing slopes with thin regolith as compared to steep, north-facing slopes mantled with thicker regolith. We suggest that the observed topographic asymmetry in these watersheds has evolved over geologic time as a result of differences in the frequency of freezethaw events between hillslopes of opposing aspect.