West, 2014

Dissertation/Thesis

Topographic Fingerprints of Hillslope Erosion in the Central Appalachians Revealed by Meteoric Beryllium-10

Nicole West (2014)
Doctor of Philosophy, Geosciences, Pennsylvania State University, p. 159  

Abstract

Regolith-mantled hillslopes are ubiquitous features of most temperate landscapes, and their morphology is a reflection of the climatically, biologically, and tectonically mediated interplay between regolith production and downslope transport. Despite this understanding, relatively few studies have been able to independently measure these processes and relate them to mechanistic controls (e.g., bioturbation, freeze-thaw). This dissertation encompasses two field-based studies and one remote data study that aim to quantify the rates of regolith production and transport, define the physical rules that dictate the shape of hillslopes, and identify the mechanisms by which topography adjusts. Here, I exploit the cosmogenic radionuclide meteoric 10Be to measure regolith residence times, erosion rates, and downslope regolith fluxes in the Susquehanna Shale Hills Critical Zone Observatory (SSHO), located in the Valley and Ridge Physiographic province of the central Appalachians. I compare these isotopic measurements with measures of hillslope morphology taken from recently acquired high-resolution LiDAR derived digital topography in order to test transport rules acting at SSHO and tease out mechanisms responsible for regolith transport.

From this work I have determined that the Shale Hills Critical Zone Observatory is approaching steady state, where the fluxes of regolith production at SSHO ridgetops are balanced by regolith erosion, within uncertainty. Additionally, regolith fluxes along hillslopes in and adjacent to SSHO suggest that the entire landscape is lowering at a rate of ~30 m/My. I have found that this condition exists despite observed aspect-related asymmetry with respect to hillslope gradient and regolith thickness. In fact, it is likely that the topographic asymmetry is a result of the landscape adjusting its morphology in order to maintain a steady flux of material off hillslopes receiving different amounts of solar radiation over geologic time. I contend that a transport mechanism that is both aspect-controlled and dependent on regolith depth is freeze-thaw cycling, which appears to occur more frequently on sun-facing hillslopes at SSHO.

Along SSHO ridgetops, where regolith is uniformly thin and aspect-related differences are minimal, regolith flux appears to be linearly dependent on local topographic gradient. Ridgetop curvature values extracted from high-resolution digital topography accurately reflect erosion rates and transport efficiency measured at SSHO, using meteoric 10Be. Applying this understanding of landscape evolution from SSHO to first-order watersheds elsewhere in the Susquehanna River Basin (n=27), reveals that, in the steadily lowering reaches of the basin, transport efficiency is everywhere equal, despite differences in lithology, and proximity to the last glacial margin. These results suggest that much of the topography of the central Appalachians has adjusted to regional climatic and tectonic perturbations occurring over the Cenozoic.

Citation

Nicole West (2014): Topographic Fingerprints of Hillslope Erosion in the Central Appalachians Revealed by Meteoric Beryllium-10. Doctor of Philosophy, Geosciences, Pennsylvania State University, p. 159.

This Paper/Book acknowledges NSF CZO grant support.