The geomorphology working group is examining three fundamental questions at the Shale Hills CZO and surrounding region (Fig. 1):

1) What are the primary processes and rate laws that set the shape of the landscape and the thickness & properties of the Critical Zone?
2) What are the dynamics of feedbacks between hydrology, biology, weathering, and physical erosion?
3) How have the processes and products of the SHO regolith mill changed in response to past climatic, base level, and land-use perturbations?

Question 1: Primary processes and rate laws
We are approaching the question of what controls soil erosion and downslope transport from both a theoretical and empirical perspective. 

Ongoing theoretical efforts include development of a hillslope sediment flux model that incorporates tree-throw and freeze-thaw creep. Model development also includes improvement of overland and channel flow formulations to simulate unsteady non-uniform flows. Validation experiments will employ LiDAR and regolith thickness data being collected onsite.

We are also conducting field-based experiments designed to quantify the rate of regolith formation, the role of tree-throw in the downslope transport of material, and the residence time of material in regolith.

  These experiments combine high-resolution topographic surveys (using terrestrial laser scanning, Fig. 2a) to characterize the spatial density of pit-and-mound topography associated with tree-throw with field calibration of the material transported per event (Fig. 2b). In addition, we are using the spatial distribution of meteoric 10Be along hillslope transects (Fig. 3) to estimate the residence time of regolith materials near the surface and the rate at which these move downslope.

Question 2: Feedbacks in the landscape/regolith system
Transport laws relating sediment transport, erosion, and deposition are being incorporated into the Penn State Integrated Hydrologic Model (PIHM) and implemented in numerical simulations designed to illuminate the nature and direction of feedbacks among processes acting within the Critical Zone. The fully integrated hydrologic/sediment flux model (PIHMSed) includes feedbacks between soil thickness, downslope sediment flux due to rate of tree-throw, rate of regolith conversion from bedrock, evolving soil moisture, and solute fluxes. Three domains (Fig. 4) all co-evolve to produce different steady-state landscapes, depending upon the external forcing conditions.

Question 3: Dynamic evolution of the SHO over geologic time
The thickness of regolith and the efficiency of the weathering engine in the SHO has been influenced by perturbations in both the geologic and recent past. Ongoing efforts are focused on understanding the response of Critical Zone processes to enhanced sediment production and hillslope transport during periglacial times.

Analysis of high-resolution digital elevation models (generated from airborne laser altimetry surveys) reveals abundant periglacial features such as solifluction lobes (Fig. 5) in the region, as well as evidence for previous periods of aggradation along fluvial systems (Fig. 1). Using a combination of coring and subsurface imaging (using ground-penetrating radar), we are currently exploring whether these processes generated deposits that are present in the subsurface of SHO, and, if so, to what degree they influence chemical and isotopic measures of weathering and soil production.

We anticipate that these investigations will inform our understanding of the degree to which antecedent conditions “preconditioned” the landscape and materials now incorporated in the Critical Zone.

  For example, hillslopes along east-west oriented hillslopes within the CZO and nearby catchments exhibit systematic differences in gradient (Fig. 7). We speculate that this difference reflects more efficient hillslope transport during vigorous periglacial activity in the past, but acknowledge that such differences may still be reflected in the modern transport and weathering regime.

Contacts: Rudy Slingerland (PI), Eric Kirby (PI), and Nicole West (Ph.D. Student)
Collaborators: Paul Bierman (University of Vermont)