Abstract

The Susquehanna Shale Hills CZO (SSHCZO) is one of the ten national CZOs in the US. It is a 0.08 km2 first order catchment in Central Pennsylvania with regolith thickness increasing downslope from about 0.3 m at the ridge top to 3 m at the valley floor. Previous studies quantified the regolith production rates, the erosion rates and the geomorphic diffusivities from ridge top to valley floor by using uranium-series isotopes (Jin et al. 2010; Ma et al. 2010) and Meteoric Beryllium 10 (West et al. 2011, 2013) revealing that the regolith production rate is greater than the erosion rate at ridge crest, implying that the regolith thickness is not at equilibrium there. However, the field measurements are restricted to a relatively small area. Additionally, previous models predicted the steady state regolith thickness increasing upslope (Rempe & Dietrich 2014; Lebedeva et al. 2007), which is opposite to the SSHCZO where the regolith thickness increases downslope. So, the questions arise: 1) Is the whole SSHCZO far from geomorphic equilibrium? 2) is the field measurements representative of the whole landscape? 3) How do the processes control the profile of the regolith thickness increasing downslope?

In order to answer these questions, two experiments are conducted at the SSHCZO and the adjacent watersheds by using landscape evoluton model-LE-PIHM. The first experiment simulates the evolution of the landscape relief and channel spacing. The second experiment focuses on the 1D analytic solution of the steady state profile of the regolith thickness based on the governing equations of LE-PIHM. The simulated steady state channel spacing and elevation relief are consistent with the current SSHCZO indicating that the SSHCZO is not far from the morphological equilibrium, and the geomorphic diffusivity on the north slope derived from the field measurement is representative of the whole watershed. The analytic solution predicts a regolith thickness profile increasing downslope, which is consistent with the regolith thickness of the current SSHCZO. The roles of bedrock weathering and regolith creep on affecting the steady state regolith thickness profile are discussed. The 1D analytic solution of the soil thickness profile is an effective tool to inversely estimate the geomorphic diffusivity across the whole landscape.

Citation

Zhang, Y., Slingerland, R.L., Duffy, C., Gu, X., Lin, H., and West, N. (2016): Geomorphic equilibrium and the spatial variation of the geomorphic diffusivity at the Susquehanna Shale Hills Critical Zone Observatory. 2016 Fall Meeting, American Geophysical Union, San Francisco, CA, 12-16 Dec..

This Paper/Book acknowledges NSF CZO grant support.