Abstract

Since the recognition that hillslope gradients adjust to balance downslope fluxes with production of regolith, workers have sought to understand the quantitative relationship between mass fluxes and hillslope topography. For much of the last century, it has been hypothesized that the downslope flux of regolith is linearly proportional to the local hillslope gradient, while the rate of surface lowering is linearly proportional to hillslope curvature. The development of new isotopic methods and the increasing availability of high resolution topographic data sets allows for testing of these simple transport models in a variety of landscapes. With the application of these tools, workers have learned that the simple linear transport rules do not always adequately describe the behavior of hillslopes. Our study presents a preliminary test of simple geomorphic models, using a combination of meteoric ¹⁰Be derived regolith erosion and flux rates and high resolution topographic data, collected using airborne laser swath mapping. Our field area, the Susquehanna Shale Hills Critical Zone Observatory (SSHO), located only 75 km due south of the Laurentide ice margin in central Pennsylvania, serves as a compelling staging ground for investigating process control on erosion, as local mechanisms for sediment transport have transitioned from freeze-thaw processes to bioturbation. Local hillslope gradients and ridgetop curvature values were extracted from a 1 m resolution digital elevation model for use in flux calculations and model testing. Meteoric ¹⁰Be concentrations in regolith along two hillslopes in SSHO were used to measure downslope flux rates, and revealed a systematic increase in flux rates from ~ 5 cm²/y near the ridge tops to ~ 30 cm²/y near the toe slopes. Comparing our measured flux rates with simple geomorphic transport rules suggests a surprising result – at SSHO, fluxes of regolith are not linearly correlated with topographic gradient. However, if we incorporate the observation that regolith also increases in depth downslope, we find that regolith flux appears to be linearly related to the product of depth and local gradient. From these comparisons, we derived a transport efficiency value (K) of ~ 45 +/- 15 cm²/y for SSHO. Ridgetop curvature data extracted from the high
resolution topographic data, when used with the transport efficiency value derived from ¹⁰Be measurements of regolith flux, predicts a ridgetop lowering rate of 12.2 +/- 5.0 m/My, a rate that agrees with previously presented rates of ridgetop lowering at SSHO, within uncertainty. This result supports the use of digital topographic data for predicting erosion rates at SSHO. The fact that the flux of mobile regolith can be predicted in part by its thickness is an exciting discovery in that it suggests that depth dependent transport mechanisms such as bioturbation have been acting at SSHO since the onset of Holocene climate.

Citation

West, N., Kirby, E. (2012): Testing hillslope transport models in the Susquehanna Shale Hills Critical Zone Observatory. AGU Annual Fall Conference Proceedings.

This Paper/Book acknowledges NSF CZO grant support.