Abstract

The critical zone depth and extent of weathering underlying hillslopes influence geomorphologic, hydrologic, and ecologic processes including the magnitude of water storage and the pace of mobile soil production. However, compared to the mobile soil layer, little is known about patterns of bedrock weathering at landscape or hillslope scales. Here we document critical zone thickness and degree of weathering at the hillslope scale to test theories for the controls on the evolution of the critical zone. We hypothesize that within a catchment underlain by uniform lithology and experiencing the same climatic and tectonic history (and thus channel incision rates), the depth to the base of the weathering profile is proportional to the hillslope relief. This "self-similar" pattern is consistent with bedrock groundwater drainage as a bottom-up control on the penetration of a weathering front. To test this hypothesis, we drilled boreholes across a sequence of repeating ridges and valleys in the upturned sedimentary Great Valley Sequence of the eastern Northern California Coast Ranges. On three different hillslopes, with ~20 and ~50 m relief (slopes of ~0.46 (m/m)), we drilled 20-45 m deep boreholes along ridges and drilled shallower holes penetrating the weathering front across the hillslope. Geochemical and porosimetry analyses on samples recovered from drilling, along with borehole and surface geophysics, reveal an upslope thickening weathering profile. Within channels that bound hillslopes, bedrock cores are relatively unweathered within centimeters of the ground surface, while at the hillslope divide, evidence of weathering extends to 10 m below the surface for the two hillslopes with ~20 m relief, and 18 m for the ~50 m relief hillslope. Our preliminary observations indicate a proportionality between hillslope relief and depth of weathering, which is consistent with the non-linear relationship predicted by a bottom-up control on bedrock weathering. By isolating lithology, vegetation, and climate, our observations indicate that the mechanisms regulating the pattern of bedrock weathering at hillslope scales are tightly linked to surface topography. Such data provide valuable constraint for models of critical zone evolution.

Citation

Pedrazas, M., Hahm, W.J., Huang, M.-H., Nelson, M.D., Bryk, A.B., Dralle, D., Fauria, K., Dietrich, W.E., and Rempe, D.M. (2019): Investigating Hillslope Self-Similarity: Field Observations of Weathering Profiles Across a Sequence of Repeating Ridges and Valleys. American Geophysical Union 2019 Fall Meeting, San Francisco, CA, 9-13 December 2019 Abstract #EP53F-2213 .

This Paper/Book acknowledges NSF CZO grant support.