Abstract

The functioning and structure of the critical zone is largely controlled by the formation of regolith - the physically and chemically altered material formed from in situ parent bedrock. Therefore, understanding how regolith production and transport respond to perturbations in climate and/or tectonic forcing remains a first-order question in critical zone science. At the Susquehanna Shale Hills Critical Zone Observatory (SSHO), high resolution LiDAR-derived topographic data and depths to hand auger refusal reveal a systematic asymmetry in hillslope gradient and mobile regolith thicknesses; both are greater on north-facing hillslopes. Hydrologic and geochemical studies of at the SSHO also suggest asymmetric sediment transport, fluid flow, and mineral weathering with respect to hillslope aspect. Here, we combine shallow seismic surveys completed along 4 hillslope transects (2 north-facing and 2-south facing), 2 ridgetops transects, and subsurface observations in boreholes to investigate the role of climate in inducing fracturing and priming the development of the observed asymmetry. Comparisons of shallow p-wave velocities with borehole and pit observations suggest the presence of three distinct layers at SSHO: 1) a deep, high velocity layer that is consistent with largely unweathered shale bedrock immediately overlain by 2) an intermediate velocity layer that is consistent with fractured and chemically altered bedrock, and 3) a shallow, slow velocity layer that is consistent with mobile material or shallow soil. Shallow p-wave velocity profiles suggest differences in thickness for both the mobile and immobile regolith material with respect to aspect. Patterns of p-wave velocities with depth are consistent with patterns of fracture densities observed in boreholes and with predictive cracking intensity models related to frost action. Similarly, p-wave velocity profiles correspond with chemical depletion profiles measured in the SSHO subsurface. These data suggest that the feedbacks between chemical weathering and the physical structure of the critical zone at SSHO may be driven by microclimate asymmetry over geologic time.

Citation

West, N., Kirby, E., Nyblade, A., and Brantley, S.L. (2016): Microclimate Controls on the Evolution of Critical Zone Architecture in 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.