We investigate aspect controls on Critical Zone (CZ) architecture, mobile regolith transport, and landscape morphology, based on a cross-CZO comparison of 1) Niwot Ridge, Boulder Creek CZO (BcCZO), a high alpine site with minimal soil/veg cover, characterized by steep S-facing and shallow N-facing hillslopes and 2) Shale Hills CZO (SSHCZO), a temperate, densely-forested, soil-mantled site with steep N-facing slopes and shallow S-facing slopes. We use high-resolution 2D seismic tomography of P- and S-wave velocities (Vp, Vs) to characterize CZ architecture and constrain depths of weathering fronts, as well as the thickness, character, and transport efficiency of mobile regolith layers. The 2D imagery allows assessment of changes in material properties both lateral and vertical (depth), thus mapping variability in CZ structures along the survey profile. The combination of Vp and Vs are used to better quantify material properties, (i.e., elastics moduli, density, fractures porosity), rock-mass strength, and weathering intensity – and when applied to the very shallow subsurface can help constrain the transport efficiency (strength or erodibility) of mobile regolith layers.
On Niwot Ridge, the depth of the weathering front and thickness of mobile regolith are substantially greater on shallower N-facing slopes, consistent with frost/freeze driven processes. However, the depth of the weathering front far exceeds modeled extents of frost-cracking depth (for past or present climates), suggesting additional processes also influence deep weathering. Mobile regolith is considerably thicker on shallow N-facing aspects and composed of large, disintegrated cobbles, however, velocity-based estimates of transport efficiency are higher on S-facing slopes composed of small talus blocks and thin soil/veg cover. Although, thin mobile regolith on S-facing slopes may be weak (slow V), the lower gradient of N-facing slopes and southward asymmetry of the ridge divide, suggests that transport efficiency is greatest on N-facing slopes. This may be explained by the dominance of frost/freeze process, which can readily lift or break, and provide a remarkably efficient process to transport the thick mobile regolith layer of large cobbles.
At SSHCZO, depths of weathering fronts are invariant with slope aspect, suggesting that aspect control is not a predominant mechanism driving regolith production. Mobile regolith thickness, however, is more than 2-fold greater on N-facing slopes. Additionally, the mobile regolith on both slope aspects is primarily composed of well-developed soils. N-facing soils are thicker with greater cohesion, moisture, and inclusion of rock fragments. This is consistent with velocity-based estimates of lower transport efficiency on N-facing slopes relative to the thin, dry, fine grained soils on S-facing slopes.
These results suggest fundamental differences in CZ architecture, weathering processes, and the influence of slope aspect between BcCZO and SSHCZO
Clarke, B.A., Kirby, E., Burbank, D.W., West, N. (2013): Cross-CZO Contrasts: Aspect Controls and Critical Zone Architecture. Abstract H43L-07presented at 2013 Fall Meeting, AGU, San Francisco, CA, 9-13 Dec..
This Paper/Book acknowledges NSF CZO grant support.