The Effect of Aspect and Elevation on Critical Zone Architecture in the Reynolds Creek Critical Zone Observatory: A Seismic Refraction Study

Abstract

In the northern hemisphere within snow-dominated mountainous watersheds north-facing slopes are commonly more deeply weathered than south-facing slopes. This has been attributed to a more persistent snowpack on the north facing aspects. A persistent snowpack releases its water into the subsurface in a single large pulse, which propagates the water deeper into the subsurface than the series of small pulses characteristic of the intermittent snowpack on south-facing slopes. Johnston Draw is an east-draining catchment in the Reynolds Creek Critical Zone Observatory, Idaho that spans a 300 m elevation gradient. The north-facing slope hosts a persistent snowpack that increases in volume up drainage, while the south-facing slope has intermittent snowpack throughout the drainage. We hypothesize that the largest difference in weathering depth between the two aspects will occur where the difference in snow accumulation between the aspects is also greatest. To test this hypothesis, we conducted four seismic refraction tomography surveys within Johnston Draw from inlet to outlet and perpendicular to drainage direction. From these measurements, we calculate the weathering zone thickness from the P-wave velocity profiles. We conclude that the maximum difference in weathering between aspects occurs ¾ of the way up the drainage from the outlet, where the difference in snow accumulation is highest. Above and below this point, the subsurface is more equally weathered and the snow accumulations are more similar. We also observed that the thickness of the weathering zone increased with decreasing elevation and interpret this to be related to the observed increase soil moisture at lower elevations. Our observations support the hypothesis that deeper snow accumulation leads to deeper weathering when all other variables are held equal. One caveat is the possibility that the denser vegetation contributes to deeper weathering on north-facing slopes via soil retention or higher rates of biological weathering.

Symmetry in Hillslope Steepness and Saprolite Thickness Between Hillslopes With Opposing Aspects

“…Another consideration that warrants future research is the magnitude of asymmetry that should be expected between hillslopes with opposing aspects in regions across the energy to water limitation gradient (Inbar et al., 2018; Nielson et al., 2021). For example, within a snow‐dominated region at the semi‐arid Reynold Creek CZ observatory underlain by extrusive igneous rock, Nielson et al.…”

“…For example, within a snow‐dominated region at the semi‐arid Reynold Creek CZ observatory underlain by extrusive igneous rock, Nielson et al. (2021) combined snowpack and soil moisture measurements with seismic refraction surveys to investigate saprolite thickness across an elevational gradient with varying snow accumulation. They observed the greatest saprolite asymmetry at the high elevation site where there was the greatest aspect‐dependent difference in snow accumulation.…”

“…Another consideration that warrants future research is the magnitude of asymmetry that should be expected between hillslopes with opposing aspects in regions across the energy to water limitation gradient (Inbar et al, 2018;Nielson et al, 2021). For example, within a snow-dominated region at the semi-arid Reynold Creek CZ observatory underlain by extrusive igneous rock, Nielson et al (2021) combined snowpack and soil moisture measurements with seismic refraction surveys to investigate saprolite thickness across an elevational gradient with varying snow accumulation. They observed the greatest saprolite asymmetry at the high elevation site where there was the greatest aspect-dependent difference in snow accumulation.…”

“…Below the soil, higher soil water content may allow for deeper recharge into the saprolite, which contributes to thicker, more weathered saprolite on NFS (Garcia‐Gamero et al., 2021; Langston et al., 2015). However, these expectations are largely based on snow‐dominated catchments where other processes, such as freeze‐thaw cycles and snowmelt, can compound the role of vegetation across aspect (Anderson et al., 2013; Befus et al., 2011; Nielson et al., 2021; West et al., 2019). Therefore, despite the expectation that NFS will have a deeper subsurface CZ, this common expectation must be tested in rain‐dominated catchments to confirm its transferability across landscapes.…”

Mapping Variations in Bedrock Weathering With Slope Aspect Under a Sedimentary Ridge‐Valley System Using Near‐Surface Geophysics and Drilling

“…While surface slope, tree density, and soil thickness have been well documented to vary based on aspect dependency (Bale et al, 1998;Inbar et al, 2018), fewer studies address the influence of aspect dependency and climate on deeper weathering transitions. Those that do focus primarily on snow-dominated systems or granite lithology (Anderson et al, 2013(Anderson et al, , 2014Leone et al, 2020;Nielsen et al, 2021).…”