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

Abstract

The structure of the critical zone (CZ) is a product of feedbacks among hydrologic, climatic, biotic, and chemical processes. Past research within snow‐dominated systems has shown that aspect‐dependent solar radiation inputs can produce striking differences in vegetation composition, topography, and soil depth between opposing hillslopes. However, far fewer studies have evaluated the role of microclimates on CZ development within rain‐dominated systems, especially below the soil and into weathered bedrock. To address this need, we characterized the CZ of a north‐facing and south‐facing slope within a first‐order headwater catchment located in central coast California. We combined terrain analysis of vegetation distribution and topography with soil pit characterization, geophysical surveys and hydrologic measurements between slope‐aspects. We documented denser vegetation and higher shallow soil moisture on north facing slopes, which matched previously documented observations in snow‐dominated sites. However, average topographic gradients were 24° and saprolite thickness was approximately 6 m across both hillslopes, which did not match common observations from the literature that showed widespread asymmetry in snow‐dominated systems. These results suggest that dominant processes for CZ evolution are not necessarily transferable across regions. Thus, there is a continued need to expand CZ research, especially in rain‐dominated and water‐limited systems. Here, we present two non‐exclusive mechanistic hypotheses that may explain these unexpected similarities in slope and saprolite thickness between hillslopes with opposing aspects.

Geologic and Tectonic Controls on Deep Fracturing, Weathering, and Water Flow in the Central California Coast Range

“…The layers include physically mobile soil, saprolite, weathered bedrock with matrix oxidation, weathered bedrock with fracture oxidation, and fractured bedrock with no oxidation. The depth of mobile soil varies from 0.3 to 0.8 m (A. M. Donaldson et al., 2023). Defining the depth of saprolite is challenging because the boundary between saprolite and weathered rock is difficult to sample and can be gradual, particularly in sedimentary rocks (Hudson Rasmussen et al., 2023).…”

“…Arbor Creek has a Mediterranean climate with cool, wet winters and warm, dry summers. The average annual rainfall from 2012 to 2021 was 600 mm and average temperature was 14°C (A. M. Donaldson et al., 2023). Vegetation at the site varies by aspect, with north to northeast facing slopes characterized by oak woodlands and south to southwest facing slopes characterized by seasonal grasses (A. Donaldson et al, 2024).…”

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

“…Other studies have also observed a lack of clear aspect-dependent saprolite thickness at sites with clear aspect-dependent vegetation density. For example, a site named Blue Oak Ranch Reserve that is ∼200 km south of Rancho Venada and shares similar lithology and climate does not show a clear aspect-dependent saprolite thickness (Donaldson et al, 2023). In addition, the south-facing slopes of the Santa Catalina Mountains in Arizona have thicker saprolite, despite a lower tree density (Leone et al, 2020).…”

Low Vp/Vs Values as an Indicator for Fractures in the Critical Zone

“…Weathering processes initiate below the surface, producing saprolite, weathered bedrock, and unweathered bedrock that can be tens of meters thick and hold clues about how physical and chemical weathering processes are affected by lithology (Bazilevskaya et al., 2013; Buss et al., 2017), fracture density (Lebedeva & Brantley, 2017, 2023; Molnar et al., 2007), foliation (Eppinger et al., 2021; Leone et al., 2020), regional and tectonic stresses (St. Clair et al., 2015), climate (Anderson et al., 2019; Chorover et al., 2011), and groundwater (Goodfellow et al., 2011; Rempe & Dietrich, 2014). In the last decade, P‐wave velocities obtained by seismic refraction surveys have become common for characterizing the deep CZ over extensive areas (Befus et al., 2011; Donaldson et al., 2023; Holbrook et al., 2014; Uecker et al., 2023). Seismic velocities have been used to quantify volumetric strains under hillslopes (Hayes et al., 2019), estimate water holding capacity (Flinchum et al., 2018a; Holbrook et al., 2014; Klos et al., 2018), and highlight essential connections between CZ structure and drought resilience (Callahan et al., 2020, 2022).…”