Anisovolumetric weathering in granitic saprolite controlled by climate and erosion rate

Abstract

Erosion at Earth’s surface exposes underlying bedrock to climate-driven chemical and physical weathering, transforming it into a porous, ecosystem-sustaining substrate consisting of weathered bedrock, saprolite, and soil. Weathering in saprolite is typically quantified from bulk geochemistry assuming physical strain is negligible. However, modeling and measurements suggest that strain in saprolite may be common, and therefore anisovolumetric weathering may be widespread. To explore this possibility, we quantified the fraction of porosity produced by physical weathering, FPP, at three sites with differing climates in granitic bedrock of the Sierra Nevada, California, USA. We found that strain produces more porosity than chemical mass loss at each site, indicative of strongly anisovolumetric weathering. To expand the scope of our study, we quantified FPP using available volumetric strain and mass loss data from granitic sites spanning a broader range of climates and erosion rates. FPP in each case is ≥0.12, indicative of widespread anisovolumetric weathering. Multiple regression shows that differences in precipitation and erosion rate explain 94% of the variance in FPP and that >98% of Earth’s land surface has conditions that promote anisovolumetric weathering in granitic saprolite. Our work indicates that anisovolumetric weathering is the norm, rather than the exception, and highlights the importance of climate and erosion as drivers of subsurface physical weathering.

Sediment size on talus slopes correlates with fracture spacing on bedrock cliffs: implications for predicting initial sediment size distributions on hillslopes

“…The first stage begins when particles are initially detached from saprolite (in soil-mantled landscapes) or fractured bedrock (when soil is absent). At any specific source location, whether at the surface or at the base of soil, a set of these newly detached sediment particles has an "initial size distribution", with "initial" defined by the moment when particles are first disturbed by processes that drive particle motion on hillslopes (Sklar et al, 2017;Roda-Boluda et al, 2018;Neely and DiBiase, 2020). The initial size distribution sets the initial condition for subsequent evolution of the size distribution by abrasion and fragmentation of particles during transport, both on hillslopes near where sediment is produced and in river channels downstream.…”

Anisotropy and Microcrack Propagation Induced by Weathering, Regional Stresses and Topographic Stresses

“…The zone of weathered and fractured regolith between fresh bedrock and mobile soil is of wide interest. It is a common feature of terrestrial landscapes (Graham et al., 1994; Riebe et al., 2021) and it forms the thickest portion of the Critical Zone—the region between the top of the forest canopy and unweathered bedrock at depth—in many regions (Buss et al., 2008; Callahan et al., 2020; Flinchum et al., 2018; Holbrook et al., 2019; Pavich et al., 1989). The thickness and material properties of this weathered layer influence the pace of landscape evolution (Dixon et al., 2009), the capacity of landscapes to retain water through drought (Salve et al., 2012), and the degree and type of landsliding a given area is subject to (Watakabe & Matsushi, 2019).…”

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The zone of weathered and fractured regolith between fresh bedrock and mobile soil is of wide interest. It is a common feature of terrestrial landscapes (Graham et al, 1994;Riebe et al, 2021) and it forms the thickest portion of the Critical Zone-the region between the top of the forest canopy and unweathered bedrock at depth-in many regions (

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Standardized field methods for fracture-focused surface processes research

“…within ~500 m - Moon et al, 2020) and on other terrestrial bodies (Molaro et al, 2020). It epitomizes mechanical weathering and the development of 'critical zone architecture', i.e., the evolving porosity, permeability, and strength of nearsurface rock (e.g., Riebe et al, 2021). Herein, we use the terms crack and fracture interchangeably to refer to any planar, open void in rock, regardless of origin or scale (more details below), acknowledging that a large body of geologic literature also refers to veins or dikes -filled with secondary minerals -as 'fractures'.…”