Weathering and erosion of fractured bedrock systems

Abstract

We explore the contribution of fractures (joints) in controlling the rate of weathering advance for a low-porosity rock by using methods of homogenization to create averaged weathering equations. The rate of advance of the weathering front can be expressed as the same rate observed in non-fractured media (or in an individual block) divided by the volume fraction of nonfractured blocks in the fractured parent material. In the model, the parent has fractures that are filled with a more porous material that contains only inert or completely weathered material. The low-porosity rock weathers by reaction-transport processes. As observed in field systems, the model shows that the weathering advance rate is greater for the fractured as compared to the analogous non-fractured system because the volume fraction of blocks is < 1. The increase in advance rate is attributed both to the increase in weathered material that accompanies higher fracture density, and to the increase in exposure of surface of low-porosity rock to reaction-transport. For constant fracture aperture, the weathering advance rate increases when the fracture spacing decreases. Equations describing weathering advance rate are summarized in the 'List of selected equations'.If erosion is imposed at a constant rate, the weathering systems with fracture-bounded bedrock blocks attain a steady state. In the erosional transport-limited regime, bedrock blocks no longer emerge at the air-regolith boundary because they weather away. In the weathering-limited (or kinetic) regime, blocks of various size become exhumed at the surface and the average size of these exposed blocks increases with the erosion rate. For convex hillslopes, the block size exposed at the surface increases downslope. This model can explain observations of exhumed rocks weathering in the Luquillo mountains of Puerto Rico. Published 2017. This article is a U.S. Government work and is in the public domain in the USA

Hillslope Hydrology in Global Change Research and Earth System Modeling

“…Going forward, multiple terrain attributes need to be considered besides steepness (e.g., Tesfa et al, ). Given the fundamental importance of rock lithology (e.g., Bazilevskaya et al, ; Lebedeva & Brantley, ), tectonic stress (e.g., St. Clair et al, ), uplift and erosion rates (e.g., Rempe & Dietrich, ), the climate (e.g., Anderson et al, ; Goodfellow et al, ), climate history (e.g., Buss et al, ), and vegetation (e.g., Brantley, Eissenstat, et al, ; Roering et al, ) as controls on CZ structure, a comprehensive approach considering these factors is needed. A useful question is whether existing global estimates of CZ depth, e.g., Pelletier et al () and Xu and Liu (), can inform global estimates of how porosity/permeability change with depth.…”

Toward catchment hydro‐biogeochemical theories

“…Weathering rates estimated from these rate laws, however, are often 2–5 orders of magnitude higher than those inferred from observed weathering profiles that record the distributions of elemental concentrations over depth (Blum & Stillings, 1995b; Drever & Clow, 1995; Navarre‐Sitchler & Brantley, 2007; Richards & Kump, 2003; Sverdrup, Warfvinge, Blake, & Goulding, 1995; White, 1995; White & Brantley, 1995; White & Brantley, 2003). For reactive transport models (RTMs) to reproduce the reaction fronts observed in weathering profiles, the effective surface area of reacting minerals often has to be set orders of magnitude smaller than the measured bulk surface area (Heidari, Li, Jin, Williams, & Brantley, 2017; Lebedeva & Brantley, 2017; Moore, Lichtner, White, & Brantley, 2012).…”

Exploring an ‘ideal hill': how lithology and transport mechanisms affect the possibility of a steady state during weathering and erosion

“…where k 1 and k 2 in (in s ‐1 ) are the corresponding effective kinetic constants, and is the equilibrium concentration of the species NaSi 2 in a porefluid. A detailed description of these approximations have been discussed previously (Lebedeva et al ., ; Lebedeva and Brantley, ).…”