Comment on “A Simple Model for Regolith Formation by Chemical Weathering” by Braun et al.: Contradictory Concentrations and a Tale of Two Velocities

Harman, Ciaran J.; Cosans, C.; Putnam, S. M.

doi:10.1002/2016jf004151

Cited by 4 publications

(4 citation statements)

References 13 publications

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“…The Figures D1 and D2 (Supporting Information Appendix D) also contradict assumptions made about the relationship between horizontal and vertical Darcy velocities in previously published models (Braun et al ., , ). Because these assumptions are crucial for their models, the results and conclusions obtained in their work should be considered with caution as noted previously (Harman et al ., ). Note that although our model does not consider the unsaturated zone, and calculated pressure heads could thus differ from those for combined saturated–unsaturated models, our model does not predict non‐physical fluxes directed into the subsurface domain through the right boundary (Figures ).…”

Section: Weathering Without Erosionmentioning

confidence: 99%

“…Many reactive transport models of weatheringincluding most of our previous articles cited laterposit that solutes move by diffusion or advection in only one direction. Recently, researchers have moved beyond simulations of weathering under the condition of one-dimensional (1D) advection to consider or speculate about the importance of 2D advection (Rempe and Dietrich, 2014;Braun et al, 2016;Pandey and Rajaram, 2016;Brantley et al, 2017;Harman et al, 2017;Lebedeva and Brantley, 2018;Anderson et al, 2019;Harman and Cosans, 2019;Harman and Kim, 2019;Reis and Brantley, 2019). Here we expand on our previous models for weathering and eroding hills and consider a model describing changes in flow conditions within eroding hills that are essentially caused by geochemical reactions.…”

Section: Rationale For Our Approachmentioning

confidence: 99%

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

Lebedeva

Brantley

2020

Earth Surf Processes Landf

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We present a model of chemical reaction within hills to explore how evolving porosity (and by inference, permeability) affects flow pathways and weathering. The model consists of hydrologic and reactive‐transport equations that describe alteration of ferrous minerals and feldspar. These reactions were chosen because previous work emphasized that oxygen‐ and acid‐driven weathering affects porosity differently in felsic and mafic rocks. A parameter controlling the order of the fronts is presented. In the absence of erosion, the two reaction fronts move at different velocities and the relative depths depend on geochemical conditions and starting composition. In turn, the fronts and associated changes in porosity drastically affect both the vertical and lateral velocities of water flow. For these cases, estimates of weathering advance rates based on simple models that posit unidirectional constant‐velocity advection do not apply. In the model hills, weathering advance rates diminish with time as the Darcy velocities decrease with depth. The system can thus attain a dynamical steady state at any erosion rate where the regolith thickness is constant in time and velocities of both fronts become equal to one another and to the erosion rate. The slower the advection velocities in a system, the faster it attains a steady state. For example, a massive rock with relatively fast‐dissolving minerals such as diabase – where solute transport across the reaction front mainly occurs by diffusion – can reach a steady state more quickly than granitoid rocks in which advection contributes to solute transport. The attainment of a steady state is controlled by coupling between weathering and hydrologic processes that force water to flow horizontally above reaction fronts where permeability changes rapidly with depth and acts as a partial barrier to fluid flow. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

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Section: Weathering Without Erosionmentioning

confidence: 99%

Section: Rationale For Our Approachmentioning

confidence: 99%

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

Lebedeva

Brantley

2020

Earth Surf Processes Landf

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“…The Philip theory indeed delivers a net infiltration that has a term proportional to the square root of the time; thus it would be possible to generate a time-dependence for chemical weathering more or less equal to what is typically observed, while preserving the proportionality of weathering to the flow rate. This publication has been criticized by Harman et al (2017) who found that the specific flow paths envisioned by Braun et al (2016) were not realistic. Since we do not specify the flow paths in the same level of detail, we do not find this criticism compelling.…”

Section: Most Closely Related Modelmentioning

confidence: 99%

Prediction of Soil Formation as a Function of Age Using the Percolation Theory Approach

Egli

Hunt

Dahms

et al. 2018

Front. Environ. Sci.

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Recent modeling and comparison with field results showed that soil formation by chemical weathering, either from bedrock or unconsolidated material, is limited largely by solute transport. Chemical weathering rates are proportional to solute velocities. Nonreactive solute transport described by non-Gaussian transport theory appears compatible with soil formation rates. This change in understanding opens new possibilities for predicting soil production and depth across orders of magnitude of time scales. Percolation theory for modeling the evolution of soil depth and production was applied to new and published data for alpine and Mediterranean soils. The first goal was to check whether the empirical data conform to the theory. Secondly we analyzed discrepancies between theory and observation to find out if the theory is incomplete, if modifications of existing experimental procedures are needed and what parameters might be estimated improperly. Not all input parameters required for current theoretical formulations (particle size, erosion, and infiltration rates) are collected routinely in the field; thus, theory must address how to find these quantities from existing climate and soil data repositories, which implicitly introduces some uncertainties. Existing results for soil texture, typically reported at relevant field sites, had to be transformed to results for a median particle size, d 50 , a specific theoretical input parameter. The modeling tracked reasonably well the evolution of the alpine and Mediterranean soils. For the Alpine sites we found, however, that we consistently overestimated soil depths by ∼45%. Particularly during early soil formation, chemical weathering is more severely limited by reaction kinetics than by solute transport. The kinetic limitation of mineral weathering can affect the system until 1 kyr to a maximum of 10 kyr of soil evolution. Thereafter, solute transport seems dominant. The trend and scatter of soil depth evolution is well captured, particularly for Mediterranean soils. We assume that some neglected processes, such as bioturbation, tree throw, and land use change contributed to local reorganization of the soil and thus to some differences to the model. Nonetheless, the model is able to generate soil depth and confirms decreasing production rates with age. A steady state for soils is not reached before about 100 kyr to 1 Myr

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“…Other hypotheses have been put forward that emphasize the importance of hillslope hydraulics as both a driver and result of critical zone evolution. Rempe and Dietrich () and Braun, Mercier, Guillocheau, and Robin () make use of hydraulic groundwater theory to build quantitative frameworks predicting the influence of lateral flow on critical zone architecture (for a discussion of Braun et al, , see Harman, Cosans, and Putnam, ). Rempe and Dietrich's () model suggests that lateral flow through the unweathered bedrock, driven by its relief above the adjacent stream, could draw reactive water into pores (and even evacuate them when water was not available) at the upper extent of the unweathered rock, initiating weathering.…”

Section: Introductionmentioning

confidence: 99%

A low‐dimensional model of bedrock weathering and lateral flow coevolution in hillslopes: 2. Controls on weathering and permeability profiles, drainage hydraulics, and solute export pathways

Harman

Cosans

2019

Hydrological Processes

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The advance of a chemical weathering front into the bedrock of a hillslope is often limited by the rate weathering products that can be carried away, maintaining chemical disequilibrium.If the weathering front is within the saturated zone, groundwater flow downslope may affect the rate of transport and weathering-however, weathering also modifies the rock permeability and the subsurface potential gradient that drives lateral groundwater flow. This feedback may help explain why there tends to be neither ''runaway weathering'' to great depth nor exposed bedrock covering much of the earth and may provide a mechanism for weathering front advance to keep pace with incision of adjacent streams into bedrock. This is the second of a two-part paper exploring the coevolution of bedrock weathering and lateral flow in hillslopes using a simple low-dimensional model based on hydraulic groundwater theory. Here, we show how a simplified kinetic model of 1-D rock weathering can be extended to consider lateral flow in a 2-D hillslope. Exact and approximate analytical solutions for the location and thickness of weathering within the hillslope are obtained for a number of cases. A location for the weathering front can be found such that lateral flow is able to export weathering products at the rate required to keep pace with stream incision at steady state. Three pathways of solute export are identified: ''diffusing up,'' where solutes diffuse up and away from the weathering front into the laterally flowing aquifer; ''draining down,'' where solutes are advected primarily downward into the unweathered bedrock; and ''draining along,'' where solutes travel laterally within the weathering zone. For each pathway, a different subsurface topography and overall relief of unweathered bedrock within the hillslope is needed to remove solutes at steady state. The relief each pathway requires depends on the rate of stream incision raised to a different power, such that at a given incision rate, one pathway requires minimal relief and, therefore, likely determines the steady-state hillslope profile.

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Comment on “A Simple Model for Regolith Formation by Chemical Weathering” by Braun et al.: Contradictory Concentrations and a Tale of Two Velocities

Cited by 4 publications

References 13 publications

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

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

Prediction of Soil Formation as a Function of Age Using the Percolation Theory Approach

A low‐dimensional model of bedrock weathering and lateral flow coevolution in hillslopes: 2. Controls on weathering and permeability profiles, drainage hydraulics, and solute export pathways

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