A simple model for regolith formation by chemical weathering

Abstract

We present here a new model for the formation of regolith on geological timescales by chemical weathering based on the assumption that the rate of chemical weathering is primarily controlled by the ability of groundwater to transport solute away from the reacting solid‐fluid interface and keep the system from reaching equilibrium (saturation). This allows us to specify the rate of propagation of the weathering front as linearly proportional to the pore fluid velocity which we obtain by computing the water table geometry in the regolith layer. The surface of the regolith layer is affected by mass transport and erosion. The main prediction of the model is that the geometry of the regolith, i.e., whether it is thickest beneath topographic highs or topographic lows, is controlled by the value of a dimensionless number, which depends on the square of the surface slope, the hydraulic conductivity, and local precipitation rate, but is independent of the chemical weathering rate. In orogenic environments, where regolith formation by chemical weathering competes with surface erosion, the model predicts that the existence and thickness of the regolith layer are controlled by the value of another dimensionless number which is the ratio between the timescale for erosion and the timescale for weathering. The model also predicts that in anorogenic areas, regolith thickness increases as the square root of time, whereas in orogenic environments, a steady state regolith thickness can be achieved, when the propagation of the weathering front is equal to erosion rate.

Colluvial deposits as a possible weathering reservoir in uplifting mountains

“…Nevertheless, Maher (2010Maher ( , 2011 and Maher and Chamberlain (2014) argued that in many mountainous situations, the weathering rate should essentially and linearly depend on the water flow in the soil, with a minor effect of temperature. In Equation 8, the linear dependency between the regolith production rate and runoff and the weaker dependence on temperature are consistent with that view, although our model clearly misses 140 the control of water flux partitioning between the surface and ground on the regolith development rate and pattern (Maher and Chamberlain, 2014;Rempe and Dietrich, 2014;Braun et al, 2016;Schoonejans et al, 2016). The drawback of Equations 7, 8 is that they are parametrical and not truly physically based.…”

“…Yet, the form of the production law controls the spatial distribution of the regolith thickness and production rate in a mountain (Carretier et al, 2014;Braun et al, 2016). We thus test the robustness of our main result by assuming an exponential regolith production rather than a humped law.…”

“…Maher, 2010;Maher and Chamberlain, 2014;Hilley et al, 120 2010; Rempe and Dietrich, 2014). Lebedeva et al (2010) and Braun et al (2016) showed that in the absence of uplift and erosion, this type of model predicts that the regolith thickens as √ t. This means that the regolith production rate w varies as 1/B. Compared with the exponential trend of Equation 7, this 1/B trend predicts a much slower attenuation of the regolith development when it thickens.…”

“…Compared with the exponential trend of Equation 7, this 1/B trend predicts a much slower attenuation of the regolith development when it thickens. This allows very thick (>100 m) regoliths to develop within a realistic period of time 125 (Braun et al, 2016). Alternatively, if the weathering front advance is controlled by the rate of mineral fracturing, then the regolith production rate is predicted to be constant (Fletcher et al, 2006).…”

“…Moreover, few models (Vanwalleghem et al, 2013;Braun et al, 2016) account for the heterogeneity of erosion and weathering during relief adaptation to uplift which may control the overall evolution of the weathering rate of a rising mountain range (Anderson et al, 2012;Cohen et al, 2013;Carretier et al, 2014). None of these models can be used to trace the weathered material through its stochastic 50 displacement from the hillslopes to the basins.…”