06 Feb 2024
 | 06 Feb 2024

A numerical model for duricrust formation by water table fluctuations

Caroline Fenske, Jean Braun, François Guillocheau, and Cécile Robin

Abstract. Duricrusts are hard elemental layers forming in climatically contrasted environments. Ferricretes (or iron duricrusts) are a type of duricrust, made of indurated iron layers. They form in tropical to semi-arid environments, but can be currently observed all around the world, in areas such as Africa, South America, India, and Australia. In most cases, they cap hills and appear to protect softer layers beneath. Two hypotheses have been proposed for the formation of duricrusts, i.e., the hydrological or horizontal model where the enrichment in the hardening element (iron for ferricretes) is the product of leaching and precipitation through the beating of the water table during contrasted seasonal cycles, and the laterisation or vertical model, where the formation of iron duricrusts is the final stage of laterisation.

In this article, we present the first numerical model for the formation of iron duricrusts based on the hydrological hypothesis. The model is an extension to an existing regolith formation model where the position of the water table is used to predict the formation of a hardened layer at a rate set by a characteristic time scale τ and over a depth set by the beating range of the water table, λ. Hardening causes a decrease in surface erodibility, which we introduce in the model as a dimensionless factor κ that multiplies the surface transport coefficient of the model.

Using the model we show under which circumstances duricrusts form by introducing two dimensionless numbers that combine the model parameters (λ and τ) as well as parameters representing external forcing like precipitation rate and uplift rate. We demonstrate that by using model parameter values obtained by independent constraints from field observations, hydrology and geochronology, the model predictions reproduce the observed conditions for duricrust formation. We also show that there exists a strong feedback from duricrust formation on the shape of the regolith and the position of the water table. Finally we demonstrate that the commonly accepted view that, because they are commonly found at the top of hills, duricrusts protect elements of the landscape is most likely an over-interpretation and that caution must be taken before using duricrusts as markers of uplift and/or base level falls.

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Caroline Fenske, Jean Braun, François Guillocheau, and Cécile Robin

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-160', John Webb, 19 Apr 2024
    • AC1: 'Reply on RC1', Caroline Fenske, 30 Apr 2024
  • RC2: 'Comment on egusphere-2024-160', Paulo Marcos Vasconcelos, 24 May 2024
    • AC2: 'Reply on RC2', Caroline Fenske, 03 Jun 2024
Caroline Fenske, Jean Braun, François Guillocheau, and Cécile Robin
Caroline Fenske, Jean Braun, François Guillocheau, and Cécile Robin


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Short summary
We have developed a new numerical model to represent the formation of ferricretes which are iron-rich, hard layers found in soils and at the surface of the Earth. We assume that the formation mechanism implies variations in the height of the water table and that the hardening rate is proportional to precipitation. The model allows us to quantify the potential feedbacks they generate on the surface topography and the thickness of the regolith/soil layer.