A numerical model for duricrust formation by water table fluctuations

IF 2.8 2区 地球科学 Q2 GEOGRAPHY, PHYSICAL
Caroline Fenske, Jean Braun, François Guillocheau, Cécile Robin
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Abstract

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.
地下水位波动形成硬壳的数值模型
摘要硬壳是在气候反差较大的环境中形成的坚硬元素层。铁锈岩(或铁硬壳岩)是硬壳岩的一种,由硬化的铁层组成。它们形成于热带至半干旱环境中,但目前在世界各地,如非洲、南美洲、印度和澳大利亚都能观察到。在大多数情况下,它们覆盖在山丘上,似乎保护着下面较软的地层。关于坚硬岩壳的形成有两种假说,一种是水文或水平模式,即硬化元素(铁锈岩中的铁)的富集是在对比强烈的季节周期中通过地下水位的跳动浸出和沉淀的产物;另一种是后期化或垂直模式,即铁质坚硬岩壳的形成是后期化的最后阶段。在本文中,我们首次提出了基于水文假设的铁质硬壳形成数值模型。该模型是对现有岩石形成模型的扩展,利用地下水位的位置来预测硬化层的形成,其速率由特征时间尺度 τ 设定,深度由地下水位的跳动范围 λ 设定。硬化会导致地表侵蚀性降低,我们在模型中引入了一个无量纲因子 κ,该因子与模型的地表传输系数相乘。我们在模型中引入了两个无量纲数字,将模型参数(λ 和 τ)以及降水率和隆起率等代表外部作用力的参数结合在一起,从而说明在什么情况下会形成硬壳。我们证明,通过使用从实地观测、水文和地质年代学中获得的独立约束条件所得到的模型参数值,模型预测结果再现了所观测到的硬壳形成条件。我们还证明,硬壳的形成对碎屑岩的形状和地下水位的位置有很强的反馈作用。最后,我们证明了人们普遍接受的观点,即由于它们通常出现在山顶,所以硬壳岩保护了地貌要素,这很可能是一种过度解读,在使用硬壳岩作为隆起和/或基底下降的标志之前必须谨慎。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Earth Surface Dynamics
Earth Surface Dynamics GEOGRAPHY, PHYSICALGEOSCIENCES, MULTIDISCI-GEOSCIENCES, MULTIDISCIPLINARY
CiteScore
5.40
自引率
5.90%
发文量
56
审稿时长
20 weeks
期刊介绍: Earth Surface Dynamics (ESurf) is an international scientific journal dedicated to the publication and discussion of high-quality research on the physical, chemical, and biological processes shaping Earth''s surface and their interactions on all scales.
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