Models of Duricrust Formation in Tropical and Subtropical Areas

Ferruginous duricrust formation takes place by dissolution and accumulation of iron during the wet season and precipitation during the dry season. However, the formation of iron duricrusts has always been the centre of much debate. They commonly form as hard iron layers in tropical and subtropical environments with strong climatic seasonality. They were first described in Africa, Australia and India in the 1950’s. They often cap landscapes, which can be explained by their extremely high iron content making them more resistant to mechanical weathering. However, in recent years, iron duricrusts were also described at depth in the regolith, like in South America.

This has led to 2 distinct formation hypotheses: the first one relies on the vertical beating of the water table most likely in a stable environment combined to lateral hydraulic transport of iron. In this scenario, iron hydroxides accumulate in one specific region after being transported from surrounding source areas through the water table, and precipitate as Fe³⁺ during the dry season. Through time, this leads to duricrusts forming within the water table beating range. This model is compatible with duricrusts forming at depth within the regolith. The second hypothesis relies on the vertical, in situ leaching of elements and resulting compaction and surface lowering. During weathering, iron nodules accumulate with the progressive leaching of soluble elements from the parent rock, leaving only the iron oxides, ultimately forming a duricrust. It implies a genetic link between the duricrust, the lateritic profile and the underlying parent rock. This model explains why duricrusts are mostly observed near the surface. Nevertheless, there is no consensus on the conditions under which either of these hypotheses prevail.

 

To address this issue, we developed two separate first-order numerical models representing the two hypotheses for duricrust formation. We used the models to demonstrate that the hypotheses lead to different scenarios of duricrust formation by running a large number of simulations. Especially, we show that the stability of a landform is a discerning element between the 2 models. In the first case, quiescent periods of time in between uplifting periods are crucial, while in the second case, constant dynamic uplift is needed to enable the accumulation of enough elements by compaction.