Degradation and dismantling of iron crusts under climatic changes in Central Africa

Abstract

In Africa, under contrasted tropical climate influences, the lateritic weathering profiles are often capped with an iron crust (Maignien, 1958 ). However, three main geographical domains can be distinguished: (1) a northern Sahelian domain under tropical arid climate where iron crusts are dismantled under the effect of mechanical erosion processes (Leprun, 1977); (2) a southern forest domain under tropical humid climate, where the iron crust dismantling (Nahon et al., 1989 ) results from chemical mechanisms identified by Beauvais ( 1991 ) and Tardy ( 1993); and (3) a transitional domain widespread between the former two domains, where iron crusts are developing under contrasted influences of tropical climate with an annual rainfall ranging from 1200 to 1800 m m (Tardy, 1993). This is the case in the Haut -Mbomou area in southeastern Central African Republic, where the climate is humid tropical with 3 months of dry season and 9 months of wet season. On average, the rainfall is 1600 m m yr -1, the temperature is close to 25 °C and the relative humidi ty of the air is ~ 80%. A long-term tectonic steady state has led to the development of horizontally and