Regions under tropical rainforest cover, such as central Africa and
Brazil are characterised by degradation and dismantling of old
ferricrete structures. In southern Cameroon, these processes are relayed
by present-day ferruginous accumulation soil facies, situated on the
middle and the lower part of hill slopes. These facies become
progressively harder towards the surface, containing from bottom to top,
mainly kaolinite, kaolinite-goethite and Al-rich goethite-hematite, and
are discontinuous to the relictic hematite-dominated ferricrete that
exist in the upper part of the hill slope. These features were
investigated in terms of geochemical differentiation of trace elements.
It appears that, in contrast to the old ferricrete facies, the current
ferruginous accumulations are enriched in transitional trace elements
(V, Cr, Co, Y, Sc) and Ph, while alkali-earth elements are less
differentiated. This recent chemical accumulation is controlled both by
intense weathering of the granodiorite bedrock and by mobilisation of
elements previously accumulated in the old ferricrete. The observed
processes are clearly linked to the present-day humid climate with
rising groundwater tables. They slowly replace the old ferricretes
formed during Cretaceous time under more seasonal climatic conditions,
representing an instructive case of continuos global change. (C) 2002
Elsevier Science B.V. All rights reserved.