Ferricrete biochemical degradation on the rainforest-savannas boundary of Central African Republic

摘要

In southeastern Central African Republic the lateritic weathering mantles are capped by 2 to 5 m thick ferricretes, which previously formed under a seasonal tropical climate. The actual humid tropical climatic conditions result in the biophysical disaggregation of the ferricretes everywhere the forest develop that lead to a soil formation composed of ferricrete relicts and ferruginous nodules embedded in a soft bioturbated micro-aggregated clay-ferruginous matrix. The potential effect of the biological activity (e.g., termites) on the soil and vegetation dynamics and therefore on the evolution of the previous consolidated ferricrete is discussed. Following its mechanical disaggregation by the forest tree roots, the ferricrete underwent a chemical degradation under the combined effect of hydration and redox conditions, which result from the biodegradation and oxidation of the organic matter. The transformation of the ferricrete into a soft bioturbated micro-aggregated clay-ferruginous soil matrix implies the hematite dissolution, and the kaolinite transformation into gibbsite, which may characterize a late "bauxitization" (secondary gibbsitization of kaolinite) of previous ferricrete profiles linked to a late Quaternary environmental (climate and vegetation) change. However, the geochemical and mineralogical patterns of the matrix not only reflect the geochemistry of the parental ferricrete but also depend on the physical transfers of quartz and heavy minerals from the lower horizons of the profile. Hence, the use of geochemical indexes such as Ti and/or Zr for mass balance calculations in the lateritic weathering profiles is precluded because these supposedly inert chemical elements are not simply accumulated by in situ chemical weathering process.