Ferrolysis induced soil transformation by natural drainage in Vertisols of sub-humid South India
Résumé
In sub-humid South India, recent studies have shown that black soil areas (Vertisols and Vertic Intergrades), located on flat valley bottoms, have been rejuvenated through the incision of streambeds, inducing changes in the pedoclimate and soil transformation. Joint pedological, geochemical and geophysical investigations were performed in order to better understand the ongoing processes and their contribution to the chemistry of local rivers. The seasonal rainfall causes cycles of oxidation and reduction in a perched watertable at the base of the black soil, while the reduced solutions are exported through a loamy sand network. This framework favours a ferrolysis process, which causes low base saturation and protonation of clay, leading to the weathering of 2:1 then 1:1 clay minerals. Maximum weathering conditions occur at the very end of the wet season, just before disappearance of the perched watertable. Therefore, the by-products of soil transformation are partially drained off and calcareous nodules, then further downslope, amorphous silica precipitate upon soil dehydration. The ferrolised area is fringing the drainage system indicating that its development has been induced by the streambed incision. The distribution of 14 C ages of CaCO3 nodules suggests that the ferrolysis process started during the late Holocene, only about 2kyr B.P. at the studied site and about 5kyr B.P. at the watershed outlet. The results of this study are applied to an assessment of the physical erosion rate (4.8 x10-3 m/kyr) since the recent reactivation of the erosion process.
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