Illuviation particulaire du sol : le rôle des paléoenvironnements froids

Particles translocation exists in both subarctic/alpine and sub-arid climate contexts. The geochemical surface properties of the particles (loess, alluvium, fossil beaches or slope deposits) induce a translocation of fine or silty clays under control of the pH. The process of illuviation corresponds to an early and precise period of soil evolution, depending on decarbonization or desaturation. It requires the percolation of a water flash: a meltwater, or a harsh rain in arid context. Any excess cations (Ca++ or Mg++ in alkaline or Al2 O3 in acid context), allows rapid flocculation and locks the leaching process. Several processes can hinder fine illuviation as in acid, calcareous or volcanic palaeoenvironments. The “Zero Point Charge” (ZPC) of the mineral surface or of clay-humic complex regulates the optimum conditions for particle dispersion or flocculation. This implies that illuvial process is an early stage in the soil evolution within a narrow pH window, but it can be reactivated by the chemical rejuvenation of the soil after erosion or by loess deposition as natural or anthropogenic superficial inputs, by burial, by changes in hydrologic functioning or in vegetation cover. The soils of our regions are the result of a complex and cumulative history since at least 50 ka or even 120 ka, modulated by the evolution of the climate and the biosphere. Clay coatings do not necessarily represent Holocene or older interglacials but can attest as well to Weichselian interstadials, even very brief. A rejuvenation or fertilization by a sedimentary contribution or a truncation will allow a very brief return of the illuvial functioning followed by a rapid return to an oligotrophic status of the surface soil. This phenomenon also makes it possible to understand the succession of illuvial phases observed in thin sections. Most Bt horizons are cumulative and often polyphased. This succession of events, in addition to the evolution of the climate and the precipitation regime, makes it possible to understand, the genesis of the current pedocomplex in function of the loessic quaternary inputs. In regions with limited sedimentary input, the soil very early acidifies, degraded and the illuvial horizon no longer evolves since at least the MIS 3, or the Last Interglacial, as in the south-west of France and the margins of the Massif central or the Vosges. An acido-complexolysis of the clays is superimposed on the leaching, especially in the glosses of the inherited fragipan.