Séquence climatique des sols anciens de la région septentrionale de Ténérife (Iles Canaries). 1ère partie: Caractéristiques morphologiques et physico-chimiques

Abstract

The study of a toposequence on the northern side of Tenerife has revealed a close relationship between the distribution of the oldest soils and the different ecological zones (of climate and vegetation). This sequence is made up successively from top to bottom of : ferralitic soils in a perhumid zone, fersiallitic soils in a sub-tropical climatic zone with two very contrasting seasons, and vertisols in the lower zone of the slope with a semi-desertic climate. In the first part of the study the morphological, physical and chemical properties of the soils are presented. The ferrallitic soils are complex, often being rejuvenated by volcanic ashes which have formed either a superficial disatured andosol, or a brown andic soil, or both successively ; certain characteristics allow them to be included with the ferrallitic soils or even with the Oxisols. The fersiallitic soils are red, moderately disatured, slightly leached and show a smail degree of rejuvenation of superficial extent ; yhey are similar to tropical fersiallitic soils and sometimes to the Alfisols. The dark coloured vertisols are typical ; they have an accumulation of calcium carbonate and a beginning of alkalinization in depth. The evolution of the different characteristics in the sequence is very marked. The second part of the study deals with the mineralogical and micromorphological characteristics. Here, the passage from ferrallitic soils with hallosyte and iron and aluminium oxy-hydroxides, for instance gibbsite, to a mixture of hallosyte and 2:I clays (montmorillonite, I-V, illite), and hematite in the fersiallitic soils, to finish exclusively with ferrferous montmorillonite in the vertisols is shown. In the same direction, the content of aluminium, iron and readily soluble silica decreases rapidly. The micromorphology shows the following evolution : from a microporous weathering structure with gibbsitans in the ferrallitic soils, it passes to a clavery illuviation structure in some fersiallitic soils, and finally to a structure of stress-cutans with multiple orientations in the vertisols, and even a structure of saline accumulation with calcitanes. Posteriorly, a summary of the typological and pedogenetic characteristics for each type of soils is made, which leads on to their respective classification (French and American systems). Finally after having summarized the evolution of the characteristics of the soils in the sequence, the problem of the relationship betwee the pedogenesis of the aged soils and the actual climatic zonality, is briefly discussed. An hypothesis concerning the recent evolution of the climate is proposed.