Non-lateritic red soils of the Brazilian drylands: pedogenic processes and landscape evolution

International audienceNon-lateritic red soils cover vast areas of the semiarid region of Northeast Brazil, or “sertão”, whereas laterites remain currently scarce in the landscape and are confined to ancient summit surfaces and/or to more humid areas along the coast. The land cover of the Brazilian drylands is dominated by dry deciduous woodland, or “caatinga”, in areas with mean annual rainfall and temperatures of 400-800 mm/y and 24-26 °C, respectively. According to morphological descriptions and analytical data, the red soils of semiarid Brazil mainly classify as Luvisols (chromic) in the WRB soil correlation system, and Alfisols in the USDA Soil Taxonomy. In all cases, their formation is strongly controlled by clay illuviation, allowing the diagnosis of a Bt (argic) horizon. Contrary to lateritic soils extending over large tracts of tropical South America, the red soils of NE Brazil are high-activity-clay soils with high cation exchange capacity and moderate to high base saturation ratios. The pedogenic interpretation for their development is that of a fersiallitic pedogenesis, which involves high contents of free iron and the prevalence of 2:1 clays in the soil mineral assemblage. In the regional setting, the red fersiallitic soils typically formed above shallow, grus-type weathering mantles developed from crystalline parent rocks, under pedoclimatic conditions of low, but irregular, deep drainage. In many ways, they are very similar to the red, decalcified fersiallitic soils formed on the crystalline shield of dryland peninsular India, but are very different from tropical ferruginous soils of semiarid West Africa developed from lateritic paleoweathering mantles. As in the Indian setting, persistent epirogenic uplift of the Brazilian margin during the Neogene steered the landscape towards the stripping of most of the paleoweathering mantle, favoring the development of a primary fersiallitic pedogenesis under the newly established semi-arid conditions