Weathering of gneiss saprolites and formation of Planosols under semiarid climate (NE Brazil)

Abstract

The Brazilian semiarid is one of the most populous regions in the world. The main income for populations in this region is agricultural production, which in turn is limited by climatic and soil conditions. This region has environmental and economic correlations with underdeveloped regions in northwestern Africa, displaying soil surface with monotonous landscapes of smooth undulating relief with large extensions of Planosols. Climatic conditions that lead to the formation of poorly developed soils and to losses of surface layers make these soils susceptible to degradation. In the Brazilian semiarid, lithology shows that felsic rocks are the main factor in pedogenesis of Planosols, conferring the main physical, chemical, and mineralogical attributes to the soil. In this study, we evaluated the pedogenesis by weathering of gneisses (felsic rocks) in two profiles of Planosols in the Cabrobó Desertification Nucleus, Pernambuco State, northeast Brazil, highlighting the role of the parent material in the soil intrinsic characteristics in the pedogenetic processes. The primary assemblage and pedological features were determined by optical microscopy in thin sections of the A, B, and C horizons from the two soil profiles. The secondary assemblage was identified by X-ray diffractometry using the technique: SEM-scanning electron microscopy. Clay formation in situ, colluvial contribution, and elutriation process contributed to the formation of the gradient textural in both Planosols, with no evidence of argilluviation. For total regolith profile, acidic metamorphic rocks present high structural variability directly influencing the evolution of saprolites, the alteration of primary minerals, and the formation routes of secondary minerals. The secondary assemblage was rather similar and comprised illite, smectite, and kaolinite in both regoliths. The saprolite inclination in P1 favors water movement in relation to P2. The regolith in P1 shows greater mineral alteration with increased pedogenic development.