Gneiss saprolite weathering and soil genesis along an east-west regolith sequence (NE Brazil)

Pressure for increase in biomass production worldwide push the boundaries of agricultural lands towards fragile soils. Such process is particularly intense in Pernambuco State, NE Brazil. Pernambuco is traversed by a moisture gradient where soils were developed from similar gneisses. The strong orientation of rock structure influenced the internal movement and distribution of water, and therefore influenced regolithogenesis. This paper presents mineralogical characterization of three regolith profiles with shallow soils (here assumed as those with the top of Cr horizon at depth 1.5 m or less) along the 8o S transect, and discusses their weathering and pedogenesis processes. Samples were investigated by microscopy (optical and SEM), XRD and total chemical analysis (ICP_AES). Weathering stages were evaluated by the indexes Xd, Ip, ɛ, Z, CIA, CIW and PIA. The presence of weathering resistant quartz veins and planes in rock structure played a central role in controlling the water dynamics and the advancement of the weathering front. Therefore, dessilication was dominantly controlled by pore network. Because of that, the weathering indexes were not contrasting among the climatic zones and both 1:1 and 2:1 types of clay were found in all profiles, although the amount of clay minerals formed and degree of dissolution of primary minerals were contrasting among climate types. From an agricultural perspective, thin soil profiles and presence of leucocratic layers are the main drawbacks of these regoliths, while the capacity to supply plant nutrients via primary mineral dissolution near the root zone can be an advantage.