Paleoenvironmental context of fluvial soils in the dryest region of Brazilian semiarid

Abstract

Paleoenvironmental analysis in the Brazilian semiarid region is challenging. The low occurrence of flooded regions limits the preservation of pollen and accumulation of organic matter. In well-drained sites, the high pH and intense erosion destroy phytoliths. The low studies performed in limestone caves indicate that the semiarid climate conditions were established in the last 5 kyr before the present in Northeastern Brazil. In these conditions, alluvial deposits are potential sources of information about the recent past evolution of semiarid regions once the size, selection and composition of sediments reflect erosion and transport. This study aimed to analyse the recent past drainage evolution in the Borborema Plateau and Sertaneja Low Surface, two of the driest regions in Brazil. Four Fluvisols and one Cambisol were described in alluvial fans. Physical, chemical and sedimentological analyses were performed. Selected horizons were dated according to the OSL method. Variability of texture, mineral chemical composition and organic carbon content indicate discontinuities in all soil profiles. Layers of very poorly selected coarse material deposited in high-energy depositional systems are interbedded with clay-rich layers deposited by low-energy energy. The abundance of Fe, Si, Ti, Ca and K indicates variation in the source of the deposited materials. The age variation between the soil profiles indicates four evolutionary stages between 2,700 and ~100 yr. The higher age variance between layers in the Borborema Plateau indicates frequent and low-magnitude sedimentation events. These events are typical of semiarid climates because they reflect low sediment transport capacity by the ephemeral rivers. On the other hand, the lower age variation between the layers and buried A horizons suggest intense aggradation intercalated by stabilized periods, when vegetation was established and bioturbation formed A horizons.