Indizi di paleopedogenesi in un suolo bruno acido della Foresta di Vallombrosa (Firenze)

Abstract

Scanning electron microscope (SEM) probes have been carried out on fine-grained residual minerals of soil samples from an acid brown soil (Umbrept) from Vallombrosa Forest near Florence (Italy). Throughout the profile, SEM textural features of the principal mineral components (quartz, felspars, biotite and muscovite) of the soil parental rock show clear signs of severe chemical weathering. This cannot be explained by recent pedogenetic processes or by the influence of acid rains, which have recently been falling in the area. Using a standard technique, studied weathering textures are compared to those observed in silicates from tropical soils subjected to deep weathering. Results obtained are discussed taking into account evidence of severe pedogenetic phases (plinthite pedogenesis) observed on mountain tops of the northern Apennines at an elevation (~1,000 m) almost equal to or lower than that of Vallombrosa. Weathering textures in the Vallombrosa soil are interpreted as resulting from a strong pedogenetic process under subtropical climatic conditions in early Quaternary times (perhaps lower Middle Pleistocene interglacial periods). An initial well-developed soil would have subsequently been reworked by erosion and crioturbation processes during one or more glacial phases.