Soil-formation in the central Mediterranean: Insight from heavy minerals

This study proposes an integrated methodology which includes pedological, petrographic, mineralogical and geochemical analyses of transparent heavy-minerals, to reconstruct the weathering patterns and pedogenetic processes in a transport-limited erosion regime. Three representative soil profiles developed on crystalline bedrocks, located in Calabria, southern Italy, were studied in detail including the complete weathering profile along the transect bedrock/grus/soil. Differences in the amounts of heavy minerals in each soil profile appear to be controlled by the nature of the local bedrocks. The addition of allochthonous parent material and a varying extent of chemical weathering, likely influenced by topography and duration of pedogenesis. The heavy mineral shapes and surface microtextures, together with the vertical distribution of major and trace elements, the values of the chemical index of alteration and the relative abundance of the clay mineral types along the profiles highlight the different weathering degrees among the profiles. These features are consistent with iron/manganese oxy-hydroxides and clay neogenesis observed on femic heavy minerals. Moreover, they mark a lithological discontinuity between the bedrock and the upper soil horizons, which led to rejuvenation by erosive and colluvial processes along the slopes and/or aeolian deposition of allochthonous material, presumably including a Late Pleistocene to Holocene pyroclastic input. Chemical weathering affecting the majority of heavy minerals is consistent with high rainfall rates, soil moisture availability and acidic pH conditions likely favored by organic acids, prone to chemical reactions and leaching processes. The clay coatings and iron/manganese segregations in the Bt horizons of all the profiles studied suggest that they developed in a warmer humid climate during the interglacial stages of the Pleistocene.