Saturday, 15 July 2006

Genesis of a Polycyclic Quaternary Terra Rossa (Bathi-Luvi Rhodic Cambisol on Rhodic Nitisol) in Central Italy.

Simone Priori1, Edoardo A.C. Costantini2, Enrico Capezzuoli3, Fabio Sandrelli3, and Giuseppe Protano4. (1) Earth Sciences Dept, Univ of Siena,, Via Laterina, 8, Siena, Italy, (2) CRA - Experimental Institute for Soil Study and Conservation, Piazza D'Azeglio 30, Florence, 50121, Italy, (3) Earth Sciences Dept, Univ of Siena, Via Laterina, 8, Siena, Italy, (4) Environmental Sciences Dept, Univ of Siena, Via Laterina, 8, Siena, Italy

The genesis of red soils on limestone in the Mediterranean environments, the so called “Terra Rossa”, is always a subject of debate. Examples have been reported of either residual or allocthonous origin of the parent material, with colluvial or aeolian contributions. Aeolianites, in particular, may be constituted of loess, ash, or Sahara dust. The aim of this work was the characterization of the pedogenetic processes and the investigation of relationships between soil horizons, parent materials and paleoclimate of a Terra Rossa soil situated in Tuscany (Central Italy). A polycyclic Terra Rossa, a Bathi-Luvi Rhodic Cambisol on Rhodic Nitisol, was studied on a plateau. The paleosol is well representative of the red and deep soils, covering an Early-Middle Pleistocene synthem (Capezzuoli & Sandrelli, 2004). This synthem is characterized by four heteropic lithofacies (clay, sand, conglomerate and limestone) reflecting a vertical and lateral evolution of a continental endorheic basin. The setting started in a palustrine environment (clay lithofacies) with mainly silty-clayey sedimentation and local lignite lenticular bodies, and evolved into a lacustrine environment, with limestone deposition. Present Terra Rossa soil distribution is closely linked to the limestone lithofacies. A detailed geological, geomorphological, and soil survey was carried out. The paleosol was investigated through field, physical, chemical, geochemical, micromorphological, sedimentological and mineralogical techniques (X-ray diffraction and SEM). The age of the soil was estimated by means of optical spectroscopy luminescence (OSL). The whole profile was divided in three pedostratigraphic levels. The first one (0 - 110 cm) was represented by an Ap horizon and two cambic horizons (Bw1 and Bw2); the second one (110 – 280 cm) by two argic horizons (2Bt1 and 2Bt2) and the last one (280 - 310 cm) by a nitic horizon (3Bt). The underlying limestone was also sampled and the insoluble residuum analyzed. The three pedostratigraphic levels showed us that there had been a succession of pedogenetic stable periods, alternated with pedogenetic unstable periods, when soil erosion and deposition of colluvial and aeolian materials occurred. From a pedogenetic point of view, the nitic horizon exhibited strong rubification and nutty structure, clayey texture (85% of clay), strong kaolinitization and enrichment of Fe, Al and heavy metals. The argic horizons showed strong clay illuviation, CaCO3 lessivage, moderate enrichment of Fe, Al and heavy metals, and presence of papules when examined under the microscope. The cambic horizons presented SiO2 enrichment, slight rubification, limestone fragments and red pedorelicts. Although the cambic and the argic horizons presented clear evidence of allocthonous material (fine sand and silt), the nitic horizon showed geochemical similitude with the insoluble residual of limestone.

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