Ricerche metodologiche sulla cronologia relativa dei suoli. Risultati preliminari di un’applicazione all’Altopiano di Poirino (Piemonte)

Methodological research on relative soil chronology. Preliminary results of an application to the Altopiano di Poirino (Piemonte)

Abstract

The present paper gives the preliminary results of an investigation on the assessment of the stage reached by the pedological evolution by the use of geochemical and mineralogical parameters. More specifically we aimed at verifying, on the one hand, the diagnostic reliability of the above parameters and, on the other hand, the soundness of a sampling method at a standard depth (20-50 cm). The latter method (alternative to the conventional creation of a pedological hole or to the exploitation of a former outcrop) has the advantage to be quite quick and not to require the identification of a definite pedological horizon. Samples were therefore taken of soils from the alluvial formations constituting the surface of the Altopiano di Poirino, near Turin (Italy). In this area it was recently identified a chronological sequence assigned, on geologic grounds, to the lower Pleistocene-Holocene. The following parameters were used in soil dating: grain size, quantitative ratios between the several iron compounds (crystalline or amorphous oxides and hydroxides), mineralogical composition of the clay fraction. Some of these parameters have already been employed and verified on a large scale, others are still at the experimental stage. The following results were obtained: a) Grain size was found to be appreciably influenced by the texture of the original sediments. b) The examination of the geochemical parameters related to the iron content proved a reliable diagnostic criterion: good correlation exists between the values of these parameters and the succession of the chronologic sequence. c) The investigation of the mineralogical composition of clays showed that, even though it was not easy to detect one highly diagnostic mineral, there clearly exists a direct relationship between the amounts of open illite and of intergrade clays (mainly of secondary origin) and the soil age; even more significant is the inverse correlation found between the amounts of chlorite and crystalline illite (probably primary minerals) and the soil age itself. The results obtained so far suggest that attempts must be made, with rewarding outcomes, to look for mineralogical and geochemical parameters as reliable indicators for soil dating; they also show that the sampling method employed can be a valid one.