Pleistocene periglacial imprinting on polygenetic soils and paleosols in the SW Italian Alps

Because of extensive Pleistocenic glaciations, which erased most of the previously existing soils, slope steepness and climatic conditions favoring soil erosion, most soils observed in the Alps (and in other mid-latitude mountain ranges) have developed during the Holocene or Late Glacial period. However, in few sites, particularly in the outermost sections of the Alpine range, Pleistocene glaciers covered only small and scattered surfaces, and ancient soils could be preserved for long periods on stable surfaces. In many cases, these soils retain good memories of Quaternary periglacial activity, which have never been characterized on the Alpine range. Based on both geomorphological and pedological interpretations, this work aims to investigate these environments, providing, therefore, new evidences to support paleoclimate reconstructions on the Alps. We described and sampled soils on stable surfaces in the Upper Tanaro valley, Ligurian Alps (Southwestern Piemonte, Italy). The sampling sites were between 600 and 1600 m a.s.l., under present day lower montane Ostrya carpinifolia, montane Fagus sylvatica forests or montane heath/grazed grassland, on quartz-rich substrata. The surface morphology often showed strongly developed fossil periglacial morphologies such as large-scale patterned ground, blockfields/blockstreams or solifluction sheets. The soils preserved in such Quaternary periglacial landforms normally showed stratification of different layers (units), separated by structural discontinuities, evidencing different depositional settings and different pedogenic development degree. A strong cryogenic granulometric sorting characterized all the observed soils/paleosols, with silt-enriched horizons and lateral differentiation of sand- and stone-rich parts and fine enriched ones; organic matter was irregularly distributed at depth as a result of past cryoturbation. Compact and dense layers with strong platy/lenticular structural aggregation, wedge casts and large-scale cryoturbations were described below fixed depths in all soil profiles. Thus, surface morphology and soil properties suggest the presence of permafrost during cold Pleistocene phases, with two main active layer thicknesses at 60–120 and 100–160 cm depths respectively