Ancient Podzols in the South-western Italian Alps

Ancient soils (pre-Holocene paleosols and vetusols) are uncommon on the Alps, because of the extensive Pleistocene glaciations which erased most of the previously existing soils, the slope steepness and climatic conditions favoring soil erosion. However, in few sites, particularly in the outermost sections of the Alpine range, Pleistocene glaciers covered only small and scattered surfaces because of the low altitude reached in the basins, and ancient soils could be preserved for long periods of time on particularly stable surfaces. We described and sampled soils on some of these stable surfaces in the Upper Tanaro valley, Ligurian Alps (Southwestern Piemonte, Italy). The sampling sites were characterized by low steepness and elevation between 800 to 1600 m, under present day montane Castanea sativa, Fagus sylvatica and Pinus uncinata forests or grazed heath/grassland, on quartzite or quartz-rich metamorphic rhiolites or shales. Podzolic soils are widespread in the area on such substrates, also on steep slopes, even if Podzols normally form on the Alps at higher altitude, under subalpine vegetation. On gentle slopes or flat surfaces in particular, the observed profiles showed signs of extremely long podzolization and/or different phases of intense pedogenesis interrupted by the deposition of periglacial cover beds. Up to four phases of intense podzolization were recognized where cover beds were developed during cold Pleistocene phases. In such cases, each cover bed underwent similar pedogenetic trends, with the exception of the less developed surface Holocene cycle. Different Podzol types formed on the different parent rocks: on quartzite, Podzols with thick E horizons (up to more than 2 m) formed. On harder and less weatherable quartzitic conglomerate, well developed placic horizons formed in each podzolization cycle (excluding the Holocene one), evidencing high humidity and waterlogging conditions; placic horizons were never found in Holocene soils and are almost absent in Alpine soils. On metamorphic rhiolite, extremely thick and hardly cemented ortstein horizons formed, evidencing better drained conditions and a less humid pedoclimate. On quartz-rich shales, no strong cementation was observed, but every pedogenic cycle created extremely thick E horizons, enriched in highly weathered secondary minerals, overlying organic carbon-rich Bhs ones. In these soils, the trends of pH, organic carbon content and Fe-Al oxyhydroxides were preserved for long times. On flat surfaces, very large stone circles and other large patterned ground features, which can be interpreted as evidence of past permafrost conditions, were preserved on coarse quartzitic conglomerate; these fossil cryogenic morphologies are accompanied by finer textured Podzols with strong cryoturbation features. These soils, together with their specific clay mineral assemblage, represent excellent pedo-signatures of different specific past climatic or environmental conditions, as a response of slightly different lithologies to well defined soil-forming environments. The precise dating and interpretation of these soils are intriguing.