Ancient soils in non-glaciated mountain areas on peridotite: the Lanzo Ultramafic Massif example (TO, NW Italy)

The Lanzo Ultramafic Massif (LUM) is one of the largest peridotite outcrops in the world; the peridotitic rocks are only slightly serpentinized, and they are often deeply weathered, and they are substituted by antigoritic serpentinite only locally, in the westernmost part of the massif. Given the low elevation, the LUM was never glaciated during the Quaternary, and the surface characterized by extremely well-developed fossil periglacial features, such as blockstreams, blockfields and large-scale patterned ground (Paro, 2011). These periglacial landforms have an age ranging the whole Quaternary, while Pliocene sediments are observed close to the base of the massif (Forno et al., 2010) The vegetation is sparse, and rich in endemic serpentine species, some of which are Ni hyperaccumulators. Inside and near the ancient landforms, extremely well developed soils are found. In particular, deep Podzolic soils are often found below 1-2 m-thick blockstreams in the highest slopes, above 1000 m a.s.l., characterized by thick E and cemented Bsm horizons; at lower elevations, soils between blockstreams are characterized by layers with increasing pedogenic indicators with depth (clay coatings, rubification and fragipan formation), with the deepest horizons strongly hardened by fragipan and deeply rubified. In the stream incisions, it is possible to observed red, strongly cemented horizons likely developed during the Pliocene, under subtropical conditions, partly resembling lateritic materials. The Fe and heavy metal concentrations is extremely high, and it is a strong ecological constraint even in these ancient soils.