Large-scale cryostructures and pedogenesis in blockstreams in the Ligurian Alps (NW Italy): Pleistocene paleoclimate implications

Blockstreams and blockfields are periglacial features induced by strong frost-action processes. As their formation can be associated with specific environmental conditions such as permafrost (in absence of an impermeable layer below), these landforms have been recognized worldwide as key factors in paleoclimate reconstructions. However, their internal morphology and origin, especially in alpine environments, were poorly investigated. In particular, the fine material at their base has almost never been characterized from the pedogenic point of view. We opened and described a 12 m-wide, 3 m-deep soil transect inside and between two blockstreams in the Ligurian Alps (NW Italy); we took samples from each pedogenic horizons and analysed them to obtain chemical, physical, mineralogical, and micromorphological data. The soil transect was characterized by deep involutions likely caused by ancient, deep cryoturbation associated to former ice-rich permafrost and its collapse during thawing (load casting). Many other macro- (i.e. large platy structure, fragic properties, vesicular porosity, wedge cast-like structures, Fe-Mn nodules above a permeable layer) and micro-morphological features (i.e. Fe-Mn concentrations, fractured clay coatings, silt caps) evidence ice-rich permafrost, while the intensity of pedogenesis points to a particularly long soil forming processes and preservation of soil materials during different Pleistocene glacial and interglacial periods.