Hidden paleosols under periglacial landforms at high elevation in the Alps (Stolenberg Plateau - NW Italy)

In the framework of climate change research, mountain soils provide valuable paleo-environmental information, representing a powerful tool for paleoclimate reconstruction. However, reconstructing soil and landscape evolution in high mountain areas can be a difficult task because Pleistocene glaciations and erosion-related processes erased most of the pre-existing landforms and soils, leading to the formation of a complex mosaic of Quaternary sediments and soils of different ages. The object of this study is the periglacial environment of the Stolenberg Plateau (LTER site Istituto Mosso), located along the border between Valsesia and Lys Valley, at the foot of the southern slope of the Monte Rosa Massif (Western Italian Alps) at an elevation around 3030 m a.s.l.. The plateau is entirely covered by a thick stone layer, organized in different periglacial features (e.g. blockfields, blockstreams, etc.), with or without patterned ground reorganization, while the plant cover reaches no more than 3-5% of the surface. Despite the strong geomorphic activity characterizing this area, extremely well developed soils were observed under the periglacial landforms. In particular, below a surface reworked stone layer, thick (between 30 and 65 cm) and dark organic C-rich horizons were observed, with the presence of several cryoturbation features, such as inclusions of different materials and convolutions along the profile. Below these umbric horizons, cambic Bw ones were often developed but discontinuous. In contrast, below more stable surfaces covered by alpine grassland (Caricetum curvulae), the soils were Cambisols with a thin (10-15 cm deep) A horizon and a moderately developed Bw, showing weak signs of cryoturbation. Despite the lack of vegetation cover, below periglacial surface stone layers, the organic C stocks were surprisingly high (over 5 kg*m-2), comparable to vegetated or even forest soils at lower elevation. Geophysical investigations (e.g. Electrical Resistivity Tomography, etc.) revealed that these soils ranged between ca. 30 and 90 cm in thickness and were widespread under the stony cover, involving a large part of the plateau surface. Radiocarbon dating and soil δ13C signatures indicated that these hidden soils were paleosols that recorded exclusively the main warming phases occurring since the end of LGM (~21 ka BP) until the beginning of Neoglacial (~5-4 ka BP). This finding suggests that the environmental conditions on the Plateau could have been suitable for alpine plant life and pedogenesis, already since the end of LGM. Our results, together with the high carbon stocks of these paleosols, the presence of strong geomorphological evidences (i.e., periglacial features such as blockstreams/blockfields), as well as the overall specific morphology, aspect and position, indicate that these hidden soils can be considered a direct evidence of a Lateglacial Alpine Nunatak. Thus, the Stolenberg Plateau recorded the transition between Pleistocene and Holocene, representing therefore a unique natural and historical archive for unravelling the post-LGM history of the high-elevation landscape of the European Alps.