Reconstructing deglaciation dynamic and environmental changes in the Italy-Switzerland transboundary area of the Val Viola Pass

The deglaciation following the Last Glacial Maximum (LGM) is one of the most important climate-driven surface processes that occurred in the Alps. The deglaciation contributed to shape the landscape of high-mountain areas. In the Lateglacial and during the Holocene, water-related, slope, and periglacial processes have interplayed with pedogenesis, shaping the landscape of the deglaciated alpine valleys. In order to reconstruct the deglaciation history and environmental changes occurred in the Val Viola area (Upper Valtellina-Poschiavo Valley), we used a multidisciplinary approach including a detailed geomorphological mapping of the area (Bollati et al., 2018) and field and laboratory characterization of post-LGM deposits and soils. As regards the analysis of deposits and soils, six soil profiles were sampled and analysed along an altitudinal transect between 2325 m and 2430 m a.s.l.; this helped elucidating the process of soil development and detecting the presence of different pedological units. Moreover, Schmidt’s hammer measurements were performed in order to establish the degree of surface weathering and thus the exposure-age of rock glacier debris and bare rock surfaces. Debris and rock surfaces suitable for Schmidt’s hammer exposure dating were selected according to the results of geomorphological mapping. Finally, two peat bog deposits and charcoals found in one soil profile were dated with AMS-14C dating. Analyses of soil profiles allowed the identification of the occurrence of different slope instability phases, which are recorded as buried surfaces, suggested by the presence of stone lines and/or granulometric discontinuity. A pristine phase of surface stability, likely characterized by increasing soil forming processes, is dated at 8120±30 BP, corresponding to the radiocarbon date of the bottom of a peat bog. Subsequently, this phase of stability has been affected by rexistasy phase probably due to the worsening of climate conditions. This instability phase occurred after the 4650 ± 30 BP, radiocarbon date of the charcoal found in a soil horizon buried below a stone line. A similar radiocarbon age (4320±30 BP) was found also at the bottom of a second peat bog. Above the stone line, covering the charcoal, a more recent soil unit testifies the occurrence of new environmental conditions promoting pedogenesis. In nearby areas, Schmidt’s Hammer was used to date bare surfaces (e.g. Scotti et al., 2017), and the regression lines obtained by the authors were used to calibrate rebound values collected in our study area. The data show that the rock weathering processes on the top of two exharated granitic gneiss outcrops started around 17 ky BP being in accordance with the timing of deglaciation since the Last Glacial Maximum and with soil developments. Schmidt’s hammer rebound values and soils analysis provided additional data useful for reconstructing the landscape evolution of the region