Morphodynamics of three active rock glaciers and its influence on spring hydrochemistry in the Swiss Alps

In the current context of climate change, intact rock glaciers represent potentially important water resources in high mountain regions, regarding the storage of both liquid and solid water. In particular, water stored in ground ice could become a valuable resource in the long term, due to slower ice melt rates occurring in rock glaciers than in surface glaciers. However, the amounts of ground ice are difficult to detect, and the related processes (i.e melting and refreezing) are complicated to monitor and therefore poorly understood. In this regard, geochemistry of water emerging from rock glaciers can help gaining more insight. In this study, morphodynamic analyses of three active rock glaciers located in the Swiss Alps were therefore combined with the physico-chemical monitoring of water emerging from these periglacial landforms. The three studied rock glaciers (Monte Prosa A, Ganoni di Schenadüi and Piancabella) are located in the Lepontine Alps (Canton of Ticino) and their ground surface temperatures and kinematics are monitored since 2009. Two of them (Monte Prosa A and Piancabella) belong to the Swiss Permafrost Monitoring Network PERMOS.

Changes in morphodynamics of the rock glaciers were investigated through repeated Unmanned Aerial Vehicle (UAV) and differential Global Navigation Satellite System (dGNSS) surveys during the warm season (i.e in early summer, late summer and early autumn). Intra-seasonal comparison between dense point clouds obtained through Structure from Motion (SfM) photogrammetry shows significant seasonal changes in elevation, especially a negative volumetric change in the rooting zone of two rock glaciers (Monte Prosa A and Ganoni di Schenadüi), with thickness losses ranging from about 0.15 to 0.55 m. Rooting zone also shows the largest seasonal horizontal displacements (up to 0.3 m) for these rock glaciers, obtained through image correlation. Furthermore, isotopic analysis (δ¹⁸O) were performed on water samples arising from rock glacier springs, precipitation, snowpack and seasonal ground ice, the latter sampled between blocks within the active layer. A seasonal increase in δ¹⁸O was observed in rock glacier springs, indicating a change in the water origin, from a supply fed mainly by snowmelt to a supply fed by a mixture of more ¹⁸O-enriched water. In addition, ion content of water samples collected from rock glacier springs and seasonal ground ice was also measured. Rock glacier springs show a seasonal increase in the solute export (e.g. SO₄^2-, Ca²⁺ and Na⁺), while high concentrations of Na⁺, K⁺ and Cl^- were found in seasonal ground ice samples. These first results show a clear seasonal pattern and indicate a probable influence of ground ice melting on both morphodynamics and chemistry of water emerging from the studied active rock glaciers.