GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 Fig. 3 - Close up of a portion of the longitudinal GPR section of Von Postbreen polythermal glacier (see Fig. 1a for positioning) where the anomalous zone is not present. a) amplitude data; b) dominant frequency; c) sweetness; d) phase. CTZ: Cold/Temperate-ice Transition Zone, WI: warm ice, CI: cold ice. Discussion Based on our analysis, some possible glaciological interpretations of the anomalous zone can be done: 1. Different thermal regimes inside the glacier are not only related to the occurrence of warm/cold ice zones and to different free water contents, but they could be also related to the existence of a capillary fringe that could extend the CTZ boundary toward the cold ice zone and detected by the GPR in terms of an anomalous phase and frequency content in turn related to variations of EM parameters like conductivity, dielectric contrast and attenuation (MacGregor et al., 2015; Matsouka et al., 1996; Matsouka et al., 1997; Mäzler and Wegmüller, 1987). 2. The interaction of the warm/cold ice zones with the morphology of the bedrock can produce a thermal effect due to the different rate of stress and strain inside the glacier. The anomalous zone occurs downstream of a local bedrock uplift (e.g. Fig. 1 black dot and Fig. 2) and this could produce a local temperature anomaly due to regelation process and heat transfer within the glacier. This effect is present in the upper part of the glacier but not at its terminus (Fig. 3). 3. A local thermal effect responsible for the formation of the anomalous zone could be related to the firnification and regelation processes because the anomaly occurs in the central portion of the glacier where we have the maximum glacier thickness and also where the glaciological ELA lies. An effect due to the transition between the