Sporadic and discontinous mountain permafrost occurrence in the Upper Engadine , eastern Swiss Alps

Special aspects of mountain permafrost distribution were investigated in the Upper Engadine, eastern Swiss Alps: discontinuous permafrost occurrence in glacier forefields at high altitude and sporadic permafrost occurrence below the timberline. The thickness of the permafrost bodies inferred through geoelectrical soundings are of similar magnitude. In contrast, the active layer of the permafrost occurrence below the timberline appears to be fairly thin, indicating the impact of the organic horizons in insulating the subsurface and controlling the ground thermal regime. The mean annual ground surface temperatures (MAGST) do not differ substantially, although about 1000 m in altitude lie in between the two investigated sites. Year-round temperature measurements show that a thick snow cover can lead to a MAGST which is higher by several degrees. Thus, seasonal snow cover characteristics are assumed to belong to the key factors for the existence of the sporadic permafrost occurrence below the timberline. Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Figure 1. d’Es-cha glacier forefield. between 2200 m and 2300 m a.s. l. The results of the field measurements presented in this paper were concentrated along a clearing on the north-exposed valley side where only small larch trees are present. The soils are poorly developed and covered by an organic layer of up to 30 cm thickness. Below the organic layer, only a few centimetres of mineral soil exists. 2.2 Near-surface temperature datalogging Miniature dataloggers (Universal Temperature Logger UTL-1) were placed at different sites in the forest at the north-exposed vegetated scree slope (embedded at the top of the organic layers) and in the glacier forefield (placed between stones) to register the temperature at the base of the snow cover in the course of several months as well as year-round near-surface temperatures. From the latter data the mean annual groundsurface temperature (MAGST) can be calculated. If the winter snow cover is sufficiently thick (at least 80 cm) and surface melting is still negligible in midto late-winter, the bottom temperature of the winter snow cover (BTS) remains nearly constant and is mainly controlled by the heat transfer from the upper ground layers, which in turn is strongly influenced by the presence or absence of permafrost. Under permafrost conditions, a colder temperature occurs. The following three classes are distinguished: probable permafrost occurrence (BTS 3°C), possible permafrost occurrence (BTS 2°C to 3°C) and improbable permafrost occurrence (BTS 2°C) (Haeberli 1973). Since permafrost exists if either the bottom temperature of the snow cover (BTS) is below 3°C and/or the MAGST is perennially below 0°C (van Everdingen 1998) the presence or absence of permafrost can be deduced from the data obtained by the miniature loggers. Additionally the evolution and roughly the thickness of the snow cover can be derived from the appearance of the temperature curve, since short-term temperature fluctuations are either damped under a thick enough snow cover or have even no impact on the temperature at the base of the snow cover. 2.3 DC resistivity soundings Geoelectrical soundings have been standardly applied in different mountain regions to confirm and characterise mountain permafrost for many years. Due to the noticeable resistivity contrast between unfrozen sediments and ground ice or ice-rich frozen sediments this method is suitable for detecting permafrost. The typical sounding curve obtained on alpine permafrost shows a three-layer model with an increase of resistivity at shallow depth representing the active layer and the permafrost underneath, followed by a sharp decrease of resistivities at greater current electrode distances (AB/2) representing the unfrozen ground beneath. Occasionally the resistivity contrast between the active layer, the permafrost layer and the unfrozen underground can be low. Resistivity values of frozen ground can vary over a wide range depending on the ice content, the temperature and the content of impurities. The dependance of resistivity on temperature is closely related to the amount of unfrozen water. Perennially frozen silt, sand, gravel or frozen debris with varying ice content show a wide range of resistivity values between 5 k m to several hundred k m (e.g. Hoekstra & McNeill 1973, King et al. 1987, Haeberli & Vonder Mühll 1996). Due to the geoelectrical principle of equivalence (e.g. Mundry et al. 1985) several calculated models can represent one sounding graph. Thus, it is recommendable to give ranges of resistivity and thickness (Vonder Mühll 1993). For the one-dimensional soundings a GGA30 Bodenseewerk instrument was used. In the present interpretations three-layer models were calculated which are assumed to represent the most probable case with respect to the local geomorphological situation. In the selected figures the sounding graphs and the interpreted models (dashed lines) are shown. 3 RESULTS AND DISCUSSION 3.1 Temperature measurements Measured near-surface temperatures from the eastexposed d’Es-cha glacier forefield and the Bever site are shown in Figures 3 and 4. All mini-loggers (ML) in the d’Es-cha forefield (Fig. 3) were obviously covered by a thick enough snow cover as there are no high-frequency temperature variations visible in the 562 Figure 2. North-exposed vegetated scree slopes in the Bever Valley.