Permafrost distribution and conditions at the headwalls of two receding glaciers ( Schladminger and Hallstadt glaciers ) in the Dachstein Massif , Northern Calcareous Alps , Austria

Permafrost distribution in rockwalls surrounding receding glaciers is an important factor for rock slope failure and rockwall retreat. The Northern Calcareous Alps of the Eastern European Alps form a geological and climatological transition 10 zone between the Alpine Foreland and the Central Alps. Some of highest summits of this area are located in the Dachstein Massif (47°28'32′′N, 13°36'23′′E) in Austria reaching up to 2995 m a.s.l. Occurrence, thickness and thermal regime of permafrost at this partly glaciated mountain massif are scarcely known and related knowledge is primarily based on regional modeling approaches. We applied a multi-method approach with continuous ground surface and near-surface temperature monitoring, measurement of bottom temperature of the winter snow cover, electrical resistivity tomography (ERT), airborne 15 photogrammetry, topographic maps, visual observations and field mapping for permafrost assessment. Our research focused on steep rockwalls consisting of massive limestone above several receding glaciers exposed to different slope aspects at elevations between c.2600-2700 m a.s.l. We aimed to quantify distribution and conditions of bedrock permafrost particularly at the transition zone between the present glacier surface and the adjacent rockwalls. Low ground temperature data suggest that permafrost is mainly found at cold, north exposed rockwalls. At southeast exposed 20 rockwalls permafrost is only expected in very favourable cold conditions at shadowed higher elevations (2700 m a.s.l.). ERT measurements reveal high resistivities (>30.000 ohm.m) at ≥1.5 m depth at north-exposed slopes (highest measured resistivity values > 100 kohm.m). Based on laboratory studies and additional measurements with small scale ERT, these values indicate permafrost existence. Such permafrost bodies were found in the rockwalls at all measurement sites independent of investigated slope orientation. ERT data indicate large permafrost bodies at north exposed sites whereas discontinuous permafrost bodies 25 prevail at northwest and northeast facing rockwalls. In summary, permafrost distribution and conditions around the headwalls of the glaciers of the Dachstein Massif is primarily restricted to the north exposed sector, whereas at the south exposed sector permafrost is restricted to the summit region.

Invisible, but also measurable, are permafrost changes in the subsurface. Formerly glacier-covered rock surfaces with former temperatures around the melting pointconditioned by temperate glacier icebecome subjected to direct local atmospheric conditions after the ice melted. Depending on slope orientation and shading effects of these rock surfaces, permafrost aggradation is possible at such sites after exposure. However, in case of cold and polythermal glaciers (with cold ice restricted to cold, high-altitude parts of the glacier; Benn and Evans, 2010) permafrost might exist even below glacier-covered areas. In 5 addition to that, glaciers might be separated from the adjacent headwall by a distinct gap or crevasse (randkluft). Such crevasses are also typical glacial features in our study area. Air can enter into this crevasse allowing a better coupling of the air and bedrock even below the glacier surfaces and also more efficient cooling during the summer season (Sanders et al., 2012). Therefore, both a polythermal glacier and a glacier with a distinct randkluft might allow permafrost aggregation below the glacier surface. 10 Changes in ground thermal conditions, permafrost extent and hydrology are all sensitive to predicted future climate change (Gobiet et al., 2014). A warming of about 0.5 to 0.8°C in the upper tens of meters of alpine permafrost between 2600 and 3400 m a.s.l at the European alps in the last century (Harris et al., 2003) effects in a vertical mean rise of the lower limit of permafrost by about 1m/year (Frauenfelder, 2005). Magnin et al. (2017), simulated the long-term temperature evolution at three rockwall sites between 3160 and 4300 m in the Mont Blanc massif from Little Ice Age (LIA) conditions to 2100 and concluded that 15 permafrost degradation has been progressing since the LIA. This ongoing degradation can potentially trigger rockwall instabilities (Wegmann et al., 1998;Sattler et al., 2011;Ravanel and Deline, 2011;Krautblatter et al., 2013;Draebing et al., 2017a, b). Therefore, acquiring knowledge on the permafrost distribution and freezing and thawing in the active layer (Supper et al., 2014) is important in high mountain areas particularly if infrastructure is potentially threatened (Kern et al. 2012). While ground surface temperature measurements in rockwalls (e.g. Matsuoka and Sakai, 1999;Gruber 20 et al., 2003;Kellerer-Pirklbauer, 2017) can provide valuable point information on rock temperature and thermal conditions of permafrost, geophysical techniques enable the visualization of subsurface permafrost characteristics in 2D-or 3D-arrays.
Several authors used electrical resistivity tomography (ERT) for permafrost investigations in sediments (e.g. Hauck, 2001;Hauck et al., 2003;Marescot et al., 2003;Laxton and Coates, 2011;Rödder and Kneisel, 2012;Stiegler et al., 2014). In contrast, in rockwalls comparable measurements are relatively scarce (e.g. Krautblatter and Hauck, 2007;25 Hartmeyer et al., 2012;Magnin et al., 2015, Draebing et al. 2017a and for rockwalls close above the present glacier surfaces (Supper et al., 2014), ERT data are widely missing. Accordingly, the aims of this study are to detect, delimit and characterize permafrost in the recently deglaciated rockwalls surrounding the two retreating Schladming and Hallstatt glaciers in the Dachstein area and thus, to contribute to the question how widespread glacier retreat will affect permafrost degradation and/or aggradation in a Mid-latitude mountain region. 30 2 Study Area

General Setting
The Dachstein Massif with its highest peak, the Hoher Dachstein (2995m a.s.l.) located at 47°28'32″N and 13°36'23″E, are a mountain range in the Northern Calcareous Alps in Austria covering an area of about 400 km 2 (Fig. 1). The study area is 35 characterized by steep rockwalls (e.g. Dachstein south wall with 850 m altitude difference within a vertical distance of some hundred meters) towering relatively flat, glacier-covered plateaus and extensive touristic infrastructure with cable cars, ski lifts and ski runs. In particular the Schladming Glacier ( Fig. 1) is intensively used for alpine skiing. The surrounding headwalls are also partly used by means of a military transmitting station, lift stations and public climbing routes.
The prevailing rock type in the study area is the very compact Dachstein Limestone (GBA, 1982;Gasser et al., 2009). The climatic conditions of the study area are dominated by west and northeast air flows. The main maximum of precipitation is during summer with a secondary maximum in winter. Air temperature measurements at the surface of the Schladming Glacier next to the Hunerkogel at 2600 m a.s.l. showed annual average temperatures (MAAT) of -2.4°C in the period 2007-2016. This MAAT value at the Dachstein massif indicates the presence of discontinuous permafrost in the study area (Humlum, 5 1998). The first evidence of the existence of permafrost in the study area was provided by measurements of bottom temperature of the winter snow cover (BTS) carried out by Schopper (1989) and Lieb and Schopper (1991) in the proglacial area of the Schladming Glacier at 2300-2400 m a.s.l. According to these authors, the lower limit of discontinuous permafrost can be expected at this elevation. More recent simulations regarding the probability of permafrost existence in Austria (Ebohon and Schrott, 2009) or in the entire European Alps (Boeckli et al., 2012 a, b) revealed that permafrost existence in the study area is 10 particularly likely at north-exposed, higher elevated slopes as well as in the proglacial area of the Schladming Glacier.
Our research focused on the lower parts of steep rockwalls of recently deglaciated areas at four different measurement sites (MS) at elevations between 2600-2700 m a.s.l. next to the Schladming and Hallstatt glaciers (Fig. 1). The Koppenkarstein site (MS-K; summit elevation 2863 m a.s.l) was chosen due to the high probability of permafrost at this radiation-sheltered position, the pronounced randkluft and the well-documented, high rates of glacier surface lowering since the LIA maximum around 15 1850. The Dirndln site (MS-D) was selected because of a distinct blowout depression between the glacier and the mountain ( Fig. 2). This causes snow-poor and ice-free conditions at the footslope of the mountain which probably reduced ice coverage even during the LIA extent of the glacier. The Gjaidstein site (MS-G) is slightly lower and oriented to the west which makes permafrost occurrence less probable. At Hunerkogel (MS-H) the cable car station is located which makes this site interesting in terms of endangered infrastructure. There are no sites oriented to the south as there is only a very small glacier facing south 20 and the probability of permafrost at this site is much lower.

Reconstruction of deglaciation
The Hallstadt Glacier and the Schladming Glacier have been subject to substantial mass loss and glacier surface lowering since the Little Ice Age/LIA (c.1850) and particularly in the last decades. Hallstatt Glacier lost about 50% of its area and 52% of its 10 length, whereas Schladming Glacier reduced by 55% in area and 48% in length until 2012. The retreat of the glaciers located at the Dachstein Massif since the LIA are well documented by Simony (1895), Moser (1997), Krobath and Lieb (2004), Helfricht (2009) or Fischer et al. (2015. New ice-free areas in the glacier forefield and the surrounding head walls afforded new touristic concepts and safety precautions over the years. A distinct randkluft exists at several places in the study area (see

Methods
We focused on the permafrost distribution in the areas of glacier retreat between 1915 and 2009 (Fig. 3). We followed a multidisciplinary approach primarily continuous ground surface temperature (GST) monitoring at the surface using miniature temperature datalogger, bottom temperature of the winter snow cover (BTS; Haeberli, 1973) and electrical resistivity tomography (ERT) profiling. 10 BTS is based on the insulating properties of sufficiently thick snow cover (> 1 m), which prevents the ground surface from short-term periodical variations in air temperature (Haeberli, 1973(Haeberli, , 1975. BTS is controlled by the heat flow of the subsurface and is distinctly lower above frozen ground. Haeberli (1973) defined temperatures < -3°C as permafrost probable, 5 measurements between -2°C and -3°C as uncertainty range (permafrost possible) and temperatures > -2°C as non-permafrost areas. A BTS thermocouple probe with a Pt100 (1/3 DIN class B) fixed to the bottom of a 3 m long steel rod (System KRONEIS, Vienna) at the lower end of a 3 m carbon tube was used. Measurements were performed at each point until constant temperature was registered for at least 2 minutes. The accuracy of measurements depends on several factors like calibration of the temperature sensor or disturbance of the temperature field by the breakthrough of the snow field by the probe. A total of 10 13 BTS-points (at each point three measurements within an area of 2 m²; cf. Brenning et al., 2005) were determined at recently glacier free areas based on the multitemporal analyses of published maps and orthophotos ( Fig. 3) in 2600 -2700 m a.s.l. In the time period around the measurement date (20-03 to 21-03-2013), the snow cover recorded by a weather station at the Hunerkogel (snowreporter, 2013) increased continuously from 1.5 m (01-12-2012) to 3.5 m (25-03-2013). During the 13 BTS measurements snow depths ranged from 2 to 3.5 m. As pointed out by Brenning et al. (2005), BTS has to be interpreted as a 15 relative measure of ground thermal state and not strictly as a permafrost indicator.

Ground surface temperature (GST)
To avoid the restrictions of short BTS measurements, additional miniature temperature data loggers (iButtons, e.g. Gubler et al., 2011) were mounted at the near bedrock surface iButtons of type DS1922L, (Maxim Integrated) with a resolution of 0.5 °C and a measurement interval of 1 h were chosen. The sensors were placed in 20 very shallow boreholes in bedrock with a 20 depth of 2 cm ( Fig. 3 and 5). Additional protection against moisture was provided by small plastic bags. Preliminary laboratory calibrations of the sensors did not show any notable effects of the used plastic bags. iButtons where placed at the measurement sites at the rock surface beneath the snow pack on 01-01-2013 and removed on 31-07-2013. Therefore, up to 7 months of data were available for analysis.
With the miniature temperature data loggers it is possible to monitor the seasonal temperature fluctuations at the uppermost 25 centimeters of the surface (e.g. Ishikawa, 2003). Such data can be used to assess for instance the thermal conditions under a seasonal snow cover. The winter equilibrium temperature (WEqT) describes temperature fluxes beneath the snow pack and is defined as the mean temperature of stable conditions during February and March. The WEqT depends on the presence/absence of permafrost and on the history of the snow cover at a given measurement site (e.g., Schöner et al. 2012, Kellerer-Pirklbauer 2019. In case of the absence of an isolating winter snow cover and, thus, thermal coupling between the atmosphere and the 30 ground, the WEqT-approach is not applicable. Interpreted threshold values of WEqT are identical to the ones for BTS (Haeberli 1973) defining WEqT temperatures < -3°C as permafrost probable and measurements between -2°C and -3°C as permafrost possible (cf. e.g., Schöner et al. 2012, Sattler et al. 2016." Another important parameter is the zero curtain period with temperatures around 0°C caused by the melting of the snow and isothermal conditions within the snow pack. The basal-ripening date (RD) at the beginning and the melt out date (MD) at the 35 end frame the zero curtain period. The RD describes the time when a frozen ground surface is warmed to 0°C by strong rainon-snow events or by percolating melt water (e.g., Westermann et al. 2011). The MD describes on the other hand the time when the snow layer is completely melted, allowing the ground surface to warm above 0°C (e.g., Schmid et al. 2012). Late dates for RD and MD as well as a long zero curtain period are regarded as favorable for permafrost conditions. Particularly, a late MD in summer implies prolonged protection of the snow-covered ground surface from solar heating.

ERT
For geophysical resistivity measurements, a constant current is applied into the ground through two 'current electrodes' and the resulting voltage differences at two 'potential electrodes' are measured (Knödel et al., 2005). From the current and voltage 5 values, an apparent resistivity value is calculated. ERT is excellently suited for permafrost detection as frozen ground is generally characterised by high electrical resistivity (due to the lack of conducting liquid water) and a strong contrast to the unfrozen surrounding Schrott and Sass, 2008). To determine the true subsurface resistivity in different zones or layers, an 'inversion' of the measured apparent resistivity must be carried out. We used the Res2Dinv software package by Loke (1999) for this inversion procedure. A GeoTom-2D system (Geolog2000, Starnberg, Germany) with 10 multicore cables was used in the field. Depending upon the local topography, between 24 and 50 electrodes were used per profile. The connection between the electrodes and the rock was established by stainless steel screws, 12 mm in diameter, which were driven into 12 mm wide and 50 mm deep boreholes. The spacing between two electrodes was 2 m. Thus, the total extent of the survey lines was between 32 and 98 m. Salt water and metallic grease were applied to improve electrical contacts. were carried out by means of Wenner array which provides a particularly sound depth resolution in the central parts of the profile (Knödel et al., 2005;Loke, 1999). We used the robust inversion modelling process in Res2Dinv. The model discretization was set to use an extended model with an increase factor of model depth range of 1.5. Robust inversion delivered very good results in terms of low absolute error (maximum 15.5%). To assess the quality of the results, the depth of investigation (DOI) method was used (Oldenburg and Li, 1999;Hilbich et al., 2009;Stiegler et al., 2014), which is given by 20 With this technique two inversions of the same data sets are carried out using equation (1), but with two different reference models with homogeneous resistivity values m01 and m02 (Hilbich et al., 2009). The first reference value (m1) is usually calculated from the average of the logarithm of the observed apparent resistivity values. The second reference resistivity value (m2) is usually set at 10 times this value. Model regions with DOI index values >0.2 are considered as unreliable (Hilbich et 25 al., 2009). This empirical method determines the effective depth of investigation (Angelopoulos et al., 2013).
Inversion artefacts are often caused by high resistivities and high resistivity contrasts between frozen and unfrozen subsurfaces and can lead to misinterpretations of the inversion model tomograms. Applying synthetic modelling can be used to confirm the hypotheses drawn from the observed internal permafrost structure of the rockwall. By using the software Res2Dmod (Loke, 1999) simulated data of the expected apparent resistivities were calculated with the same measurement setup as in the field. 5 30 % Gaussian noise was added to the apparent resistivities to simulate field conditions (Hauck, 2001;Stiegler et al., 2014). The robust inverted synthetic model was compared to the real inverted data. The modelling process continued until both inverted data sets had similar tomograms. The final synthetic model was used as a possible representation of the subsurface (Hilbich et al., 2009;Stiegler et.al., 2014).

Resistivity category definition 35
To determine the thermal condition within the rockwall, the resistivity values have to be grouped into different categories. In this study the results of a small-scale geoelectric monitoring station used for rock moisture and frost weathering research nearby to the ERT-profiles at the Koppenkarstein (Fig. 5, MS-K) were used to classify the resistivities. The same GeoTom-2D system with multicore cables for 68 electrodes was used. The connection to the rock was established by stainless steel screws, 5 mm in diameter, which were driven into 4 mm wide and 1 cm deep boreholes, each 6 cm apart. Thus, the total extent of the survey line was 4.08 m. Additional temperature sensors (Pt1000, Geoprecision) at 0, 2, 6, 12 and 18 cm depth gave simultaneous information about the temperature behaviour within the rock. The combined analysis of resistivity and temperature changes at different depths caused by freeze thaw events provides the necessary information to define the rock 5 resistivity characteristics at different temperatures. The mean resistivities along the whole profile at 2, 6, 12 and 18cm depth were compared with the temperature results. Like the laboratory results of Krautblatter et al. (2010), a rapid increase in resistivity from 13 kohm m to 30 kohm m was observed in the temperature range between -0.5 and -1°C (Fig. 4). The unfrozen rock was characterized by resistivities of up to 13 kohm m, the transition zone with still unfrozen layers ranged from 13-30 kohm m and frozen rock had resistivities exceeding 30 kohm m. Similar thresholds were used by Krautblatter et al. (2007)

BTS and GST
The ground temperature curves from January to July 2013 display that the winter equilibrium temperature (WEqT) was reached, as an indicator for permafrost and could be measured at all sites by GST. (Fig. 5 a-d). At the north exposed MS-K ( Fig. 5a) the mean WEqT of nine GST measurements is -3.9°C, the mean of the six BTS measurements is -5.0°C. At MS-H ( Fig. 5b) the WEqT (iB-12) and BTS in northeast aspect is with -5.2°C resp. -5.6°C significantly lower than the measured 20 values at the east exposed rockwall. There, the mean WEqT (iB-13, iB-11) is -3.1°C, the mean of the BTS values is even higher (-2.2°C, maximum value of all temperature measurements). The later beginning of RD at this site in July is connected to ski run work with snow redistribution at this site during spring. At MS-D (Fig. 5c) the mean WEqT of the two iButtons in northeast exposure is -3.6°C while the mean BTS, measured north exposed, is -4.2°C. For MS-G (Fig.5d) the results show some more fluctuation in temperature at the beginning of the year (iB-19, February), probably because of less insulation due 25 to a shallow snowpack. Mean WEqT of all GST measurements carried out at the foot of the west to northwest exposed slope is -4.3°C, the mean BTS is -4.5°C. The WEqT of the southeast exposed iB-14 is more than 1 K higher (-2.9°C), which can be explained by much higher direct solar radiation.
Furthermore, the longest durations of the zero curtain period were measured at sites MS-D and MS-G ( Fig. 5c and 5d) indicating long and more-or-less continuous snow cover depletion at those sites. In addition, the melt out date (MD) at the 30 measurement sites reveal substantial differences in snow-cover disappearance between the different sites. The earliest date of differences conditions at least when it comes to the moment when the ground temperature measurements sites got exposed to atmospheric warming.  Table 2 gives an overview of the 6 ERT profiles and their respective range of resistivities. The temperature/resistivity classification from Fig. 4 constitutes the base for the interpretation of permafrost existence.

Significance of ERT data for permafrost detection 5
ERT permafrost investigations in bedrock may be error-prone because the resistivity contrast is small between ice, air and certain rock types, as all three nearly behave as an electrical insulator with very high resistivities . Furthermore, the resistivity values for subzero ground span a wide range from about 13 kohm m to more than 30 kohm m depending on the ice content (Hilbich et al., 2009). At all six ERT profile lines, areas with resistivities higher than 100 kohm m up to 500 kohm m (Table 2) were measured which, in all probability, represent frozen ground. These exceptionally high 10 electrical resistivities could also be caused by air-filled cavities in the rock; due to karstification of the Dachstein limestone, the existence of caves or small karstified cavities cannot be ruled out. However, the known caves usually occur in pronounced horizontal cave floors, and no cave entries can be found at the elevation of the study sites. Furthermore, the geometrical distribution of high resistivities particularly at MS-K1 and the position of the resistivity anomalies beneath an active layer of +/-consistent depth make the interpretation as cavities extremely improbable. Furthermore, the interpretation is backed by GST and BTS temperatures.
The use of salt water and conductive grease at the drilled-in screws lowered contact resistances between electrodes and rock and provided satisfactory data quality in terms of RMS errors. The use of the DOI method showed, that mainly at high resistivity changes (between thawed and frozen layers) some areas with DOI > 0.2 occur and should be discussed with caution 5 (Hilbich, 2009). However, these zones are in positions where they do not affect the general interpretation. On the whole, the DOI analyses showed that all ERT profiles yield reliable results. To exclude resistivity misinterpretations regarding frozen vs. unfrozen conditions, we performed resistivity measurements at a small-scale geoelectric profile combined with temperature measurements at different depths. Krautblatter et al., (2010) performed systematic temperature/resistivity investigations in the laboratory and found a distinct resistivity increase with subzero temperatures. They determined 30 kohm m as the threshold 10 value from which on the rock (the very similar Wetterstein limestone) is very probably frozen. We were able to confirm these findings in a natural setting. The knowledge of the resistivity range of frozen and unfrozen rock at our sites puts the interpretation on a solid basis. The transition zone with a mixture of liquid water and ice with values between 13 and 30 kohm m is characterized by the rapid increase of resistivity at the temperature change from positive to negative (starting around -0,5° C). During constant freezing and temperatures below -0.5 °C, values higher than 30 kohm m. were measured. 15

General distribution of permafrost
Investigations on permafrost distribution in cirque headwalls are scarce due to limited accessibility. Almost all of the measured BTS and GST temperatures point to the existence of permafrost in the recently deglaciated zones of the upper glacier margins which have been subject to glacier retreat since the LIA maximum. This is confirmed by all 2D-geoelectic profiles (excluding MS-G) that indicate permafrost at some meters depth. At all four study sites permafrost layers with resistivities higher than 30 20 kohm m occurred. Highest resistivities were found at the MS-K north face, followed by the MS-D site, while at the MS-G and MS-H the resistivities were lower and the borders to the permafrost zones are not so pronounced (Fig. 6). The GST results support this resistivity order, with deeper WEqT temperatures beneath -5 °C at MS-K and WEqT between -5 and -3°C at the three other sites. modelling approach. According to the APIM approach, permafrost in our study area is to be found in mostly cold to only in very favorable conditions. A comparison of our field data with the APIM model leads to the conclusion that our field data support the model (Fig. 5-7). According to the GST/BTS and WEqT classification defining temperatures < -3 °C as areas with probable permafrost (Haeberli, 1973), all of our sites should be affected by permafrost in favorable conditions. Although the Evidence of permafrost was found below and above the LIA glacier margin with the lowest active layer thickness at the very line of the former LIA glacier surface which means that the imprint of the LIA glacier margin can be found in the resistivity profiles. At MS-K, the thinnest active layer and the highest resistivities of the deeper subsurface were found in the approximate middle of the vertical profiles, corresponding with the 1850 glacier surface. This is particularly well visible at the K1 profile.
The flattening of the rockwall at K3 in the elevation of the 1850 surface probably enabled accumulation of infiltrated moisture and the development of massive ice below the surface (-2 to -10m). Generally higher resistivities (>50 kohm m) were found in the part of the rockwall above the 1850 margin which has been ice-free for more than 150 years. At the ERT-sites near the 5 present glacier surface (MS-K2, MS-D1, MS-G1 and MS-H1) which have been ice-free for a much shorter time, resistivities are lower and the active layer is thicker. The increasing active layer depth below the LIA margin and the generally very high resistivities around the LIA margin are among the most important observations of the study (see 5.3).
At MS-D it is difficult to determine the historically highest surface of the glacier, because the glacier surface at the Dirndl mountain is influenced by the mentioned blowout depression at the footslope of this mountain (cf. Fig. 5). The investigated 10 part of the rockwall at MS-D1 was probably ice free even before 1850 (Simony, 1884) and thus exposed to atmospheric conditions for much longer than at MS-K2, MS-G1 and MS-H1. The absence of insulating glacier ice could be the reason for the well-established frozen layers at the left and in the middle of the profile beneath the active layer. However, between those two frozen parts thawing processes occur with resistivities between 10 and 20 kohm m, mirrored by a wet and fractured rock surface in the field. 15

Degradation or aggradation of permafrost?
Significant areas of the study region were affected by glacier recession and glacier surface lowering at the glacier forefield and the surrounding headwalls. The thermal regimes of surface ice and frozen ground can be interconnected and are influencing each other (Suter et al., 2001;Otto and Keuschnig, 2014) Evans, 2010), permafrost might exist under the cold-based areas of glacier ice. In this case, the active layer which developed since deglaciation would indicate current permafrost degradation. As the thermal conditions at the base of the Schladming Glacier are not yet known, this question cannot be definitively clarified, and further research is needed. However, at profile MS-K1 the active layer thickness decreases from the lowest point of the profile upwards and reaches its minimum at the elevation of the 1850 line. This finding strongly supports interpretation 'a' because the higher areas had more time for 30 aggradation than the lower parts. If 'b' was right, we would expect the active layer to be thicker where the time span for permafrost thawing was longer with even stronger degradation above the LIA line. An alternative interpretation (c) is that some permafrost existed below the marginal LIA ice cover (e.g. through transverse heat conduction, ice cleft cooling, pronounced randkluft, etc.) without the strict precondition of cold-based marginal ice. However, as the detected permafrost reaches at least 20 m under the former LIA glacier surface, this interpretation is considered unlikely. The timescales involved 35 in building up permafrost are rarely addressed in the literature; however, it is known from glacier forefields that permafrost can form few years after glacier retreat (Kneisel, 2003). Furthermore, Magnin et al. (2017Magnin et al. ( , p. 1821) modelled permafrost degradation rates of approx. 5-10 m in 20 years in vertical rockwall settings. Considering these time scales, permafrost aggradation in the up to 150 years after LIA appears to be realistic.
The higher parts of the vertical ERT profiles have presumably been ice free at least since the onset of the Holocene as judged from general glacier evolution during the entire Holocene in the Eastern Alps (Wirsig et al., 2016). Thus, permafrost in these areas results from significantly different conditions than in the lower parts which have been ice free for a much shorter period of time; smaller, warmer permafrost layers with lower resistivities should be expected. This assumed pattern is realized at MS-K3 in an ideal way. However, the pattern is opposite at MS-K1 with much higher resistivity in the assumedly ‚younger' 5 permafrost zone. The reason might be drier conditions above the 1850 line and the supply of meltwater below the line leading to more massive ice formations. As further changes in the shallow subsurface might become apparent after some years of observation, repeated measurements might clarify the question of degradation or aggradation of permafrost.
The conditions at the investigated headwalls at Dachstein are typical of many high-mountain cirque settings in which, according to our findings, transient permafrost aggradation is to be expected during glacier surface lowering. Enhanced frost 10 cracking and rockfall around glacier margins has frequently been found or hypothesized (e.g. Matsuoka & Sakai, 1999;Sanders, 2012). As permafrost occurrence increases the sensitivity to frost weathering by increasing cryostatic pressures (Murton et al., 2001, Sass, 2010, Krautblatter et al., 2013, aggradation might provide an additional mechanism for temporarily increased rockfall intensity and cirque erosion.

Conclusions and Outlook 15
The used methods have proven their applicability for permafrost mapping and have delivered novel and valuable information on permafrost distribution around the upper margins of retreating glaciers in the Dachstein area. Permafrost was found in all investigated north facing rockwalls between 2600 and 2800 m a.s.l. that were subject to glacier retreat since the LIA maximum.
Permafrost preservation (or even aggradation, see below), is thus possible in favourable cold conditions in north faces with MAAT below -2.5 °C (in 2013). Slightly less radiation-exposed sites oriented northwest and northeast show degradation effects 20 with very heterogeneous subsurface ERT tomograms indicating frozen and unfrozen parts. At the only west-facing site, no permafrost could be confirmed. The ERT data are of good quality. The resistivity calibration by using data of a small-scale ERT profile line proved to be a helpful method to delimit frozen from unfrozen rock which may aid the interpretation also in other study regions. The ERT interpretation is backed by GST and BTS data.
The most significant finding is the imprint from the LIA ice cover in the vertical ERT profiles K1 and K3 reflected by 25 particularly thin active-layer thicknesses. The existence of permafrost at the former ice-covered positions could be due to slow degradation of permafrost that already existed under polythermal glacier ice, or to aggradation of permafrost after glacier retreat. Evidence from resistivity distributions in ERT profiles (downslope increasing active layer thickness) rather points to aggradation which would be an important finding for research in comparable cirque settings. However, longer-term observations are still necessary to underpin this conclusion. 30 To clarify the open questions of aggradation vs. degradation, repeated ERT and temperature measurements are necessary, together with temperature measurements at the glacier base to confirm warm-based or polythermal conditions.

Acknowledgements
This study was supported by the project 'ROCKING ALPS -Rockfall and Weathering in the Eastern Alps' financed by the Austrian Science Fund (FWF) through project no. FWF: P24244. Further thanks to the following organisations: Dachstein 35 cable car company (E. Schnepfleitner and L. Traninger), Austrian Federal Forests, "snowreporter" for private climate and snow depth data, as well to G. K. Lieb for fruitful discussions. Many thanks to Johannes Stangl, Eric Rascher, Reinhold Schöngrundner, Patrick Zinner and Eduard Rode for their help during fieldwork. Mitteilungen der Versuchsanstalt für Wasserbau, Hydrologie u. Glaziologie der ETH Zürich, 17, Zürich, 221, 1975.