The Impact of Recent Glacier Fluctuation and Human Activities on Permafrost Distribution, Stelvio Pass (Italian Central-Eastern Alps)

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The impact of recent glacier fluctuation and human activities on permafrost distribution, Stelvio Pass (Italian Central-Eastern Alps)

N. Cannone Department of Natural and Cultural Resources, University of Ferrara, Italy M. Guglielmin Arpalombardia, Viale Restelli Milan, Italy C. Hauck Graduiertenkolleg Natural Disasters & Institute for Meteorology and Climate Research, University of Karlsruhe, Germany D. Vonder Mühll University of Basel & Institute for Geography, University of Zurich, Switzerland

ABSTRACT: The relationships between permafrost and glacier fluctuations were analysed near Stelvio Pass (Italy). The glacial evolution was reconstructed by comparing historical documentation, vegetation surveys and geomorphology. Patchy permafrost occurrences, detected by geophysical techniques (VES, resistivity tomography, seismic, EM-31) correspond well with ecological indications derived from vegetation surveys. Meteorological data, with a reconstructed MAAT lower than 2°C since 1927, indicate a suitable environment for permafrost aggradation. Differences in permafrost thickness (10–20 m) and active layer thickness (3–5 m) were determined through geophysical surveys. These differences can be explained by varying snow distribution, topographic characteristics of the terrain and by different times since deglaciation. In addition, disturbance of the terrain in 1987 may have influenced the ground thermal regime.

1 INTRODUCTION 1937 when the front reached a minimum altitude of 2820 m a.s.l. (Pelfini, 1992). Despite its limited extent The relationship between glacier fluctuations and per- (ca. 3 km2), the area is rich in periglacial features: sev- mafrost aggradation or degradation is a key question in eral types of patterned ground, solifluction lobes and mountain permafrost research, especially in the context two active rock glaciers. One of the rock glaciers was of glacier retreat in the last century as consequence of destroyed in 1987 to build a ski track. climate change. In alpine environments, in many cases The broad lineaments of the vegetation were drawn there are strong interactions in space and in time by Giacomini and Pignatti (1955) and, more recently, by between glacier evolution and permafrost distribution. Cannone (1997). The study area is located in the alpine The subglacial thermal regime and the climatic condi- and in the nival belts (Ellemberg, 1988), the latter tions after glacier retreat are responsible for permafrost colonised by discontinuous pioneer communities, espe- distribution in recently deglaciated areas. Earlier results cially by Androsacetum alpinae, Oxyrietum digynae, indicate that permafrost may persist from previous sub- and by the more evolved Luzuletum spadiceae. glacial conditions underneath cold-based glaciers or In the alpine belt, the climatic condition is reflected by may form after the retreat of glaciers (Guglielmin and the continuous alpine meadow Caricetum curvulae and Vannuzzo, 1995; Kneisel, 1998, Guglielmin et al., 2001), while in other situations the Little Ice Age (LIA) expansion seems to have degraded permafrost.

2 STUDY AREA

The Stelvio area is a summer ski area, located between Stelvio Pass (2,758 m a.s.l.) and M. Scorluzzo (3,095 m a.s.l.) close to the border between Italy and Switzerland in the Central Alps. The area is characterised by bedrock outcrops, as well as some Holocene till and talus deposits (Fig. 1). Notwithstanding the abundance of historical documents, the Holocene gla- Figure 1. Geographical location of the study area. Legend: cial evolution of this area is not very well known. The glaciers; * PACE borehole; 1 Vedretta Piana presence of Scorluzzo Glacier was documented until Glacier.

125 in chionophilous conditions, by Salicetum herbaceae. The permafrost distribution in this area was first inves- tigated during the EU-PACE project (Hauck et al., 2001), except for the Braulio Valley (Guglielmin, 1994).

3 METHODS

An integrated approach, based on a geomorphological- geological survey, geophysical exploration and a vegetation survey, was used to examine the environmental evolution of the area and to detect interactions between permafrost, glaciers and vegetation.

3.1 Geomorphological and geological mapping Figure 2. Simplified geological and geomorphological map Geomorphological and geological mapping of the area with the different glaciers’ fluctuations and the distribution was carried out first through photo-interpretation of of vegetation associations. Legend: 1 Pre-LIA deposits; aerial photographs at a scale 1:20,000, followed by 2 LIA deposits Stage II; 3 LIA deposits Stage III; 4 Rock glacier boundaries; 5 Post-LIA deposits 1910 field survey to map the different landforms and types A.D.; 6 Post-LIA deposits 1937 A.D.; 7 Pioneer associ- of deposits. The deposits were mapped according their ations; 8 Evolved associations (Luzuletum spadiceae); sedimentological facies, stratigraphic relationships and 9 Climax associations (Salicetum herbaceae). weathering profile, or chronologic criteria where dat- ing was possible (e.g. 14C method). To reconstruct the Table 1. Vegetation elements used as ecological indicators. recent evolution of Scorluzzo and Vedretta Piana Environmental factor/effect Vegetation indicator Glaciers, historical cartographic documents were col- lected, georeferenced and included in a GIS (Fig. 2). High soil disturbance Androsacetum alpinae Thlaspeetum rotundifoliae Low soil disturbance Salicetum herbaceae 3.2 Vegetation mapping and permafrost modelling Luzuletum alpino-pilosae Long snow persistence Arenaria biflora (8 months per year) Arabis alpina The spatial distribution of vegetation communities was Ranunculus glacialis analysed, focusing on their degree of evolution and Veronica alpina development and on the indications of the time since Alchemilla pentaphyllea colonisation. 112 vegetation surveys were carried out Gnaphalium supinum Leucanthemopsis alpina applying the phytosociological method (Braun Blanquet, Luzula alpino-pilosa 1964) and using the nomenclature of vascular plants Salix herbacea proposed by Pignatti (1982). To determine the perma- Sedum alpestre frost distribution over a larger area, a model based on Taraxacum alpinum the ecological indications concerning factors related to permafrost occurrence (snow cover) and the effects of different snow persistence and the vegetation communi- permafrost (soil disturbance) was used and derived from ties used to define the soil disturbance. All the commu- the vegetation surveys. The model considers two differ- nities occurring in these environments are characterised ent indicators derived by vegetation: snow persistence by the presence of chionophilous species indicating at the ground and soil disturbance. The first indicator is snow persistence, with differences in their number, obtained by the abundance (percentage) of particular coverage and frequency (Giacomini and Pignatti, vascular plant species that have been related to snow 1955; Credaro and Pirola, 1975). Based on the combi- cover (longer than 8 months per year), as the persistence nation of the frequency and coverage of these species, of snow cover is a limiting factor on plants’growing sea- three optimal categories, representing different condi- sons (Giacomini and Pignatti, 1955; Billings and Bliss, tions of snow persistence were derived. In particular, 1959). The second is obtained by the degree of evolution communities with more than 80% of chionophilous of vegetation communities. Within the same ecological species (Polytrichetum sexangularis, Salicetum belt and on surfaces of the same age it has been related herbaceae salicetosum), are compatible with very long mainly to the soil disturbance being able to stop vegeta- snow persistence (more than 10 months); communities tion development at pioneer stages (Carbiener, 1966; with 40–80% (Salicetum herbaceae alchemilletosum, Somson, 1984; Evin, 1991; Cannone, 1999). Listed in Luzuletum spadiceae) indicate long snow persistence Table 1 are the vascular plants used as indicators of (8–10 months) while communities with less than 40%

126 of chionophilous species (Oxyrietum digynae, reached 2610 m a.s.l. During this phase, the Vedretta Androsacetum alpinae) are compatible with shorter Piana Glacier crossed the Stelvio Pass and joined with snow persistence (less than 8 months). The indicators the Scorluzzo glacier. A small separate glacier deposited derived by vegetation have been combined empiri- its morainic ridge just some hundred meters northward cally to define three classes of probability of per- of the Scorluzzo active rock glacier. The frontal mafrost occurrence (absent, possible or present), morainic ridge at 2690 m a.s.l. represents the maximum according to the following criteria: advance of the Scorluzzo glacier during the Little Ice Age. From the historical documents it is possible to say Permafrost absent: low soil disturbance with all • that this position is lower than that reached in 1866 A.D. snow conditions or high soil disturbance with more and documented by Payer (1868), which seems to cor- than 80% of plants indicating long snow persistence; respond to the frontal morainic ridge of 2705 m a.s.l. Permafrost possible: high soil disturbance with • Geomorphological evidence of more recent changes is plants indicating long snow persistence ranging practically undetectable and the glacier fluctuations between 80 and 40%; have been reconstructed using historical and icono- Permafrost present: high soil disturbance with plants • graphical documentation. Around 1910 there were still indicating long snow persistence less than 40%. two small tongues, reaching 2720 and 2730 m a.s.l., but only the more western was still present in 1937, when its 3.3 Geophysical surveying front reached the minimum altitude of 2820 m a.s.l. (Pelfini, 1992) (Fig. 2). The reconstruction of the glacial Geophysical investigations were carried out in a limited history, especially the more recent, was completed by part of the area for verification and comparison with including the results of the vegetation survey. The the other methods. The methods included DC resistiv- occurrence of communities belonging to different suc- ity, refraction seismics and electromagnetic induction. cessional stages, indicating different evolution degree Three vertical electrical soundings (V.E.S.), using the and time since colonisation, allowed us to recognise symmetric Schlumberger array (Guglielmin, 1994) and four types of surfaces with different ages (Fig. 2). two DC resistivity tomographies were carried out on Stage I is the oldest surface, characterised by the the Scorluzzo active rock glacier and along the slope of occurrence of evolved communities, typical of surfaces the old Scorluzzo glacier, where the ground was partly stabilised for a long time. The dominant communities reworked for the construction of a ski run. Both meth- are Salicetum herbaceae alchemilletosum (a mature ods are well suited for the distinction between frozen facies of Salicetum herbaceae) and its transitions with and unfrozen ground and have often been used individ- Luzuletum spadiceae, with high coverage (average ually or in combination, for the study of permafrost 80%), and characterised by the occurrence of species (e.g. Hauck and Vonder Mühll, 1999; Kneisel et al., typical of evolved seral stages (Alchemilla pentaphyl- 2000). A refraction seismic survey was carried out lea, Gnaphalium supinum, Homogyne alpina). These along the DC resistivity survey line on the ski run. The deposits are located in the lower part of the study area P-wave velocity, determined by measuring the travel- along three different morainic ridges; time of a seismic wave from a source point to a spread Stage II: these surfaces are younger than those of of geophones at the surface, provided a complementary Stage I, but older than Stage III and are characterised indicator to resistivity for the presence of frozen mate- by the occurrence of Luzuletum spadiceae with high rial. Finally, electromagnetic conductivity mapping of coverage; a 300 200 m area within the lateral moraines of the Stage III: these surfaces are intermediate between old Scorluzzo Glacier was undertaken with a Geonics Stage II and Stage IV and are colonised by a mature EM-31 instrument. This method was used to rapidly Oxyrietum digynae, with medium to high coverage determine the bulk conductivity (1/resistivity) of the (45–80%, average 60%), and, partially, by transi- uppermost 6 m of the subsurface (e.g. McNeill, 1980). tion communities between Oxyrietum digynae and This method has been commonly used in the Arctic Thlaaspeetum rotundifoliae. These associations occur (e.g. Scott et al., 1990), but much less in alpine envi- in the central part of the study area and colonize both ronment to date (Hauck et al., 2001). the W and the E cirques of Mount Scorluzzo. Stage IV: these are the youngest surfaces, with pioneer communities dominated by Androsacetum 4 RESULTS alpinae and Thlaspeetum rotundifoliae and their transitions to Oxyrietum digynae. These communities are 4.1 Glacial reconstruction distributed mainly in the upper part of both cirques. The east cirque of Mt. Scorluzzo and the upper part of From the geological and geomorphological surveys, the the west cirque are characterised by the occurrence of maximum extension of the glaciers during the Holocene transitional communities with calcareous associations

127 Thlaspeetum rotundifolii and Arabidetum caerulae the location of the DC resistivity survey line. The results (Fig. 2, areas A, B). This is surprising because the indicate that the distribution of the anomalies shown in Scorluzzo cirques are characterised by the occurrence Figure 3 is almost parallel to the altitudinal lines. In the of siliceous rocks and by a total absence of calcareous upper part (in the southern part in Fig. 4), above the DC material. These transitional communities document the resistivity line, conductivities decrease even more, presence of carbonate rocks transported by the Vedretta which may indicate a decreasing active layer thickness. Piana Glacier when it was joined with the Scorluzzo Finally, Fig. 5 shows the inversion result for a survey and crossed over the ridge Platigliole Pass-Stelvio Pass. along the rock glacier some 300 m to the west of the ski run. The resistivity maximum, corresponding to an ice 4.2 Permafrost distribution body, is comparatively thin (15–20 m) and has values of up to 500 km. This resistivity maximum does not In Figs. 3–5 three examples of the geophysical field- extend to the rock glacier tongue (distances 30–80) indi- work conducted in summers 1998 and 1999 are cating a much smaller ice content and/or a much deeper presented. Fig. 3 shows the inversion results of a 2- active layer in the frontal part. The three V.E.S. carried dimensional DC resistivity survey along a 200 m sur- out in 1993 confirm these findings, showing a slightly vey line on the ski run. Two regions of higher resistivity smaller maximum resistivity of 100 km. The active (10–30 Km) can be seen on the upper and lower layer thickness of the main part of the rock glacier may ends of the profile. be determined as 3–5 m, similar to that of the ski run. Due to the fine-grained characteristics of the surface cover, the causes of these high-resistive anomalies are believed to be subsurface permafrost occurrences. The vertical extent of the anomalies is between 10 m (upper) and 20 m (lower). Below and throughout the centre of the profile, resistivities of less than 2 km are found. Active layer thickness is in the order of 3–5 m, but is difficult to determine due to the coarse resolution of the measurement geometry used (5 m spacing between the electrodes). The interpretation of the high-resistive regions as permafrost was confirmed through refraction seismic analysis yielding a velocity around 3500 m/s (the velocity of ice) between horizontal distances 30–50 and a velocity of less than 2000 m/s between distances 10 and 20 (not shown here). In order to determine the spatial representativeness of this result, an EM-31 conductivity mapping survey was conducted within a major part of the area between Figure 4. EM-31 conductivity survey of the old ski run the two lateral moraines (Fig. 4). The grey line marks near Stelvio Pass. The black line marks the location of the DC resistivity survey line shown in Fig. 3.

Figure 3. DC resistivity tomography survey of the old ski Figure 5. DC resistivity tomography survey of the Stelvio run near Stelvio pass. The root-mean-square error of the rock glacier. The root-mean-square error of the inversion inversion results is given in the left corner. results is given in the top part.

128 4.3 Integrative permafrost modelling and the more internal morainic ridge of 2705 m a.s.l. (Stage III), suggest that the maximum advance of LIA The analysis of the permafrost distribution was inte- was in this area comparable with the 14C at 1580 A.D grated by comparing the geophysical results with the age obtained in the close Trafoi Glacier (only 2 km to permafrost map (Fig. 6) obtained by combining the eco- the east), instead of 1850 previously identified by logical indications of vegetation, according to the Pelfini (1992). In fact, Luzuletum spadiceae (Stage II) empirical model presented here. There is a high degree starts to develop at least 30 years after the develop- of correspondence between this map and the distribu- ment of Oxyrietum digynae (Stage III) (Pirola and tion determined by geophysical prospecting. Credaro, 1994). The occurrence of the Stage IV is The areas with “permafrost absent” are mainly limited within the extension of the glacier documented located in the lower part of the study area, while areas until 1910, while in the area covered by glaciers in with “possible permafrost” occurrences are distributed 1937, vegetation is absent. Concerning the permafrost all over the study area, without any particular link to distribution, considering the available climatic data altitude, even though the highest percentage of this (since 1927) and using a lapse rate of 0.6°C/100 m, category occurs between 2720 and 2750 m a.s.l. the MAAT for this area was almost always lower “Permafrost present” has a patchy distribution. In par- than 2°C (with an average of 2.3°C) until 1988, ticular, in the ski area, permafrost occurs both in the when there was a significant warming (aver- upper and in the lower part, the middle area being age 1.6°C). According to most authors (e.g. Cheng “possible permafrost”. On the rock glacier, permafrost 1983) a MAAT lower than 1°C is favourable for is mainly in an area between 2700 and 2750 m a.s.l. permafrost occurrence. The climatic conditions and on a little patch on the western side of the very between 1927 and 1988 were hence suitable for per- upper part (close to 2800 m a.s.l.). mafrost aggradation. Permafrost thickness around 20 m is compatible with the theoretical MAGT (mean annual ground temperature) of 0.6°C, resulting 5 DISCUSSION from applying heat-conduction theory and using thermal conductivity values of 2.8 Wm1°C1 and a heat There is a total concordance between the different ages flow of 0.085 Wm 2 (Vonder Mühll and Haeberli, of the late-Holocene deposits as determined by geo- 1990). The MAGT recorded since 1999 at the PACE morphological and geological surveys (Pre-LIA, LIA borehole at 3000 m a.s.l. (Fig. 1) is around 2°C. The and post-LIA) and the four stages identified by vege- differences of permafrost distribution detected by tation. In fact, the Pre-LIA deposits correspond to geophysical prospecting seem to be linked to the snow Stage I of vegetation colonization, the LIA moraine distribution, as demonstrated by vegetation and by the deposits with Stages II and III, and the Post-LIA topographical characteristics of the terrain (e.g. aspect deposits to Stage IV. The differences of vegetation and slope) that influence both the snow distribution from the morainic ridges at 2690 m a.s.l. (Stage II) and solar radiation. One of the possible explanations of the occurrence of thicker permafrost in the lower part of the ski run may be the longer time since deglaciation and hence the longer exposure to air temperatures inducing permafrost aggradation. The map derived from the vegetation ecological indicators (Fig. 6) shows that permafrost occurs all along the undisturbed area on the eastern side of the ski run, while its presence is limited only in the higher and lower parts on the western reworked side. On the west side of the ski run the permafrost probability is high from the lower up to the higher parts, where solifluction lobes are present. This fact suggests that the terrain disturbance due to the digging and levelling operations for the ski run preparation in 1987 has indeed influenced the ground thermal regime, even if, at the moment, there are not enough data to evaluate its Figure 6. Map of ecological indicators derived by vegetation survey and geographical location of geophysical sur- full impact. A high impact of human activities is well veys. Legend: 1 Permafrost absent; 2 Permafrost documented by the vegetation which is completely possible; 3 Permafrost present; 4 Vertical Electrical lacking in all the areas that suffered the works to build Soundings; 5 Electric Tomography; 6 Electromagnetic the ski run and all the related ground roads in 1987 Conductivity EM-31. (Fig. 2). The absence of vegetation in the reworked

129 area indicate that 15 years are insufficient for the Giacomini, V. & Pignatti, S. 1955. Flora e vegetazione restoration of natural vegetation in this environment. dell’Alta Valle del Braulio con speciale riferimento ai pascoli d’altitudine. Mem. Soc. It. Sc. Nat., 11: 47–238. Guglielmin, M. 1994. Permafrost e morfodinamica peri- 6 CONCLUSION glaciale nelle Alpi centrali italiane. Metodologie per l’individuazione del permafrost e dei processi e forme An integrated approach was applied to detect the ad esso collegate. Doctorate Thesis, University of interactions between permafrost, glaciers and vegeta- Parma, unpublished. tion. The vegetation survey allowed the maximum Guglielmin, M. & Vannuzzo, C.. 1995. Studio della dis- LIA advance in this area to be dated to 14C 1580 AD, tribuzione del permafrost e delle relazioni con i ghiac- instead of 1850 AD. The boundaries of the glacier ciai della Piccola Età Glaciale nell’Alta Valtournenche (Valle d’Aosta, Italia) Atti Ticinensi, XXXVIII: 119–127 respectively in 1910 and 1937 were also reconstructed Guglielmin, M., Cannone, N. & Dramis, F. 2001. by vegetation, in the absence of any geomorphologi- Permafrost-Glacial Evolution during the Holocene in cal evidence. Geophysical prospecting indicated a the Italian Central Alps. Permafrost and Periglacial patchy permafrost distribution with an active layer Processes, 12(1), 111–125. ranging from 3 to 5 m, and a permafrost thickness Hauck, C. & Vonder Mühll, D. 1999: Using DC resistivity between 10 and 20 m. An empirical model to map per- tomography to detect and characterise mountain permafrost distribution was developed using the vegeta- mafrost. In: Proceedings of the 61. Europ. Association tion ecological indicators. The results of geophysical of Geoscientists and Engineers (EAGE) conference, mapping were integrated with vegetation ecological 7.-11. June 1999, Helsinki, Finland. 2–15. indications and were applied to the whole study area. Hauck, C., Guglielmin, M., Isaksen, K. & Vonder Mühll, D. 2001: Applicability of frequency- and time-domain Permafrost aggradation was induced by climatic con- electromagnetic methods for mountain permafrost stud- ditions from 1927 to 1988. The permafrost distribution ies. Permafrost and Periglacial Processes 12 (1): 39–52. appears to relate to different snow cover persistence Kneisel, C., 1998. Occurrence of surface ice and ground and to topographic parameters. A longer deglaciation ice/permafrost in recently deglaciated glacier forefields, time, with a correspondingly longer exposure to cold St. Moritz area, Eastern Swiss Alps. 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