Long-term monitoring of sporadic permafrost at the eastern margin of the European Alps (Hochreichart, Seckauer Tauern range, Austria)

Abstract

Delineating the spatial extent and the altitudinal lower limit of mountain permafrost is difficult due to complex topo-climatic and variable ground thermal conditions within short distances. Little information exists regarding sporadic permafrost existence, its thermal characteristics and its long-term changes at the eastern margin of the European Alps. To reduce this gap, permafrost monitoring was initiated in 2004 in the Seckauer Tauern mountains, Austria. Research was carried out in the summit region of Mt Hochreichart (2416 m a.s.l.) and at several nearby cirques and valleys, all with rock glaciers. Geomorphic mapping, numerical permafrost modeling, measurements of the bottom temperature of the winter snow cover, continuous ground temperature monitoring, electrical resistivity tomography and optical snow cover monitoring were applied. Results indicate sporadic permafrost occurrence in the summit region with mean annual ground temperatures slightly below 0°C at the surface and -1.4°C at 2.5 m depth. Permafrost lenses also exist in the transition zone between the rock glacier and the talus slope behind attributed to coarse-grained, blocky material causing additional ground cooling. Thanks to long-term data, statistically significant trends of atmospheric and ground warming were observed in 2000-2018. Permafrost at this site will presumably disappear within the next few decades.

Keywords: climate change; coarse debris layer; eastern European Alps; ground thermal regime; long‐term permafrost monitoring; rock glacier types.

Figure 1

Map of the study area. (a) Overview map with location of the Hochreichart area (H) in Austria. (b) Location of the relevant subareas shown in Figure 3, modeled permafrost distribution,2, 11 extent of rock glaciers, bedrock geology (simplified from ref.12) and different automatic monitoring sites (for explanation of codes see Table 2). (c) Extent of glaciers during the Last Glacial Maximum in the Seckauer Tauern (based on refs 13, 14). (d) Geomorphological overview map of the Hochreichart area with modeled permafrost extent [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 2

Terrestrial images of the study area Hochreichart: (a) Mt Hochreichart (2,416 m a.s.l.) and the Reichart cirque with the uppermost part of a polymorphic rock glacier as seen from the site of the remote digital camera (RDC); (b–d) summit area of Mt Hochreichart with coarse‐grained autochthonous blockfields with evidence of material sorting by frost action (note the vegetation patch with hiking stick in b); (e) Höll valley with late‐lying snow patches (cf. Figure 3h); (f) Reichart cirque with the upper part of a polymorphic rock glacier. Locations of relevant ground and temperature monitoring sites and profiles where geoelectrical measurements were carried out are indicated. Dashed line delineates rock glacier parts visible in the images. Photographs by a. Kellerer‐Pirklbauer [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 3

Maps of subareas of the Hochreichart area with location of monitoring sites (Tables 2), spatial extent of modeled permafrost distribution, late‐lying seasonal snow patches (July conditions), and results of nine different BTS campaigns between 2004 and 2012 (for details see Table 1). Modeled permafrost distribution are based on the literature2, 11 [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 4

Reichart cirque: BTS measurements and interpolated temperatures in 2004, 2005, 2007, 2008 and 2009 (a–e) and summary box‐plot diagram for a 0.04‐km² area in the rooting zone of a large polymorphic rock glacier for these five years (f) [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 5

MAAT and MAGT at the surface (000) and at different depths (003–250; in cm) during the period 2005/06 to 2017/18 for all sites with respective data: (a) for the summit areas Hochreichart and Kleinreichat; (b) for the Reichart cirque. For location of sites refer to Figure 3 [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 6

Ground thermal conditions at the coarse‐grained rock glacier site R‐G1 between October 2014 and August 2018. (a) Mean daily ground temperature at the surface and at three different depths. (b) Subsurface isotherms (3°C intervals) based on mean monthly values. The dashed line marks the 0°C isotherm [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 7

Pictures of the Reichart and (partly) Brandstätter cirques as seen from the automatic RDC in late winter (a and b) and mid‐winter (c) indicating typical snow cover conditions during those times. Only the relevant cutout of the RDC images is shown. Note the snow distribution pattern in the cirque, the talus slope behind and the rockwalls around [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 8

Surface offsets in the Hochreichart area based on adjacent ground surface and air temperature monitoring sites: comparison of annual air temperature, ground surface temperature and resulting surface offset values for the summit areas of Hochreichart (a, b) and Kleinreichart (c), Reichart cirque (d) and the frontal area of the rock glacier which originates in the Reichart cirque (e); comparison of mean monthly air temperature, ground temperature and resulting surface offset values for the summit area of Hochreichart (f) and the Reichart cirque (g) [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 9

Evolution of ground temperature (H‐G1 at 2,416 m a.s.l.; G), air temperature and precipitation (both R‐A2 at 1,512 m a.s.l.; A and P) between 2000 and 2018 in the Hochreichart area for annual and selected monthly (January, June) time series. Linear trends for annual means/sum are indicated. Significance: **P < 0.01, *P < 0.05, ns = not significant [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 10

Electrical resistivity tomography inversion results for 13 profiles measured in the study area. For technical details refer to Table 3. Locations of profiles are shown in Figure 3. “X” indicates the location where two ERT profiles intersect. Note that the spatial scale varies between profiles [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 11

Rock glacier types as proposed previously.37, 38 The rock glacier in the Hochreichart cirque is considered to be pseudo‐relict, and hence a rock glacier which appears to be visually relict but still contains patches of permafrost (modified after 38). Changes of the timber line are a proxy for changes in vegetation [Colour figure can be viewed at http://wileyonlinelibrary.com]