The Triglav Glacier (South-Eastern Alps, Slovenia): Volume Estimation

Pure Appl. Geophys. 2016 Springer International Publishing Pure and Applied Geophysics DOI 10.1007/s00024-016-1348-2

The Triglav Glacier (South-Eastern Alps, Slovenia): Volume Estimation, Internal Characterization and 2000–2013 Temporal Evolution by Means of Ground Penetrating Radar Measurements

1,3 2 3 4 COSTANZA DEL GOBBO, RENATO R. COLUCCI, EMANUELE FORTE, MICHAELA TRIGLAV Cˇ EKADA, 5 and MATIJA ZORN

Abstract—It is well known that small glaciers of mid latitudes 1. Introduction and especially those located at low altitude respond suddenly to climate changes both on local and global scale. For this reason their monitoring as well as evaluation of their extension and volume is Small glaciers and glacierets, unlike the larger ice essential. We present a ground penetrating radar (GPR) dataset bodies are important indicators of short term varia- acquired on September 23 and 24, 2013 on the Triglav glacier to tions in the climate system, both on local and on identify layers with different characteristics (snow, firn, ice, debris) global scale due to their fast response to climate within the glacier and to define the extension and volume of the actual ice. Computing integrated and interpolated 3D using the changes (Kuhn 1995; Hughes et al. 2006). The very whole GPR dataset, we estimate that at the moment of data small glaciers are particularly sensitive to topocli- 2 3 acquisition the ice area was 3800 m and the ice volume 7400 m . matic conditions (Hughes and Woodward 2009), and Its average thickness was 1.95 m while its maximum thickness was slightly more than 5 m. Here we compare the results with a pre- thus they often exhibit large mass balance fluctua- vious GPR survey acquired in 2000. A critical review of the tions even in a very short time (Hughes 2008, 2010). historical data to find the general trend and to forecast a possible Although these glaciers are very small they still have evolution is also presented. Between 2000 and 2013, we observed relevant changes in the internal distribution of the different units to be considered because they are so numerous that at (snow, firn, ice) and the ice volume reduced from about 35,000 m3 regional scale, they can contain a significant amount to about 7400 m3. Such result can be achieved only using multiple of ice. In such context, if we consider just all glaciers GPR surveys, which allow not only to assess the volume occupied larger than 1 km2, this could result in errors in the by a glacial body, but also to image its internal structure and the actual ice volume. In fact, by applying one of the widely used order of ±10 % in the regional estimations (Bahr and empirical volume-area relations to infer the geometrical parameters Radic´ 2012). In the European Alps, to obtain evalu- of the glacier, a relevant underestimation of ice-loss would be ations with smaller errors, certainly essential for achieved. regional water resource quantification and exploita- 2 Key words: 3D GPR, 4D analysis, ice melting, time moni- tion, an inventory of all ice bodies down to 0.01 km , toring, climate changes, Triglav glacier, Slovenia, South-eastern or even smaller, would be necessary (Bahr and Radic´ Alps. 2012; Pfeffer et al. 2014). In addition, the alpine cryosphere represents an important drinking water reserve as well as a key factor in the landscape evolution and biodiversity conservation. Differences 1 Present Address: Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innrain 52f, 6020 Innsbruck, between small glaciers, glacierets, ice patches and Austria. snow patches are in fact based on their dynamics 2 Department of Earth System Sciences and Environmental (motion or non motion), internal materials (ice, firn, Technologies, ISMAR-CNR, Viale R. Gessi 2, 34123 Trieste, Italy. snow, folds) and genesis (glacial or nival). For this 3 Department of Mathematics and Geosciences, University of Trieste, Via Weiss 2, 34128 Trieste, Italy. E-mail: [email protected] reason, it is increasingly crucial to understand not 4 Geodetic Institute of Slovenia, Jamova 2, 1000 Ljubljana, only the extension, but even more important, the Slovenia. internal structure and evolution of such smaller ice 5 Anton Melik Geographical Institute, Research Centre of the Slovenian Academy of Sciences and Arts, Gosposka ulica 13, 1000 masses for a correct evaluation of their characteristics Ljubljana, Slovenia. and genesis (Serrano et al. 2011). C. Del Gobbo et al. Pure Appl. Geophys.

The Triglav glacier (4622042.52N 1350011.86E, Alps and also in Slovenia. Its location and the sur- Fig. 1), known in Slovenian as Zeleni sneg (Green rounding morphologies ensure a fairly high insulation snow), lies at an altitude between about 2400 and rate especially during summer. It has been regularly 2500 m.a.s.l. on the north-eastern slope of Mount measured, observed, and monitored since 1946 by the Triglav (2864 m a.s.l.), the highest peak in the Julian Anton Melik Geographical Institute of the Research Triglav Glacier Evolution by Means of GPR

b Figure 1 (e.g., 2009, 2011), and since 2003 the ice was no Location map of the study area (a) and a detail around Mount longer outcropping like in the previous seasons Triglav (b). The blue area corresponds to the limit of the surface covered by snow (with some snow free patches inside) at the (Gabrovec et al. 2013). moment of the 2013 GPR survey. The yellow dots show the Unlike such long and detailed historical data on position of the GPR profiles. In c a photo taken from the helicopter the areal extent, no precise information is available on September 23, 2013 with the approximated limit of the snowfield as in (b) and the location of the tectonic discontinuity about the volume of the Triglav glacier and of its later discussed in the text. On the bottom of the photo, several evolution through time. This is indeed a crucial typical glacial morphologies (roche mountonneé) are apparent aspect for any water equivalent estimation and for realistic forecasts about the future evolution of any Centre of the Slovenian Academy of Sciences and glacial body (Bahr et al. 2015). Such problem affects Arts (Gabrovec et al. 2014), although information the glaciology because even if there are several about the area evolution derived by different pho- empirical equations and correlations between the area togrammetric techniques are available since 1897 and volume extension of glaciers, many pitfalls are (Triglav Cˇ ekada et al. 2014). The Triglav glacier has reported, especially for smallest size glaciers (Bahr some similarities in terms of altitude, latitude and et al. 2015). Moreover, even if the volume changes size with other small glaciers and ice patches in the with time can be estimated by both photogrammetric Julian Alps (Colucci 2016). All these ice bodies are and LiDAR techniques (e.g., Triglav Cˇ ekada and generally located on the north facing slopes and Gabrovec 2013), the overall volume of frozen mate- develop on carbonate rocks which are typically rials (and their characteristics) are a more challenging characterized by high albedo owing to light-colored issue. The use of ground penetrating radar (GPR) rock types (Hughes 2007). techniques in glaciological studies at different scales In 1946, when scientific campaigns on the Triglav has a quite long history due to the low electric con- glacier started, it covered an area of 0.144 km2 ductivity of frozen materials, which allows to reach (Verbicˇ and Gabrovec 2002). During the Little Ice investigation depths that would be difficult to reach Age (LIA) the area of the glacier was about 0.4 km2 otherwise (e.g., Arcone 1996). GPR surveys were in (Gabrovec et al. 2014) and it still had an extension of fact traditionally applied to image the ice stratigra- about 0.3 km2 at the beginning of the 1900s and phy, measure the snow/ice thickness and evaluate the values within the range 0.10–0.27 km2 until the end volume of glaciers. On the other hand, GPR studies of the 1970s (Triglav Cˇ ekada et al. 2014). After that focusing on time monitoring of subsurface evolution period the Triglav glacier showed a dramatic and (i.e., 4-D analyses) are still challenging due to logistic continuous shrinking, mainly due to the higher sum- problems and to the varying topography, which mer temperatures and dryer winters. Since the end of makes quite difficult to compare data acquired in the 1970s the glacier had no more movement also different periods. In recent years, some examples on testified by the absence of crevasses (Gabrovec et al. ice caps and glaciers of different size both with ter- 2013). In the last decades, the average summer tem- restrial and airborne surveys have been provided peratures increased significantly (Tosˇic´ et al. 2016) (Machguth et al. 2006; Saintenoy et al. 2013; Colucci and the glacier undergone a marked thinning espe- et al. 2015), but the full potential of GPR for cially in the middle part, because the snow tends to glaciological monitoring is probably still unexploited. accumulate in the lower part of the glacier, where it The main purpose of this work is to estimate the can remain until the next season. Since the mid 1980s volume of the Triglav glacier and evaluate its internal the ice body is divided in some isolated ice patches structure determining the actual presence of ice and separated by rocky outcrops. This is a typical its volume through a dense high resolution GPR behavior of the recession of glaciers resulting in a dataset. Such new information is compared with data transition from ice bodies into ice patches, with already available for the Triglav glacier and for the melting on nonmoving residual ice masses (Serrano other glaciers of the Eastern Alps to make more et al. 2011). In the 2000s, the negative trend stopped constrained evolutional models and insert the glacial even showing a certain area recovery in some years changes in the wider context of climate changes. A C. Del Gobbo et al. Pure Appl. Geophys.

4D analysis of GPR data acquired in 2000 and 2013 2. Methods in the same location is also provided, giving evidence of the strong internal changes of the glacier in the Ground penetrating radar (GPR) is a geophysical analyzed time period. technique based on the transmission of electromagnetic waves (EM) into the ground and on the registration of the reflections and diffractions gener- 1.1. Study Area ated by the subsurface electromagnetic impedance The Julian Alps are located at the Italian-Slove- contrasts. The GPR is a noninvasive and high reso- nian border (Fig. 1) and according to the lution tool, widely used in various fields of International Standardized Mountain Subdivision of application spanning from geology, engineering, and the Alps (Marazzi 2005), which gives a subdivision archeology among the others. The use of the GPR in of the Alpine Chain from the geographic and glaciology is particularly effective, because snow, toponomastic point of view, they belong to the firn and ice, as well as permafrost levels are high section 34 of the south-eastern Alps. Area extends on electrical resistivity materials. In these materials, the 1853 km2. They are predominantly characterized by electromagnetic radiation undergoes a limited atten- carbonate massifs, reaching the highest elevations at uation, allowing a maximum penetration depth of the Mount Triglav (2864 m a.s.l.). several tens or even hundreds of meters, depending The highest parts of the Julian Alps around Mount on the frequencies used, the free water content and Triglav (Fig. 1) are composed of massive Carnian the ice characteristics (Jol 2009). limestone and belong to the so called Zlatna Nappe up Ice, firn and snow are considered dielectrics, to 450 m thick, which is overthrusted on the bedded having an electrical conductivity less than 0.1 mS/m Norian-Rhaetian Dachstein limestone (Hrvatin et al. (Annan and Cosway 1994). Below 0 C the ice 2005;Planicˇar et al. 2009;Sˇmuc and Rozˇicˇ 2009). structure is almost totally devoid of free water and The morphology is dominated by the glaciokarst losses are further limited. If water is present, dis- landscape (e.g., Zˇ ebre and Stepisˇnik 2015) and the persive phenomena could happen, due to the karstic topography provides opportunities for the relaxation of the water at the typical frequencies of buildup of thick snow accumulations in niches and the GPR. This occurs in case of temperate glaciers, hollows, promoting the survival of perennial snow where the component of free water is not negligible and ice. For this reason, the Triglav area is extremely in the warmer periods of the year, determining an important for Slovenia being one of the only exam- increasing of the overall electrical conductivity (and ples of recent glacial and periglacial forms of this so of the EM attenuation) and a change in the sub- Alpine sector where carbonatic landforms of various surface EM velocities. Such strong relation between origins coexist (Hrvatin et al. 2005). EM parameters (especially electrical conductivity The climatology of Triglav area is well estab- and dielectric permittivity) and frozen material lished owing to the presence of the Kredarica weather physical parameters like density and free water con- observatory located at 2515 m.a.s.l. in close proxim- tent, beyond scientific researches has been also ity to the glacierized area (Fig. 1). The mean annual exploited for different practical applications like air temperature (MAAT) averaged over the period avalanche’s forecasts and water equivalent estima- 1981–2010 is -1.0 ± 0.6 C being February the tions (e.g. Godio 2009; Previati et al. 2011; Forte coldest month (-8.1 ± 2.9 C) and July the warmest et al. 2013). one (6.9 ± 1.3 C). Mean annual precipitation On the Triglav glacier, a GPR survey was carried (MAP) is equal to 2071 mm w.e. and mean winter out on July 5 and 6, 2000, when the surface of the snow accumulation (cw) is set in 5.14 m over the glacier was almost completely covered by snow. same period of time. In the Julian Alps precipitation Such data allowed to estimate a maximum thickness is much more abundant on the western side where of about 9.5 m in the central part of the glacier and a MAP[3000 mm w.e. are measured (e.g. Colucci and volume of about 35,000 m3 (Verbicˇ and Gabrovec Guglielmin 2015). 2002). Triglav Glacier Evolution by Means of GPR

We acquired a new GPR dataset on September 23 path previously fixed and measured with the GNSS and 24, 2013 not only to estimate the total volume of system. the Triglav glacier, but also to determine its internal The whole recorded GPR dataset has been pro- layering and obtain detailed information about the ice cessed using a standard, but very effective processing volume variations with time. The time of the year in flow including: DC-removal and time drift correction, which the two surveys were conducted is different but spectral analysis and bandpass filtering, amplitude since the main objective of the research was to esti- recovery, velocity analysis on the diffraction hyper- mate the changes in ice volume and not the whole bolas, topographic (static) corrections, migration and volume change (combined ice and firn), such differ- depth conversion. We used Prism (Radar System) ence is not so relevant, especially as ice was not software for data editing and filtering and SeisSpace outcropping in both measurements. ProMax suite (Halliburton), which is originally We used a ProEx system (Mala˚ Geoscience) developed for reflection seismic data, for velocity equipped with a 500 MHz shielded antenna pair, a analysis and migration. In detail, we used an Ormsby constant 0.18 m offset and a monostatic array. We bandpass filter with corner frequencies equal to choose such antennas after dedicated field tests, 200–250–800–1200 MHz. The upper side of the filter because they give the best tradeoff between reso- was quite smoothed to avoid artifacts in the data due lution and penetration depth. The instrument was to Gibbs effects. connected with an electromechanical odometer for The most crucial steps were the topographic cor- automatic triggering every 0.1 m. The odometer’s rection and the FK time migration aimed to wheel had a rough surface to prevent slippering on reconstruct the correct location and shape of the ice- the frozen surface. A time window of 308 ns, with bedrock contact and of the glacier’s internal struc- a vertical stack of 16 and a sampling interval of tures, also focusing the diffraction hyperbolas. We 0.239 ns has been set. Data has been acquired along fixed a reference plane (datum) at a constant height a grid of 14 sub-parallel profiles, irregularly spaced equal to 2505 m.a.s.l., and then we relocated each and with length from 40 to 82 m (Fig. 1b). These trace at its own correct elevation. No special pro- profiles were along the same paths used in the 2000 cessing algorithms were necessary due to the very GPR campaign, in which the same antennas have high quality of the original data. been used (Verbicˇ and Gabrovec 2002). The 2013 In addition to the geophysical survey, a 1.75 m GPR measuring points have been positioned by real deep snow pit for direct density measurements was time kinematic global navigation satellite system digged contemporary to the GPR data collection, (RTK GNSS) measuring method, which enabled obtaining densities of 375 kg m-3 in the first 20 cm the definition of the same locations as in 2000. and values close to 600 kg m-3 from 30 cm down to A Trimble R8 instrument was used for RTK GNSS the pit bottom. The velocities estimated by the few measurements, applying GPSurvay software for available diffraction hyperbolas fitting have been data editing. In GNSS measurements, the vertical integrated with the velocity calculated from the error is usually greater than positional error, but we density measures in the snow pit, by the Looyenga obtained average positional and vertical errors of a empirical equation (Looyenga 1965) and by the few cm (average errors not greater than 5 cm) even velocity analysis performed on four common mid- in such mountainous environment. By comparing point gather (CMP) acquired on selected positions of the length of the GPR profiles measured by the the glacier. We decided to use a constant 0.20 m/ns odometer with the distances fixed by the GNSS a velocity (corresponding to a density of 600 kg m-3) maximum difference of 1.4 % has been achieved. for the firn layer and a constant 0.17 m/ns velocity This is a quite good result considering the obvious (density equal to 900 kg m-3) for the ice, for both logistic problems intrinsic to both methods when depth conversion and data migration. Obviously, applied on steep frozen surfaces with mountains all some local variations from these velocity values are around, and the possible real small discrepancies expected, but such trend is confirmed also by the between the actual path of the antenna pairs and the migration results (Fig. 2). In fact, using this C. Del Gobbo et al. Pure Appl. Geophys.

Figure 2 Example of an unmigrated (a) and migrated (b) GPR profile (P720). In the migrated version no relevant artifacts due to under- or over- migration effects are present, testifying that the simplified adopted velocity field is accurate enough. The profile is shown without any topographic (static) correction for a better visual clarity simplified velocity field the imaging is quite good most probably related to the snow and firn of the (Fig. 2b) without apparent under-migration (grims) or previous years in which the layering due to the over-migration (smiles) effects. original snow deposition was still detectable; the unit (2) was interpreted as ice, locally encompassing internal debris which is responsible of the EM scat- 3. Results tering. The ice bottom (in yellow on Fig. 3)is characterized by high amplitude quite discontinuous We first analyzed each 2D GPR profile picking reflections and diffractions testifying the presence of the most apparent horizons, i.e., the glacier bottom, local basal moraines and high fractured rocks. the ice top and the topographic surface (Fig. 3b), then After the horizons have been picked, we created integrating the whole dataset with the software Petrel continuous surfaces interpolating all the available (Schlumberger), which is originally developed for geophysical data and using the frozen material sur- reflection seismic data interpretation (Fig. 3c). Two face border as a lateral limit for the interpolation different units characterized by peculiar geophysical (Fig. 4). This procedure is often reported in the lit- signatures have been imaged: (1) a clearly layered erature as 3D interpolation, but it is more appropriate level with several internal sub-parallel horizons, just to name it a pseudo-3D or 2.5-D model (Forte et al. below the topographic surface (where during the 2013) because the dataset is composed by single acquisition there was a 15–20 cm fresh snow layer discrete profiles rather than a full 3D acquisition. having a quite low density equal to 375 kg m-3); (2) Nowadays, the latter is still challenging for GPR an almost transparent unit, without clear internal surveys, especially in areas characterized by difficult layering and with local diffractions. The unit (1) is logistic conditions, like in the present case. Triglav Glacier Evolution by Means of GPR

Figure 3 Example of an uninterpreted (a) and interpreted (b) GPR proﬁle (P721); (c) 3D perspective view of the whole 2013 GPR dataset. The horizons in light blue, dark blue and yellow, respectively, mark the topographic surface, the ﬁrn bottom and the ice bottom

Analyzing both the 2D proﬁles and 3D pseudo maximum depth is close to 8 m (P719) in corre- volume it was possible to estimate the average spondence of a central band, where the bedrock thickness of the glacier, which is about 3 m, while its shows a deepening of several meters. Anyway, such C. Del Gobbo et al. Pure Appl. Geophys.

Figure 4 Thicknesses of ice (a) and firn (b) inferred by the 2013 GPR data. The blue dots mark the location of the fault zone interpreted on the GPR profiles, while ‘‘R’’ indicates a zone where the bedrock was outcropping at the data acquisition time. See text for further details data refers to the whole (ice, firn and snow) measured specific GPR reflector due to the vertical resolution volume, while the ice occurrence is limited in a limits of the used antennas. smaller and thinner zone (Fig. 4a). In fact, the ice reaches its maximum thickness of about 5 m, in the north-western part of the glacier, 4. Discussion where there is an ice patch having an area extension of about 70 m (north–south) by 50 m (east–west). During the LIA the glacier covered an area of The ice extends in the south-eastern part only by a about 0.4 km2. A documented glacier advance occur- narrow band no more than 2–3 m thick. In the red in the 1920s when the glacier was larger than in northernmost (P723 and P724) and southernmost previous years, but at present the glacier almost profiles (P711) the ice is absent. completely melted away and its dramatic shrinkage in At the time of data acquisition, the ice (locally area and volume was especially evident during the containing some internal debris) had a total area of 1980s and the 1990s (Gabrovec et al. 2014). The cli- 3800 m2 and a volume of 7400 m3. The obtained mate sensitivity of the Triglav glacier to summer average thickness is therefore equal to 1.95 m. temperature (Triglav Cˇ ekada and Gabrovec 2013), and Looking at the map of the firn thickness (Fig. 4b), the amount of solar radiation (Gabrovec and Zaksˇek it is possible to note that it has a quite uniform depth 2007) can explain the faster and most dramatic of about 3 m, thinning only near the rocky outcrops. shrinkage of Triglav glacier with respect to other very We did not calculate the snow ? firn volume because small glaciers and ice patches of the Julian Alps, the acquisition has been performed on a smaller area which still preserve thicker ice bodies (Colucci 2016). than the one covered by snow at the moment of the In fact, even if they are located at sensibly lower geophysical survey (Fig. 1b, c). elevation (Montasio glacier 1820–1950 m.a.s.l.; Canin The maximum density in the snow pit (620 glacier 2150–2375 m.a.s.l.) they lie in a more mar- kg m-3) was measured at the bottom (175 cm), fur- itime environment and are more shaded by the ther validating the GPR interpretation. Within the topography. snow pit two centimetric ice lenses at 80 and 120 cm Indeed, even under present and continuous have been found, but they cannot be related to any warming trend, the present phase of slight glacier Triglav Glacier Evolution by Means of GPR stabilization started around mid 2000s and it is be quite different because the area in 2000 caused by a strong increase in winter precipitation, (September 12) was 1.1 ha (Triglav Cˇ ekada and which is counteracting warmer and longer summers Gabrovec 2013) and in 2012 (September 18) 0.4 ha (Colucci and Guglielmin 2015). (Gabrovec et al. 2013). Such apparent discrepancies One intrinsic problem of such interpretive between the estimated area extension and the volu- approach is that any glacier evolution beside meteo- metric variations that occurred in the period rological, climatological and morphological 2000–2013 are mainly due to a great difference in parameters is driven by the ice volume/thickness and terms of frozen units within the ice body. In July the ice type or, in other words, by the frozen materials 2000, the ice was covered by only 20–30 cm of fresh density. In fact, the total water content (i.e., the water snow. In September 2013, GPR data allowed to equivalent) is often even more important than the assess that the ice was instead present just in some glacier extension, especially for temperate low lati- localized areas, under a firn layer about 3 m thick tude glaciers (Huss et al. 2014). In several cases, the (below a snow layer 15–20 cm thick). Estimating the main drawback is a lack of accumulation measure- geometrical parameters of the glacier using some of ments typically obtained by the time-consuming the empirical volume-area relations such as the ones excavation of snow pits. Therefore, if accumulation proposed by Chen and Ohmura (1990) and Bahr et al. data are under-represented, its spatial variability is (1997) and widely applied in glaciology, would pro- often not correctly resolved. To compensate for this duce a great underestimation of ice-loss. drawback, an adequate complement to conventional The noticeable withdrawal phase of the glacier accumulation measurements is necessary, which ended between 2003 and 2007, when even under should preferably be nondestructive and low time- continuous global warming leading to longer and consuming like the GPR (Sold et al. 2015). warmer summers in the south-eastern Alps, a strong In this study, we focused on the ice volume (and increase in winter precipitation produced a series of area) estimation because we hypothesized that the years with positive mass balance. Therefore, even if ice, not having been in contact with the atmosphere in surface melting phenomena prevailed during sum- recent years, has undergone lesser volume changes mer, there was enough accumulation to create a thick compared to the overlying layers of firn and snow. By firn layer at the end of the ablation season and the ice comparing the results of the 2013 GPR survey with was not outcropping anymore. Hughes (2008) the ones obtained in 2000 (Verbicˇ and Gabrovec reported a similar behavior for the Debeli Namet 2002), we observed (Fig. 5) a relevant glacier change glacier (Durmitor massif, Montenegro), which sur- not only in terms of its area as already reported, but vived two of the hottest summers on record in 2003 also on the whole volume and the different frozen and 2007, although it experienced significant retreat. units. From 2000 to 2013, the volume of ice reduced However, during the intervening years (2004–2006), from about 35,000 m3 to about 7400 m3, while the such glacier increased in size and advanced forming average thickness changed from 3 m (maximum new moraines. thickness equal to 9.5 m) to 1.95 m (maximum The main question is what exactly occurred thickness equal to 5 m). before this renewed phase of ice body stability and As deduced from the 2013 geophysical data and overall between 2000 and 2013. We can hypothesize considering that no evidence of ice flow has been two different scenarios. recently detected (the crevasses were visible until the The glacier size reduced until when higher cw 1970s—Gabrovec et al. 2014), the Triglav ice body formed a firn layer that isolated the upper surface of following the international terminology (Cogley et al. the ice, thus stopping the withdrawal phase. In this 2011) should be now reported as a glacieret (Gab- case, we consider the ice-rock contact as an interface rovec et al. 2013) or better, as a glacial ice patch where energy and mass exchanges are negligible and following the classification proposed by Serrano et al. main variations occur on the shallowest portion of the (2011). On the other hand, if we analyze only the data ice body. When the accumulation started to be pre- related to the estimated extension, the results would vailing the ice becomes insulated and is conserved. C. Del Gobbo et al. Pure Appl. Geophys.

Figure 5 Comparison between the GPR profiles acquired along the same path in 2000 (a) and 2013 (b). Profile on a is from Verbicˇ and Gabrovec 2002. Dashed lines mark different horizons: in yellow the 2000 surface (superimposed for a visual comparison also on the 2013 profile), in orange the 2013 surface, in green the snow/firn-ice contact, in blue the top of the bedrock. SF snow and firn, Ic ice, F fault zone Triglav Glacier Evolution by Means of GPR

Otherwise, the reduction of ice does not stop where the ice-bedrock interface is not clearly immediately after the firn layer begins to form; the imaged. In fact, the reflected signal appears to be volume of ice continues to reduce even with the strongly dependent on the quantity of water and presence of an overlying firn layer. Indeed, this is the sediments above the bedrock and by the dip of such usual behavior of temperate ice bodies, as the Triglav interface (Forte et al. 2015). A high reflective ice- glacier is, but the presence of karstified bedrock in an bedrock contact was reported for some Alpine gla- area with relevant tectonic structures is able to ciers and glacierets developed on a carbonatic quickly drain away melt water reducing the possible environment with relevant karstic phenomena (Forte effect of thermal storage systems (e.g., cirque lakes, et al. 2013; Colucci et al. 2015) preventing or lim- stagnation of water in hollows). On the other hand, iting basal water accumulation. The drainage is basal melting phenomena could be locally induced further made easier in highly tectonized areas like even by the presence of cave-karstic systems. In this the Triglav one. About this point, analyzing the ice case, air ventilation would likely be particularly thickness map (Fig. 4b), we can see that the maxi- effective in case of stagnant ice without dynamics, mum thickness is reached in correspondence of a similar to what has been recently observed in some central band having approximately a NW–SE trend. alpine ice caves of Austria (Hausmann and Behm All profiles in this area show several diffraction 2011) as well as of Julian Alps (Colucci et al. 2016). hyperbolas and bedrock deepening. We interpreted Indeed GPR surveys did not highlight localized such zone (blue dots on Fig. 4b) as a tectonic dis- melting phenomena. The area covered by the geo- continuity (Fig. 1b, c). It is interesting how this physical survey allowed us to precisely investigate discontinuity markedly affects the bedrock mor- the ice extension, but the volume of firn would be phology, producing a quite continuous hollow, filled underestimated, since the firn boundary reached areas by ice up to 4–5 m thicker respect to the sur- not covered by the geophysical survey. On the other rounding areas (Fig. 4b). hand, the direct estimate of the glacier area and On the other hand, looking at the map of the firn volume is complicated by the presence of different thickness (Fig. 4b), it is possible to note that it has a materials within the glacial body (ice, firn, snow and quite uniform thickness, with no sharp lateral varia- debris) and by the not easy identification of the tions. It is sub-parallel to the topographic surface, transition between firn and snow, both using geo- thinning only near the rocky outcrops. We hypothe- physical and direct techniques. Similar problems sized that this happens because the ice fills the have been recently discussed by Godio and Rege hollows and flattens the morphologies, producing a (2016) for an extensive GPR survey in the basin of quite flat top surface having a distinct concave shape. Breuil-Cervinia (Western Alps). In addition, ice, firn, Such fact also testifies the dramatic melting phase, snow and debris are not present on the whole and that at present, the avalanches do not play a extension of the glacier, so we decided to separately relevant role in the glacier dynamics. In fact, an draw maps that might represent the location of each eventual area characterized by a remarkable ava- of such materials. lanches contribution should be related to a thicker firn In the GPR profiles, the transition between pure zone finally showing a chaotic internal structure on ice and firn is not always clear and well defined. On the GPR profiles. The continuous reflectors imaged the contrary, the marked contrast in electrical inside the shallowest layers by the GPR seem to be properties of ice and rock generates a high ampli- rather related to the metamorphism of the snow tude signal, typical of the ice-bedrock or firn- redistribution by the wind, which has to have a bedrock interface. The bedrock is also often easy to dominant role. Such result differs from the observa- recognize because it presents diffraction hyperbolas tions in 1960s and 1970s when several avalanches and high reflections. Anyway, this behavior is not were observed reaching the lower part of the glacier reported for all the Alpine glaciers since there are where the slope is less dipping (Gabrovec et al. examples obtained with low frequency antennas 2014). C. Del Gobbo et al. Pure Appl. Geophys.

5. Conclusion general interest because even when the volume of frozen materials does not change (or has just small The Triglav glacier lies in a karst landscape, variations), relevant mass changes can occur. The where vertical karst drainage of subglacial waters is areal extension and the volume are obviously active. This can result in a different functioning of important parameters of any glacier, but their internal glaciers compared to nonkarstic landscapes, but the mass distribution and stratigraphy, especially in the relationship between glaciers and karst is still poorly small glaciers, are even more essential to make known (Zˇ ebre and Stepisˇnik 2015). accurate water equivalent estimations and realistic The goal of this work was the thickness and forecast about their future evolution. These findings volume assessment of the Triglav glacier using GPR are important in areas occupied by small glaciers techniques and analyzing the glacier evolution where geodetic mass balance calculations are per- between 2000 and 2013. The GPR data interpretation formed, and highlight the importance of integrated has allowed us to define: measurements not limited to the area monitoring. • Altitude and morphology of the bedrock which is generally evident thanks to the great amplitude and Acknowledgments the high lateral continuity of the GPR signal. • The elevation of the firn-ice interface, where This research was funded by the ‘‘Finanziamento di present. Such contact is not always imaged because Ateneo per progetti di ricerca scientifica—FRA 2012 the ice is localized only in some portions of the and 2014’’ of the University of Trieste and by analyzed area. research Program No. P6-0101 of the Slovenian • A continuous tectonic structure correlated with the Research Agency. We gratefully acknowledge Hal- rocky outcrops. In correspondence of such discon- liburton and Schlumberger through the University of tinuity, the maximum depth of the glacier has been Trieste academic grants, respectively, for SeisSpace detected. ProMax and Petrel interpretation packages. We also • The maximum depth of the whole frozen body thank the editor in chief Carla Braitenberg, and four (firn ? ice) was equal to approximately 8 m, while anonymous reviewers for their useful suggestions. on average it does not exceed 3 m. • The surface occupied by ice and its volume at the time of data acquisition (September 23 and 24, REFERENCES 2013) the ice itself (which in some areas contains 2 Annan, A.P., & Cosway, S.W. (1994). GPR frequency selection. In rocky debris) had a total area of 3800 m and a Proceeding of the 5th International Conference on Ground 3 volume of 7400 m . The average thickness is Penetrating Radar, (GPR’94) 12–16 June, Kitchener, Ontario, consequently resulted equal to 1.95 m while the Canada, pp. 747–760. maximum observed thickness is equal to 5 m. Arcone, S. A. (1996). High resolution of glacial ice stratigraphy: A ground-penetrating radar study of Pegasus Runway, McMurdo With such conclusions we can confirm that the Station, Antarctica. Geophysics, 61, 1653–1663. Bahr, D. B., Meier, M. F., & Peckham, S. D. (1997). The theo- Triglav ice body cannot be considered as a glacier retical basis of volume-area scaling. Journal of Geophysical any more, but rather a glacial ice patch without evi- Researches, 102(B9), 20355–20362. dence of flow structures, absent since the late 1970s. Bahr, D. B., Pfeffer, W. T., & Kaser, G. (2015). A review of volume-area scaling of glaciers. Reviews of Geophysics, 53(1), Moreover, the comparison between the 2000 and 95–140. 2013 GPR data acquired at the exact same location, Bahr, D. B., & Radic´, V. (2012). Significant contribution to total allowed us to infer the evolution of the ice body. mass from very small glaciers. The Cryosphere, 6, 763–770. Important changes in the internal distribution of the Chen, J., & Ohmura, A. (1990). Estimation of Alpine glacier water resources and their change since the 1870s, hydrology in moun- different materials occurred between the two com- tainous regions, hydrological measurements, the water cycle. In pared years, with a dramatic ice volume reduction of Proceedings of two Lausanne Symposia, August 1990, Interna- about 80 % from 35,000 to 7400 m3. This result has tional Association of Hydrological Sciences, 193, 127–135. Triglav Glacier Evolution by Means of GPR

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(Received October 5, 2015, revised June 30, 2016, accepted July 1, 2016)