The sources of high airborne radioactivity in cryoconite holes from the Caucasus (Georgia)

Cryoconite granules are mixtures of mineral particles, organic substances and organisms on the surface of glaciers where they decrease the ice albedo and are responsible for formation of water-filled holes. The contaminants are effectively trapped in the cryoconite granules and stay there for many years. This study evaluates the contamination level of artificial and natural radionuclides in cryoconite holes from Adishi glacier (Georgia) and identifies the sources of contamination based on activity or mass ratios among artificial radionuclides. Results revealed high activity concentrations of fallout radionuclides reaching 4900 Bq/kg, 2.5 Bq/kg, 107 Bq/kg and 68 Bq/kg for 137Cs, 238Pu, 239+240Pu and 241Am, respectively. The main source of Pu is global fallout, but the low 240Pu/239Pu atomic ratios also indicated local tropospheric source of 239Pu, probably from the Kapustin Yar nuclear test site. Also, high activity ratios of 241Am/239+240Pu could originate from Kapustin Yar. The natural radionuclides originate from the surrounding rocks and were measured to control the environmental processes. 210Pb in cryoconite granules comes predominantly from the atmospheric deposition, and its activity concentrations reach high values up to 12000 Bq/kg.

glaciers [16][17][18] . In addition, they usually form in ablation zones of glaciers. Also, due to the relatively high concentration of nutrients to the availability of liquid water, the ablation zones represent a hotspot for biodiversity in glacial environments 4,18,20,21 . Micro-fauna found in the cryoconite holes, such as rotifers (Rotifera) and tardigrades (Tardigrada), play the role of grazers and may accumulate pollutants as apex consumers 11,14 . Several papers have been published in recent years on the diversity of the organisms flourishing in cryoconite, on their potential negative impact as pathogens, on the interaction between ice and organisms, on the darkening of ice and on the connection with algal blooms 4,10,[21][22][23][24][25] . Other aspects which were investigated related to the biotechnological and astrobiological potentials of cryoconite holes and their inhabitants [26][27][28] . Polar glaciers are intensively investigated in both Arctic and Antarctic; however, little attention has been given to the mountain glaciers 29,30 , especially in the Eurasian area. Those glaciers are projected to lose 80% of their volume by the end of 2100, and some of them are expected to disappear within decades at current climatic conditions 31 . The best example is the Caucasus glaciers, with mostly glaciological papers published so far [32][33][34] , and only one single paper studies the microorganisms and analyses the total microbial 16 S rRNA gene in cryoconite sediments, ice and gravel 30 . The analysis of radionuclide contaminations in the inland Caucasus glacier (Fig. 1) is essential because the position of Caucasus is near many of the most important sites where nuclear activities are not explored sufficiently. The artificial radionuclides that are studied in this article ( 137 Cs, 238,239,240 Pu, 241 Am, 90 Sr) were introduced into the environment in the second half of the 20th century. However, the estimates of their atmospheric deposition in Eastern Europe are fragmented and inaccurate. Therefore, there is a need for methodical and primary research on the contents of these radionuclides in cryoconite which can preserve information on their atmospheric deposition. Moreover, the Fukushima Power plant accident in 2011 has revived the interest of the researchers and the general public in the consequences of the releases and global spreading of radioactive contamination.
Besides the global fallout from nuclear explosions, which is the result of a worldwide, mainly stratospheric, transport of contaminants, the influence of a tropospheric transport occurring at much shorter distances may also be considered. Isotopic ratios characteristic for the nuclear test site can differ from those for the global fallout. Among the radioactive contaminants, the most dangerous are believed to be alpha emitters ( 241 Am and Pu isotopes) due to their long physical half-life and high biological toxicity. 241 Am originates mainly from the decay of 241 Pu released into the environment, and its activity increases with time 35 . The main objective of this study is to determine the activity concentrations of anthropogenic ( 137 Cs, 238,239,240 Pu isotopes, 241 Am, 90 Sr) and natural radionuclides ( 210 Pb, 230,232 Th and 234,238 U isotopes) and identify their sources on Adishi glacier in the Caucasus.  36 reported activity concentrations of 137 Cs significantly lower (maximum value is 144 Bq kg −1 ).

Results and Discussion
The highest activity concentrations for all considered artificial radionuclides were observed in sample 2, except 90 Sr, where the highest activity concentration was in sample 1. Such high activity concentrations of these radionuclides were reported only in cryoconite samples from other sites (Alpine glaciers, Svalbard glacier) 11,13,40,41 . The ability of cryoconite material to retain and concentrate the airborne radionuclides and metals could be related to metal binding properties of extracellular substances that are excreted by microorganisms (cyanobacteria) to immobilize metallic contaminants 42,43 . In cryoconite holes on Hans Glacier (SW Spitsbergen) along with radionuclides and heavy metals, high densities of micro-animals were detected 11 . In turn, on the edge of Greenland Ice Sheet, low radionuclides content seems to be the result of strong flushing, lack of animals and erosion of granules 14 . On Adishi Glacier, micro-fauna were detected in the holes 30 , and they, along with the presence of granules (mostly cyanobacteria), may store contaminants. Differences in activity concentrations of airborne radionuclides ( 238,239,240 Pu, 241 Am, 90 Sr and 210 Pb) found among the samples seems to reflect the location of sampling sites and the amount of organic matter. Samples 1 and 2 contain the highest value of organic matter (11 and 16%) and 137 Cs, Pu isotopes and 241 Am in these samples were the highest. These sampling points (1 and 2) were located closer to the moraine (Fig. 1), and both sampling points were surrounded by debris covered ice surfaces. Sample 1 resembles typical oval cryoconite hole but sampling point 2 was characterized by the presence of gravel at the cryoconite hole bottom. The highest activity concentrations of 210 Pb are detected in samples 5, 6, 7 and 8. These samples were collected from typical cryoconite holes with dark and dense cryoconite material on the bottom (Fig. 1C,D). The role of the cryoconite systems in sediment transfers downstream the glacier is not fully understood. Regardless of the lifespan of individual cryoconite holes, their collapse does not imply removal of cryoconite from glacier surface as the dispersed cryoconite granules initiate formation of new holes 16 . The cryoconite material could be retained in the porous weathering crust that develops on the surfaces of non-temperate glaciers 44,45 . Migration of supraglacial streams was proposed as the only effective mechanism of cryoconite evacuation from glacier surfaces 4 . Once washed down by a stream, the cryoconite granules enter the supraglacial or subglacial drainage system and, even if deposited in the glacier forefront, they become diluted by the prevailing sediments with low artificial radionuclide contents. Occurrences of the cryoconite-derived material with high radionuclide contents in the glacier forefronts indicate that the cryoconite granules can be retained on the glacier surface or in a deeper ice layer and deposited at the glacier terminus after the ice melts out. Samples 5-8 were collected between ice waterfall and the edge of ice tongue where the flatty surface dominated and higher deposition of 210 Pb was observed. In these samples, Pu isotopes and 241 Am show much lower activity concentrations. The cryoconite granules could disintegrate on the surface of the glacier; therefore, the activities of Pu isotopes, 241 Am and 90 Sr are diluted, while activity concentrations of 210 Pb are much higher due to the long exposure of the material to the atmosphere. Levels of 210 Pb and artificial radionuclides contents in cryoconite granules reflect the temporal patterns of their exposure to atmospheric deposition. Because of the constant delivery of 210 Pb from the atmosphere concentration of this radionuclide in cryoconite material should be proportional to the exposure time, while high concentrations of the artificial radionuclides indicate significant contribution of material that was exposed to the stratospheric or tropospheric fallout. Cryoconite granules have infrequent, often patchy distribution which supports long residence on the ice surface of some glaciers 4,44 . Prolonged exposure of cryoconite to the atmospheric dust may then lead to the build-up of the radionuclide contents to high levels. Using ice cores samples, Segawa et al. 46 proved that cyanobacteria found on glaciers remained unchanged in last 12,500 years. Moreover, Segawa et al. (2018) showed that cyanobacterial population sizes increased during the Holocene 46 , corroborating with our hypothesis regarding the prolonged exposure and increasing accumulation of contaminants.
The artificial radionuclides could originate from various sources mentioned previously; however, the fresh and relatively intense fallout occurs in isolated events. Between them (Pu isotopes, 241 Am, 137 Cs and 90 Sr), mostly traces are deposited from resuspension. In contrast, airborne natural radionuclides, such as 210 Pb, are deposited at a more constant rate. The diminishing content of 210 Pb suggests that the majority of material in samples 1-4 was already removed with melt waters, or these materials can be covered by ice and had no contact with the atmosphere. Samples 1 and 2 had the highest activity concentrations of artificial radionuclides which originate from the original deposition. After this event, these materials can be redeposited in places where the deposition of 210 Pb was limited. In contrast, sampling points 1 and 2 were localised close to the moraine, and this glacial area is usually covered with debris originating from the moraine. Thus, artificial radionuclides may come from moraine material that is removed from the glacier tongue during geomorphological processes and may be stored in water reservoirs in the debris-covered area of the glacier.
The activity ratios of 238 Pu/ 239+240 Pu, 239+240 Pu/ 137 Cs and 241 Am/ 239+240 Pu, as well as atomic ratios of 240 Pu/ 239 Pu, were calculated to distinguish the sources of these radionuclides in cryoconite samples. The correlation factors between plutonium isotopes and 137 Cs and 241 Am concentrations (activity ratios of 238 Pu/ 239+240 Pu, 239+240 Pu/ 137 Cs and 241 Am/ 239+240 Pu) in cryoconite samples are presented in Table 2 (Supplementary data) and Fig. 3A-C and Fig. 4A. The 238 Pu/ 239+240 Pu activity ratios varied between 0.023 ± 0.002 (sample 2) and 0.043 ± 0.005 (sample 7). There are significant differences in activity concentrations between the lowest and the highest values of 239+240 Pu; therefore, linear regression is an appropriate method to evaluate the correlation between investigated isotopes. The 238 Pu and 239+240 Pu activity concentrations are plotted versus each other (Fig. 3A). The slope of the best fit line (R 2 = 0.97) equals 0.023 ± 0.002.
There are two main sources of plutonium in the Northern Hemisphere: global fallout with a 238 Pu/ 239+240 Pu activity ratio of 0.027 (calculated for 2014 47 ) and spent fuel sources (e.g. waste from nuclear fuel reprocessing plants or Chernobyl accident). There is no clear and reasonable way to explain how the waste from reprocessing plants could reach mountain glaciers and influence the composition of cryoconite. The only probable source other than the global fallout from atmospheric tests is the Chernobyl accident, and it is characterized by the 238 Pu/ 239+240 Pu activity ratio of 0.45 in 2014 48 or 0.33 (calculated from data given by Kudryashov et al. 47 ). The activity ratio of 238 Pu/ 239+240 Pu was used to evaluate the percentage of Chernobyl-derived 239+240 Pu activity. The model assumes that the 238 Pu/ 239+240 Pu activity ratios for global fallout and Chernobyl plutonium are about 0.027 and 0.45 (0.33), respectively. Calculations were performed for each of the studied samples, and the fraction of global fallout varied from 96 ± 1% to 100 ± 1%. Differences resulting from using different 238 Pu/ 239+240 Pu activity ratio for Chernobyl contribution (0.45) are approx. 1%, demonstrating that vast majority of the total Pu found in the samples comes from the nuclear weapons explosions (stratospheric global fallout and perhaps local tropospheric fallout) with only negligible fraction which may be attributed to the Chernobyl nuclear accident.
The analysis of atomic ratios of 240 Pu/ 239 Pu provides important information which allows more precise identification of the origin of Pu isotopes in environmental samples. The average atomic ratios of 240 Pu/ 239 Pu from global fallout are about 0.180 ± 0.007, and they depend on weapon design or explosion yield 49,50 . In addition, 240 Pu/ 239 Pu atomic ratios found in cryoconite samples varied from 0.132 ± 0.001 to 0.177 ± 0.003 (Fig. 4B) with an average of 0.163 ± 0.014 (1σ), indicating that there may be two different sources of the plutonium derived from weapon explosions in the analysed cryoconite samples. The measured ratios could be the result of mixing of both the stratospheric and tropospheric fallouts. The presence of the tropospheric fallout can be assumed by the presence of samples with 240 Pu/ 239 Pu atomic ratios close to 0.13, explaining the overall lower than 0.18 values for this ratio, especially those ranging from 0.132 to 0.154. The low 240 Pu/ 239 Pu atomic ratios potentially point to the explosion of a low-yield nuclear devices 51 , where the production of 240 Pu is limited due to a relatively low neutron flux. The Soviet Union conducted a few low-yield, high-altitude nuclear tests in the Kapustin Yar nuclear test site, which is located relatively close to the Caucasus. These nuclear explosions, apparently tested as potential anti-aircraft warheads, may be responsible for this slightly unusual fallout. In addition, the presence of plutonium of such low 240 Pu/ 239 Pu atomic ratio could also potentially partially mask (compensate) the higher ratio Pu of the Chernobyl origin; however, the low 238 Pu/ 239+240 Pu activity ratio shows that the Chernobyl Pu in this case is insignificant. The 239+240 Pu/ 137 Cs activity ratios in the cryoconite samples range between 0.003 ± 0.001 to 0.024 ± 0.003. The slope of the best fit line (R 2 = 0.44) in the 239+240 Pu-137 Cs correlation plot equals 0.016 ± 0.007 (Fig. 3C). The ratios are much lower than the decay-corrected value of 0.031 in the global fallout expected for the year 2014 52 , but the possibility that some of the 137 Cs activity coming from the Chernobyl accident could not be excluded. Because of the long distance from Chernobyl where the ratio between 137 Cs and 239+240 Pu was high, the measurable amount of 137 Cs in the analysed samples did not increase Pu contamination enough to influence the observed 239+240 Pu/ 137 Cs activity ratio. This ratio observed near the Chernobyl zone was estimated as 0.0088 53 . However, over the distance between Chernobyl and Central Europe, the ratio dropped to the level of 10 −5 54 . The radionuclide ratio in the Chernobyl clouds depended on the release history, physical-chemical nature of the released matter and the atmospheric transport conditions, which mainly are the difference in aerosols diameters  transporting Pu and Cs 55 . 239+240 Pu/ 137 Cs activity ratio for analysed samples are also lower than in cryoconite samples from Svalbard 11 , where the ratio varied between 0.011 to 0.030. The 241 Am/ 239+240 Pu activity ratios range between 0.64 ± 0.06 and 1.03 ± 0.09 (Fig. 4A). The slope of the best fit line (R 2 = 0.96) in the 241 Am -239+240 Pu correlation plot equals 0.62 ± 0.05 (Fig. 3B). Such high activity ratios (about 1) was found in the North-western Black Sea 37 , but in this region, 238 Pu/ 239+240 Pu activity ratios and 240 Pu/ 239 Pu atom ratios also exceeded the stratospheric fallout ratio because of the riverine transport mechanism. The transfer of the airborne contaminants in cryoconite can occur only by atmospheric deposition. The 241 Am/ 239+240 Pu activity ratio for global fallout is 0.45, but for the Chernobyl accident, the ratio is 2.2 (calculated for 2014 from data given by Kudryashov et al. 47 ). No data is available on the Am/Pu ratio for Kapustin Yar explosions, but there is no clear reason why it should be higher than for global fallout. Since the presence of plutonium of Chernobyl origin was negligible in the samples, this should be similar for americium.
The mean activity ratios of 90 Sr/ 137 Cs and 90 Sr/ 239+240 Pu were 0.023 ± 0.014 and 2.06 ± 0.96, respectively. These ratios are much lower than the global fallout signatures of 0.641 for 90 Sr/ 137 Cs and 36 for 90 Sr/ 239+240 Pu. Strontium can be quickly removed from cryoconite samples because of its high solubility in water. Apparently, radiocesium and plutonium isotopes have much higher concentrations in cryoconite material then strontium due to the presence of both organic matter and traces of fine mineral fraction. The obtained results are also lower than in cryoconite samples from Arctic glacier 11 . Natural radionuclides. Data on natural radioisotopes ( 210 Pb, 234,238 U, 230,232 Th and activity ratio of 234 U/ 238 U) for the cryoconite samples are presented in Table 3 (Supplementary material) and Fig. 5. Activity concentrations range from 1400 ± 100 to 12000 ± 600 Bq kg −1 for total 210 Pb, 27 ± 2 to 39 ± 3 Bq kg −1 for 234 U, 293 to 44 ± 4 Bq kg −1 for 238 U, 49 ± 4 to 64 ± 5 Bq kg −1 for 230 Th and 35 ± 3 to 54 ± 5 Bq kg −1 for 232 Th. Activity concentrations of natural radionuclides (U and Th isotopes) show little variability between samples and do not differ from values reported for soils globally 56 . Activity concentrations of these lithogenic radionuclides are related to their contents in the source minerals 57 with the exception of 210 Pb which originates from two sources: in situ production from 226 Ra decay products (supported) and from atmospheric deposition (unsupported). The unsupported 210 Pb originates in the atmosphere from radioactive decay of the gaseous 222 Rn, attaches itself to aerosol particles and is finally deposited with both wet and dry precipitation in a similar way to the artificial fallout radionuclides. The highest activity concentration for 210 Pb is in sample 8 (Fig. 2B). Most of the airborne radioactivity of 210 Pb is attached to aerosol particles, and that is why the activities accumulated in cryoconite granules, mosses and lichens, which could be exposed to atmospheric wet/dry deposition for a long time, may reach very high values 58 . Our previous paper 41 showed similar results for the concentration of 210 Pb (from 4000 to 9500 Bq kg −1 ) in cryoconite granules from the Werenskiold glacier (Svalbard, Arctic), while on the neighbouring Hans glacier 11 , they reach up to around 4600 Bq kg −1 . Similarly, high 210 Pb activities were also observed in cryoconite samples from a Swiss glacier, with a maximum of 4200 ± 240 Bq/kg as well as in other environmental matrices 13,58 . The presences of uranium and thorium are the main elements contributing to natural terrestrial radioactivity. Uranium isotopes ( 234 U and 238 U) in terrestrial samples (rocks, soils and sediments) are usually present in radioactive equilibrium. This equilibrium may be biased for samples from marine or freshwater environments. The main reason for radioactivity disequilibrium is greater mobility of 234 U resulting in enriched 234 U concentrations in waters and depleted 234 U/ 238 U activity ratios being observed for submerged solid samples. The main source of uranium in the natural environment is the atmospheric precipitation of terrigenic material, soil resuspension, rock weathering. The concentration of uranium can be increased by human activity (e.g. industry, fossil fuel combustion and industrial sewage) 59 . The value of the 234 U/ 238 U activity ratio in analysed cryoconite granules varies between 0.9 ± 0.1 to 1.1 ± 0.1, suggesting a state of radioactive equilibrium. Few studies have been performed on the equilibrium conditions of the 232 Th decay chain in soils, but such equilibrium may be expected in most natural materials 60 . The activity concentrations of 232 Th are also in agreement with cryoconite samples from Swiss glaciers 13 . Summary. Present study and previously published papers 11,13,40 indicate that cryoconite holes are glacial reservoirs for heavy metals and radionuclides. Thus, new knowledge gaps have appeared; for example, the effect of glacial morphology on effective trapping and storing of radionuclides.
The differences in the concentrations of radionuclides between sampling points and the lack of clear differences in the elevation gradient from terminus towards icefall may reflect the heterogeneous topography of the glacier tongue. The surface of Adishi is wavy, with glacial wells, mills, and ablation forms 32 . There are many small grooves and gorges which may affect the accumulation of radioactivity on the surface.
The combination of the results obtained from different isotopic and mass ratios allowed us to investigate the proportion of Cs, Pu and Am from different sources. The main source of Pu is the global fallout, but the low 240 Pu/ 239 Pu atomic ratios seem to suggest the possibility of another, more local tropospheric source of 239 Pu probably from the Kapustin Yar nuclear test site. High activity ratios of 241 Am/ 239+240 Pu could originate also from the local fallout from Kapustin Yar.

Methods
Sample collection and preparation. The structure and geological features of the Caucasian region of the Black Sea-Caspian Sea are determined by their location between the converging Eurasian and Africa-Arabian lithosphere plates within a zone of continent-continent collision 61 . The Caucasus Mountains located in this area are one of the main centers of mountain glaciation in Europe. The Greater Caucasus mountain range is located along the territory of Georgia, and it is divided into three parts, Western, Central and Eastern Caucasus 33,34 . At present, there are 637 glaciers in Georgia, and contemporary glaciers are mainly concentrated in the Enguri, Rioni, Kodori and Tergi river basins 32,33 . Adishi is a valley glacier with south-western exposition surrounded by roughed mountains (Fig. 1B). The area of Adishi glacier was 10.5 km 2 and tongue was terminated on 2330 m. asl, in 1960. Adishi Glacier covers the area of 9.5 km 2 and the terminus at 2,485 m asl 62,63 . The Glacier is divided into three parts: with the firn valley above 3800 meters, which is surrounded by the high peaks, grandiose icefall (~1000-1300 meters in height) and the classic ice tongue with a terminus at 2,485 m asl. The shape of the glacier changes dramatically from the ice base (2650 m), and the tongue is slightly inclined (~10°-15°) 64,65 . The surface of glaciers is wavy, with numerous glacial wells, mills and ablation forms. There are many small grooves and gorges in its surface formed by the melting water. Observation of the aerial images shows that the amount of weathered material has increased since 1960 32 .
Analytical procedures for all isotopes. 137 Cs and 210 Pb activities were determined using a planar HPGe (high-purity germanium) detector (home-made by the Institute of Nuclear Physics PAS Krakow and electronics by Silena S.p.A.). The activities of 137 Cs were determined using its emission peak at 662 keV, and its emission peak at 46.6 keV was used to determine the activities of 210 Pb. The absolute efficiencies of the detector were determined using calibrated sources and sediment samples of known activity. Also, corrections were made to measure the effect of self-absorption of low-energy γ-rays (46.6 keV) within the sample, although these corrections were insignificant because the masses of the samples were low. The activities of the 238 Pu, 239+240 Pu, 241 Am, 234,238 U, 230,232 Th and 90 Sr dried samples were determined 0.94 and 1.93 g. Organic matter in the samples was decomposed by heating in a Muffle oven at 600 °C for 6 hours.
The samples were dissolved using concentrated HF, HNO 3 , HCl, and a small addition of H 3 BO 3 . Details of the sequential radiochemical procedure used to determine 238 Pu, 239+240 Pu, 241 Am, 234,238 U, 230,232 Th and 90 Sr are described in previous publications 64,65 . Also, the measurements of plutonium and americium isotopes activities were determined using alpha particle spectrometers with passivated planar silicon (PIPS) detectors (Canberra) on a Silena Alphaquattro spectrometer (Silena S.p.A). 90 Sr was measured using a Wallac 1414 Guardian LSC spectrometer for the equilibrated 90 Sr-90 Y fraction after determining the chemical recovery of 85 Sr by gamma-spectrometry. The full sequential radiochemical procedure and gamma analyses were verified using soil reference material produced by the International Atomic Energy Agency (IAEA 447). After the alpha-spectrometric measurements, the Nd(Pu)F 3 alpha-spectrometric sources in the form of polyvinyl chloride filters glued to stainless steel planchettes were removed by immersing them in a small volume of warm water. After the filters were separated from the supporting metal disks, the disks were removed and the water was evaporated to dryness. To the dried filters, 0.5 g of solid H 3 BO 3 and the portions of Aqua Regia (5 ml each) were added and evaporated to dryness. The remainder of the filters was ignited at 450 °C to remove the organic material, and the residue was further attacked with approx. 0.5 ml of concentrated HClO 4 . After evaporation to dryness, the samples were dissolved in 5 ml of concentrated HCl, 10 mg of Fe carrier was added and the samples were transferred to centrifuge tubes followed by the MQ water wash to ensure the quantitative transfer. Pu was pre-concentrated by Fe(OH) 3 precipitation using ammonia solution. The precipitate was then separated from the liquid by centrifuging and the supernatant was discarded. The residual precipitate was dissolved in 9 M HCl and loaded onto an anion exchange resin (Eichrom 1 × 8 Cl form, 100-200 mesh). Pu was retained on the column and washed with 30 ml of 9 M HCl followed by 2 × 15 ml of 8 M HNO 3 . Finally, an additional 10 ml of 9 M HCl was loaded to the column to converted back to Cl form, and Pu fraction was eluted using 30 ml of freshly prepared 9 M HCl/NH 4 I solution. The purified Pu fractions were evaporated to dryness by adding 5 ml of concentrated HNO 3 to remove the excess iodide presence and transferring them into analytical vials using 1 ml of 2% (v/v) nitric acid. The samples were later analysed using Neptune MC-ICP-MS and calibrated with standards of known 239 Pu and 240 Pu concentrations. The reference data for 210 Pb, 137 Cs and 90 Sr activity August 2014 year. The percentage of the global fallout of Pu isotopes versus Chernobyl fallout was estimated using the following formula described by Mietelski and Wąs 66 : where f G represents the fraction of plutonium isotopes from global fallout sources (%), A R represents the 238 Pu/ 239+240 Pu ratio of Chernobyl sources (0.45), A M represents the measured 238 Pu/ 239+240 Pu activity ratio in cryoconite samples and A G represents the 238 Pu/ 239+240 Pu activity ratio of global fallout (0.025).