Cryoconite as an efficient monitor for the deposition of radioactive fallout in glacial environments

. Cryoconite is extremely rich in natural and artificial radionuclides, but a comprehensive discussion about its 20 ability to accumulate radioactivity is lacking. A characterization of cryoconite from two Alpine glaciers is presented and discussed. Results confirm that cryoconite is among the most radioactive environmental matrices, with activity concentrations exceeding 10,000 Bq kg -1 for single radionuclides. Atomic and activity ratios of Pu and Cs radioactive isotopes reveal that the artificial radioactivity of Alpine cryoconite is mostly related to the stratospheric fallout from nuclear weapon tests and to the 1986 Chernobyl accidents. The signature of cryoconite radioactivity is thus influenced by both local 25 and more widespread events. The extreme accumulation of radioactivity in cryoconite can be explained only considering the glacial environment as a whole, and particularly the interaction between ice, meltwater, cryoconite and atmospheric deposition. Cryoconite is an ideal monitor to investigate the deposition and occurrence of natural and artificial radioactive species in glacial environment.


Introduction
Radioecological research is primarily focused on Earth surface environments, where continuous atmospheric deposition of fallout radionuclides (FRN), both natural and artificial, are accumulated.The most common FRNs are cosmogenic nuclides, the twentieth century, as a consequence of nuclear test explosions and accidents.Hundreds of thousands of PBq were spread in the high troposphere and stratosphere from the 1940s to the 1960s, allowing for global dispersion and contamination at the Earth surface.The extent and impact of FRN deposition on the Earth surface is monitored through the analysis of different environmental matrices, which are used to reconstruct FRN deposition (Steinhauser et al., 2013) and understand their environmental mobility and distribution (Avery et al., 1996;Yasunari et al., 2011).
Among the matrices used in the study of FRNs, those that receive the greatest attention share common features: their composition is directly influenced by airborne deposited impurities; the contribution from environmental compartments other than atmosphere is limited; they are widespread, accessible and preferably easy to sample.Given these attributes, lichens, mosses and peat are commonly used to study the distribution of FRNs and establish depositional inventories (Nifontova, 1995;Kirchner and Daillant, 2002).
In recent years cryoconite began drawing attention in the field of radioactive environmental monitoring as an alternative environmental matrix.Cryoconite is the dark, incoherent sediment that is found on the surface of glaciers worldwide (Takeuchi et al., 2001).It forms exclusively at the ice-atmosphere interface and in presence of abundant meltwater.It can be found as a dispersed material or as a deposit accumulated on the bottom of characteristic water-filled holes melted into ice (cryoconite holes, see Fig. 1).Cryoconite forms out of the interaction between the mineral particles present on the ice surface (both allochthonous and autochthonous), and the complex microbial communities that develop on the surface of glaciers (Cook et al., 2015).Among the microbes that are present on glaciers, cyanobacteria play a major structural role.
During the ablation season, when liquid water is available at the surface of glaciers, cyanobacteria develop films and filaments that promote the formation of aggregates composed of mineral sediments and organic matter, resulting in cryoconite (Takeuchi et al., 2001).The composition of cryoconite is dominated by a mineral component accounting for 85-95 % of its mass, whereas the remnant fraction is comprised of both living and dead organic matter and is responsible for its dark colour (Cook et al., 2015).The formation of cryoconite holes is attributable to the dark colour, and thus low albedo, of cryoconite, which enhances the absorption of incoming solar radiation and locally increases ice melting to foster the development of holes in the ice surface.Due to its contribution to ice surface melting, it's diverse composition, and the role in biodiversity, cryoconite has been studied by a range of disciplines, including glaciology, microbiology, biogeochemistry and ecology.More recently cryoconite has been the subject of renewed interest due to its ability to accumulate specific substances, including anthropogenic contaminants.
To the best of our knowledge, the first evidence of the accumulation of radionuclides in cryoconite was reported in 1996 by Tomadin et al. who found high levels of anthropogenic radioactivity in cryoconite samples from the European Alps.The following year Meese et al. analysed cryoconite formed on the surface of multi-year Arctic sea ice, measuring high radioactivity values.Since these early findings, no other studies (to our knowledge) have focused on the radioactivity of cryoconite until the presentation of an extensive characterization of samples from an Austrian glacier in 2009 (Tieber et al.,

2009
).That study has showed that cryoconite is contaminated not only by 137 Cs, a common artificial radionuclide spread into the environment, but also by several other species, both artificial and natural in origin.The reported activity levels in this study are extremely high, in some cases exceeding 10,000 Bq kg -1 , comparable to soil samples from nuclear incident and explosion sites (Abella et al., 2019;Steinhauser et al., 2014).Subsequent studies, carried out in different regions of global cryosphere, have corroborated the ability of cryoconite to efficiently accumulate radionuclides in the European Alps, the Caucasus, and the Svalbard (Baccolo et al., 2017;Łokas et al., 2016, 2018), with radioecological consequences concerning the presence of FRNs that are not limited to glaciers, but extend to the pro-glacial areas (Łokas et al., 2017).In addition to FRNs, cryoconite has also been shown to accumulate other anthropogenic contaminants including heavy metals (Baccolo et al., 2017;Łokas et al., 2016;Singh et al., 2017), artificial organic compounds (Ferrario et al., 2017;Weiland-Bräuer et al., 2017), and even microplastics (Ambrosini et al., 2019).However, among the species found in cryoconite, radionuclides show by far the highest concentration with respect to other environmental matrices.The considerable activity concentrations of artificial FRNs found in cryoconite, has allowed for the application of cutting-edge radiological techniques, offering important insight to the sources and distribution of radioactivity deposited within the glaciated regions of the Earth (Łokas et al., 2018, 2019), and making novel contributions to the emerging field of environmental nuclear forensics (Steinhauser, 2019).Even though the link between cryoconite and environmental radioactivity is now indisputable, several important questions remain.Firstly, is not clear why and how cryoconite accumulates radioactivity and to what extent this accumulation is related to processes specific to glacial environments.Previous studies (Osburn Jr., 1963;Pourcelot et al., 2003) have focused on the role of snowmelt in accumulating radionuclides in residual snow patches during summer, suggesting that nival and melt processes could encourage local accumulation of radioactivity in a similar fashion to what has been observed in cryoconite on glaciers.Secondly, it is not fully understood where this radioactivity comes from and if its signature is local, regional or even more widespread.This paper aims to present cryoconite as a promising tool for radioecological monitoring in high latitude and high-altitude areas and shed light on some of the open issues related to the themes explored above.Data concerning cryoconite from two European Alpine glaciers are presented and compared in detail to data from previous studies on both cryoconite and other environmental matrices used for radioactive monitoring.

Study site and sampling strategy
The Morteratsch and Forni Glaciers are located in the central sector of the European Alps and are situated ~50 km apart (Fig. 2).Both glaciers are among the biggest and most studied in the Alps.The Morteratsch is a Swiss glacier and the largest in the Bernina range, spanning an altitudinal range of roughly 2000 m and with a terminus located at 2100 m a.s.l..It has an area of ~7.5 km 2 , but until few years ago the value exceeded 15 km 2 due to its connection with a tributary glacier that has since detached.A similar setting characterizes the Forni Glacier, the biggest glacier of the Ortles-Cevedale range in Italy.
Forni is a north-facing valley glacier presenting a glacial tongue that was, until recently, fed by three accumulation basins.
Owing to retreat of the glacier, the connections between the basins have become weaker and the glacier is now fragmented in two distinct parts (Azzoni et al., 2017).The Forni glacier ranges between 2500 to 3750 m a.s.l. and is characterized by a surface area of ~10 km 2 considering both ice bodies.From a climatic perspective, the glaciers are similar; they are both characterized by a continental climate and in the last years experienced a darkening of their ablation zones because of the accumulation of impurities on the ice surface and the progressive emergence of detritus from medial and lateral moraines (Di Mauro et al., 2017;Fugazza et al., 2019).These processes are extremely favourable for the formation of cryoconite, and these two glaciers are two of the most studied in the Alps in terms of cryoconite and its components (Ambrosini et al., 2019;Baccolo et al., 2017;Di Mauro et al., 2017;Fugazza et al., 2019;Pittino et al., 2019).
Samples considered in this work were collected in summer 2015 (July and September) and 2016 (July) from the ablation zones of both glaciers (Fig. 2).Each sample represents a distinct cryoconite hole.The sampling was carried out using clean  pipettes or spoons and samples were kept at 0°C during the field campaign and successively stored at -20°C in the EUROCOLD laboratory of the University Milano-Bicocca, until preparation for the geochemical analyses.Twelve samples have been gathered on the Morteratsch Glacier and ten on the Forni Glacier.Where possible, multiple analyses have been carried out on the same samples, but this has not been always possible, due to limited sample availability.Part of the dataset emerging from the fieldwork on the Morteratsch glacier (data concerning gamma spectroscopy) has already been published (Baccolo et al., 2017), however the remaining samples and results are presented here for the first time.

Radioactivity measurements
The activity of natural and artificial radionuclides present in cryoconite, was measured using a number of techniques. 137Cs, 241 Am, 207 Bi, 40 K and 238 U and 232 Th decay chain nuclides were analysed through γ-spectroscopy about six months after sampling, full details are found in the supplementary material.Aliquots dedicated to γ-spectroscopy consisted in ~1 g of dry material (dried until constant weight at 50°C, 2 mm sieved) sealed in polyethylene vials.The acquisition of the γ-spectra took place at least two weeks after the sealing, to allow the secular equilibrium between 222 Rn and its progenies to be attained.Each sample was counted for about one week using a customized high purity germanium well detector (Ortec).
Details about the instrument, calibration and analytical performances are presented elsewhere (Baccolo et al., 2017).
Aliquots dedicated to Pu analyses (~1 g of dry material) were ashed at 600° C to remove organic matter.The ash was dissolved with mineral acids and the resultant liquid samples underwent radiochemical separation and concentration of Pu isotopes.The procedure is extensively described elsewhere by Łokas and colleagues (2016).Activities of 239+240 Pu and 238 Pu were determined through α-spectrometry after Pu isotopes co-precipitation with NdF3, using Canberra 7401 and Ortec Alpha Duo spectrometers.After further radiochemical purification procedures, the 240 Pu/ 239 Pu atomic ratio was measured through MC-ICP-MS (Thermo Fisher Scientific Neptune spectrometer), in accordance with Łokas et alii (2018). 238U and 232 Th activities were not directly measured but were estimated considering the total content of U and Th.

Instrumental neutron activation analysis
The Th and U composition of cryoconite samples was assessed through instrumental neutron activation.Samples were irradiated at the LENA laboratory, where a TRIGA Mark II nuclear reactor is available for research.The irradiation lasted for six hours under a thermal neutron flux of 2.4 ± 0.2 • 10 12 neutron s -1 cm -2 .To determine the concentration of Th and U in the samples, the following nuclear reactions and γ-emissions were exploited: 232 Th (n,γ) 233 Th → 233 Pa (analyzed emissions: 300.3 and 312.2 keV) and 238 U (n,γ) 239 U → 239 Np (analyzed emissions: 228.2 and 277.6 keV).Each irradiated sample was counted for six hours a few days after the irradiation, using the same well detector applied for γ-spectrometry.The quantification of the elemental concentrations was carried out in accordance to a relative method, comparing irradiated samples with irradiated reference materials.Full details are given in a previous publication (Baccolo et al., 2017).

Carbonaceous content
A thermo-optical analyzer (Sunset Lab Inc. analyzer) was used for the determination of organic and elemental carbon content (OC and EC respectively), following the protocol adopted in Baccolo et al. (2017).Cryoconite samples were suspended on clean quartz fiber filters and analyzed.The mass concentration of OC and EC was obtained combining the information relative to filter superficial concentrations and the mass of cryoconite deposited on the latter, determined using an analytical microbalance (precision 1 µg) which was operated inside an air-conditioned room (T = 20 ± 1 °C; relative humidity = 50 ± 5 %).

Statistics
To evaluate the degree of correlation between the variables and the samples, two multivariate statistical tools were applied.
Multidimensional scaling (MDS) was used to appreciate the degree of correlation between the radionuclides, for an overview of the method and of the required calculation please see the work from Diaconis et al. (2008).MDS was applied to a similarity metric derived from the correlation matrix (Pearson correlation) of the original data, following Eq. 1, where the distance d between two variables v1 and v2 is obtained considering their Pearson correlation coefficient (r).In accordance to this metric distance (van Dongen and Enright, 2012), two perfectly correlated (or anticorrelated) variables (r = ± 1) have a null distance, while two uncorrelated variables (r = 0) have a maximum distance equal to 1.
Eq. 1 The correlation between samples and the differences between the two glaciers were evaluated applying the principal component method to standardized data.The first two components (which explain 65 % of the total variance) were taken into consideration.

Cryoconite natural radioactivity
The ability of cryoconite to accumulate radioactivity is now recognized within a number of previous research efforts and multiple locations around the world (Baccolo et al., 2017;Łokas et al., 2016, 2018;Tieber et al., 2009).Results from the Forni and Morteratsch glacier samples further support this process of accumulation, with anomalously high activities found for the majority of the analysed radionuclides.The common factor shared by the enriched radionuclides is their primary source.Only FRNs, whose distribution is related to atmospheric transport, are actually accumulated in cryoconite, not the lithogenic ones.This feature can be observed in Fig. 3, where the activity of lithogenic radionuclides is presented.A substantial secular equilibrium characterizes the nuclide belonging to the 238 U and 232 Th decay chains, except for 210 Pb (t1/ 2 = 22.3 yr). 210Pb presents an excess with respect to the other 238 U-related nuclides.Excluding this nuclide, the average 238 U and 232 Th chain activities are 70 ± 15 and 52 ± 8 Bq kg -1 respectively for the 238 U chain (Morteratsch and Forni samples, ± standard deviation) and 50 ± 10 and 55 ± 10 Bq kg -1 for the 232 Th chain.These values, as seen in Fig. 3, are comparable to the average 238 U and 232 Th radioactivity of upper continental crust (UCC) reference (Rudnick and Gao, 2003).An analogous situation concerns the primordial radioactive nuclide 40 K (t1/2 = 1.28•10 9 yr).Its activity in the samples from the Morteratsch and Forni glaciers (810 ± 55, 770 ± 200 Bq kg -1 ) is of the same order of magnitude of the average value for UCC  concentration of K, i.e. 720 Bq kg -1 , calculated from the UCC reference (Rudnick and Gao, 2003).This points to a crustal origin for the natural lithogenic radionuclides measured in cryoconite and to the absence of accumulation and/or dilution processes.An exception to this is 210 Pb, which, although being a decay product of 238 U progeny, shows activity levels two orders of magnitude higher than the other 238 U-chain nuclides.The average activities in the samples from the Morteratsch and Forni glaciers are 2,800 ± 800 and 6,200 ± 1,900 Bq kg -1 respectively and are statistically different (Student's t test: p- value < 0.01; degree of freedom = 20; t-value = 5.9) within the two glaciers.Finding such high 210 Pb activities in samples collected on the surface of glaciers is not completely unexpected.It is common to observe an excess of 210 Pb in Earth surface environments, due to its dual source.A fraction of 210 Pb is present in materials of geologic origin because of the internal decay of 238 U progeny (supported 210 Pb); a second fraction (unsupported 210 Pb) is found in samples exposed to the atmosphere and is attributable to the scavenging by precipitation of atmospheric 210 Pb, produced from the decay of the gaseous 222 Rn released into the atmosphere from rocks and soils.Given its relatively long half-life (22.3 yr), precipitated 210 Pb concentrates in surficial environments, but typically its activity doesn't exceed tens or a few hundreds of Bq kg -1 in matrices strongly influenced by atmospheric deposition and rich in organic matter, for which Pb is particularly affine (Strawn and Spark, 2000).In Fig. 4, cryoconite radioactivity is compared to data from literature concerning other environmental matrices.With respect to lichens and mosses, which are known to be efficient in accumulating radioactive atmospheric species (Kirchner and Daillant, 2002), cryoconite shows a 210 Pb activity that is, on average, higher by one order of magnitude.Two hypotheses are made to explain the excess found in cryoconite: 1) the two glaciers considered here are located in areas where the atmospheric deposition of 210 Pb is enhanced; and 2) cryoconite is more efficient at concentrating atmospherically derived radionuclides than lichens and mosses.At the Morteratsch Glacier a comparison has been made between cryoconite and samples collected from the surface of the moraines surrounding the glacier (Baccolo et al., 2017).
The moraine sediments have had a mean activity of 145 ± 30 Bq kg -1 for unsupported 210 Pb, while in cryoconite it has exceeded 2,500 Bq kg -1 .This evidence rejects the first hypothesis: if an anomaly of atmospheric 210 Pb deposition was present in the Morteratsch valley, it should impact both the surface of the moraines and the surface of the glacier.Several studies have reported high unsupported 210 Pb activity in cryoconite from different regions of the Earth (Baccolo et al., 2017;Łokas et al., 2016, 2018;Tieber et al., 2009), suggesting that high 210 Pb activity is related to characteristics of cryoconite and to interactions with processes occurring on the surface of glaciers.

Anthropogenic radioactivity in cryoconite and other environmental matrices
In Fig. 4, the comparison between radioactive contamination of worldwide lichens, mosses, soils and sediments from other studies (full information is found in the supplementary material), is extended to all of the radionuclides that were found in excess in cryoconite in this study.Out of all of them, 210 Pb is the only natural occurring species while the others are anthropogenic in origin.In descending order of average activity in cryoconite, they are: 137 Cs (t1/2 = 30.1 yr), 239+240 Pu (t1/2 = 24,110 and 6,536 yr respectively), 241 Am (t1/2 = 432.2yr), 207 Bi (t1/2 = 31.6yr), 238 Pu (t1/2 = 87.7 yr).These nuclides have been released into the environment as a consequence of nuclear incidents and explosions and have been atmospherically transported and deposited globally.For all radionuclides, the activities measured in cryoconite are always higher than those of other environmental matrices (see Fig. 4).To find samples with activities comparable to the ones found in cryoconite, it would be necessary to consider sites within the vicinity of nuclear tests or incidents.The mean ratios between the activity levels found in cryoconite and in lichens for 137 Cs, 239+240 Pu, 241 Am, 207 Bi, 238 Pu are 9.5, 58, 39, 35, 7 respectively, and the values are even higher when matrices less efficient in accumulating radioactivity are considered.This supports the  The accumulation capability of cryoconite can be observed not only for the most common artificial radionuclides, such as 137 Cs and 239,240 Pu, but also for less abundant species, such as 241 Am, 207 Bi and 238 Pu. 137 Cs is among the most common longlived fission products from 235 U and has been released in the environment due to commercial reactor failures and fission bomb test explosions.The plutonium isotopes 239 and 240 also originate from atmospheric weapon tests and nuclear accidents, but the relative contribution from atmospheric tests is larger, since 239 Pu has been the most common fissile material used in pure fission bombs and for igniting fusion devices.Because of their widespread dispersion, 137 Cs and 239,240 Pu are the most abundant artificial nuclides found in cryoconite.Their mean activities in the samples from Morteratsch and Forni Glaciers are 2,600 ± 3,800 and 1,900 ± 2,900 Bq kg -1 for 137 Cs, 80 ± 75 and 4.9 ± 0.9 Bq kg -1 for 239,240 Pu (average activities and standard deviations for the Morteratsch and Forni cryoconite respectively).Less abundant nuclides are present in cryoconite with lower concentration, including 241 Am (30 ± 35 and 4 ± 1.5 Bq kg -1 ), 207 Bi (9 ± 7 and 6 ± 2 Bq kg -1 ) and 238 Pu (2.5 ± 2.5 and 0.22 ± 0.08 Bq kg -1 ).Despite being low, such activities are still significant and among the highest ever found in the environment with respect to these nuclides.Typical environmental activities usually do not exceed 1 Bq kg -1 for 241 Am and 207 Bi, and 0.1 Bq kg -1 for 238 Pu (Fig. 4), and their rarity is due to their production mechanisms.The presence of 241 Am in the environment is not primarily related to direct deposition (it is present in nuclear power plant spent fuel); it is mostly produced in situ, from the decay of its parent nuclide ( 241 Pu, t1/2 = 14.3 yr), which has been released into the environment alongside other Pu isotopes.Thanks to 241 Pu decay, the environmental activity of 241 Am globally is increasing and will peak around year 2100 (Thakur and Ward, 2018). 238Pu is one of the rarest plutonium isotopes produced by commercial reactors and nuclear explosions, and its diffusion is mostly related to the atmospheric re-entry of satellites powered by pure 238 Pu thermoelectric generators (Łokas et al., 2019) and to a smaller degree by the release from nuclear fuel reprocessing plants into marine environment (Bryan et al., 2008). 207Bi has been released as a consequence of a few high yield thermonuclear explosion tests (Noshkin et al., 2001) and has rarely been observed within the environment.Finding easily detectable activities in cryoconite for these rare radionuclides, many of which have been released decades ago, is both surprising and unprecedented.Studies focused on these rare radionuclides usually require the application of preconcentration and separation procedures, but for cryoconite a direct measure of activity was sufficient.These results highlight the potential of this environmental matrix for radioecological monitoring of glaciated areas.
Looking in detail at the two Alpine glaciers considered here, only the activity of Pu isotopes is significantly different between the two sites, with higher values found in the samples from the Morteratsch Glacier (Student's t test: 0.01 < p-value < 0.02; degree of freedom = 12; t-score = 2.99 for both 238 Pu and 239+240 Pu). 241Am is also more abundant in the samples from the Morteratsch Glacier, but not significantly because of the large standard deviation.The ratios between the mean activity of the Morteratsch and Forni samples are 15.9, 11.6 and 6.8 for 239+240 Pu, 238 Pu and 241 Am respectively.

Sources of anthropogenic radioactivity in cryoconite
To infer the potential sources of the radioactivity found in cryoconite from Alpine glaciers, isotopic and activity ratios between Pu and Cs isotopes have been calculated (Fig. 5).The use of such ratios has been used as an efficient tool for estimating the provenance of environmental radioactivity, since specific signatures are associated to different sources (Steinhauser, 2019).The atomic ratio 240 Pu/ 239 Pu and activity ratio 238 Pu/ 239+240 Pu show that, on average, the plutoniumrelated radioactivity of Morteratsch and Forni cryoconite is compatible with the worldwide signal from global radioactive fallout (Fig. 5a).The latter reflects the composition of the stratospheric Pu reservoir, primarily established in the '60s as a consequence of atmospheric nuclear weapon testing.On average, more than 99 % of the Pu found in cryoconite from the Morteratsch glacier is from global fallout, while for the Forni glacier the average contribution is 95 %, suggesting a nonnegligible influence from the Chernobyl accident (~5 %).
By comparing the 240 Pu/ 239 Pu ratio of global fallout and of modern snow deposited in the Alps (Gückel et al., 2017), it is possible to further discuss the Pu sources in cryoconite.Modern Alpine snow has a slightly higher ratio than global fallout (0.21 vs. 0.18), probably because of the partial influence of re-suspended Chernobyl radioactive fallout, which is more enriched in 240 Pu than global fallout (Ketterer and Szechenyi, 2008).Only two cryoconite samples (one from Forni and one from Morteratsch) show a Pu isotopic composition pointing to the Chernobyl influence.They show a ratio of 0.286 ± 0.006 and 0.24 ± 0.02 respectively, which is even higher than the that of modern Alpine snow.The occurrence of only two samples bearing a partial Chernobyl signature can be explained by the presence of fallout particles from Chernobyl nuclear fuel in these specimens.The non-volatile constituents of nuclear fuel, such as Pu, were not scattered into the environment homogenously, as in the case of the more volatile 137 Cs, but as micrometric and highly radioactive particles (Sandalls et al., 1993).The presence of even one of such particles in the two samples could be sufficient to explain the anomalies.The other samples present a global signature fully compatible with global fallout and not with modern Alpine snow.This implies that Pu accumulated in Alpine cryoconite dates back to when the deposition of radionuclides was dominated by global stratospheric fallout, approximately from 1960 to 1980 (Hirose et al., 2008).The deposition of plutonium which is still occurring on the Alpine snowpack, is too weak to influence the isotopic fingerprint of cryoconite.Pu activity in fresh snow ranges from 0.4 to 11.5 μBq kg -1 (Gückel et al., 2017), roughly six orders of magnitude lower than the average activity of cryoconite, which is 41 Bq kg -1 .This has important implications because it suggests that cryoconite is more influenced by a historic atmospheric fallout rather than the contemporary one, at least when considering Pu.The only source that can provide to cryoconite FRNs from past atmospheric deposition is ice accumulated within the period of maximum deposition of atmospheric radioactivity, when the latter was dominated by global stratospheric fallout.The presence of 207 Bi in cryoconite also supports this hypothesis.In the northern hemisphere this radionuclide was mostly produced during the explosion of the Tzar thermonuclear device in 1961 in Novaja Zemlya (Aarkrog and Dahlgaard, 1984).A few years after this event the 207 Bi atmospheric contamination decreased until reaching non-detectable levels (Kim et al., 1997).If a considerable amount of  Comparing our results with the data obtained for cryoconite collected in the Caucasus and in regions of the Arctic (Łokas et al., 2018, 2019), it is possible to see that there are small variations in the radioactive signatures, pointing to secondary local influences, despite the general features are compatible with global stratospheric fallout (Fig. 5).Caucasian and Arctic samples are characterized by a lower 240 Pu/ 239 Pu atomic ratio than the Alpine samples (Fig. 5a and b).Such a signature is compatible with the influence of weapon grade Pu, depleted in 240 Pu (Cagno et al., 2014).It has been argued that samples from the Caucasus were influenced by the debris spread from the Semipalatinsk Test Site (Kazakhstan), where hundreds of nuclear test explosions have been carried out (Łokas et al., 2018).The effects of high latitude nuclear polygons (Novaya Zemlya) and of the reentry of 238 Pu powered satellites, could explain the non-global fallout contribution observed in the Arctic cryoconite, which is enriched in both 239 Pu and 238 Pu with respect to the Morteratsch and Forni samples (Łokas et al., 2019).The latter, showing a good agreement with the global fallout reference, rule out the possibility that a fraction of the Pu found in Alpine cryoconite was produced during the Algerian atmospheric nuclear tests carried out by France between 1960 and1961.By studying the ratio between 137 Cs and 239+249 Pu activities (Fig. 5c), it is possible to infer the potential sources of 137 Cs, whose activity is by far the highest among the artificial radionuclides found in cryoconite.While global fallout has been demonstrated as the main source of Pu, the same is not for 137 Cs.On average, the 137 Cs found in the Morteratsch Glacier samples associated with global fallout is 41 % with respect to total 137 Cs.For the Forni samples the value is even lower (12 %).The non-global fraction of 137 Cs found in Alpine cryoconite is attributable to the radioactive contamination released during the Chernobyl event.The European Alps, and in particular the Eastern Alps, were among the most heavily impacted areas by Chernobyl fallout, where 137 Cs was one of the main components (Steinhauser et al., 2014).This is confirmed by the radioactive signature of cryoconite from two Austrian Alpine glaciers (Eastern Alps) (Tieber et al., 2009;Wilflinger et al., 2018) whose 137 Cs content is dominated by Chernobyl contamination (more than 90 %).Samples from the Caucasus also show a dominant Chernobyl contribution with respect to 137 Cs, while cryoconite from Svalbard is anomalous in being characterized by a primary influence from global fallout (56 %).This is, however, not unexpected since, among the glaciers considered in Fig. 5, the Waldemarbreen (Svaldbard) is the farthest from Chernobyl.
While Pu has a dominant global source, 137 Cs is related both to global and Chernobyl-related fallouts.Pu, together with the other actinides, is highly non-volatile and its transport mostly takes place through the dispersion of micrometric particles from nuclear fuel, fission and activation products (Sandalls et al., 1993), while Cs is more volatile and its atmospheric mobilization during nuclear accidents requires relatively low temperatures.The Pu contamination from Chernobyl was limited to few hundreds of km from the emission site and could not be efficiently transported for long distances, while 137 Cs transport was widespread, leaving a strong signature all over Europe (Steinhauser et al., 2014), as also supported by the radioactive signature of Alpine cryoconite.

Carbonaceous content
Results of carbon analyses are fully presented in Fig. 6.On average (± standard deviation), the carbonaceous composition of the Morteratsch Glacier samples was 4.7 ± 0.7 % m/m in terms of organic carbon and 0.50 ± 0.25% m/m for elemental carbon.Cryoconite from the Forni Glacier contains a lower concentration of both species: 3.6 ± 0.4 % for organic carbon and 0.2 ± 0.2 % for elemental carbon.Organic carbon content has been converted into organic matter content, following the convention by Pribyl (2010).The mean estimated organic matter concentration for the Morteratsch Glacier is 9.4 %, while for the Forni it is 7.2 %.These values are compatible with data in the wider literature, where organic matter in cryoconite has been reported to vary between 2 and 18 % (Cook et al., 2015).Very limited information is available about the elemental and/or black carbon composition of cryoconite, despite a great deal of attention having been given to the carbonaceous impurities present in snow in relation to their effect on ice/snow surface darkening (Di Mauro et al., 2017).Our results show that elemental carbon is efficiently accumulated in cryoconite with respect to Alpine snow, where typical concentrations are at least four orders of magnitude lower (Jenk et al., 2006).Only contaminated urban soils present an elemental carbon concentration comparable to Alpine cryoconite samples () Lorenz et al., 2006.These findings support the hypotheses by Hodson (2014), who have suggested that cryoconite plays a role in extending the residence time of black and elemental carbon on the surface of glaciers, with implications for the accumulation of hydrophobic contaminants, which have an affinity for these carbonaceous species.

Considering radioactivity as a whole
To analyse possible relationships between the different radionuclides, 2-D multidimensional scaling (MDS) and principal component analysis (PCA) have been applied on our data.The first tool has been used to represent the degree of similarity and dissimilarity between the radionuclides (Fig. 7a).In the MDS 2-D domain, the radionuclides are grouped within three clusters which are interpreted as: 1) artificial radionuclides; 2) 238 U-chain nuclides; 3) 232 Th-chain nuclides.Despite 40 K not belonging to any of these groups, its distance from the 238 U-and 232 Th chain clusters is limited, confirming that K, Th and U in cryoconite are all associated to its lithogenic component.The most isolated of the nuclides is unsupported 210 Pb, in accordance to its peculiar biogeochemical cycle.
MDS is able to highlight the different sources of the radionuclides considered in this study: 1) the artificial radionuclides, whose presence on the glaciers is mostly related to stratospheric fallout; 2) the lithogenic radionuclides which are present in the mineral fraction of cryoconite; 3) and 210 Pb which is deposited onto the glacier from the lower troposphere by precipitation.This partitioning is useful for interpreting the differences observed between the two glaciers considered here.
At the Morteratsch Glacier the activity of the stratospherically derived radionuclides (Pu, Am, Bi) is higher than on Forni Glacier; for 210 Pb the opposite is true (Fig. 4 and Errore.L'origine riferimento non è stata trovata.).This pattern may be related to the altitude of the glaciers.The Morteratsch Glacier basin has a maximum altitude of 4,049 m a.s.l. and an average elevation higher than 3000 m a.s.l., while the Forni basin is delimited by peaks whose maximum altitude spans from 3200 to 3400 m a.s.l. and only occasionally exceed 3500 m a.s.l..The lower altitude could explain the higher amount of 210 Pb found in cryoconite from the Forni Glacier, since the maximum atmospheric scavenging of 210 Pb occurs in the lower troposphere, below 4,000 m (Guelle et al., 1998).In contrast, the Morteratsch basin, given its high elevation, is more exposed to stratospheric fallout, perhaps explaining why the cryoconite from this glacier is highly contaminated with Pu isotopes, 241 Am and 207 Bi.
Results from PCA allow for the distinction of cryoconite sampled from the two glaciers.As seen in Fig. 7b-c, the first two components, mostly the second one, separate the Morteratsch and Forni samples.The nuclides diagnostic for the separation in PC2 are the anthropogenic ones, which define the negative scores of the Morteratsch samples, and unsupported 210 Pb, which is linked to the positive scores of the Forni cryoconite.One sample from the Forni Glacier is an outlier with respect to the others, being characterized by low concentration activity for most of the radionuclides, in particular the artificial ones.

The age of cryoconite and its relationship with ice surface processes
Natural and artificial FRNs are widely used to constrain chronologies in sedimentary environments.Among the nuclides considered in this work, the most common ones applied for dating are 210 Pb and 137 Cs, while 207 Bi and 241 Am have been rarely used to mark the period of maximum FRN deposition from atmospheric weapon tests (Appleby, 2008;Kim et al., 1997).Given the high concentration of radionuclides in cryoconite, it would be interesting to assess if they could be used to estimate the age of cryoconite itself.The most important issue that makes any attempt at dating challenging, is the complete absence of a stratigraphic record in cryoconite.In studying cryoconite it is only possible to obtain a set of distinct and uncorrelated samples.Wilflinger and colleagues (2018)  made: once cryoconite is formed, its radioactive content starts decreasing following the decay law.Based on this hypothesis, cryoconite is viewed as a sort of pure concentrated airborne material which is extremely rich in atmospheric derived contaminants, as FRNs.A highly radioactive sample of airborne sediments extracted from fresh snow, was interpreted as a sort of time-zero reference (a primordial cryoconite material), however no further details were given about this specimen.
Comparing the 210 Pb activity of cryoconite to the reference, the mixing age of the samples was thus inferred.According to this conceptual model, older cryoconite presents lower 210 Pb activity in the light of the fact that the more time has passed from its formation, the more profound should be 210 Pb depletion due to the exponential radioactive decay.The estimated ages ranged from a few years to more than a century (Wilflinger et al., 2018).The glacier considered by Wilflinger et al. (2018) is small (less than 1 km 2 ) and is undergoing significant retreat and fragmentation (Kaufmann et al., 2013).The distribution of cryoconite on glaciers is extremely dynamic and is influenced by meteorological processes, local ice morphology, and supraglacial melting and runoff.It has been observed that within only a few days, single cryoconite holes can form, deepen and collapse (Takeuchi et al., 2018).The transience of surficial glacial environments is also confirmed by glacier moss balls (conglomerations of mineral debris, moss and organic matter forming on the surface of glaciers), whose lifespan was observed not to exceed few years (Hotaling et al., 2019).We thus find it unlikely that a fraction of the We present an alternative hypothesis to link the content of FRNs in cryoconite and its formation age.Our conceptual model arises from an assumption opposite to that of Wilflinger and coauthors (2018): cryoconite is not a static material, its composition changes with time because of the processes taking place on the surface of glaciers.In light of this, the radioactive content of cryoconite is not only subjected to decay, but also to a build-up derived from absorption.
Consequently, the older the cryoconite is, the higher is its 210 Pb content, because it has had a longer time within which to accumulate the radionuclide, which is continuously deposited on the glacier with snow and rain.We hypothesize that the build-up of radioactivity in cryoconite is derived from the interaction between ice, meltwater and cryoconite.During summer ice and snow melt, mobilizing their radionuclide content, including unsupported 210 Pb, which is always present in relatively recent ice (given its lifetime, it is not present at detectable concentrations in ice older than 150-200 years).The interaction between cryoconite and meltwater containing 210 Pb, explains why the latter is always found at high concentrations in cryoconite, regardless of the geographic context.For artificial FRNs the case is different since they are not continuously deposited on the surface of glaciers; however, they are still present in cryoconite with high activities.Each year during the melting season part of the ice dating back to the peak of atmospheric nuclear tests and to major nuclear incidents, melts out, releasing the artificial nuclides which are transported by meltwater.As for unsupported 210 Pb, cryoconite, which only forms if meltwater is available (Cook et al., 2015;Takeuchi et al.,2001), retains such nuclides and accumulate a load of artificial radioactivity even if decades have passed since its original deposition on glaciers.The extreme ability of cryoconite is likely related to the affinity of organic matter and the sticky extra-cellular polymeric substances produced by cyanobacteria for radionuclides (Chuang et al., 2015;Gadd 1996).
One observation might corroborate our hypothesis.Wilflinger et al. (2018) reported about high activity of 7 Be (t1/2 = 53 d) in their samples, of up to 34,000 Bq kg -1 . 7Be is a short-lived cosmogenic radionuclide, deposited from the atmosphere with precipitation.We observed 7 Be within our samples, but we could not properly quantify it because six months passed between sampling and γ-spectrometry.Finding an excess of 7 Be in cryoconite, implies that, given its lifetime, the absorption by cryoconite took place in the weeks just before sampling and not when the cryoconite originally formed.The presence of short-lived nuclides suggests that cryoconite continuously accumulates radioactive species through the interaction not only with meltwater but also with rain, where 7 Be is always present.
According to our interpretation, cryoconite containing higher concentrations of radionuclides should be older than the cryoconite with lower activities.Beyond this, however, we believe it is difficult to attempt a more precise dating of cryoconite through radioactive decay, even if it remains an interesting task.Too many processes are poorly understood to make a rigorous attempt at present, we first should understand the relationships which exist between the formation of cryoconite, the geometry of the glacier, the age and displacement of ice, and in particular the exchanges between ice, meltwater and cryoconite.

Conclusions and future perspectives
We have described the capability of cryoconite to accumulate both artificial and natural FRNs.A comprehensive comparison against other environmental matrices revealed that cryoconite is, excluding samples from nuclear test and incident sites, one of the most radioactive natural substance found in Earth surface environments, with activities for single radionuclide that can exceed 10,000 Bq kg -1 , making cryoconite a potentially hazardous material with respect to many legislations.
Our study focused on cryoconite samples from the European Alps but results from other regions of the global cryosphere confirm our findings, proving that the accumulation of radioactivity is not a local phenomenon, but involves worldwide glaciated areas.The capability of cryoconite to accumulate FRNs is so efficient that it has even allowed for a relatively easy detection of not common FRNs.The high activities detected also made it possible to determine elemental and activity ratios.
Cryoconite is thus an extremely promising tool in the fields of radioecology and environmental nuclear forensics.
The use of diagnostic ratios shed light on the sources of radioactivity found in cryoconite.Our analysis revealed that multiple sources, both regional and global, influenced its radioactive signature.Pu related nuclides (Pu and Am isotopes) revealed a dominant source of their presence to be the global stratospheric fallout, associated with atmospheric nuclear tests carried out in the second half of the 20 th century.In contrast, the major contribution for 137 Cs was determined to have come from the 1986 Chernobyl accident.The capability of recording both regional and planetary events, was also suggested by a comparison with literature data concerning cryoconite from other geographic contexts.Some differences were observed in terms of radioactivity signatures and they could be explained considering the impact of local events.It is important to note that currently no information exist about the radioactivity of cryoconite from the Southern Hemisphere.To build a comprehensive picture of radioactivity in the global cryosphere this is a geographic gap that it would be valuable to close.
There is evidence to suggest that the fundamental process which makes cryoconite a "sponge" for radioactivity, is the interaction between ice and cryoconite itself, through the mediation of meltwater.When glaciers melt, they release and mobilize with meltwater the radionuclides originally preserved in snow and ice layers.Due to the organic matter content and its sticky properties, cryoconite efficiently absorbs the impurities contained in meltwater, in particular those with an affinity for organic substances, including radionuclides.
This study has focused strictly on the glacial environment, ignoring the fate of cryoconite once it is released by glaciers and transported into the downstream ecosystems.It is likely that owing to meltwater discharge, the radioactivity accumulated in cryoconite is promptly diluted, avoiding any health and ecotoxicological risk.However, caution should be taken considering Half a century ago, when nuclear atmospheric testing was a common practice, no one could expect that, thanks to the unique features of glacial environments, the ultimate legacies from those activities would have been maximally concentrated on the surface of glaciers around the world.

Fig. 1
Fig. 1 Cryoconite sampled on the Morteratsch and Forni glaciers.Panels A and B: two cryoconite holes sampled on the surface of the Forni glacier (A) and the Morteratsch glacier (B).Panel C: a snapshot from the Morteratsch glacier at the beginning of the melt season (early July).The ice surface has been recently exposed to the atmosphere after snow melting, and cryoconite is preferentially accumulating within the early meltwater channels.Panel D: a second snapshot from the Morteratsch glacier at the end of the melt season (late September).The ice surface has experienced three months of severe melting and is largely covered by cryoconite by this time.Panel E: this cryoconite deposit was located few meters downstream from a melting Spring snow patch rich in Saharan dust.Cryoconite acted as a filter and separated the dust particles from meltwater, retaining and accumulating them.

Fig. 2
Fig. 2 The geographic setting of the present work.Satellite images (ESA Sentinel-2, dates are reported) of the two glaciers considered in this study (panels A and B).The blue triangles highlight the highest peak point within each of the two catchments, and the blue lines define the ablation areas where cryoconite samples were collected.Panel C: a wider view of the central sector of the European Alps.The black line represents national borders, while the orange line, when not coincident with the black one, represents the Northern-Southern Alpine watershed dividing line.

Fig. 3
Fig. 3 Activity of the radionuclides belonging to the decay chains of 238 U and 232 Th and of 40 K.The upper row (panels A-C) refers to the cryoconite samples from the Morteratsch glacier, the lower ones (panels D-F) to the samples collected on the Forni glacier.Red bars represent detection limits, and green bars measured activities.The activity of 210 Pb was divided into supported (green bar) and unsupported fractions (grey bar), considering the upper 238 U decay chain as reference for the supported fraction.Solid (black) and dashed lines correspond to average and standard deviation activity of the decay chains respectively (not considering 210 Pb) and of 40 K (in this case single sample data are shown), and yellow lines to average upper continental crust activity of 238 U, 232 Th and 40 K, gathered from the average UCC elemental concentrations reported by Rudnick & Gao (2003).

Fig. 5
Fig. 5 Defining the fingerprint of cryoconite radioactivity.Panel A and B: Pu isotopic composition of cryoconite samples (panel B is an enlargement of panel A). 238Pu/ 239+240 Pu is expressed as an activity ratio, and 240 Pu to 239 Pu as an atomic ratio.Panel C: 137 Cs to 239+240 Pu activity ratio of cryoconite.For each glacier the average 137 Cs contribution from global fallout (right x-axis) is shown.Coloured and dashed bars represent the average ratios and the standard deviations respectively.In addition to the Forni and https://doi.org/10.5194/tc-2019-176Preprint.Discussion started: 23 August 2019 c Author(s) 2019.CC BY 4.0 License.

Fig. 6
Fig. 6 Carbonaceous content of cryoconite samples.Panel A refers to organic carbon (and to estimated organic matter), and panel B to elemental carbon.Mean values are depicted alongside standard deviations (coloured and dashed lines).
used 210 Pb to infer the mixing age (intended as an approximate mean age) of cryoconite samples from an Austrian glacier, the Stubacher Sonnblickkees.To attempt the dating, an assumption was https://doi.org/10.5194/tc-2019-176Preprint.Discussion started: 23 August 2019 c Author(s) 2019.CC BY 4.0 License.

Fig. 7
Fig. 7 Multivariate statistical analysis applied to Alpine cryoconite radioactivity data.Panel A: multidimensional scaling applied to the correlation matrix related to different nuclides.Panel B and C refer to scores and loadings of the first two principal components respectively, calculated through principal component analysis.OC is organic carbon; EC is elemental carbon.
https://doi.org/10.5194/tc-2019-176Preprint.Discussion started: 23 August 2019 c Author(s) 2019.CC BY 4.0 License.cryoconite sampled on the surface of a small and steep glacier as the Stubacher Sonnblickkees, could form at the end of the 19 th century and persist there since then.
https://doi.org/10.5194/tc-2019-176Preprint.Discussion started: 23 August 2019 c Author(s) 2019.CC BY 4.0 License.those pro-glacial areas in close proximity to the ice, where the dilution could be limited, and some risks could exist.Given the global relevance of this phenomena, further research should focus on the extra-glacial fate of cryoconite and the contaminants contained within it.