Cryoconite is rich in natural and artificial
radioactivity, but a discussion about its ability to accumulate
radionuclides is lacking. A characterization of cryoconite from two Alpine
glaciers is presented here. Results confirm that cryoconite is significantly
more radioactive than the matrices usually adopted for the environmental
monitoring of radioactivity, such as lichens and mosses, with activity
concentrations exceeding 10 000 Bq kg-1 for single radionuclides. This
makes cryoconite an ideal matrix to investigate the deposition and
occurrence of radioactive species in glacial environments. In addition,
cryoconite can be used to track environmental radioactivity sources. We have
exploited atomic and activity ratios of artificial radionuclides to identify
the sources of the anthropogenic radioactivity accumulated in our samples.
The signature of cryoconite from different Alpine glaciers is compatible
with the stratospheric global fallout and Chernobyl accident products.
Differences are found when considering other geographic contexts. A
comparison with data from literature shows that Alpine cryoconite is
strongly influenced by the Chernobyl fallout, while cryoconite from other
regions is more impacted by events such as nuclear test explosions and
satellite reentries. To explain the accumulation of radionuclides in
cryoconite, the glacial environment as a whole must be considered, and
particularly the interaction between ice, meltwater, cryoconite and
atmospheric deposition. We hypothesize that the impurities originally
preserved into ice and mobilized with meltwater during summer, including
radionuclides, are accumulated in cryoconite because of their affinity for
organic matter, which is abundant in cryoconite. In relation to these
processes, we have explored the possibility of exploiting radioactivity to date
cryoconite.
Introduction
Radioecological research is primarily focused on the Earth surface, where
continuous atmospheric deposition of fallout radionuclides (FRNs), both
natural and artificial, is accumulated. The most common FRNs are cosmogenic
nuclides, 222Rn progeny and artificial products. The last ones of these
have been released into the environment since the second half of the
20th century, as a consequence of nuclear test explosions and
accidents. Hundreds of thousands of petabecquerels 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, 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 monitoring as an alternative environmental matrix. Cryoconite is
the dark, unconsolidated sediment that is found on the surface of glaciers
worldwide (Takeuchi et al., 2001). It forms exclusively at the
ice–atmosphere interface and in the presence of meltwater. It can be found as a
dispersed material on the ice surface or as a deposit accumulated on the
bottom of characteristic water-filled holes melted into ice, usually
aggregated into granules (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 (Langford et al., 2010). 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
granules (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 color (Cook et al., 2015). The formation of
cryoconite holes is attributable to the dark color, and thus low albedo, of
cryoconite, which enhances the absorption of solar radiation and locally
increases ice melting to foster the development of holes in the ice surface.
Due to its contribution to ice melting, its diverse composition and the
role in biodiversity, cryoconite has been studied by a range of disciplines,
including glaciology, microbiology, biogeochemistry and ecology (Takeuchi et
al., 2001; Langford et al., 2010; Cook et al., 2015; Ferrario et al., 2017;
Pittino et al., 2018a). More recently cryoconite has been the subject of
renewed interest due to its ability to accumulate specific substances,
including anthropogenic contaminants (Pittino et al., 2018b).
Cryoconite from the Morteratsch (a) and Forni (b) glaciers. Panel (c) shows the beginning of the melt season at the Morteratsch glacier. The ice surface
has been exposed to the atmosphere after snow melting, and cryoconite
preferentially accumulates within the early meltwater channels. Panel (d) shows the end of the melt season at the Morteratsch glacier. The ice surface has
experienced months of melting and is now largely covered by cryoconite.
Panel (e) shows a cryoconite deposit (at the Morteratsch glacier) located
few meters downstream from a melting snow patch rich in Saharan dust.
Cryoconite aggregates acted as a filter, retaining and accumulating dust
particles.
To the best of our knowledge, the first evidence of the accumulation of
radionuclides in cryoconite was reported by
Tomadin et al. (1996), who found high levels of anthropogenic radioactivity in
cryoconite samples from the European Alps. Meese et al. (1997) analyzed
cryoconite formed on the surface of multi-year Arctic sea ice, measuring
high radioactivity values. Similar observations were successively reported
by Cota et al. (2006), who founded unexpectedly high radioactivity
burdens (related to 137Cs, 210Pb and 7Be) in cryoconite from
the surface of multi-year sea ice in the Canadian Arctic. Since then 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 137Cs, a common
artificial radionuclide spread into the environment, but also by several
other species, artificial and natural in origin. The reported activity
levels in this study are extremely high and comparable to soil samples from
nuclear incident and explosion sites (Steinhauser et al., 2014; Abella et
al., 2019). Subsequent studies, carried out in different regions of the global
cryosphere (Alps, Caucasus, Svalbard and Canada), corroborated the ability
of cryoconite to accumulate radionuclides (Baccolo et al., 2017; Łokas et
al., 2016, 2018; Owens et al., 2019), with radioecological consequences
concerning the presence of FRNs that extend to the proglacial areas (Łokas et al., 2017; Owens et al., 2019). In addition to FRNs, cryoconite has
also been shown to accumulate other anthropogenic contaminants including
heavy metals (Nagatsuka et al., 2010; Łokas et al., 2016; Baccolo et al.,
2017; Singh et al., 2017; Huang et al., 2019), artificial organic compounds
(Ferrario et al., 2017; Weiland-Bräuer et al., 2017) and 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 have allowed for the application of
cutting-edge radiological techniques, offering important insight into 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, it is not clear
why and how cryoconite accumulates radionuclides and other anthropogenic
impurities and to what extent this accumulation is related to processes
specific to glacial environments. Previous studies (Osburn, 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.
This paper aims to present cryoconite as a promising tool for
radioecological monitoring in high-latitude and high-altitude areas and
sheds light on some of the open issues related to the themes explored above.
Data concerning cryoconite from two Alpine glaciers are presented and
compared 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 European Alps and are
situated ∼50 km apart (Fig. 2). Both
glaciers are among the biggest and most studied in the Alps (Azzoni et al.,
2017; Di Mauro et al., 2019). 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 km2, but until 2015 the value exceeded 15 km2
due to its connection with a tributary glacier that has since detached. A
similar setting characterizes the Forni glacier, the biggest one in 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 parts (Azzoni et al., 2017). The Forni glacier ranges between 2500 and
3750 m a.s.l. and is characterized by a surface area of ∼10 km2 considering both ice bodies. From a climatic perspective, the
glaciers are similar; they are 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 surface and the progressive emergence
of detritus from medial and lateral moraines (Di Mauro et al., 2017; Fugazza
et al., 2019). The abundance of supraglacial debris is favorable for the
formation of cryoconite, and in fact the two glaciers are well studied in
terms of cryoconite and its components (Baccolo et al., 2017; Di Mauro et
al., 2017; Pittino et al., 2018a; Ambrosini et al., 2019; Fugazza et al.,
2019). We have also considered these glaciers because of the relatively easy
access, which makes them ideal sites for annual sampling campaigns.
The geographic setting of the present work. Satellite images (ESA
Sentinel-2) of the considered glaciers (a, b). The highest peak point
within the two catchments is highlighted; the blue lines define the
ablation areas where cryoconite has been collected. Panel (c) shows 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, shows the
northern–southern Alpine watershed dividing line. In the box a map shows
the sites cited in the discussion: Alpine (1), Svalbard (2) and Caucasus (3)
glaciers where cryoconite has been studied from the radiological
perspective (blue dots); Chernobyl (Ukraine, 1); Novaya Zemlya (Russia, 2),
Semipalatinsk (Kazakhstan, 3) and Kapustin Yar (Russia, 4) nuclear testing
areas (red dots).
Samples considered here were collected in summer 2015 (July and September)
and 2016 (July) from the ablation zones of
the glaciers (Fig. 2). Each sample represents a distinct cryoconite
hole. The sampling was carried out using sterile disposable pipettes and
ethanol-cleaned spoons; samples were kept in sterile tubes at 0 ∘C
during the field campaign and stored at -20∘C in the EUROCOLD
laboratory of the University of Milano-Bicocca, until preparation for the
geochemical analyses. We selected the most abundant cryoconite deposits, so
as to also have material available for other analyses: 12 samples have
been gathered on the Morteratsch glacier (between 2100 and 2300 m a.s.l.)
and 10 on the Forni glacier (between 2600 and 2800 m a.s.l.), each one
consisting of 10–40 g of wet cryoconite. Where possible, multiple analyses
have been carried out on the same samples, but this has not been always
possible. Part of the data concerning gamma spectrometry applied to
cryoconite from the Morteratsch samples have already been published (Baccolo
et al., 2017).
Materials and methodsRadioactivity measurements
The activity of natural and artificial radionuclides in cryoconite has been
measured using a number of techniques. 137Cs, 241Am, 207Bi,
40K, and 238U and 232Th decay chain nuclides were analyzed
through γ-spectrometry about 6 months after sampling; full details
are found in the Supplement. Aliquots dedicated to γ-spectrometry consist of ∼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 2
weeks after the sealing, to allow the secular equilibrium between 222Rn
and its progenies to be attained. Each sample has been counted for 1 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) have been ashed at 600 ∘C to
remove organic matter. The ash has been dissolved with mineral acids and the
resultant liquid samples have undergone radiochemical separation and
concentration of Pu. The procedure is extensively described elsewhere by Łokas et al. (2016). Activities of 239+240Pu and 238Pu have
been determined through α-spectrometry after Pu co-precipitation
with NdF3 using Canberra 7401 and Ortec Alpha Duo spectrometers.
After further radiochemical purification, the 240Pu/239Pu atomic
ratio was measured through multicollector inductively coupled mass spectrometry (MC-ICP-MS) (Thermo Fisher Scientific Neptune
spectrometer), in accordance with Łokas et al. (2018). 238U and
232Th activities have not been directly measured but have been
estimated considering the total content of U and Th.
Instrumental neutron activation analysis
The Th and U composition of cryoconite was assessed through
instrumental neutron activation. Samples were irradiated at the LENA
laboratory, where a TRIGA Mark II nuclear reactor is available. The
irradiation lasted for 6 h under a thermal neutron flux of 2.4±0.2×1012 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: 232Th (n, γ) 233Th→233Pa (analyzed emissions: 300.3 and 312.2 keV)
and 238U (n, γ) 239U→239Np (analyzed emissions:
228.2 and 277.6 keV). Irradiated samples were counted for 6 h a
few days after the irradiation, using the same well detector applied for
γ-spectrometry. The quantification of concentrations was
carried out comparing samples and reference materials (Baccolo et al.,
2017).
Carbonaceous content
A thermo-optical analyzer (Sunset Lab Inc. analyzer) has been 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 have been suspended on clean quartz fiber filters and
analyzed. The mass concentration of OC and EC has been 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 has been operated inside an
air-conditioned room (T=20±1∘C; relative humidity
=50±5 %). Mass concentration of OC has been converted into
organic matter content (Pribyl, 2010).
Statistics
To evaluate the degree of correlation between variables and samples, two
multivariate statistical tools have been applied. Multidimensional scaling
(MDS) has been used to appreciate the degree of correlation between the
radionuclides (Diaconis et al., 2008). MDS has been applied to a similarity
metric derived from the correlation matrix (Pearson's correlation
coefficient) of the original data, following Eq. (1), where the
distance d between variables v1 and v2 is obtained considering
their Pearson correlation coefficient (r) (van Dongen and Enright, 2012).
dv1,v2=1-rv1,v22
The correlation between samples and the differences between the two glaciers
have been evaluated applying the principal component analysis (PCA) method to standardized
data. The first two components (which explain 65 % of the total
variance) have been taken into consideration.
Results and discussionCryoconite natural radioactivity
Our results confirm the ability of cryoconite to accumulate radioactivity
and in particular FRNs. In fact only FRNs, whose environmental occurrence is
related to atmospheric transport, are actually accumulated in cryoconite
from the Morteratsch and Forni glaciers, not the lithogenic ones. This can
be observed in Fig. 3, where the activity of lithogenic
radionuclides is presented. A substantial secular equilibrium characterizes
the nuclide belonging to the 238U and 232Th decay chains, except
for 210Pb (t1/2=22.3 years), which presents an excess with
respect to the other 238U-related nuclides. Excluding it, the average
238U and 232Th chain activities are 70±15 and 52±8 Bq kg-1, respectively, for the 238U chain (Morteratsch and Forni
samples, ± standard deviation) and 50±10 and 55±10 Bq kg-1 for the 232Th chain. These values, as seen in Fig. 3, are slightly higher than the average 238U and 232Th
radioactivity of upper continental crust (UCC) reference (Rudnick and Gao,
2003), which is 34 and 43 Bq kg-1 for 238U and 232Th,
respectively. The difference is probably related to the accumulation in
cryoconite of heavy minerals, where U and Th are typically enriched, because
of hydraulic sorting related to meltwater flow (Baccolo et al., 2017). The
activity of the primordial radioactive nuclide 40K (t1/2=1.28×109 years) in the samples from the Morteratsch and Forni
glaciers (810±55, 770±200 Bq kg-1) is of the same order
of magnitude as 40K UCC activity, i.e., 720 Bq kg-1 (Rudnick and
Gao, 2003). Such results point to a typical crustal origin for the
lithogenic radionuclides measured in cryoconite. An exception to this is
210Pb, which, although being a decay product of 238U progeny,
shows activity levels 2 orders of magnitude higher than the other
238U-chain nuclides. The average activities in the samples from the
Morteratsch and Forni glaciers are 2800±800 and 6200±1900 Bq kg-1, respectively, and are statistically different (Student's t test:
t20=5.9; p value < 0.001) within the two glaciers. Finding
such high 210Pb activities in samples collected on the surface of
glaciers is not completely unexpected. It is common to observe an excess of
210Pb in Earth surface environments, due to its dual source. A fraction
of 210Pb is present in materials of geologic origin because of the
internal decay of 238U progeny (supported 210Pb); a second
fraction (unsupported 210Pb) is found in samples exposed to the
atmosphere and is attributable to the scavenging by precipitation of
atmospheric 210Pb, produced from the decay of the gaseous 222Rn
released into the atmosphere from rocks and soils. Given its relatively long
half-life (22.3 years), precipitated 210Pb concentrates in surficial
environments, but typically its activity does not exceed tens or a few
hundred of becquerels per kilogram in matrices strongly influenced by atmospheric
deposition and rich in organic matter, for which Pb shows a particular affinity
(Strawn and Spark, 2000).
Activity of the radionuclides belonging to the decay chains of
238U and 232Th and of 40K. Panels (a), (b) and (c) refer to cryoconite
from the Morteratsch glacier and panels (d), (e) and (f) to cryoconite from the Forni
glacier. Red bars represent detection limits and green bars measured
activities. The activity of 210Pb is divided into supported (green bar)
and unsupported fractions (grey bar), considering the upper 238U decay
chain as reference for the supported fraction. Crustal references are
calculated from Rudnick and Gao (2003).
In Fig. 4, cryoconite radioactivity is compared to data 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 210Pb activity that
is, on average, higher by 1 order of magnitude. Two hypotheses are made to
explain the excess found in cryoconite: (1) the glaciers considered here are
located in areas where the atmospheric deposition of 210Pb is enhanced;
(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 210Pb, while in cryoconite it has exceeded 2500 Bq kg-1. This evidence rejects the first hypothesis: if an anomaly of
atmospheric 210Pb deposition was present in the Morteratsch valley, it
should impact both the moraine and glacial surfaces. Several studies
have reported high unsupported 210Pb activity in cryoconite from
different regions of the Earth (Tieber et al., 2009; Baccolo et al., 2017;
Łokas et al., 2016, 2018), suggesting that high 210Pb activity is
related to specific characteristics of cryoconite.
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 in the Supplement) is extended to all of the
radionuclides that have been found in excess in cryoconite in this study.
Out of all of them, 210Pb is the only natural occurring species while
the others are anthropogenic in origin. In descending order of average
activity in cryoconite, they are 137Cs (t1/2=30.1 years),
239+240Pu (t1/2=24110 and 6536 years, respectively),
241Am (t1/2=432.2 years), 207Bi (t1/2=31.6 years) and
238Pu (t1/2=87.7 years). 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 137Cs, 239+240Pu, 241Am, 207Bi and
238Pu are 9.5, 58, 39, 35 and 7, respectively, and the values are even
higher when matrices less efficient in accumulating radionuclides are
considered. This supports the hypothesis that cryoconite accumulates
atmospherically derived artificial radionuclides more efficiently than other
matrices, as already suggested by exploring unsupported 210Pb.
The accumulation ability of cryoconite can be observed not only for common
artificial radionuclides, such as 137Cs and 239,240Pu, but also
for less abundant species, such as 241Am, 207Bi and
238Pu. 137Cs is among the most common long- lived fission
products from 235U 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 239Pu has been the most common fissile material used in
fission bombs and for igniting fusion devices. Because of their widespread
dispersion, 137Cs and 239,240Pu are the most abundant
artificial nuclides found in cryoconite from the two glaciers, confirming
previous results (Tieber et al., 2009; Łokas et al., 2018, 2019). Their
mean activities in the samples from Morteratsch and Forni glaciers are 2600±3800 and 1900±2900 Bq kg-1 for 137Cs and 80±75 and 4.9±0.9 Bq kg-1 for 239,240Pu (average
activities and standard deviations for the Morteratsch and Forni cryoconite,
respectively). Less abundant nuclides are present in cryoconite with lower
concentration, including 241Am (30±35 and 4±1.5 Bq kg-1), 207Bi (9±7 and 6±2 Bq kg-1) and
238Pu (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. Typical environmental activities usually do not
exceed 1 Bq kg-1 for 241Am and 207Bi and 0.1 Bq kg-1
for 238Pu (Fig. 4), with their rarity being related to their
production mechanisms (Shabana and Al-Shammari, 2001; Bossew et al., 2006).
The presence of 241Am 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 (241Pu,
t1/2=14.3 years), which has been released into the environment
alongside other Pu isotopes. Thanks to 241Pu decay, the environmental
activity of 241Am globally is increasing and will peak around the 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 reentry of satellites powered by pure
238Pu thermoelectric generators (Łokas et al., 2019) and to a smaller
degree by the release from nuclear fuel reprocessing plants into the 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 were released decades ago, is both surprising and
unprecedented. Studies focused on them 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.
Radionuclides presenting anomalously high activities in cryoconite
compared to other environmental matrices. Activity in cryoconite (green
boxes) is compared to data from literature concerning the contamination in
other matrices sampled in surficial environments (yellow boxes). The number
of considered samples is shown in the lower part of each plot. Given the
number of publications from which the displayed data were sourced, they have
been listed individually in the Supplement. All activities were
corrected for decay at June 2017, with the exception of 210Pb, which,
being continuously produced in the atmosphere, did not require any
adjustment.
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: t12=2.99; p value =0.010 for both 238Pu
and 239+240Pu). 241Am is also more abundant in Morteratsch
cryoconite, but not significantly because of the large standard deviation
(Student's t test: t20=2.24; p value =0.018). The ratios between
the mean activity of the Morteratsch and Forni samples are 15.9, 11.6 and
6.8 for 239+240Pu, 238Pu and 241Am, 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 to
estimate the provenance of environmental radioactivity, since specific
signatures are associated with different sources (Steinhauser, 2019). The
atomic ratio 240Pu/239Pu and activity ratio
238Pu/239+240Pu show that the plutonium-related radioactivity of
Morteratsch and Forni cryoconite is compatible with the worldwide signal
from global radioactive fallout (Table 1 and Fig. 5a, b).
The latter reflects the composition of the stratospheric reservoir,
established in the 1960s 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 non-negligible influence
from the Chernobyl accident (∼5 %).
Data about the fraction of Pu and 137Cs related to global
fallout in cryoconite samples from the Morteratsch and Forni glaciers and
from other glaciers. To calculate global fractions, reference ratios defined
by Ketterer et al. (2008), Cagno et al. (2014), Gückel et al. (2017) and
Wilflinger et al. (2018) have been used.
Glacier (sample nr.)Geographical locationPu from global fallout (%)137Cs from global fallout (%)Morteratsch (7)Alps99±241±18this studyForni (6)Alps95±312±6this studyStubacherAlpsn.a.5±4Sonnblickkees (19)Tieber et al. (2009)Hallstätter (8)Alpsn.a.10±8Wilflinger et al. (2018)Adishi (8)Caucasus99±236±19Łokas et al. (2018)Waldemarbreen (9)Svalbard93±456±25Łokas et al. (2019)
By comparing the 240Pu/239Pu 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 resuspended Chernobyl radioactive fallout,
which is more enriched in 240Pu 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 that of modern Alpine snow. The
occurrence of only two samples with 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 137Cs, 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 6 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 past atmospheric
fallout rather than contemporary fallout, at least when considering Pu. The
only source that can provide cryoconite FRNs from past atmospheric
deposition is ice accumulated within the period of maximum deposition of
atmospheric radioactivity, when it was dominated by global
stratospheric fallout. The presence of 207Bi in cryoconite also
supports this hypothesis. In the Northern Hemisphere 207Bi was produced
during the explosion of the Tzar thermonuclear device in 1961 in Novaya
Zemlya (Aarkrog and Dahlgaard, 1984). A few years after this event the
207Bi atmospheric contamination decreased until reaching undetectable
levels (Kim et al., 1997). If a considerable amount of 207Bi is present
in cryoconite, it means that the cryoconite has had the possibility to
interact with ice deposited shortly after 1961.
The fingerprint of cryoconite radioactivity. (a, b) Pu
isotopic composition of cryoconite samples (panel b is an enlargement of
panel a). 238Pu/239+240Pu is expressed as an activity ratio,
240Pu to 239Pu as an atomic ratio. (c)137Cs-to-239+240Pu activity ratio of cryoconite. In addition to the Forni and
Morteratsch glacier samples, data from the Austrian Alps (Stubacher
Sonnblickkees and Hallstätter glaciers; Tieber et al., 2009; Wilflinger
et al., 2018), Svalbard (Waldemarbreen; Łokas et al., 2019) and from the
Caucasus (Adishi glacier; Łokas et al., 2018) are included. Reference
ratios (blue square for global fallout, red square for Chernobyl fallout,
yellow square for weapon-grade Pu, green square for modern Alpine snow) are
from literature (Ketterer et al., 2008; Cagno et al., 2014; Gückel et
al., 2017; Wilflinger et al., 2018). All values are corrected for decay to
June 2017. A geographic setting of the data presented here is found in Fig. 2.
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 variations in the radioactive signatures,
pointing to secondary regional influences, despite the fact that the general features are
compatible with global stratospheric fallout (Fig. 5). Caucasian
and Arctic samples are characterized by a lower 240Pu/239Pu atomic
ratio than the Alpine samples (Fig. 5a and b). Such a
signature is compatible with the influence of weapon-grade Pu, depleted in
240Pu (Cagno et al., 2014). It has been argued that samples from the
Caucasus were influenced by the debris spread from the Semipalatinsk
(Kazakhstan) and Kapustin Yar (Russia) test sites, where hundreds of nuclear
explosions have been carried out (Łokas et al., 2018). The effects of high-latitude nuclear polygons (Novaya Zemlya) and of the reentry of 238Pu-powered satellites explain the non-global fallout contribution observed in
the Arctic cryoconite, which is enriched in both 239Pu and 238Pu
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
in the 1960s.
By studying the 137Cs and 239+249Pu activity ratio (Table 1 and Fig. 5c), it is possible to infer the potential sources of
137Cs, 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 true for 137Cs. On
average, the 137Cs fraction found in the Morteratsch glacier samples
from global fallout is 41 % with respect to total 137Cs. For the
Forni samples the value is lower (12 %). The non-global fraction of
137Cs found in Alpine cryoconite is attributable to the radioactive
contamination released during the Chernobyl event. The Alps, and in
particular the eastern Alps, were among the most heavily impacted areas by
Chernobyl fallout, where 137Cs was a dominant component (Steinhauser et
al., 2014). This is confirmed by the radioactive signature of cryoconite
from two Austrian Alpine glaciers in the eastern Alps (Tieber et al., 2009;
Wilflinger et al., 2018), whose 137Cs content is dominated by Chernobyl
contamination (more than 90 %). Samples from the Caucasus also show a
dominant Chernobyl contribution with respect to 137Cs, 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, Waldemarbreen (Svalbard) is
the farthest from Chernobyl.
While Pu has a dominant global source, 137Cs is related to both 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 volatile and its
mobilization during nuclear accidents requires relatively low temperatures.
The Pu contamination from Chernobyl was limited to a few hundreds of kilometers from
the emission site and could not be efficiently transported for long
distances, while 137Cs transport was widespread, leaving a strong
signature all over Europe (Steinhauser et al., 2014), as is also
supported by Alpine cryoconite.
Carbonaceous content
Results of carbon analyses are presented in Fig. 6. On average
(± standard deviation), the carbonaceous composition of the
Morteratsch glacier samples is 9.4±1.4 % mass ratio for organic matter
and 0.50±0.25 % mass ratio for elemental carbon. Cryoconite from the
Forni glacier contains a lower concentration of both species: 7.2±0.8 % for organic matter and 0.2±0.2 % for elemental carbon.
Values for organic matter are compatible with 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 the effect on ice and snow darkening (Di Mauro et al.,
2017). Our results show that elemental carbon is accumulated in cryoconite
with respect to Alpine snow, where typical concentrations are 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 has 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 and for ice
darkening.
Cryoconite from the Morteratsch glacier presents a higher concentration of
both organic and elemental carbon than that from the Forni glacier
(Student's t test: t19=3.80; p value < 0.001 for organic
carbon concentration; Student's t test t19=3.10; p value =0.003 for elemental carbon). We hypothesize that elevation
has a role in explaining the difference. Cryoconite from the Morteratsch
glacier has been sampled at an elevation between 2100 and 2300 m a.s.l.,
while samples from the Forni glacier have been collected between 2600 and
2800 m a.s.l. A higher elevation implies lower temperatures, a shorter
summer season and thus a less pronounced biochemical activity, which is in
accordance with the lower organic carbon content observed in cryoconite at
the Forni glacier.
Carbonaceous content of Alpine cryoconite: organic carbon (and
estimated organic matter) is shown in panel (a), elemental carbon in panel (b).
Mean values are depicted alongside standard deviations (colored and dashed
lines).
Considering radioactivity as a whole
To analyze possible relationships between the different radionuclides, MDS
and PCA have been applied to our data. The first tool has been used to
represent the degree of similarity and dissimilarity between the
radionuclides (Fig. 7a). In the two-dimensional domain of MDS, the
radionuclides are grouped within three clusters which are interpreted as (1) artificial radionuclides, (2) 238U-chain nuclides and (3) 232Th-chain
nuclides. Despite 40K not belonging to any of these groups, its
distance from the 238U and 232Th chain clusters is limited,
confirming that K, Th and U in cryoconite are all associated with lithogenic
components. The most isolated of the nuclides is unsupported 210Pb, in
accordance with 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
glaciers is mostly related to stratospheric fallout, (2) the lithogenic
radionuclides which are present in the mineral fraction of cryoconite and (3) 210Pb 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 210Pb
the opposite is true (Fig. 4 and Supplement). This
pattern may be related to the altitude of the glaciers. The Morteratsch
glacier basin has a maximum altitude of 4049 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 exceeds 3500 m a.s.l. The lower altitude could explain the
higher amount of 210Pb found in cryoconite from the Forni glacier,
since the maximum atmospheric scavenging of 210Pb occurs in the lower
troposphere, below 4000 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, 241Am and 207Bi.
Another factor that should be considered to explain the stronger
contamination of cryoconite from Morteratsch is the higher concentration of
carbonaceous compounds in cryoconite from this glacier, for which
radionuclides show a particular affinity (Gadd, 1996; Fowler et al., 2010; Kim
et al., 2011; Chuang et al., 2015).
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 Morteratsch samples, and unsupported
210Pb, which
is linked to the positive scores of 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.
Multivariate statistical analysis applied to Alpine cryoconite
radioactivity data. Panel (a) results from multidimensional scaling. Panels (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.
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 here, 210Pb and
137Cs are commonly applied for dating, while 207Bi and 241Am
have been rarely used to mark the period of maximum FRN deposition from
atmospheric weapon tests (Kim et al., 1997; Appleby, 2008). 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 et al. (2018) used 210Pb 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 made: once cryoconite is formed, its radioactive content
starts decreasing following the decay law, regardless of the aggregation and
dissolution processes that affect cryoconite granules (Takeuchi et al.,
2001). Based on this hypothesis, cryoconite is viewed as a sort of pure
concentrated airborne material which is rich in atmosphere-derived
contaminants, such as FRNs, and maintains its composition despite the dynamism of
the supraglacial environment. In Wilflinger et al. (2018) 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 210Pb activity of cryoconite to the reference, the mixing
age of the samples has been thus inferred. According to this conceptual
model, older cryoconite presents lower 210Pb activity in the light of
the fact that the more time that has passed from its formation, the more profound 210Pb depletion due to the exponential radioactive decay
should be. 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 km2) 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, scattering cryoconite granules downstream on the
glacier (Takeuchi et al., 2018). In addition, it is known that cryoconite is
far from being a static sediment: its granules are in fact subjected to
uninterrupted changes, such as aggregation and breakup, and their lifetime
on glaciers does not exceed a few years (Takeuchi et al., 2010). In Antarctica,
where cryoconite holes are usually covered by a permanent ice lid and
supraglacial hydrology is poor, the isolation age (i.e., the time period
during which a single cryoconite hole has remained isolated from glacial
hydrology) of single cryoconite holes has been estimated through a
biogeochemical method: it never exceeds a few years (Fountain et al., 2004;
Bagshaw et al., 2007). The transience of surficial glacial environments is
further 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 a few years (Hotaling et al., 2019). Given
this evidence, we find it unlikely that a fraction of cryoconite sampled
on the surface of a small and steep glacier such as the Stubacher Sonnblickkees
could form at the end of the 19th century and persist there since then
without being subjected to significant compositional changes.
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 et al. (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 buildup derived from continuous accumulation. Consequently, the older
the cryoconite is, the higher its 210Pb content is, 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 buildup of radioactivity in cryoconite is derived from the
interaction between ice, meltwater and cryoconite granules. During summer,
the radionuclide content of ice and snow is mobilized through melting,
including unsupported 210Pb, 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 granules and meltwater containing 210Pb 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 its
artificial nuclide burden which is transported by meltwater. As for
unsupported 210Pb, cryoconite granules retain such nuclides owing to
their biogeochemical properties and accumulate a load of artificial
radioactivity even if decades have passed since its original deposition on
glaciers. The ability of cryoconite is likely related to the presence of
organic matter and extracellular polymeric substances which are affine for
heavy metals, including the radioactive ones (Gadd, 1996; Fowler et al.,
2010; Kim et al., 2011; Chuang et al., 2015). An additional support for the
importance of organic matter in this process is also given by previous
studies showing that the organic fraction of cryoconite and snow algae
accumulates heavy metals associated with anthropogenic atmospheric emissions
(Fjerdingstad, 1973; Nagatsuka et al., 2010; Łokas et al., 2016; Baccolo
et al., 2017; Owens et al., 2019; Huang et al., 2019).
One observation might corroborate our hypothesis. Wilflinger et al. (2018)
reported about high activity of 7Be (t1/2=53 d) in their
samples, up to 34 000 Bq kg-1. 7Be is a short-lived cosmogenic
radionuclide, deposited from the atmosphere with precipitation. We observed
7Be within our samples, but we could not properly quantify it because
months passed between sampling and γ-spectrometry. Finding an excess
of 7Be in cryoconite, implies that, given its lifetime, the absorption
by cryoconite granules took place in the weeks just before sampling and not
when the cryoconite originally formed. The presence of short-lived nuclides
suggests that cryoconite granules continuously accumulate radioactive
species through the interaction not only with meltwater but also with rain,
where 7Be is always present.
According to our interpretation, cryoconite containing higher concentrations
of radionuclides has formed on the glacier before cryoconite, presenting
lower activities. Beyond this, however, we believe it is difficult to
attempt a more precise dating 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 and of cryoconite granules,
the geometry of the glacier, the age and displacement of ice, and in
particular the exchanges between ice, meltwater and cryoconite granules.
Conclusions and future perspectives
We have described the ability 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. Our study is 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 accumulation
of FRNs in cryoconite is so efficient that it has even allowed for a
relatively easy detection of uncommon FRNs. Cryoconite is a promising tool
in the fields of radioecology and environmental nuclear forensics.
The use of diagnostic ratios has shed light on the sources of radioactivity
found in cryoconite. Results show that multiple sources, both regional and
global, influence the radioactive signature of Alpine cryoconite. Pu-related nuclides reveal a dominant source of their presence to be the global
stratospheric fallout from atmospheric nuclear tests carried out in the
second half of the 20th century. In contrast, the major contribution
for 137Cs is determined to have come from the 1986 Chernobyl accident.
The ability to record both planetary and more regional events is also
suggested by a comparison with literature concerning other geographic
contexts. Differences are observed and can be explained considering the
impact of regional events. It is important to note that currently no
information exists about the radioactivity of cryoconite from the Southern
Hemisphere. Building a comprehensive picture of radioactivity in the global
cryosphere 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 impurities in glacial environments, including
radionuclides, 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 binds and accumulates 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 those proglacial areas in close proximity to the ice, where the
dilution could be limited, and some risks could exist. Given the global
relevance of this phenomenon, further research should focus on the
extra-glacial fate of cryoconite and the contaminants contained within it.
Data availability
All data are available in the Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/tc-14-657-2020-supplement.
Author contributions
GB conceived the idea of this study, interpreted the data and wrote the
manuscript with contributions from all the coauthors; RA, GB, BDM,
AF and RSA collected the samples; GB, EŁ, PG and MN performed the
radioactivity measurements and outlined the potential sources of
radioactivity; DM and PP determined the carbonaceous content of
cryoconite; GB, MN and MP carried out neutron activation analysis; CC,
BD, VM and EP helped in the interpretation of the data; VM handled
funding acquisition.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank Herbert Lettner for sharing the data
about cryoconite from Austrian glaciers. We also thank the two reviewers for the useful and constructive comments.
Financial support
This study has been supported by the Project of Strategic Interest NEXTDATA, funded by the Italian National Research Programme PNR 2011-2013, and by the MIAMI (Monitoraggio
Inquinamento Atmosferico della Montagna Italiana) project, funded by “Dipartimento per gli affari regionali e le autonomie della
Presidenza del Consiglio dei Ministri”. Plutonium isotopic analyses have been supported by the Polish National Science Center (grant no. 2016/21/B/ST10/02327).
Review statement
This paper was edited by Ruth Mottram and reviewed by Elizabeth Bagshaw and one anonymous referee.
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