This study reports on the glaciochemistry of a deep ice
core (182 m long) drilled in 2009 at Mount Elbrus in the Caucasus, Russia.
Radiocarbon dating of the particulate organic carbon fraction in the ice
suggests that the basal ice dates to 280±400 CE (Common Era).
Based on chemical stratigraphy, the upper 168.6 m of the core was dated by
counting annual layers. The seasonally resolved chemical records cover the
years 1774–2009 CE, thus being useful to reconstruct many aspects of
atmospheric pollution in south-eastern Europe from pre-industrial times to
the present day. After having examined the extent to which the arrival of large
dust plumes originating from the Sahara and Middle East modifies the chemical
composition of the Elbrus (ELB) snow and ice layers, we focus on the
dust-free sulfur pollution. The ELB dust-free sulfate levels indicate a 6-
and 7-fold increase from 1774–1900 to 1980–1995 in winter and summer,
respectively. Remaining close to 55±10 ppb during the 19th
century, the annual dust-free sulfate levels started to rise at a mean rate
of ∼3 ppb per year from 1920 to 1950. The annual increase
accelerated between 1950 and 1975 (8 ppb per year), with levels reaching a
maximum between 1980 and 1990 (376±10 ppb) and subsequently
decreasing to 270±18 ppb at the beginning of the 21st
century. Long-term dust-free sulfate trends observed in the ELB ice cores
are compared with those previously obtained in Alpine and Altai (Siberia)
ice, with the most important differences consisting in a much earlier onset and a
more pronounced decrease in the sulfur pollution over the last 3 decades
in western Europe than south-eastern Europe and Siberia.
Introduction
It is now well recognized that the present climate change is not only
related to the change of long-lived greenhouses gases but also of short-lived
climate forcers, with one of the most important components being aerosol,
particularly at regional scales. In this way, it has been suggested that
aerosols may have weakened the rate of global warming during the second
part of the last century (Andreae et al., 2005). However, uncertainties
still exist in quantifying the climatic impact of aerosols. The spatial
distribution of aerosols is very heterogeneous and therefore requires
numerous observations to make these parameters useful as inputs and
constraints for transport and chemistry models. An important gap is also
related to the fact that direct atmospheric observations are available only
from long after (starting with the appearance of the acid rain phenomena in
the late 1960s) man-made activities started to impact the
pre-industrial atmosphere. However, to predict future climate, knowledge
of atmospheric changes in aerosol load and composition from the present-day
polluted atmosphere back to pre-industrial times is required. Chemical
records of species trapped in snow deposited on cold glaciers provide a
unique and powerful way to reconstruct past atmospheric chemistry changes
including aerosol load and composition (see Legrand and Mayewski, 1997, for
a review).
In Eurasia, a largely industrialized continent, ice cores were extracted
from high-elevation glaciers located at various places, including the Alps
(Preunkert and Legrand, 2013; Schwikowski et al., 2004), the continental
Siberian Altai (Eichler et al., 2009, 2012; Olivier et al.,
2006), and Kamchatka (Kawamura et al., 2012). In the Alps, intimately
connected to western European emissions, ice cores have been investigated at
Col du Dôme (CDD, Mont Blanc, Preunkert, 2001), Fiescherhorn (Bernese
Alps, Jenk et al., 2006), and Colle Gnifetti (CG) in the Monte Rosa region
(Schwikowski, 2006; Wagenbach et al., 2012) in order to examine various
aspects of atmospheric pollution. The exceptionally high net snow
accumulation at the CDD site permitted the extraction of seasonally resolved
records of various chemical species over the last 100 years (Preunkert and
Legrand, 2013). In older ice layers preservation of winter layers at CDD
becomes very limited and summer layers become very thin. Conversely, the low
and incomplete net snow accumulation rate at CG, which is controlled by wind
erosion, highly limits the preservation of winter ice layers (Wagenbach et
al., 2012) but is low enough to provide access to an extended time period,
at least over the last millennium. Using the EMEP (European Monitoring and
Evaluation Programme) regional chemistry–transport model and past emission
inventories of SO2 in Europe, observed CDD long-term trends of sulfate were
fairly well reproduced, leading Fagerli et al. (2007) to conclude that the
seasonal changes seen at the CDD alpine site are associated with
geographical changes in source regions impacting the site. This is a strong
argument for a separate examination of summer and winter data, extracted
from alpine ice cores. However, until now, Alpine ice cores document only
the last hundred years (at the best back to 1890, Legrand et al., 2018) on a
seasonal basis, whereas the early stage of the industrialization time
period, which is generally considered to have started around 1850, is
missed. An ice core recently extracted from the Elbrus (the highest summit
of the Caucasus) indicated excellent preservation of summer and winter
layers at least back to 1820 (Mikhalenko et al., 2015). Thus, the ELB ice
may contain very valuable information on past atmospheric pollution in
south-eastern Europe since the beginning of industrialization.
Here we report on the glaciochemistry of a deep ice core in 2009 in the
Caucasus, Russia. Seasonally resolved chemical records were obtained back to 1774 (i.e. well prior to the onset of the industrial period). Data are
discussed in two companion papers of which this one. The present paper
examines first of all the impact of large dust plumes, which arrive
sporadically from the Sahara and Middle East, on the chemical composition of the
Elbrus (ELB) snow and ice layers. It then focuses on long-term dust-free
sulfate trends in relation to growing sulfur pollution. The long-term summer
and winter trends of dust-free sulfate are discussed with respect to past
SO2 emissions in south-eastern Europe and compared to those extracted
at the Alpine site of CDD and the Siberian Altai (Belukha glacier) in
relation to SO2 emissions from western and eastern Europe,
respectively. The second paper focuses on the calcium (a dust tracer) long-term
trend (Kutuzov et al., 2019), discussing its past changes in relation
to natural variability, as well as climatic and land use changes in the
dust source regions of the Middle East and North Africa.
Methods and dating
A deep ice core was drilled to bedrock (182.6 m, i.e. 142.1 m water
equivalent, m.w.e.) in 2009 on the western plateau of Mt Elbrus
(43∘21′ N, 42∘26′ E; 5115 m above sea
level) in the Caucasus, Russia (Fig. 1). Glaciological settings of the drill
site are detailed in Mikhalenko et al. (2015). In brief, the surface of the
glacier plateau is about 0.5 km2, and the surface snow accumulation at
the drill site is about 1.5 m.w.e. yr-1. Ice-penetrating radar
measurements made in 2007 and 2009 revealed a maximum glacier thickness of
255±8 m at the central part of the plateau and minimum values of
∼60 m near the western border of the glacier. Borehole
measurements indicated temperatures of -17 ∘C at 10 m depth and
-2.4 ∘C at 181.8 m depth. Occasionally, melting of surface snow can occur; however, the thickness of the infiltration ice layers, which do not
form every year, does not exceed 10 mm. After the overall presentation from
Mikhalenko et al. (2015), two other studies of the ELB ice core were
dedicated to black carbon (Lim et al., 2017) and water stable isotope
composition on the 126 m upper layers (Kozachek et al., 2017).
Discrete subsampling of firn and ice, as well as chemical analysis
Ice cores were subsampled and decontaminated at -15 ∘C using the
electric plane tool methodology described in Preunkert and Legrand (2013).
In brief, in a first step, ice samples were cut with a band saw. After that,
all surfaces of the cut samples were decontaminated by removing
∼3 mm with a pre-cleaned electric plane tool under a clean
air bench. A total of 3724 subsamples were obtained along the upper 168.6 m
(131.6 m.w.e.) of the Elbrus core. As expected from glacier ice flow (e.g.
Paterson and Waddington, 1984), a decrease in the annual ice thickness with
depth is observed (Fig. 2). Annual layer thickness decreases from 1.5 m.w.e.
(0.8 m.w.e. in summer and 0.7 m.w.e. in winter) near the surface to 0.46 m.w.e. at
∼100 m (i.e. 75 m.w.e.) (0.35 m.w.e. in summer and 0.11 m.w.e. in
winter), 0.21 m.w.e. at 155 m (i.e. 120 m.w.e.) (0.12 m.w.e. in summer and 0.09 m.w.e.
in winter), and 0.18 m.w.e. (0.15 m.w.e. in summer and 0.03 m.w.e. in winter) at 157 m
(i.e. 122 m.w.e.) depth. To minimize the loss of temporal resolution with depth
along the core, the sample depth resolution was decreased from 10 cm at the
top to 5 cm at 70 m (47 m w.e.) and 2 cm at 157 m (122 m.w.e.) depth and below.
In this way, an average of 9 summer samples per year were sampled at 157 m
depth (compared to 15 summer samples per year near the surface). The large
decrease in the net snow accumulation in winter below 155 m (Fig. 2), likely
due to more wind erosion of winter than summer snow layers upstream of the
drill site as already observed at other high-altitude glacier sites (e.g.
Preunkert et al., 2000), leads to a more pronounced loss of resolution in
these winter layers compared to the surface layers (12 samples per winter
near the surface and 1–2 samples per winter at 157 m depth).
Map showing the location of the Elbrus site (red star) and dust
sources (orange shading, based on Kutuzov et al., 2019, and references
therein).
For measurements of cations (Na+, K+, Mg2+, Ca2+, and
NH4+), a Dionex ICS-1000 chromatograph equipped with a CS12
separator column was used. For anions, a Dionex 600 equipped with an AS11
separator column was run with a quaternary gradient of eluents (H2O and NaOH at 2.5 and 100 mM, as well as CH3OH). A gradient pump system allows the
determination of inorganic species (Cl-, NO3-, and
SO42-) as well as short-chain carboxylates. Investigated
carboxylates include formate (HCO2-), lactate
(CH3CHOHCO2-), acetate (CH3CO2-), glycolate
(CH2OHCO2-), glyoxylate (CHOCO2-), oxalate
(C2O42-), malate (CO2CH2CHOHCO22-),
malonate (CO2CH2CO22-), succinate
(CO2(CH2)2CO22-), and glutarate
(CO2(CH2)3CO22-). Details on working conditions are
reported in Legrand et al. (2013). For all investigated ions, blanks of the
ice decontamination procedure were found to be insignificant with respect to
the respective levels found in the ice cores.
During the drill operations, an incident occurred at the depth of 31 m and a
fluid (Havoline XLC, Texaco company) was poured in the hole to liberate the
drill device. This has led to the contamination of the firn at 31 m down to the
firn–ice transition located at 55.7 m depth. Samples covering the 1983–1997
years were contaminated for sodium (124±87 ppb compared to 26±28 ppb over the 16 preceding years) and potassium (35±25 ppb compared to 16±15 ppb over the 1966–1982 years). One
core section (denoted ELB-140) that covers winter 1875/1876, summer 1876, and
winter 1876/1877 was not analysed. Finally, a part of the ELB-138 ice core
section that covers winter 1877/1878 was of poor quality (split ice).
(a) and (b) show mean summer (a) and winter (b) half-year ice
thickness along the Elbrus deep ice core. (c) shows mean annual ice thickness and
(d) summer-to-winter ratio of ice thickness.
Annual layer counting
As discussed by Mikhalenko et al. (2015), dating of the Elbrus ice can be
done by annual layer counting on the basis of the stratigraphic ammonium and
succinic acid records, both exhibiting well-marked winter minima. As
previously seen in alpine ice cores, ammonium reveals a well-marked maximum
in summer due to a maximum of NH3 emission together with an efficient
upward transport in summer (Maupetit et al., 1995; Preunkert et al., 2000,
Eichler et al., 2000). Succinic acid is a light dicarboxylic acid for which
a strong summer maximum and a quasi-null winter level can be observed in the
present-day atmosphere in Europe (Legrand et al., 2007). The very low winter
levels of this organic compound are related to the absence of a winter
source of this species, which is mainly photochemically produced from
biogenic precursors. Mikhalenko et al. (2015) assumed a concentration limit
of 100 ppb ammonium and 5 ppb of succinate to separate winter and summer in
the upper layers down to 75.6 m depth (i.e. 1963). To account for an
observed decreasing trend of ammonium concentrations with depth (i.e. due
to a post-1950 increase as also seen for species like nitrate and sulfate;
see below), the ammonium winter criterion was adjusted to 50 ppb between
75.6 and 86.8 m depth (i.e. 1950–1963) and 30 ppb below. Since no
systematic change of succinate with depth is observed, the succinate
concentration limit of 5 ppb was also applied in deeper layers. Requiring
that at least one of the criteria is fulfilled and the value of the other is
below or close to the limit (< 10 % above), the annual counting was
found to be very accurate dating (a 1-year uncertainty) over the last
hundred years when anchored with the stratigraphy of the 1912 Katmai
horizon (Mikhalenko et al., 2015). A good agreement (a 2-year discrepancy)
was also found when comparing this dating with the chronology achieved by
annual layer counting of the water stable 18O isotope back to 1900
(Kozachek et al., 2017). Though the annual counting becomes less evident
prior to 1860, Mikhalenko et al. (2015) reported an ice age of 1825 at 156.6 m depth, which is still consistent with the presence of a volcanic horizon at
around 1833–1840 such as Cosigüina (1835).
We here extended the annual counting down to 168.5 m depth (i.e. 131.6 m.w.e.)
by considering a concentration limit of 30 ppb for ammonium and 5 ppb for
succinate. With that, ice dates to 1774 CE at the depth of 168.5 m. In Fig. 3 we report the obtained chronology for three different sequences including
the deepest one (1774–1784). It could be seen that, in the years prior to
1850, quite often a winter layer is made up of only one or two samples, whereas
summer layers are made up of more than six samples. Below 168.5 m depth,
the ice core quality becomes rather bad (numerous small pieces of broken
ice) rendering subsampling and ice decontamination not evident. Furthermore,
as seen in Fig. 4, in contrast to what is observed above 168.5 m, the 30 subsamples obtained along a 1 m long core section at 176.3–177.3 m depth
reveal an absence of samples with ammonium and succinate concentrations
below the applied winter criterion. This hampered the dating of the basal
ice layers of the core by annual counting, and therefore another dating
approach based on 14C of particulate organic carbon (POC) was made for this
part of the core.
ELB ice chronology at depth intervals of 166.2 to 168.5 m (a, b),
154.4 to 156.5 m (c, d), and 99.8 to 107.3 m (e, f), based on the
ammonium and succinate stratigraphy. Vertical red lines denote yearly
dissection based on identification of winter layers (see Sect. 2.2). For the
two oldest time periods (1773–1782 and 1850–1859), each sample was 2 cm long,
whereas for the most recent time period (1925–1934) one sample was on
average 4 cm long. Note that, although not coherent with the intra-seasonal precipitation distribution (see Kozachek et al., 2017), we here
assumed that the accumulation is equally distributed within the summer and
winter seasons.
Basal ice dating
Four ice samples located along the deepest 6 m of the core (bottom at 182.65 m, 142.1 m.w.e.) were analysed for radiocarbon in particulate organic carbon
(PO14C). The lowest 0.5 m of the core was not analysed due to a large
presence of macro-size inorganic particles. To minimize the time interval
covered by each sample, sample lengths were kept as short as possible with
respect to the detection limit related to working conditions during sampling
and analysis. A typical sample length of 26–40 cm (available ice core
section of 14–21 cm2) was used, leading to an initial ice sample mass of
430–580 g. After having decontaminated the ice sample (using a DOC
decontamination method according to Preunkert et al., 2011), melted ice was
filtered and combusted at 340 ∘C at the Institut de
Géophysique Externe (Grenoble) using the in-line
filtration–oxidation unit REFILOX (Reinigungs-Filtrations-Oxidationssystem,
Hoffmann et al., 2018). Hereby, the resulting ice mass was reduced to
260–320 g containing 4.4 to 6.5 µgC of POC (Table 1). After cryogenic
extraction of the CO2 content, radiocarbon analyses were done at the
accelerator mass spectrometer facility at the Curt Engelhorn Centre for
Archaeometry (CEZA) in Mannheim equipped with a Gas Interface System (GIS)
(Hoffmann et al., 2017). Calibration of the retrieved 14C ages was done
using OxCal version 4.3 (Bronk Ramsey, 1995, 2009).
Distribution of succinate and ammonium concentrations observed in
the deepest ice layers for which annual counting was possible (i.e. above
168.5 m depth, a, b) and 10 m below (c, d). The vertical red dashed bars
denote the values of the winter criteria.
Overview of masses (corrected for blanks but not for
combustion efficiency) and conventional 14C ages of the Elbrus ice core
samples combusted in the REFILOX system. Calibrated date ranges are shown at
68.2 % confidence level and are rounded according to Millard (2014).
To test the reliability of the DOC decontamination method (Preunkert et al.,
2011) for POC analysis, a 340 ∘C REFILOX mass combustion
comparison was made between ultrapure water and decontaminated blank ice (three samples of 200 to 500 mL). To achieve an impurity-free solid ice the
ultrapure water was slowly frozen in polyethylene (PE) foil (Hoffmann et
al., 2018, and references therein). The comparison showed that whatever the
blank ice volume, the blank values were in the same range as the ultrapure
water POC blanks (∼0.4±0.25µgC) which were
determined during the course of the ELB radiocarbon sample measurements.
Thus, the ice decontamination procedure used for DOC ice measurement is also
valid for POC ice decontamination.
Since the CO2 collection line was recently extended to allow sample
pooling, we were now able to directly determine the fraction of modern carbon
(F14C) in blanks done with ultrapure water. A F14C value of 0.71±0.07 was found, measured on three blank samples in total (each
consisting of four pooled samples). This value is in agreement with
F14C blank values found in previous studies (as reviewed and adopted in
Hoffmann et al., 2018). In Table 1, we report ice sample data after blank
correction, including correction of the F14C value as well as
correction of the extracted POC mass with the respective ultrapure water
blank determined before each ice sample extraction.
Using a mean mass-related combustion efficiency of the device of 0.7
(Hoffmann et al., 2018), the mean POC concentration of the four samples
obtained by combustion at 340 ∘C is 25.4±3.1 ngC
g-1 with a highest concentration of 29.0 ngC g-1 for the lowest
sample analysed at 141.6 m.w.e. (182.0 m). These values are in good agreement
with those observed by Hoffmann et al. (2018) in the 340 ∘C POC
fraction of the lowest 8 m.w.e. of a CG ice core (37±16 ngC g-1).
Since a similarity between CG and ELB was also observed for their
pre-industrial black carbon content (Lim et al., 2017), we thus exclude
significant age errors due to a POC contamination during 14C sample
preparation and analysis.
As seen in Table 1, the mean age of the ELB-178-03 sample (1530 yr cal BP,
i.e. 1530 years before 1950) is older than the mean age of sample ELB-181-01
located 2.3 m below the ELB-178-03 sample. However, given age uncertainties,
it is difficult to conclude that, as observed previously at other
mid-latitude glacier sites such as CG (Hoffmann et al., 2018), the ELB
radiocarbon ages do not increase monotonically with depth as would be
expected from well-behaved ice flow. PO14C measurements suggest that
the ELB ice core extends to ∼1670±400 yr cal BP
(Table 1). This is younger than basal ice ages found at Alpine sites, i.e.
∼5000 yr BP for Col du Dome (Preunkert et al., 2019),
∼4000 yr BP (Hoffmann et al., 2018) and > 10 000 yr BP (Jenk et al., 2009) for two CG ice cores, and ∼7000 yr BP for Mt Ortles (3905 m a.s.l.) (Gabrielli et al., 2016). From the observed
temperature gradient in the borehole ELB site, Mikhalenko et al. (2015)
calculated a heat flux at the bottom glacier that is presently 4–5 times
larger than the mean value for the Earth's surface, possibly due to a heat
magma chamber of the Elbrus volcano, leading to potential basal ice melting
when ice thicknesses exceeds 220 m. Though the ice at the drill site and
upstream is at present frozen to bedrock, we can not exclude that in the
past, assuming a more active heat chamber due to the eruption of 50±50 CE (located 1.6 km away from the eastern Elbrus plateau), a temporary
basal ice melting had occurred at the drill site. If so, that may explain
the young age of basal ice at the volcanic crater site compared to other
non-volcanic mountain glaciers. The age of the basal ELB ice is nevertheless
largely greater than expected by ice flow model calculations, estimating a
basal ice age of less than 400 years at the drill site (Mikhalenko et al.,
2015). Figure 5 summarizes the extended depth–age relation, including annual
layer counting back to 1774 CE, the prominent time horizons, and PO14C
data. To interpolate data to a continuous age–depth relation, a
two-parameter fit (based on a simple analytical expression for the decrease in the annual layer thickness with depth) was used (Nye, 1963; Jenk et al.,
2009; Preunkert et al., 2019).
Depth (in m.w.e.) and age relation of the ELB ice core derived from
annual layer counting, prominent time horizons, and mean blank-corrected and
calibrated PO14C data with 1σ age ranges. To interpolate data
to a continuous age–depth relation a two-parameter fit was used following
Nye (1963).
The effect of large dust events on the chemistry of ELB ice
Large dust plumes originating from the Middle East and less frequently from
the Sahara reach the Caucasus (Kutuzov et al., 2013). As seen in the Alps, these
dust events modify the chemistry of snow deposits, in particular with
calcium-rich alkaline snow layers (Wagenbach et al., 1996). Depositions of
these plumes enhance the level of numerous chemical species in Alpine ice
because either they are present in dust at the emission stage or, being
acidic, they were taken up by the alkaline dust material during transport
(Usher et al., 2003). Preunkert (2001) showed that the arrival of dust
plumes at CDD enhanced depositions of several cations (sodium, potassium,
magnesium, and sodium) as well as acidic anions (sulfate, nitrate, chloride,
fluoride, and carboxylates). To identify these layers in the ELB snow and
ice we have estimated the acidity (or alkalinity) of samples by checking the
ionic balance between anions and cations with concentrations expressed in
micro-equivalents per litre, µEq L-1):
1H+=F-+Cl-+NO3-+SO42-+MonoAc-+DiAc2--Na++K++Mg2++Ca2++NH4+,2withMonoAc-=HCO2-+CH3CHOHCO2-+CH3CO2-+CH2OHCO2-+CHOCO2-,3andDiAc2-=C2O42-+CO2CH2CHOHCO22-+CO2CH2CO22-+CO2CH22CO22-+CO2CH23CO22-.
In this work, samples that are below the 25 % quartile of a robust spline
through the calculated raw acidity profile and contain more than 120 ppb of
calcium were considered to be impacted by dust events. In this way, 616 (out of a total of 2524) summer and 67 (out of a total of 1150) winter samples were
considered to contain a large amount of dust. Note that the results are
quite similar when changing the calcium concentration criteria from 120 ppb
to 100 or 140 ppb. Since the frequency of these events has changed over time
(Kutuzov et al., 2019) the significance of their impact on the
deposition of chemical species is examined. Figure 6 shows the mean ionic
budget of samples considered to be contaminated by dust and containing a high and moderate content of calcium compared to samples assumed to be free of dust
events, over the 1950–1980 period. For the largest events (Ca2+
> 30 µEq L-1, Fig. 6a), the increase in calcium,
accompanied by a strong increase in the alkalinity, reaches a factor of 7.4
compared to dust-free samples (Fig. 6c). In addition, this calcium
enhancement is accompanied by an increase of a factor close to 8 for
chloride, sodium, potassium, and magnesium, whereas ammonium, nitrate,
sulfate, and carboxylates are, at the most, enhanced by a factor of 2. When
comparing dust samples containing weaker calcium contents (i.e. Ca2+ < 30 µEq L-1, Fig. 6b) with dust-free samples (Fig. 6c),
in addition to the increase in calcium (factor of 2) the most significant
changes are seen for magnesium (×1.5), sodium and chloride (×1.6), and
potassium (×1.3).
Mean ionic content of ELB layers deposited between 1950 and 1980.
textbf(a) and (b) mean composition of dust event samples containing more and less
than 600 ppb (i.e. 30 µEq L-1) of calcium. (b) and (c) mean
composition of dust event samples containing less than 600 ppb of calcium
compared to samples free of dust. Abbreviations Monoac. and Diac. stand for
C1–C3 monocarboxylates and C2–C5 dicarboxylates,
respectively (see Eqs. 2 and 3 in Sect. 3).
To evaluate the effect of dust events on the deposition of chemical species,
we compare in Table 2 averaged chemical concentrations of all samples with
those not impacted by dust (denoted Xred.). Averages were obtained on
the base of half-year summers over the half-decades 1996–2000 and 1974–1978
characterized by high and low dust content, respectively (Kutuzov et al.,
2019). Though the main impact of dust is as expected, on cations
(except ammonium) and chloride, i.e. the constituents of dust particles, the
impact is also significant for acidic species like nitrate and sulfate. For
instance, the increase in nitrate from 1974–1978 to 1996–2000 CE (144 ppb) in
the whole data set is largely (three quarters) related to the increase in
dust as indicated by the smaller increase after removal of dust samples
(increase of 31 ppb for the NO3-red. value). Same is true
for sulfate, for which the apparent increase between the two periods (90 ppb) is not due to an increase in pollution but of dust, as indicated by the
drop of values when the dust-free data set is considered (decrease of 48 ppb
for the SO42-red. value). The effect of changing dust
inputs over time therefore has to be considered when discussing long-term
trends in order to relate them to growing anthropogenic emissions (see Sect. 5 for sulfate in relation to SO2 emissions). Finally, the large
effect of dust seen for formate (HCOO-) and not for acetate
(CH3COO-) is in agreement with previous observations made by
Legrand et al. (2003) in Alpine ice and by Legrand and De Angelis (1995) in
Greenland ice. These studies showed that the presence of formate and acetate
in ice follows the uptake of formic and acetic acid from the atmospheric gas
phase and that the incorporation of these weak acids into hydrometeors is
pH dependent with a stronger dependence for formic acid, which is a stronger
acid than acetic acid. In conclusion, large dust events significantly
influence the chemistry of ELB ice for many species, requiring a case-by-case examination depending on the nature of the dust contribution: primary
emissions for sodium and other cations (except ammonium), neutralization of
the alkaline material during atmospheric transport for nitrate, and both
primary gypsum emissions and neutralization of the alkaline material by
acidic sulfur during atmospheric transport for sulfate, for instance.
Mean chemical composition of snow layers deposited in
periods characterized by low and high dust inputs (1974–1978 and 1996–2000,
respectively). Values in parentheses are mean values calculated after
removal of samples containing dust (denoted red.). Δ refers to the
difference between total and dust-reduced (red.) values (i.e., [X]-[Xred.] for the X species).
A few outliers of unknown origin were observed in the sulfate raw data set
including 22 ppm at 166.65 m depth (summer 1780 CE), 2248 ppb at 146.38 m
depth (summer 1862 CE), 1080 ppb at 146.11 m depth (summer 1863 CE), and 864 ppb at 154.73 m depth (winter 1833/1834 CE) (Fig. 7). These individual values
were removed when calculating the corresponding half-year summer and winter
means reported in Fig. 8. In addition, single winter samples with sulfate
levels of 815 ppb at 160.62 m depth (winter 1810/1811 CE) and three data points
from 150 to 230 ppb corresponding to the winters 1786–1787, 1827–1828, and 1844–1845
were not considered, and corresponding half-year winter values were not
reported.
Raw sulfate data. The red dots denote the outliers, which were
removed prior to calculation of the half-year summer and winter means. We
also indicate samples possibly impacted by the volcanic eruptions of
Cotopaxi and Shiveluch (see text).
Individual summer (red) and winter (blue) half-year means of
sulfate along the ELB ice core. Solid circles refer to values calculated
considering all samples. Open circle data (SO4red.) were calculated after
having removed samples considered to be impacted by large dust events (see
Sect. 3).
A few ELB snow and ice layers are impacted by known volcanic eruptions. As
discussed by Mikhalenko et al. (2015), ice layers dated to 1911 and 1913
were probably impacted by the 1912 CE Katmai eruption, and summer layers of
1836 and 1837 were probably impacted by the 1835 CE Cosigüina eruption. In addition,
we suspect the 1854 CE Shiveluch eruption to have impacted the summer 1854 ice
layer and finally, although less evident since this part of the core is made
up of split ice (see Sect. 2), the Cotopaxi 1877 CE eruption may have
influenced the winter 1877/1878 layer. To discuss the long-term trends of
sulfate in relation to the growing SO2 emissions, these half-year summer
and winters means, suspected to contain volcanic debris, were discarded in
Fig. 9. To minimize the effect of year-to-year variability due to
meteorological transport conditions we added the first component of single
spectra analysis (SSA) with a 5-year time window in Fig. 9.
As discussed in Sect. 3, large dust events significantly enhanced the
sulfate level of the ELB ice. Since, as detailed in Kutuzov et al. (2019), their occurrence has changed in the past with more frequent
events after 1950, we have examined to what extent they contribute to the
sulfate trend. It is difficult to accurately directly correct sulfate
concentration from the large dust event contribution since the amount of
sulfur trapped by the alkaline material during atmospheric transport towards
the site would be very different from event to event. For instance, Koçak et
al. (2012) reported non-sea-salt-sulfate to non-sea-salt-calcium mass ratios
in aerosol collected in the eastern Mediterranean ranging between 0.25 (in
the case of direct arrival at the site of air mass from the Sahara) and 1.15
(when mineral dust passed through polluted sites located in the Balkans and
Turkey before arriving at the site). Instead of corrected sulfate values for
the large dust events, we therefore have reported in Fig. 8 individual
values of total sulfate and sulfate calculated after having removed from the
average samples suspected to contain a large amount of dust
(SO42-red. values). The influence of large dust events on the
long-term SSA winter trend is rather insignificant and if existing remains limited (i.e.
effect of < 10 ppb) around 1870 and to the
2000–2010 decade (Fig. 9). Remaining negligible prior to 1850,
the large dust event effect on the summer trend gradually increases after
1950, reaching often 100 ppb after 1960. This change in large dust events
results from the change in the occurrence of drought in North Africa and the Middle
East regions (Kutuzov et al., 2019).
ELB half-year summer and winter sulfate trends from 1774 to 2010
(the first SSA component with a 5-year time window; see text). The black
lines refer to raw sulfate values, the dashed lines to sulfate means
calculated after removal of dust samples (SO4red.), and the solid red and
blue lines to sulfate values after having corrected SO4red. values from
the background dust contribution (see Sect. 3). Samples suspected to be
impacted by volcanoes were removed.
In addition to the enhanced frequency of large dust events after 1950, the
calcium background concentrations (i.e., Ca2+red.) also changed
over time with an increase from 68±21 ppb prior to 1900 CE to 194±61 ppb after 1960 CE. As discussed by Kutuzov et al. (2019),
this change may result from changes in precipitation and soil moisture
content in the Levant region (Syria and Iraq). In order to discuss the dust-free
sulfate changes with respect to anthropogenic emissions, we make an attempt
to correct the sulfate record from this increase in the background level of
dust. To do so, we examined in Fig. 10 the relationship between calcium and
sulfate concentration in individual summer samples corresponding to
pre-industrial time (1774–1900 CE). Despite being poor (R2=0.32),
the correlation suggests a mean slope of the linear SO4red-Cared
relationship close to 1. The use of this value to correct sulfate from
background dust emissions would lead to an overestimation of the sulfate
dust contribution. Indeed, as seen in Fig. 10, there are numerous samples
that contain more sulfate than what is expected with respect to the presence
of pure calcium sulfate (gypsum; see the blue line reported in Fig. 10),
likely to due the presence of sulfate as ammonium sulfate or sulfuric acid.
To correct sulfate from the background dust contribution we here have used a
sulfate-to-calcium ratio close to 0.63 (see the red line drawn as the lower
envelope of the relationship in Fig. 10) and subtracted this contribution
from the SO42-red. values by using the Ca2+red. values. As seen in Fig. 9, the derived mean summer and winter
pre-industrial dust-free sulfate levels in ELB ice (taken as the mean value
observed from 1774 to 1850) are 70 and 40 ppb, respectively. In summer
as in winter, the dust-free sulfate values remained close to the
pre-industrial values until 1910–1920 (81 ppb instead of 70 ppb in summer,
47 ppb in winter). After 1920, dust-free sulfate levels increased at a mean
rate of 5 ppb yr-1 in summer and 1 ppb yr-1 in winter. The sulfate
increase then accelerated between 1950 and 1975 (10 ppb yr-1 in summer,
5 ppb yr-1 in winter), until a maximum of 530 ppb in summer (255 ppb in
winter) was reached at the end of the 1980s. After 1990, sulfate levels
decreased to 390 ppb in summer (154 ppb in winter) during the first decade
of the 21st century.
Sulfate versus calcium concentrations in summer samples free of
large dust events (red. values) in ice deposited prior to 1900. The blue
line refers to a pure gypsum composition. The red line illustrates a lower
envelope of the sulfate-to-calcium relationship.
Dust-free sulfate (annual values in black, summer in red, and
winter in blue) trends at the CDD (a), ELB (b), and BEL (c) sites. For CDD
we report the records derived from the C10 (dashed lines) and CDK (solid
lines) ice cores (see Sect. 5.1). Green areas indicate the decades in which
the sulfate levels reached their maxima.
Comparison between Elbrus, Alpine, and Siberian Altai long-term sulfate trendsThe Alpine CDD and Siberian Altai (Belukha, BEL) ice core sulfate
records
The ELB dust-free sulfate long-term trend is compared with those previously
extracted from the Alpine CDD site (ice cores denoted C10 and CDK in Fig. 11). C10 sulfate data were presented in Preunkert et al. (2001) and those
from CDK in Legrand et al. (2013). Since winter data from CDD are more
limited (only a few pure winter layers are available between 1890 and 1930,
Legrand et al., 2018), we here focus on the comparison of summer levels. The
two CDD cores were dated by annual layer counting using the pronounced
seasonal variations of ammonium. The two chronologies were in excellent
agreement over their overlapping period from 1925 to 1990 (Legrand et al.,
2013; Preunkert et al., 2000). A re-evaluation of the C10 chronology based
on very recently made continuous measurements of heavy metals, as well as a
comparison to a well-dated Greenland ice core record (McConnell and Edwards,
2008), resulted in a revised C10 chronology (Legrand et al., 2018). As for
C10, continuous measurements of heavy metals are also available in the
lowest part of CDK (Preunkert et al., 2019). It was thus possible to
identify the distinct Greenland increases in thallium, lead, and cadmium
associated with the widespread start of coal burning at the beginning of the
Industrial Revolution in 1890 CE also in the CDK core (at 117.8 m, 90.5 m.w.e.). This time marker was then used to constrain a revised annual layer
counting in the early 20th century part of the CDK record. The
dust-free ELB sulfate long-term trend is also compared with the one
previously extracted from the Siberian Altai (Belukha glacier, denoted BEL
in Fig. 11) by Eichler et al. (2009, 2012).
SO2 emissions (from 1850 to 2005) from various countries
located around the Caucasus (a) and the Alps (b).
GEO&AZE&IRK&SYR denotes emissions from Georgia, Azerbaijan, Iraq,
and Syria. Data are from Smith et al. (2011).
Comparison of the dust-free sulfate summer trends (SO42-red.
values) from two CDD ice cores (C10 and CDK) (a) and the ELB ice core (b)
with SO2 emissions from countries suspected to contribute to sulfate
depositions at the two sites (c for CDD, d for ELB; see discussions in Sect. 5.2). (a, b) Solid lines refer to the first SSA component with a
5-year time window. Horizontal dashed blue lines refer to the respective
pre-industrial sulfate levels. (c)SO2 emissions from France, Italy, Spain,
Switzerland, and half from Germany. (d)SO2 emissions from Turkey, Georgia,
and Azerbaijan; half from Russia; and a quarter from Ukraine, Bulgaria, and
Iran. Green areas indicate the decades in which the sulfate levels reached
their maxima.
ELB versus CDD and BEL sulfate trends
Since they are available at all three sites, we compared in Fig. 11 the annual
long-term trends of dust-free sulfate from ELB, CDD, and BEL. The ELB and
CDD annual values were calculated as the arithmetic mean from 5-year-SSA winter
and summer records, whereas the BEL annual data refer to 5-year-averaged raw
data. Examination of the three annual records reveals three major differences
between the three sites: (1) an impact of anthropogenic emissions already
significant in 1910–1930 at CDD but neither at ELB nor at BEL, (2) a maximum
of the anthropogenic perturbation which is reached in 1970–1980 at CDD and
later at the two other sites (10 years after at ELB, and a few years after
at BEL), and (3) a far less pronounced re-decrease at the beginning of the
21st century at ELB compared to CDD. The re-decrease in sulfate over
the very recent decades is somewhat stronger at BEL than at ELB.
Using data from Smith et al. (2011), available at http://sedac.ciesin.columbia.edu/data/set/haso2-anthro-sulfur-dioxide-emissions-1850-2005-v2-86 (last access: 30 July 2015),
we report in Fig. 12 emissions of SO2 from countries located near the
ELB site: Georgia, Azerbaijan, Syria, Iraq, Turkey, Russia, Iran or located
further north (Ukraine) and west (Bulgaria). In these countries SO2
emissions became significant after 1930 and reached a maximum in the late 1980s
or later (for Turkey and Iran). This feature clearly differs from the
situation at CDD where emissions from countries located around the site
(France, Italy, Spain, Switzerland, and Germany) were already significant in
1930 and exhibited a maximum between the early 1970s and the early 1980s (Fig. 10). For BEL, Eichler et al. (2009) demonstrated the importance of emissions
from eastern Europe for the dust-free sulfate annual record at that site.
Finally, we tentatively examine the cause of the recent decrease in sulfate,
focusing on the summer season for which the most relevant source regions are
limited to countries located near the site. As discussed by Kutuzov et al. (2019), 10 d backward air mass trajectories calculated for the ELB
site using the NOAA HYSPLIT-4 model suggest that, in summer, air masses
arriving at ELB mainly originate from Georgia, Azerbaijan, Syria,
and Iraq, as well as from Turkey, southern Russia, and north of Iran. As previously
discussed by Fagerli et al. (2007), the CDD site in summer is mainly
influenced by emissions from France, western Germany, Italy, and Spain.
Consistent with SO2 emission changes, the recent sulfate decrease is
more pronounced at CDD than ELB, with a recovered 2005 level (316 ppb) close
to the 1950 one (296 ppb) (Fig. 13). At the ELB site this is not the case;
here the 2005 level (380 ppb in 2005) is found to be still almost 2 times
higher than the one of 1950 (227 ppb in 1950). An intermediate pattern is
seen at BEL, likely due to a weak impact of countries like Turkey that
significantly contribute to the ELB record but not the BEL one, as well as a stronger contribution of emissions from Russia at BEL (see also Eichler et
al., 2009) than at ELB.
Conclusions
Based on the ammonium and succinate stratigraphy, the upper 168.6 m of the
deep ice core extracted at Mt Elbrus (Caucasus) in 2009 was dated by
counting annual layers back to 1774 CE. The derived seasonally resolved
chemical records cover the years 1774–2009, making this ice core particularly
useful to reconstruct many aspects of atmospheric pollution in south-eastern
Europe from pre-industrial times (1850 CE) to the present day. Below 169 m depth
the annual counting is not possible, but radiocarbon analysis of the
particulate organic carbon fraction in the basal ice of the glacier suggests
an age of ∼1670±400 cal yr BP. We have examined the
impact on the chemical composition of the Elbrus ice layers of arrival at
the site of large dust plumes originating from the Sahara and Middle East. We
then report on the dust-free sulfate records. The ELB dust-free sulfate
record indicates a 6- and 7-fold increase from prior to 1900 to
1980–1995 in winter and summer, respectively. Still moderate at the
beginning of the 20th century, the sulfate increase accelerated after
1950, with dust-free annual levels reaching a maximum in 1980–1990 (376±10 ppb) and subsequently decreasing to 270±18 ppb at the beginning
of the 21st century. These long-term sulfate changes observed in the
ELB ice cores are compared with those previously obtained in Alpine and
Siberian Altai ice. Consistent with past SO2 emission inventories, a
much earlier onset and a more pronounced decrease in the sulfur pollution
over the last 3 decades are observed in western Europe than in south-eastern and eastern Europe.
Data availability
Sulfate and calcium data can be made available for scientific purposes upon
request to the authors (contact: suzanne.preunkert@univ-grenoble-alpes.fr or
michel.legrand@univ-grenoble-alpes).
Author contributions
SP and ML performed research, analysed ice samples and
data, and wrote the original manuscript. SK performed research,
analysed data, and commented on the original manuscript. PG and VM
performed the analysis and commented on the original manuscript. RF analysed
ice samples and commented on the original manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We would like to thank
Simon Escalle for preparing the blank ice for the PO14C measurements.
We thank Margit Schwikowski and the anonymous reviewer for their very useful comments,
which significantly improved the quality of the manuscript.
Financial support
This research has been supported by the Russian Science Foundation (grant no. 17-17-01270), the CNRS programme “Evolution séculaire de la charge et composition de l'aérosol organique au dessus de l'Europe” with the support of “Agence de l'Environnement et de la Maîtrise de l'Energie” (grant no. 1262c008 – ESCCARGO), and the French–Russian International Associated Laboratory – VOSTOK.
Review statement
This paper was edited by Frank Dentener and reviewed by Margit Schwikowski and one anonymous referee.
ReferencesAndreae, M. O., Jones, C. D., and Cox, P. M.: Strong present-day aerosol
cooling implies a hot future, Nature, 435, 1187–1190,
10.1038/nature03671, 2005.Bronk Ramsey, C.: Radiocarbon Calibration and Analysis of Stratigraphy: The
OxCal Program, Radiocarbon, 37, 425–430, 10.1017/S0033822200030903,
1995.
Bronk Ramsey, C.: Bayesian analysis of radiocarbon dates, Radiocarbon,
51, 337–360, 2009.Eichler, A., Schwikowski, M., Gaeggeler, H., Furrer, V., Synal, H.-A., Beer,
J., Saurer, M., and Funk, M.: Glaciochemical dating of an ice core from
upper Grenzgletscher (4200 m a.s.l.), J. Glaciol., 46, 507–515,
10.3189/172756500781833098, 2000.Eichler, A., Brütsch, S., Olivier, S., Papina, T., and Schwikowski, M.: A
750 year ice core record of past biogenic emissions from Siberian boreal
forests, Geophys. Res. Lett., 36, L18813, 10.1029/2009GL038807, 2009.Eichler, A., Tobler, L., Eyrikh, S., Gramlich, G., Malygina, N., Papina, T.,
and Schwikowski, M.: Three Centuries of Eastern European and Altai Lead
Emissions Recorded in a Belukha Ice Core, Environ. Sci. Technol., 46,
4323–4330, 10.1021/es2039954, 2012.Fagerli, H., Legrand, M., Preunkert, S., Vestreng, V., Simpson, D., and
Cerqueira, M.: Modeling historical long-term trends of sulfate, ammonium,
and elemental carbon over Europe: A comparison with ice core records in the
Alps, J. Geophys. Res., 112, D23S13, 10.1029/2006JD008044, 2007.Gabrielli, P., Barbante, C., Bertagna, G., Bertó, M., Binder, D.,
Carton, A., Carturan, L., Cazorzi, F., Cozzi, G., Dalla Fontana, G., Davis,
M., De Blasi, F., Dinale, R., Dragà, G., Dreossi, G., Festi, D.,
Frezzotti, M., Gabrieli, J., Galos, S., Ginot, P., Heidenwolf, P., Jenk, T.
M., Kehrwald, N., Kenny, D., Magand, O., Mair, V., Mikhalenko, V., Lin, P.
N., Oeggl, K., Piffer, G., Rinaldi, M., Schotterer, U., Schwikowski, M.,
Seppi, R., Spolaor, A., Stenni, B., Tonidandel, D., Uglietti, C.,
Zagorodnov, V., Zanoner, T., and Zennaro, P.: Age of the Mt. Ortles ice
cores, the Tyrolean Iceman and glaciation of the highest summit of South
Tyrol since the Northern Hemisphere Climatic Optimum, Cryosphere, 10,
2779–2797, 10.5194/tc-10-2779-2016, 2016.Hoffmann, H., Friedrich, R., Kromer, B., and Fahrni, S.: Status report:
Implementation of gas measurements at the MAMS 14C AMS facility in
Mannheim, Germany, Nucl. Instrum. Meth. B, 410, 184–187, 10.1016/j.nimb.2017.08.018,
2017.Hoffmann, H., Preunkert, S., Legrand, M., Leinfelder, D., Bohleber, P.,
Friedrich, R., and Wagenbach, D.: A New Sample Preparation System for
Micro-14C Dating of Glacier Ice with a First Application to a High Alpine
Ice Core from Colle Gnifetti (Switzerland), Radiocarbon, 60, 517–533,
10.1017/RDC.2017.99, 2018.Jenk, T., Szidat, S., Schwikowski, M., Gaeggeler, H., Brütsch, S., Wacker, L., Synal, H.-A., and Saurer, M.: Radiocarbon analysis in an Alpine ice core: Record of anthropogenic and biogenic contributions to carbonaceous aerosols in the past (1650–1940), Atmos. Chem. Phys., 6, 5381–5390, 10.5194/acpd-6-5905-2006, 2006.Jenk, T. M., Szidat, S., Bolius, D., Sigl, M., Gäggeler, H. W., Wacker,
L., Ruff, M., Barbante, C., Boutron, C. F., and Schwikowski, M.: A novel
radiocarbon dating technique applied to an ice core from the Alps indicating
late Pleistocene ages, J. Geophys. Res.-Atmos., 114, 1–8,
10.1029/2009JD011860, 2009.Kawamura, K., Izawa, Y., Mochida, M., and Shiraiwa, T.: Ice core records of
biomass burning tracers (levoglucosan and dehydroabietic, vanillic and
p-hydroxybenzoic acids) and total organic carbon for past 300 years in the
Kamchatka Peninsula, Northeast Asia, Geochim. Cosmochim. Ac., 99, 317–329,
10.1016/j.gca.2012.08.006, 2012.Koçak, M., Theodosi, C., Zarmpas, P., Séguret, M. J. M., Herut, B.,
Kallos, G., Mihalopoulos, N., Kubilay, N., and Nimmo, M.: Influence of
mineral dust transport on the chemical composition and physical properties
of the Eastern Mediterranean aerosol, Atmos. Environ., 57, 266–277,
10.1016/j.atmosenv.2012.04.006, 2012.Kozachek, A., Mikhalenko, V., Masson-Delmotte, V., Ekaykin, A., Ginot, P.,
Kutuzov, S., Legrand, M., Lipenkov, V., and Preunkert, S.: Large-scale
drivers of Caucasus climate variability in meteorological records and Mt
El'brus ice cores, Clim. Past, 13, 473–489, 10.5194/cp-13-473-2017, 2017.Kutuzov, S., Shahgedanova, M., Mikhalenko, V., Ginot, P., Lavrentiev, I., and
Kemp, S.: High-resolution provenance of desert dust deposited on Mt. Elbrus,
Caucasus in 2009–2012 using snow pit and firn core records, Cryosphere, 7,
1481–1498, 10.5194/tc-7-1481-2013, 2013.Kutuzov, S., Legrand, M., Preunkert, S., Ginot, P., Mikhalenko, V., Shukurov, K., Poliukhov, A., and Toropov, P.: The Elbrus (Caucasus, Russia) ice core record – Part 2:
history of desert dust deposition, Atmos. Chem. Phys., 19, 14133–14148, 10.5194/acp-19-14133-2019, 2019.Legrand, M. and De Angelis, M.: Origins and variations of light carboxylic
acids in polar precipitation, J. Geophys. Res.-Atmos., 100, 1445–1462,
10.1029/94JD02614, 1995.Legrand, M. and Mayewski, P.: Glaciochemistry of polar ice cores: A review,
Rev. Geophys., 35, 219–243, 10.1029/96RG03527, 1997.Legrand, M., Preunkert, S., Wagenbach, D., Cachier, H., and Puxbaum, H.: A
historical record of formate and acetate from a high-elevation Alpine
glacier: Implications for their natural versus anthropogenic budgets at the
European scale, J. Geophys. Res.-Atmos., 108, 2001–2015,
10.1029/2003JD003594, 2003.Legrand, M., Preunkert, S., Oliveira, T., Pio, C. A., Hammer, S.,
Gelencsér, A., Kasper-Giebl, A., and Laj, P.: Origin of C2–C5
dicarboxylic acids in the European atmosphere inferred from year-round
aerosol study conducted at a west-east transect, J. Geophys. Res., 112,
D23S07, 10.1029/2006JD008019, 2007.Legrand, M., Preunkert, S., May, B., Guilhermet, J., Hoffman, H., and
Wagenbach, D.: Major 20th century changes of the content and chemical
speciation of organic carbon archived in Alpine ice cores: Implications for
the long-term change of organic aerosol over Europe, J. Geophys. Res.-Atmos., 118, 3879–3890, 10.1002/jgrd.50202, 2013.Legrand, M., McConnell, J. R., Preunkert, S., Arienzo, M., Chellman, N.,
Gleason, K., Sherwen, T., Evans, M. J., and Carpenter, L. J.: Alpine ice
evidence of a three-fold increase in atmospheric iodine deposition since
1950 in Europe due to increasing oceanic emissions, P. Natl. Acad. Sci. USA,
115, 12136–12141, 10.1073/pnas.1809867115, 2018.Lim, S., Faïn, X., Ginot, P., Mikhalenko, V., Kutuzov, S., Paris, J.-D., Kozachek, A., and Laj, P.: Black carbon variability since preindustrial times in the eastern part of Europe reconstructed from Mt. Elbrus, Caucasus, ice cores, Atmos. Chem. Phys., 17, 3489–3505, 10.5194/acp-17-3489-2017, 2017.Maupetit, F., Wagenbach, D., Weddeling, P., and Delmas, R.: Seasonal fluxes
of major ions to a high altitude cold alpine glacier, Atmos. Environ., 29,
1–9, 10.1016/1352-2310(94)00222-7, 1995.McConnell, J. R. and Edwards, R.: Coal burning leaves toxic heavy metal
legacy in the Arctic, P. Natl. Acad. Sci. USA, 105, 12140–12144,
10.1073/pnas.0803564105, 2008.Mikhalenko, V., Sokratov, S., Kutuzov, S., Ginot, P., Legrand, M.,
Preunkert, S., Lavrentiev, I., Kozachek, A., Ekaykin, A., Fain, X., Lim, S.,
Schotterer, U., Lipenkov, V., and Toropov, P.: Investigation of a deep ice
core from the Elbrus western plateau, the Caucasus, Russia, Cryosphere,
9, 2253–2270, 10.5194/tc-9-2253-2015, 2015.Millard, A. R.: Conventions for Reporting Radiocarbon Determinations,
Radiocarbon, 56, 555–559, 10.1017/S0033822200049596, 2014.
Nye, J.: Correction factor for accumulation measured by the thickness of the
annual layers in an ice sheet, J. Glaciol., 4, 785–788, 10.3189/s0022143000028367, 1963.Olivier, S., Blaser, C., Brütsch, S., Frolova, N., Gäggeler, H. W.,
Henderson, K. A., Palmer, A. S., Papina, T., and Schwikowski, M.: Temporal
variations of mineral dust, biogenic tracers, and anthropogenic species
during the past two centuries from Belukha ice core, Siberian Altai, J.
Geophys. Res., 111, D05309, 10.1029/2005JD005830, 2006.
Paterson, W. S. B. and Waddington, E. D.: Past precipitation rates derived
from ice core measurements: methods and data analysis, Rev. Geophys.
Space Phys., 22, 123–130, 1984.
Preunkert, S.: L'histoire de la pollution atmosphérique Européenne
reconstituée à partir des carottes de glace Alpine, Univ. Joseph
Fourier de Grenoble, France, 240 pp., 2001.Preunkert, S. and Legrand, M.: Towards a quasi-complete reconstruction of
past atmospheric aerosol load and composition (organic and inorganic) over
Europe since 1920 inferred from Alpine ice cores, Clim. Past, 9,
1403–1416, 10.5194/cp-9-1403-2013, 2013.Preunkert, S., Wagenbach, D., Legrand, M., and Vincent, C.: Col du Dôme
(Mt Blanc Massif, French Alps) suitability for ice-core studies in relation
with past atmospheric chemistry over Europe, Tellus B,
52, 993–1012, 10.3402/tellusb.v52i3.17081, 2000.Preunkert, S., Legrand, M., and Wagenbach, D.: Sulfate trends in a Col du
Dôme (French Alps) ice core: A record of anthropogenic sulfate levels in
the European midtroposphere over the twentieth century, J. Geophys. Res.,
106, 31991–32004, 10.1029/2001JD000792, 2001.Preunkert, S., Legrand, M., Stricker, P., Bulat, S., Alekhina, I., Petit, J.
R., Hoffmann, H., May, B., and Jourdain, B.: Quantification of Dissolved
Organic Carbon at Very Low Levels in Natural Ice Samples by a UV-Induced
Oxidation Method, Environ. Sci. Technol., 45, 673–678,
10.1021/es1023256, 2011.Preunkert, S., MConnell, J., Hoffmann, H., Legrand, M., Wilson, A.,
Eckhardt, S., Stohl, A., Chellman, N., Arienzo, M., and Friedrich, R.: Lead
and antimony in basal ice from Col du Dome (French Alps) dated with
radiocarbon: A record of pollution during Antiquity, Geophys. Res. Lett., 46, 4953–4961,
10.1029/2019GL082641, 2019.Smith, S. J., van Aardenne, J., Klimont, Z., Andres, R. J., Volke, A., and Delgado Arias, S.: Anthropogenic sulfur dioxide emissions: 1850–2005, Atmos. Chem. Phys., 11, 1101–1116, 10.5194/acp-11-1101-2011, 2011.Schwikowski, M.: Paleoenvironmental reconstruction from Alpine ice cores,
PAGES news, 14, 16–18, 10.22498/pages.14.1.16, 2006.Schwikowski, M., Barbante, C., Doering, T., Gaeggeler, H. W., Boutron, C.,
Schotterer, U., Tobler, L., Van de Velde, K., Ferrari, C., Cozzi, G.,
Rosman, K., and Cescon, P.: Post-17th-Century Changes of European Lead
Emissions Recorded in High-Altitude Alpine Snow and Ice, Environ. Sci.
Technol., 38, 957–964, 10.1021/es034715o, 2004.
Usher, C. R., Michel, A. E., and Grassian, V. H.: Reactions on Mineral Dust,
Chem. Rev., 103, 4883–4940, 10.1021/cr020657y, 2003.
Wagenbach, D., Preunkert, S., Schäfer, J., Jung, W., and Tomadin, L.:
Northward Transport of Saharan Dust Recorded in a Deep Alpine Ice Core, in:
The Impact of Desert Dust Across the Mediterranean, edited by: Guerzoni, S.
and Chester, R., Springer Netherlands, Dordrecht, 291–300, 1996.Wagenbach, D., Bohleber, P., and Preunkert, S.: Cold, alpine ice bodies
revisited: what may we learn from their impurity and isotope content?,
Geogr. Ann. Ser. A, 94, 245–263,
10.1111/j.1468-0459.2012.00461.x, 2012.