Ice cores are one of the most valuable paleo-archives. Records from
ice cores provide information not only about the amount of dust in the
atmosphere, but also about dust sources and their changes in the past. In
2009, a 182 m long ice core was recovered from the western plateau of Mt
Elbrus (5115 m a.s.l.). This record was further extended after a shallow ice
core was drilled in 2013. Here we analyse
Atmospheric dust is the most important aerosol emitted to the atmosphere in terms of mass (Knippertz and Stuut, 2014) and impacts. Despite the significance of atmospheric dust and its impacts on the planetary radiation balance, atmospheric chemistry, biosphere and human health (Middleton, 2017), knowledge of its regional variability and long-term trends over past centuries is still poor (Mahowald et al., 2010). Dust concentration in the atmosphere depends on specific meteorological conditions, which may also be influenced by large-scale circulation patterns (e.g. ENSO, NAO). Long-term trends are controlled by changes in precipitation and vegetation cover in dust source regions, with the vegetation cover being dependent on both natural (climatic changes) and anthropogenic (land cover change) causes. The complexity of dust emission, atmospheric transport and deposition mechanisms can result in large uncertainties in atmospheric dust models (Mahowald et al., 2007). The discrepancies between models are partly explained by limited observations of past dust variability, which has limited possibilities for evaluating models' capability of reproducing of the dust cycle. However, reliable information on multiannual dust variability dating back to 1980 is now available from satellite data (e.g. Gautam et al., 2009; Chudnovsky et al., 2017; Li and Sokolik, 2018).
Analyses of recent aerosol patterns over different land and ocean regions show that, despite significant trends over some major continental source regions, average values demonstrate little change in the past three decades (1980–2009) because opposite trends cancel each other out in the global average (e.g. Chin et al., 2014). Recent broad-scale assessments of changes in dust emissions show a doubling of the dust deposition in many sedimentary archives since the mid-18th century, which was attributed to anthropogenic land use and short-term climate variability (Hooper and Marx, 2018). Globally, anthropogenic dust sources account for 25 % of emissions, but this value varies and can be considerably higher – up to 75 % in some regions (Ginoux et al., 2012). Climate–aerosol model simulations with the ability to separate natural and anthropogenic dust sources show that there was a 25 % increase in dust emissions between the 19th century and today. These changes are attributed to climate change (56 %) and anthropogenic land cover change (40 %) although the model underestimates dust concentrations in Asia, the Middle East and the US (Stanelle et al., 2014).
Records of past changes in dust concentrations are essential to better constrain interconnections between dust emissions and both natural and anthropogenic environmental changes. In this respect, proxy data are fundamental. Ice cores are natural archives of past concentrations of various impurities present in the atmosphere, including dust (e.g. Legrand and Mayewski, 1997). Beyond dust, these records also grant insight into the strength of the particular dust sources and their changing magnitude through the time. Polar ice cores from Greenland and Antarctica reconstruct the changes in dust content in the atmosphere over hundreds thousands of years at a hemispherical scale (e.g. De Angelis et al., 1997; Delmonte et al., 2002; Legrand, 1987; Petit et al., 1999; Ruth et al., 2003). In contrast, data from ice cores drilled at mid-latitude mountain glaciers proximal to arid areas reconstruct local- to regional-scale dust aerosol emission histories over shorter timescales (e.g. Grigholm et al., 2015, 2017; Kaspari et al., 2009; Osterberg et al., 2008; Thompson et al., 2000; Bohleber et al., 2018).
Mineral dust from North Africa and deserts of the Middle East is regularly
deposited on glaciers in the Caucasus Mountains
(Kutuzov et al., 2013).
Due to its high elevation (over 5000 m a.s.l.) and proximity to arid and
semi-arid areas, the Caucasus are a natural trap for desert dust. The absence
of meltwater infiltration near the summit of Elbrus ensures the
preservation of a climatic record in an ice core while high accumulation
rates promote greater temporal resolution
(Mikhalenko
et al., 2015). A study focusing on the long-term trend of black carbon in
the Elbrus ice
(Lim et
al., 2017) is currently underway as well as several studies investigating
additional chemical species, including investigations of calcium and of
sulfate. Data are discussed in two companion papers including the
glaciochemistry of the deep Elbrus ice core drilled in 2009 (Preunkert et
al., 2019). Here, we report changes in
The Caucasus are situated between the Black and Caspian seas and generally
trend east–south-east, with the Greater Caucasus range often considered as
the divide between Europe and Asia. The 2020 glaciers in the Caucasus cover
an area of
Location of Elbrus (red star) and dust sources (orange polygons) based on Ginoux et al. (2012). Annual NOAA HYSPLIT_4 10 d backward trajectory density plots for the period 1948–2013 using NCEP/NCAR Reanalysis. Trajectories were run every 6 h.
To characterize possible sources of aerosols deposited on glaciers, we
calculated three-dimensional backward trajectories of air parcels
(elementary air particles) arriving at the Elbrus plateau (5100 m a.s.l.)
using the NOAA HYSPLIT_4 trajectory model
(Draxler and Hess, 1998; Stein et
al., 2015) and NCEP/NCAR Reanalysis data on
To identify potential dust source contributions, we also analysed vertical distribution of the backward trajectories. An objective criterion was chosen to extract locations of possible dust entrainment along the trajectories. The criterion is met when the air parcel is close to the ground (i.e. within the well-mixed boundary layer), allowing the uptake of mineral aerosols (Sodemann et al., 2006). Density plots were calculated only for 10 d backward trajectories which descended below mixed layer depth. The depth is calculated by HYSPLIT_4 (using NCEP/NCAR Reanalysis data) for each point of backward trajectory as the height exceeding potential air temperature over surface air temperature by 2 K (Draxler and Hess, 1998).
During August–September 2009, an ice core measuring 181.8 m in length was
recovered at the western plateau of Elbrus in the central Caucasus
(43
We determined cations (
Seasonal ice core stratigraphy of chemical parameters and ice core dating
based on annual layer counting of the deep Elbrus ice core is described in
detail in
Mikhalenko
et al. (2015). The seasonal oscillations of
Due to the glacier compression with depth, we applied a variable sampling resolution of 10 cm from 0 to 157 m and then a sampling resolution of every 2 cm below 157 m depth. As a result (Fig. 3), the temporal resolution remains relatively consistent throughout the core, with 12 samples per summer over the 1950–2010 CE time period to 14 samples each summer over the 1900–1950 CE time period.
It should be noted that Fig. 2 shows the thickness of annual layers and does not represent the linear change in accumulation rate. In order to obtain an accumulation rate, layer thickness must be corrected for any compression which might have occurred following deposition (e.g. Paterson and Waddington, 1984), which is beyond the scope of this paper.
In addition, In June 2013 the existing ice record was expanded to include the years 2009–2012 CE via a 20.5 m long ice core which was extracted at the same drill site. In total, 515 samples of the firn core were analysed (85 samples per year).
For the 2013 ice core the succinic acid data were not available. We
therefore used a combination of
In order to assess which factors drive the variations in dust content in the
Elbrus ice core, we compared the dust concentrations with various climatic
parameters in the potential dust sources. For climate data which may
potentially influence the dust emission (temperature, precipitation, wind
speed, soil moisture) we used ERA-Interim reanalysis fields for the period
of 1979–2013 CE (Dee et al., 2011). Since the variables used in this study
are continuous and normally distributed, the Pearson correlation
coefficient was used. The statistical significance of the correlation was
checked using Student's
NOAA HYSPLIT_4 10 d backward trajectories density
plots for the period 1948–2013 for December–February
The Standardised Precipitation-Evapotranspiration Index (SPEI) was selected for use as a drought proxy (Vicente-Serrano et al., 2010) as its capacity for investigating the relationship between dust emission and drought has been well documented (Achakulwisut et al., 2018). We considered SPEI calculated over periods of 1, 2, 3, 6 and 12 months. The time series of SPEI were obtained by averaging over the regions of interest.
We also investigated the
The amount of dust in ice cores depends on many factors and corresponds to
the presence of dust particles in the atmosphere. Primarily, dust emissions
are influenced by the characteristics of the dust source (soil type,
geomorphology, soil moisture) as well as by meteorological conditions
(surface winds). Once dust clouds are uplifted to the mid-troposphere, their
transport depends on the main circulation patterns. In mountainous areas
with high snow accumulation, wet deposition defines annual and seasonal
aerosol concentrations. During spring and summer, the majority of air masses
arrive to the Elbrus site from arid areas with calcareous soils. If all of
the
The total dust deposited at Mt Elbrus may have three different natural
components: (i) dust from local sources (nunataks, rock outcrops), (ii) sporadic arrival of large aeolian desert dust events and (iii) large-scale
background continuous terrigenous aerosol emissions. Apart from regions
strongly impacted by sea-salt aerosols, the presence of calcium in aerosols
in continental atmospheres is expected to mainly originate from exposed
continental sediments. Even in polar regions (Antarctica or Greenland),
calcium in ice mainly comes from dust where only a small fraction is related
to sea salt emitted from the ocean
(De
Angelis et al., 1997; Legrand, 1987). Assuming that
The volcanic rocks of Elbrus near the drilling site do not contain calcite;
therefore, we can assume that
The majority of the calculated backward trajectories show a south-west origin with the highest frequency over the Middle East, eastern Mediterranean and North Africa in all seasons. In winter (December–February), air masses tend to come from more remote locations, whereas summer (June–August) reveals possible transport from the Caspian Sea region and southern Russia (Fig. 3).
Large dust plumes reach the Caucasus Mountains, which originate from the Middle East – and less frequently from the Sahara (Kutuzov et al., 2013). As seen in the Alps, these events impacts the chemistry of snow deposits to create calcium-rich alkaline snow layers (Wagenbach et al., 1996). Deposition of these plumes increases the concentrations of numerous chemical species in Alpine ice due to either their presence in the dust at its emission stage or, being acidic, their interaction with alkaline material during transport (Usher et al., 2003). Deposition of light-absorbing impurities (in particular black carbon and dust) plays an important role in changes in the snow and glaciers and enhances the response of the mountain cryosphere to climate changes via snow–albedo feedbacks (Ginot et al., 2014; Gabbi et al., 2015; Di Mauro et al., 2017; Skiles et al., 2018).
In this work, as well as in Preunkert et al. (2019), Elbrus samples are considered to be impacted by dust events if two criteria are met: (1) they contain more than 120 ppb of calcium and (2) they fall below the 25 % quartile of a robust spline calculated through the raw acidity profile. These selection criteria result in 616 dust deposition summer samples (from a total of 2524) and 67 winter samples (from a total of 1150). Similar results were obtained when changing the calcium concentration criteria from 120 ppb to 100 or 140 ppb.
Using ammonium and succinate stratigraphy to separate the winter and summer seasons (Sect. 3.2), we determined half-year summer and winter means from 1774 to 2010 CE (Fig. 4). In Fig. 5, we report the seasonal cycle of calcium and ammonium averaged over two different periods of the 20th century (1900–1950 and 1950–2010 CE).
The mean concentration of
As seen in Fig. 6, most of long-term calcium trend is detected in summer with (1) more frequent arrivals of large dust events after 1950 and (2) an increase of 100 ppb of the calcium background level after 1950. The maximum annual concentrations were found in 1999 and 2000 annual layers (980 and 850 ppb). There was a pronounced period of increased dust content in the 1960s with a following decrease in late 1970s. When compared to the background, exceptional peaks occurred during the following years: 1802, 1957, 1863, 1917, 1933, 1963, 1984, 1989, 1999, 2000, 2008 and 2009.
Total and background (grey lines)
Warm season layers contain an increase in
In recent decades (1950–2010 CE), over which time period more dust events were detected in the Elbrus ice, a clear spring maximum of the dust fraction is observed (Fig. 5). These dust peaks are consistent with the timing of the arrival of large dust plumes at the site (Kutuzov et al., 2013). A major long-range dust outbreak event recently occurred over North Africa, as well as the eastern Mediterranean and Caucasus on 22 and 23 March 2018 (Solomos et al., 2018), which resulted in significant dust deposition on glaciers.
Magnesium versus calcium concentrations (in ppb) in dust ice samples from Elbrus.
The calcium peaks in Elbrus ice containing dust are accompanied by an
increase in magnesium (Fig. 7). Interestingly, the mean
[
Composition of aerosol collected at Erdemli during dust events from
the Middle East and at Heraklion during dust events from the Sahara (from Koçak
et al., 2012). The magnesium sea-salt contributions were calculated via the
Figure 6 demonstrates an increase in the calcium background concentrations
after 1950. This increase may be influenced by human activities, such as
coal combustion and cement production, thereby contributing to the
background calcium trend detected in the Elbrus ice. Similar impacts of
anthropogenic emissions on various species in natural dust emissions
(including calcium) were reported by Kalderon-Asael and
colleagues (2009); this demonstrates that, under strong stratification in the
lower atmosphere in Israel, part of the atmospheric calcium may be
anthropogenic in nature. At the scale of Europe, Lee and Pacyna (1999)
estimated that 0.8 Tg of anthropogenic calcium is emitted per year, with coal combustion contributing 60 % and cement contributing 30 % of the total. However,
to date, these anthropogenic calcium emissions remain 1 order of magnitude
weaker than dust calcium emissions from north-eastern Africa (12 Tg yr
Particles emitted during both coal combustion and cement production are rich
in calcium (calcite). Therefore, we may expect a weaker
[
[
Individual summer means of background levels of
De Angelis and Gaudichet (1991) presented additional evidence to suggest that cement production and use (despite its growing impact) does not represent the dominant contribution to background calcium levels at Elbrus derived in their study of calcium and aluminium at the Col du Dôme site, Mont Blanc, France. The authors demonstrate that the increase in both dust arrival frequency in the 1980s and the background dust levels occurred without any coinciding decrease in the aluminium to calcium ratio. Given the low aluminium content of cement as compared to desert dust, these observations suggest that the Col du Dôme site is not significantly impacted by the growing use of cement. We may expect that this impact is even weaker at the Elbrus site.
Two major dust sources contribute mineral particles to the Elbrus glaciers: the Sahara and the Middle East. It was established that a majority of the small-scale dust sources in the Middle East are located in northern Mesopotamia (northern Syria–north-western Iraq) and the Syrian Desert (Kutuzov et al., 2013). The Levant is a major source of atmospheric dust (Middleton, 1986) with natural, anthropogenic and hydrological (intermittent streams and lakes) sources. The area between the Tigris and the Euphrates in Iraq contains natural desert dust sources, while the Nineveh region in Iraq was recently identified as the most active dust source in the Middle East (Moridnejad et al., 2015). In the northern Sahara, strong sporadic dust storms originating in the Libyan desert and the foothills of the Hoggar Mountains in eastern Algeria sometimes reach the Caucasus in the spring (Kutuzov et al., 2013).
Spatial correlation of Elbrus
A statistically significant spatial correlation occurs between
The
As evidenced by the Elbrus ice core record, the frequency of dust events and
total
Two periods of maximum
Our findings are supported through analysis of the frequency of droughts in
Syria. For the period between 1961 to 2009 CE, droughts were observed in 25
of the examined years, resulting in
Anthropogenic land use and changes in land cover impact the soil erodibility and dust emission. The magnitude of such impacts is highly uncertain as both climatic and anthropogenic processes occur simultaneously (Webb and Pierre, 2018). Unsustainable agricultural practices, overgrazing and deforestation may significantly increase the area of the dust sources. It should be noted that only around 5 % of the land in North Africa and the Middle East is suitable for agriculture; the rest consists of pastures, forests, shrubs, urban zones, badlands, rocky areas, and deserts (Sivakumar and Stefanski, 2007).
The general increase in dust concentrations in the Elbrus ice core is accompanied by a quasi-decadal variability. The relationship of precipitation in the Middle East region to the different large-scale circulation patterns is summarized in a recent review of the droughts in the Middle East and south-western Asia (Barlow et al., 2016, and references therein). The precipitation and occurrence of droughts in the Middle East region may be influenced by several major climatic features such as the NAO, south-west Asian and Indian monsoon, global-scale variability associated with ENSO and state of the western Pacific which together determine the strength of the general atmospheric circulation (Barlow et al., 2016).
For the 33 year time period (1979–2012), significant correlations exist
between
PDO
Correlation coefficient of the Elbrus
A significant negative correlation was found between atmospheric dust concentrations in Syria and the PDO in springtime during 2003–2015 CE. It was shown that a positive geopotential height anomaly is formed over the Arabian Peninsula and North Africa during the positive phases of PDO (Pu and Ginoux, 2016). A positive PDO is characterized by an increase in cyclonic activity over the northern Pacific and northern Atlantic, which occurs together with the intensification of subtropical anticyclones. Despite an increase in geopotential height, the positive PDO increases the probability of moisture transport to North Africa and Middle Eastern regions in the spring (Dai, 2013; Pu and Ginoux, 2016). This moisture is due to increased advection from the Mediterranean Sea which causes deep convection due to unstable stratification in the lower troposphere. The negative PDO phases on the other hand are associated with a negative geopotential height anomaly in the middle troposphere over the Mediterranean and the Middle East and cyclonic activity in the region. Low-pressure anomalies over Europe, the southern Arabian Peninsula, and north-eastern to eastern Africa create favourable conditions for westerly winds from North Africa and increase the probability of dust transport to the Caucasus.
Studies of interannual decadal variability of dust activity in the Arabian peninsula and Fertile Crescent suggest that the occurrence of severe droughts and increased dust emission were influenced by the La Niña phase amplified by the negative PDO (Notaro et al., 2015). There are still large uncertainties in such connections due the lack of long-term observations (Pu and Ginoux, 2016). The correlation between Pacific circulation indices and the amount of dust in the Elbrus ice core supports these previous findings and may indicate an increase in the frequency of extreme El Niño and La Niña events with climate warming (Cai et al., 2015), which can then influence circulation over the Pacific Ocean and extend to the Middle East.
Our results are also in line with previous conclusions about the possible
influence of large circulation patterns on the aridity of the tropics. Dai (2011) suggests that the El Niño–Southern Oscillation, tropical Atlantic
sea surface temperature (SST) and Asian monsoons played a significant role
in the increase in global aridity since the 1970s over Africa, southern
Europe, East and South Asia, and eastern Australia while recent warming has
increased atmospheric moisture demand contributing to the drying. It is
expected that due to anthropogenic climate change the tropical belt may
expand toward the poles and shift precipitation patterns, which ultimately
will lead to an increase in the territory affected by droughts
(Seidel et al., 2008). Model studies show that
under global warming the large-scale circulation systems (jet streams and
storm tracks) may shift poleward (Mbengue and Schneider, 2017).
The size and intensity of the Hadley cell and the associated shift of the
subtropical anticyclone zone are likely to occur over the 21st century,
which should primarily affect the precipitation regime in subtropical
latitudes (Lu et al., 2007). An expansion of the global
tropics since 1979 by 1 to 3
The first record of dust deposition history in the Caucasus was obtained by
Davitaya in 1962 (Davitaya, 1969). This work was based
on sampling of the firn layers from a crevasse located at the Kazbek plateau
(4600 m a.s.l.). Based on dust layer counting Davitaya estimated that sampled
firn layers covered the period between 1793 and 1962. Dust concentrations
clearly increased by a factor of 3 since the late 1920s. This increase was
attributed to various reasons, including local dust influence due to glacier
retreat, industry development, fires, World War II and volcanic activity.
Despite different methodology and location, we qualitatively determined a
similar long-term dust trend in the Elbrus ice core record. The average
An overall increasing trend in dust content and
A review of the dust paleo-records demonstrates that 16 of the 25 compiled
sedimentary archives from across the globe depict a doubling in dust
emissions over the past
Information about variations in dust concentrations over the Caucasus leads to a better understanding of the climate change consequences and atmospheric circulation patterns in the dust source regions. Arid and semi-arid regions in North Africa and the Middle East are unstable under the recent climatic changes. Temperature and hydrological anomalies during the last millennium have led to large variations in human migration patterns and agricultural production, with precipitation variability as a key factor in the productivity in the Middle East region (Kaniewski, 2012). The Elbrus ice core record confirms previous findings (Cook et al., 2016; Kelley et al., 2015) that the recent droughts in 1998–2012 CE were the most severe over at least the past three centuries.
This paper presents the
Calcium data can be made available for scientific purposes upon request to the authors (contact kutuzov@igras.ru, suzanne.preunkert@univ-grenoble-alpes.fr or michel.legrand@univ-grenoble-alpes).
The supplement related to this article is available online at:
SK performed field research, analyzed the data and wrote the original manuscript. ML analyzed ice samples, assessed data and contributed to the original manuscript. SP analyzed ice samples and provided comments on the original manuscript. PG and VM performed analysis and provided comments on the original manuscript. KS calculated backward trajectories and provided comments on the original manuscript. AP and PT analyzed climatic data, assessed atmospheric circulation patterns and provided comments on the original manuscript.
The authors declare that they have no conflict of interest.
We thank Natalie M. Kehrwald and Forrest S. Schoessow for valuable comments on the original manuscript and English language editing. We thank two anonymous reviewers for their useful comments, which significantly improved the quality of the manuscript.
This research was supported by the Russian Science Foundation (RSF, grant no. 17-17-01270). The Les Enveloppes Fluides et l'Environnement – Chimie Atmosphérique (CNRS) programme entitled “Evolution séculaire de la charge et composition de l'aérosol organique au dessus de l'Europe” provided funding for analysis in France, with the support of Agence de l'Environnement et de la Maîtrise de l'Energie. Backward trajectory analysis was supported by the Presidium of the Russian Academy of Sciences (grant no. 20). Interpretation of the shallow core was supported by the President Grants for Government Support of Young Russian Scientists and the Leading Scientific Schools of the Russian Federation (grant no. 2508.2017.5).
This paper was edited by Yves Balkanski and reviewed by two anonymous referees.