History of desert dust deposition recorded in the Elbrus ice core

Ice cores are one of the most valuable paleoarchives. 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. A 182 m long ice core was recovered from the western plateau of Mt. Elbrus (5115 m.a.s.l.) in 2009. This record was extended with a shallow ice core 15 drilling in 2013. Here we analyse Ca 2+ concentrations, a commonly used proxy of dust, recorded in the Elbrus ice core over the time period of 1774-2013 CE. The Ca 2+ record reveals a quasi-decadal variability with a generally increasing trend. Using multiple regression analysis, we found a statistically significant spatial correlation of the Elbrus Ca 2+ summer concentrations with precipitation and soil moisture content in the Levant region (specifically Syria and Iraq). The Ca 2+ record also correlates with drought indices in North Africa (r=0.69 p<0.001) and Middle East regions (r=0.71 p<0.001). Dust 20 concentrations prominently increase in the ice core over the past 200 years confirming that the recent droughts in the Fertile Crescent between 1998-2012 CE were the most severe aridity for at least the past two centuries. For the most recent 33 years (1979-2012 CE), significant correlations between Ca 2+ and Pacific circulation indices (Pacific Decadal Oscillation, Southern Oscillation Index and Niño 4) may indicate that the increased frequency of extreme El Niño and La Niña events due to a warming climate extended their influence to the Middle East. It was shown that the increase of Ca 2+ concentration in the ice 25 core cannot be attributed to human activities, such as coal combustion and cement production.


Introduction
Dust is the most important aerosol emitted to the atmosphere by mass (Knippertz & Stuut, 2014). Atmospheric dust affects planetary radiation balance, atmospheric chemistry, biosphere and human health (Middleton, 2017). Despite the significance of atmospheric dust, knowledge of its regional variability and long term trends over past centuries is still poor (Mahowald et 30 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 result in large uncertainties in atmospheric dust models (Mahowald et al., 2007). The discrepancies between models are partly explained by very limited observations of dust variability over the past and therefore limited possibilities to evaluate the model's reproducibility of the dust cycle. However, reliable information on multiannual dust variability back to 5 1980 is now available from satellite data (e.g. Gautam et al., 2009;Chudnovsky et al., 2017;Li and Sokolik, 2018).
Analysis of recent aerosol variations and trends 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  because opposite trends cancel each other out in the global average (e.g. Chin et al., 2014). Recent broad-scale assessments of changes of dust emissions show a doubling of the dust deposition in many sedimentary achieves since the mid-18 th 10 century which was attributed to anthropogenic land use and short term climate variability (Hooper & Marx, 2018). Globally anthropogenic dust sources account for 25% of emission but this value vary 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 19 th century and today. These changes are attributed to climate change (56%) and anthropogenic land cover change (40%) although the model underestimates dust 15 concentrations in Asia, Middle East, and the US (Stanelle et al., 2014).
Records of past changes in dust concentrations are essential to better constrain connections 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 & Mayewski, 1997). The particle concentrations in ice cores provide information not only on the concentrations of atmospheric dust in the past but 20 also give insight into the strengths of the dust sources and their changes through the time. Polar ice cores from Greenland and Antarctica reconstruct the changes of 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).
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 is a natural trap for desert dust. The absence of melt water infiltration near the summit of Elbrus ensures the preservation of a climatic record in an ice core. High accumulation rates assure high temporal resolution of the ice-core data (Mikhalenko et 30 al., 2015). In addition to a study focusing on the long-term trend of black carbon in the Elbrus ice (Lim et al., 2017), several studies of other chemical species are currently in progress, including studies 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., this issue).
Here, we report changes in Ca 2+ concentrations recorded in the Elbrus ice core between 1774-2012 CE and connections with natural variability, climatic and land use changes in the dust source regions.

Location, climatology, and backward air-mass trajectories
The Caucasus are situated between the Black and Caspian seas, and generally trend east-southeast, with the Greater Caucasus range often considered as the divide between Europe and Asia. The 2020 glaciers in the Caucasus cover an area of 5 1193 ± 27 km 2 (Tielidze & Wheate, 2018). Elbrus mountain glaciers contain about 10% of Caucasus ice volume and cover an area of 112.6 km 2 (Kutuzov, et al., 2015) (Fig. 1). Glaciers cover an altitudinal range from 2800 to 5642 m a.s.l. with the coldest conditions present above 5200 m a.s.l. where mean summer air temperature stays below 0°C.
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 10 model (Draxler & Hess, 1998;Stein et al., 2015) and NCEP/NCAR Reanalysis data on 2.5×2.5 degree grids (Kistler et al., 2001) for the 1948-2013 period. 10 day backward trajectories were calculated for every 6 hours for the whole period, resulting in a total of ~100k modelled backward trajectories. Over this time period, air particles transport is defined by the westerlies. Elbrus glaciers receive air particles highly likely polluted with aerosols from sources located in Turkey and the Mediterranean, Eastern and Central Europe, the Middle East, North Africa and Southern Russia (Fig. 1b). 15 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 day 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 20 trajectory as the height exceeding potential air temperature over surface air temperature by 2 K (Draxler & Hess, 1998 (Mikhalenko et al., 2015). Ice cores were packed in insulated core boxes and shipped in a frozen state to the cold laboratory of the Institute des Géosciences de l'Environnement (IGE) in Grenoble, France for analyses. A total of 3724 samples were prepared from the surface (2009) down to 168.6 m of the Elbrus core. Cores were subsampled and decontaminated at -15°C using the pre-cleaned electric plane tool methodology described in . In brief, in a first step, ice 30 samples were cut with a band saw. After that, all surfaces of the cut samples were cleaned under a clean air bench by using a pre-cleaned electric plane tool over which the ice was slid. Sampling was continuous along the core. To control the decontamination efficiency process blank ice samples, consisting of ultrapure frozen MilliQ water were preceded regularly.
We determined cations (Na + , K + , Mg 2+ , Ca 2+ , and NH 4 + ) using a Dionex ICS-1000 chromatograph equipped with a CS12 separator column. For anions, a Dionex 600 equipped with an AS11 separator column was used with an eluent mixture of 5 H 2 O, NaOH at 2.5 and 100 mM, and CH 3 OH. A gradient pump system allows the determination of inorganic species (Cl -, NO 3 -, and SO 4 2-) as well as short-chain carboxylates . For all investigated ions, ice decontamination blanks were insignificant compared to respective concentrations in the ice cores.

Dating the ice core
Seasonal ice-core stratigraphy of chemical parameters and ice-core dating based on annual layer counting of the deep Elbrus 10 ice core is described in detail in (Mikhalenko et al., 2015). The seasonal oscillations of NH 4 + and succinic acid allow dating the core with seasonal resolution. Based on the ammonium and succinic acid profiles, each annual layer was divided into two parts corresponding to snow deposition under winter conditions and spring-summer-autumn conditions (Preunkert et al., 2000;Legrand et al., 2013;Mikhalenko et al., 2015). The annual counting was confirmed with 1 year uncertainty over the last one hundred years by reference horizons from a tritium peak (1963) and the Katmai 1912 horizon (Mikhalenko et al., 15 2015). Though the annual counting becomes less straight forward prior to 1860, Mikhalenko et al. (2015) reported an ice age It show be noted that Fig. 2 show the thickness of annual layers and does not represent the linear change in accumulation 25 rate. In order obtain an accumulation rate the layer thickness must be corrected for the compression which occurred since it was deposited (e.g. Paterson and Waddington, 1984) which is out of the scope of this paper.
In addition, a 20.5 m long ice core was extracted in June 2013 at the same location to expand the existing record from 2009 to 2012. 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 NH 4

Climatic data analysis
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 (Dee et al., 2011). Since the variables used in this study are continues and normally distributed 5 the Pearson's correlation coefficient was used. The statistical significance of the correlation was checked using Student's tdistribution for probability. The correlations between Ca 2+ concentrations in the ice core and each climatological parameter fields were computed.
It was shown that this index is preferable among the other drought indices to investigate the relation of dust emission to 10 droughts (Achakulwisut et al., 2018). We considered SPEI calculated over 1, 2, 3, 6 and 12 month. The time series of SPEI were obtained by averaging over the regions of interest.
To identify the possible factors which influence the dryness in the dust sources and dust transport to the Caucasus Mountains, we also investigated the Ca 2+ record in relation with various circulation indices.

Results and discussion 20
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 25 areas with calcareous soils. We consider that all of the Ca 2+ in the Elbrus ice core is of natural origin and therefore Ca 2+ concentrations can be considered as a proxy for dust.
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 30 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 Na + present in the Elbrus melted ice samples is only related to sea-salt emissions, and considering the [Na + ]/[Ca 2+ ] ratio of 0.038 in seawater, we find that only 1.0± 0.7 % in summer (1.40 ± 1.0 % in winter) of total Ca 2+ may be attributable to sea-salt emissions. That percentage is clearly an upper limit since, in precipitation deposited at continental free tropospheric sites (e.g. Legrand, 2002), Na + is not only related to sea-salt due to the presence of leachable sodium in alumino-silicate particles but also Na + from halide evaporates present in 5 the deserts.
The volcanic rocks of Elbrus near the drilling site do not contain calcite and we can therefore assume that Ca 2+ present in the Elbrus ice archive information on past dust emissions, including continuous background emissions and large dust plumes reaching the site. In the following sections, we attempt to separate these two possible calcium sources.

Calcium concentrations during desert dust events and background conditions 10
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 while summer (June-August) reveals possible transport from the Caspian Sea region and Southern Russia (Fig. 3).
Large dust plumes originating from the Middle East and less frequently from the Sahara reach the Caucasus (Kutuzov et al., 15 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 because either they were present in dust at the emission stage or, being acidic, they interacted 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 of the snow and glaciers and enhance the response of the mountain cryosphere to climate changes 20 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. (this issue) Elbrus samples are considered to be impacted by dust events if they contain more than 120 ppb of calcium and are also below the 25% quartile of a robust spline through the calculated 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 25 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 (Fig. 4). In Fig. 5, we report the seasonal cycle of calcium and ammonium averaged over two different periods of the 20 th century (1900-1950 and 1950-2010).
The mean concentration of Ca 2+ was 145 ppb with a maximum of 5506 ppb. Most of the dust from long distance transport is 30 deposited on Elbrus during warm seasons. The average background summer Ca 2+ concentration was 103 ppb compared to 44 ppb in winter layers. Most of the large dust deposition events also occur during the warm season which further increases the difference between summer (178 ppb) and winter (54 ppb) layers (Table 1).
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 5 the 1960s with a following decrease in late 1970s. Exceptional peaks compared to the background occur during the following years: 1802, 1957, 1863, 1917, 1933, 1963, 1984, 1989, 1999, 2000, 2008 and 2009.

Increasing frequency of desert dust events
Warm season layers contain an increase in Ca 2+ throughout the entire investigated time period. The most pronounced increase in peak Ca 2+ concentrations occurred after 1950s (Fig. 6). It is important to emphasize that this appearance of more 10 frequent calcium peaks after 1950 compared to the preceding decades cannot be attributed to a decreasing time resolution of the record with a smoothing of events with depth ( Fig. 2). More specifically, the enhanced occurrence of summer calcium peaks between 1960 and 1970 compared 1950-1960 does not correspond to a significant decrease of the temporal resolution (from 9 samples over the 1960-1970 years compared to 11-12 samples over the 1950-1960 years).
In recent decades , over which time period more dust events were detected in the Elbrus ice, a clear spring 15 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). Recent major long-range dust outbreak event over North Africa, eastern Mediterranean, and Caucasus occurred on 22 and 23 March 2018 [Solomos et al., 2018] and resulted in a significant dust deposition on glaciers.
The calcium peaks in Elbrus ice containing dust are accompanied by an increase of magnesium (Fig. 7). Interestingly, the 20 mean [Mg 2+ ]/[Ca 2+ ] ratio in Elbrus samples containing dust (0.035, Fig. 7) is similar to those in atmospheric dust aerosols from the Sahara or Middle East. For instance, Koçak et al. (2012) reported dust event related aerosol concentrations of sodium, magnesium, and calcium from two Eastern Mediterranean sites, i.e. from Erdemli (Turkey) with dust arriving from Middle East and from Heraklion (Crete) with dust from Sahara. It is important to emphasize that, as for the Elbrus ice core, the atmospheric concentrations of these cations correspond to their water-soluble fraction (not the total fraction), which were  (Table 2). A similar value is obtained for aerosol at Heraklion during a Saharan dust event (0.043, Table 2). The content of Mg 2+ in Elbrus samples impacted by dust is therefore very consistent with what is observed in atmospheric aerosol from the Eastern Mediterranean region during dust events. Figure 6 demonstrates an increase in the calcium background concentrations after 1950. This increase may be influenced by 5 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, was reported by (Kalderon-Asael et al., 2009) demonstrating that, under strong stratification in the lower atmosphere in Israel, part of the atmospheric calcium may be anthropogenic nature. At the scale of Europe, Lee and Pacyna (1999) estimated that 0.8 Tg of anthropogenic calcium are emitted per year, coal combustion contributing for 60% and 10 cement for 30% of total. To date, these anthropogenic calcium emissions remain however one order of magnitude weaker than dust calcium emissions from North-East Africa (12 Tg yr -1 ) or West Asia (12.7 Tg yr -1 ) .

Enhanced background concentrations
Particles emitted during both coal combustion and cement production are rich in calcium (calcite). We therefore may expect a weaker [Mg 2+ ]/[Ca 2+ ] ratio in particles emitted by these anthropogenic processes compared to that in natural dust particles.
Examination of Mg 2+ and Ca 2+ in Elbrus ice layers that not impacted by dust events helps to determine a possible 15 contribution of anthropogenic activities to the increasing background calcium trend in Elbrus ice. As seen in Fig. 8 ] to a growing impact over the recent decades of calcium from cement (neglecting the presence of soluble magnesium in cement), the mean increase of Mg 2+ red. level (10.8 ppb over the 1774-1920 years and 17.6 ppb over the 1960-2010 years) would lead to a Ca 2+ red. increase of 55 ppb from 1774-1920 to 1960-2010. This 20 value is half of the observed Ca 2+ red. increase between the two time periods (71 ppb over 1774-1920 and 183 ppb over 1960-2010). These calculations suggest that half of the observed increase of background calcium concentration after 1950 are attributable to anthropogenic activities. As seen in Fig. 9, even prior to 1950 the [Mg 2+ red. ]/[Ca 2+ red. ] ratio sometimes dropped to values as low as 0.1 or less (around 1820, 1850, 1865, or 1908 Gaudichet, 1991). The authors demonstrate that both the increase of the frequency of dust arrival in the 1980s and the background dust levels occurred without any coinciding decrease of the aluminium to calcium ratio. Given the low aluminium content of cement compared to desert dust, these observations suggest that the Col du Dome site is not significantly impacted by growing use of cement. We may expect that this impact is even weaker at the Elbrus site.

Climatic factors
Two major dust sources contribute mineral particles to the Elbrus glaciers, the Sahara and the Middle East. It was established that majority of the small scale dust sources in the Middle East are located in northern Mesopotamia (northern Syrianorth-5 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 Neinava 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 Ahaggar Mountains in eastern Algeria may reach the Caucasus in the 10 spring (Kutuzov et al., 2013).
A statistically significant spatial correlation occurs between Ca 2+ and precipitation as well as between soil moisture content in the Middle East (Fig. 10). Unlike in the Middle East, no spatial correlation was found between Ca 2+ and the amount of precipitation in North Africa. It should be noted, however, that the observation network is very sparse in the arid areas and there are much larger uncertainties in Global Precipitation Climatology Centre (GPCC) datasets in North Africa. 15 The Ca 2+ record was also compared with drought indices in potential dust sources in the Middle East (32-37 N; 38-45 E) and in North Africa (20-35 N; 0-35 E) (Fig. 10). Ca 2+ significantly correlates with drought indices for both regions. The highest correlations were found for the SPEI 3 index which is determined by aridity over the three previous months. The correlation coefficient for the Middle East is statistically significant (p<0.001) and reaches 0.71. The correlation remains significant after trend removal (r=0.48, p<0.001 for 1904-2012 and 0.63 (p<0.001) for 1970-2012) (Fig. 11b, d). Periods of 20 dryer conditions in the Middle East region coincide with the increased Ca 2+ concentrations and vice versa. During the period of droughts in both regions, a greater amount of mineral particles are emitted into the atmosphere, and transported to Caucasus glaciers during the spring and summer. The general increasing trend in dust content corresponds to negative trends in precipitation and increased dryness of the soil.
Ca 2+ correlates significantly with the SPEI 3 drought index for the North African region (r=0.69 p<0.001) but most of the 25 correlation is defined by similar tendencies in two series. Drier conditions at the dust source region correspond to a positive trend in dust concentrations in the Elbrus ice core. The correlation weakens for the full detrended series (r=0.30, p<0.05) and increases in the later period, reaching 0.51 (p<0.001) in 1970-2012 (Fig. 11a, c). Large portions of the annual dust flux in Elbrus can be deposited during a single strong deposition event. Such events are more often associated with dust transport from the Sahara (Kutuzov et al., 2013). Dust emitted from North Africa can also mix with dust from Middle Eastern sources 30 during transport (Shahgedanova et al., 2013).
As evident from the Elbrus ice core record, the frequency of dust events and total Ca 2+ concentrations has increased, where the most notable trend occurs after the 1950s. This increase corresponds with the recent analysis of centennial climate change trends in Africa which depict a significant northward expansion of the Saharan desert in the winter (Thomas & Nigam, 2018). The desert conditions are observed across larger territories in recent years and are associated with a slightly negative trend in winter precipitation and an increase in surface air temperatures. In coastal North Africa, dry conditions dominated and rainfall decreased since the 1980s (Nicholson et al., 2018). The number of dust days in the eastern Mediterranean increased over the period 1958-2006(Ganor et al., 2010. 5 Two periods of maximum Ca 2+ concentrations in the ice core correspond to the two most severe drought episodes in the Middle East since the1940s, and occur during 1998-2000(Barlow et al., 2016. The significant precipitation decline (up to 70%) during these years is explained by the dominance of the high pressure systems over the eastern Mediterranean during the winter and spring months (Trigo et al., 2010). 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 & Pierre, 2018).
Unsustainable agricultural practices, overgrazing, and deforestation may significantly increase the area of the dust sources. It should be noted however that only around 5% of the land in North Africa and the Middle East is suitable for agriculture; the 20 rest consists of pastures, forests, shrubs, urban zones, badlands, rocky areas, and deserts (Sivakumar & Stefanski, 2007).

Atmospheric circulation patterns
The general increase in dust concentrations in the Elbrus ice core are accompanied by a quasi-decadal variability.
The relationship of precipitation in the Middle East region to the different large-scale circulation patterns are summarized in a recent review of the droughts in the Middle East and Southwest Asia (Barlow et al., 2016 and references therein). The 25 precipitation and occurrence of droughts in the Middle East region may be influenced by several major climatic features such as the NAO, southwest 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 , significant correlations exist between Ca 2+ and PDO, SOI and Niño 4 in the preceding winter period (Table 3). A significant correlation (r=0.47 p<0.01) for the Niño 4 index also exists for Dec-June in 30 1948-2012. The relationship between dust concentrations and circulation indices of the Pacific Ocean is illustrated by the spatial correlation of Ca 2+ with sea surface temperatures ( Supplementary Fig. 1). 23 out of 47 years of high dust deposition recorded in Elbrus ice core coincide with negative Niño 4 phases in 1900-2012. After the 1950s 19 out of 23 years of increased dust concentration correspond to the negative Niño 4 and negative PDO (Fig. 12). No statistically significant correlation was found between the dust in the Elbrus ice core and the NAO index. Ca 2+ also correlates with the SOI index of the preceding autumn and winter (Nov-Jan) over the last 50 years. It is still unclear whether circulation in the Pacific played a significant role in the precipitation patterns in these dust source regions before the mid-20 th century or not. The uncertainties in reanalysis data in this period are large and therefore do not allow us to draw any conclusions about the 5 stability of the revealed correlations in time.
A significant negative correlation was found between atmospheric dust concentrations in Syria and the PDO in springtime during 2003-2015. 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 & Ginoux, 2016). A positive PDO is characterised by an increase of cyclonic activity over the northern Pacific and northern Atlantic which occurs together with the intensification of subtropical 10 antyciclones. 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 & 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 15 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 20 & 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), 25 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, shift precipitation patterns, and lead to increasing 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 & 30 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 21 st 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° latitude was identified in both hemispheres although the mechanisms behind this tropical expansion are still unclear (Lucas et al., 2014).

Comparison to other paleo records
The first record of dust deposition history in the Caucasus was obtained by F.F. 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 asl). 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 1920's. This increase was attributed to various reasons including local dust 5 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 Ca 2+ concentration over the same periods increases by a factor of 2.5 from 88 ppb in 1793-1925 to 224 ppb in 1925-1962. An overall increasing trend in dust content and Ca 2+ exists in Colle Gnifetti (Alps) ice cores after 1870 (Thevenon et al., 10 2009). Unlike in the Elbrus Ca 2+ records, no significant long-term trend over the past 300 years exists in the Alps (Bohleber et al., 2018). In Central Asia, negative trends in dust concentrations were found in ice cores in Tien Shan (Grigholm et al., 2017) and in Tibetan Plateau (Grigholm et al., 2015). After the 1950s, calcium concentrations and multiannual and decadal variability significantly decrease in both Central Asian regions due to changes in dust storm frequency in arid areas related to regional trends in reduced zonal wind strengths (Grigholm et al., 2015). 15 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 ~250 years which was attributed to the impact of anthropogenic activity in source regions (Hooper and Max, 2018). Previous estimates for the atmospheric dust variability also suggests that desert dust doubled during the 20 th century with some exceptions (Mahowald et al., 2010). All of the existing regional and global estimates of atmospheric dust variations in the past have large uncertainties due to limited and unevenly distributed 20 paleodata records, which is especially true for the important dust sources of the North Africa, East Asia and Middle East/Central Asia (Mahowald et al., 2010). Tree ring reconstructions of the June-August drought variability over the past 900 years in the Mediterranean revealed that 1998-2012 was the driest period in the Levant since the 12 th century (Cook et al., 2016). Similarly, the latter half of the 20 th century was found to be one of the driest in the last nine centuries in north-western Africa (Touchan et al., 2011). 25 Understanding 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 occupation 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 30 findings (Cook et al., 2016;Kelley et al., 2015) that the recent droughts in 1998-2012 were the most severe over at least the past three centuries.

Conclusions
This paper presents the Ca 2+ record from the Elbrus ice core and provides a valuable archive of regional atmospheric dust variability in the past. Ca 2+ concentrations in the Elbrus ice core are a proxy for atmospheric dust transported over long distances from primary dust sources in North Africa and the Middle East. The Ca 2+ record shows that recent decades were characterized by the highest dust emission activity since 1774. From the 1970s onward, Saharan dust 5 deposition events significantly impact the dust deposition in the Caucasus. The increase in frequency and intensity of dust deposition events is related to general trend towards the dryer conditions and therefore higher probability of dust emission in the dust sources. Ca 2+ significantly spatially correlates with precipitation and soil moisture content in the Middle East. The correlation between circulation indices and amount of dust in the Elbrus ice core indicate that the influence of circulation over the Pacific Ocean extended to the Middle East. 10

Data availability
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).