1 Introduction

Dust transport from the Saharan region to Europe is linked to the state of the climate system (Middleton & Goudie, 2001), yet variability in the intensity of these events is poorly constrained for the past two millennia during which both natural and anthropogenic climate change has occurred. Saharan dust influences terrestrial and ocean biogeochemical ecosystems, human health, and radiative properties of t2001). Due to the importance of Saharan dust events (SDEs) to natural systems and human environments, and given the uncertainty of future occurrences of these events in a changing climate, several studies have looked to the past as an analog to understand how SDEs are connected to climate (e.g., Angelis & Gaudichet, 1991; Bohleber et al., 2018, Maupetit & Delmas, 1994, Schwikowski et al., 1995; Schwikowski et al., 1999; Thevenon et al., 2009; Wagenbach et al., 1996; Wagenbach & Geis, 1989). To further investigate SDE transport to Europe and their response to changing climates, we present in this paper the longest, continuous, ultrahigh‐resolution ice core record yet produced reflecting Saharan dust transported to the European Alps. This record was developed using an innovative technique of compiling continuous measurements from ultrahigh resolution (120‐μm) laser ablation‐inductively coupled plasma‐mass spectrometer (LA‐ICP‐MS) technology, applied to the Colle Gnifetti ice core below the firn‐ice transition.

Europe and surrounding regions in the Northern Hemisphere (NH) experienced several anomalous global climate shifts during the late Holocene including the Medieval Climate Anomaly (MCA; ~900–1300 CE) (e.g., Berner et al., 2011; Bradley et al., 2003; Lamb, 1965; Mann et al., 2009; Mann & Jones, 2003; Xoplaki et al., 2011), and the following colder era, the Little Ice Age (LIA; ~1350–1900 CE) (e.g., Bradley & Jonest, 1993; Grove, 1988; Mann et al., 2008; Moberg et al., 2005). Hypothesized causal mechanisms of these long‐term climate events include changes in total solar irradiance (Renssen et al., 2006; Shindell et al., 2001), volcanic aerosol loading into the atmosphere (Crowley, 2000; Shindell et al., 2003; Otterå et al., 2010), and ocean circulation (Broecker, 1991; Crowley, 2000). Atmospheric aerosol composition and air mass transport changes provide both a mechanism for and an indicator of transitions in and out of anomalous climate periods and can be measured through chemical fingerprinting of air masses captured in ice cores (Meeker & Mayewski, 2002). Saharan dust deposition reconstructions from 240 ka marine sediment records show low latitude dust deposition does not parallel the variability of glacial‐interglacial changes seen in mid‐latitude and high latitude dust emission. Rather, Skonieczny et al. (2019) identified a high correlation to summer insolation on millennial timescales, indicating a significant African monsoon influence on low latitude dust transport. However, relatively little is known about Saharan dust transport during significant climate events of the Common Era such as the LIA and MCA (e.g., Thevenon et al., 2009, 2011; Bohleber et al., 2018), and understanding the variability and causality of past climate anomalies can offer essential insights for future climate change (e.g., Antoine & Nobileau, 2006).

Saharan dust transported across the Mediterranean to Central Europe originates from North African sources, predominantly during the spring and summer months, and is typically carried at least 5,000 m above sea level (m a.s.l.; Prospero, 1996; Prospero et al., 2005). Major modern source regions of Saharan dust to Europe include three main areas: Western Sahara, Moroccan Atlas, and northern/central Algeria, confirmed through trajectories analyses from northeastern Spain to their area of origin (Avila et al., 1997), also noted in Scheuvens et al. (2013). The transport and deposition of Saharan dust over the Mediterranean in the summer depends on favorable conditions for entrainment and transport of dust above the boundary layer (Gaetani & Pasqui, 2014). Past research has shown dust emissions are highest when peak surface heating moves from the Sahel to the central Sahara during the summer months during the northward extension of the Intertropical Convergence Zone (ITCZ), a heavy precipitation band caused by converging northeasterly and southeasterly winds (Engelstaedter et al., 2006). The ITCZ has a southernmost position in the winter months and northernmost in the summer, causing dry convection and convergence, thereby enhancing near surface turbulence and facilitating dust transport (Engelstaedter et al., 2006). Favorable conditions for SDE incursions into Europe during the summer months are influenced by a steepening of the pressure gradient along a strong subtropical high (Azores High), coupled with a northeastward shifting of the Saharan High, located on the northern boundary of the Sahara, into the Mediterranean and a southeast shift in the Icelandic Low (Barkan et al., 2005). Longer transport pathways are also documented from back trajectories reaching out over the North Atlantic prior to transport to Europe (e.g., Schwikowski et al., 1995; Collaud Coen et al., 2004; Sodemann et al., 2006; Thevenon et al., 2011). Although infrequent, Saharan dust has the potential of reaching as far north as the British Isles following an anticyclone over western Europe (Wheeler, 1986; Coudé‐Gaussen, 1989).

The variability of summer SDEs on an interannual to decadal time scale during the modern era has been explained by anthropogenic forcings and ocean‐atmospheric teleconnections such as the North Atlantic Oscillation (NAO), El Niño Southern Oscillation (ENSO), the ITCZ, and the Atlantic multidecadal oscillation (AMO). Human‐induced soil degradation and increased droughts in the Sahel are linked to increases in dust emissions across the Atlantic (Moulin & Chiapello, 2006). However, anthropogenic influences (e.g., desertification, land‐use changes, and climate change) on Saharan dust emissions are difficult to resolve precisely due to large natural variability in the arid Saharan region (Moulin et al., 1997; Middleton & Goudie, 2001). For example, we do not yet have the capability to estimate the percentage of anthropogenic dust (Engelstaedter et al., 2006). According to Moulin et al. (1997), enhanced dust transport across the Mediterranean is linked with an increased pressure gradient (r = 0.66, p = 0.027) associated with the winter NAO index, the difference in normalized winter sea level pressures between Lisbon, Portugal, and Stykkisholmur, Iceland (Hurrell, 1995), along with eastward transport over the North Atlantic (r = 0.50, p = 0.097). Years of high (low) NAO indices are identified by a stronger (weaker) Azores high coupled with a below (above) normal Icelandic Low‐pressure system, causing drier (wetter) conditions over southern Europe, the Mediterranean Sea and northern Africa, therefore increasing (decreasing) dust mobilization and transport across the Mediterranean and North Atlantic. Shifts in the ENSO are coupled with changes in lower tropospheric atmospheric circulation over North Africa, causing stronger winds towards the Atlantic Ocean, and therefore enabling SDEs (DeFlorio et al., 2016; Prospero & Nees, 1986; Prospero & Lamb, 2003; Rodríguez et al., 2015). A southward displacement in the ITCZ, along with a colder North Atlantic Ocean (negative phase AMO; Wang et al., 2012), prompts a decrease in precipitation and increased surface winds over dust‐producing regions in the Southern Sahara leading to amplified SDEs over the North Atlantic, therefore also contributing to Mediterranean transport (Doherty et al., 2012; Doherty et al., 2014). Periods of increased aridity in the Sahara (Middleton, 1985; Littmann, 1991) and decreased rainfall in the Sahel (Prospero & Lamb, 2003) caused by ocean‐atmospheric teleconnections can also play a major role in dust production.

In agreement with Evan et al. (2016), we speculate that any phenomenon that facilitates the transport of dust can lead to an increase in SDEs. Saharan dust, along with other natural and anthropogenic aerosols transported across the Mediterranean, are deposited at high elevation glaciers in the European Alps (e.g., Angelis & Gaudichet, 1991; Maupetit & Delmas, 1994; Schwikowski et al., 1995; Schwikowski et al., 1999; Wagenbach & Geis, 1989; Wagenbach et al., 1996). Because of the close redundant proximity of the alpine glaciers to population centers, ice core records from these areas capture and preserve unique records of natural and anthropogenic aerosol transport not available through polar ice cores. Ice cores from this region contain the southernmost such records available for the North Atlantic. These records of past dust and sea‐salt aerosols offer a history of atmospheric circulation, including strength of wind and pressure systems and changes in air mass sources. Previous ice core studies of aerosols throughout the European Alps include: Col du Dôme, Mont Blanc (Preunkert et al., 2000); Fiescherhorn, Bernese Alps (Schwerzmann et al., 2006); Ortles, Eastern Alps (Gabrielli et al., 2016); and Colle Gnifetti and Colle del Lys in the Monte Rosa region (e.g., Wagenbach et al., 2012, and references therein).

Situated in the heart of the European Alps, Colle Gnifetti (CG) glacier (4,500 m a.s.l) stands out as the only nontemperate site where net snow accumulation is low enough to record environmental signals over at least the last two millennia (Bohleber et al., 2018). Previous studies of CG demonstrate it is an ideal site to examine past changes in natural and anthropogenic source aerosols to the European Alps region (e.g., Bohleber et al., 2018; Loveluck et al., 2018; Lugauer et al., 1998; Meola et al., 2015; More et al., 2017; Schotterer et al., 1985; Schwikowski et al., 1995; Thevenon et al., 2009; Wagenbach & Geis, 1989). Due to transport and deposition in relation to meteorological conditions, the primary and most consistently deposited aerosol transported to CG is Saharan dust (Figure 1; Schotterer et al., 1985). Precise source locations of dust to the CG region have not been well identified, however back trajectories in concert with chemical tracer measurements by Thevenon et al. (2011) specify regions of Algeria and north‐central to north‐western part of the Saharan desert (i.e., Morocco, Tunisia, Libya, and Mali), following similar conclusions by Schwikowski et al. (1995). Lugauer et al. (1998) noted the most pronounced aerosol variability captured in modern snow at CG is accumulated during the summer months with some variability recorded during fall and spring and minimal amounts in the winter months.

Figure 1 Open in figure viewer PowerPoint 1995 2011 Summer trajectories for Saharan dust transport to Colle Gnifetti. Idealized modern summer trajectories for Saharan dust from Hysplit back‐trajectories in Schwikowski et al. () and Thevenon et al. () (brown arrow) and marine airmass transport (blue arrow) to the Colle Gnifetti ice core site in the Swiss‐Italian Alps (red star). Map created using the Basemap module in Python 3.6 and ESRI_Imagery_World_2D from arcgis.

Building upon the pioneering framework established by Schotterer et al. (1985), Wagenbach and Geis (1989), Schwikowski et al. (1995), and Lugauer et al. (1998) on Saharan dust transport to the European Alps, our research facilitates an advance in understanding aerosol transport to the CG region by utilizing a novel ultrahigh‐resolution, continuously sampled, two millennia‐long, subannually dated dust record, from a core collected in 2013 (Bohleber et al., 2018). Previous studies on the 2013 CG ice core have formed a reliable ice core chronology through the combination of annual layer counting paired with previously well‐known horizons and newly discovered tephra layers (Bohleber et al., 2018; Loveluck et al., 2018; Luongo et al., 2017; More et al., 2017). Based on annual layer counting and cryptotephra analysis, the CG record goes back to at least 1 CE at 61 m (abs) depth with the possibility to extend back even further (Bohleber et al., 2018; Loveluck et al., 2018).

We investigate the recent portion (1780–2006 CE) of the 2013 CG ice core record using annually resolved discretely sampled inductively coupled plasma‐mass spectrometer (ICP‐MS) measurements for signatures of potential air mass sources, then calibrate to modern climate reanalysis data and known SDEs. Using this recent portion of the record as an analog for SDEs, we extend our record back to 1 CE using subannual (0.02 year) and annually averaged LA‐ICP‐MS 56Fe, followed by the identification of major dust episodes at 121‐μm resolution. The Climate Change Institute's W. M. Keck Laser Ice Facility LA‐ICP‐MS sampling system was used to collect a total of 316,000 data points for dust element 56Fe over 20‐m of core (1–1820 CE). For the LA‐ICP‐MS 56Fe raw data, an average of 570 data points was collected per year for the first 100 years (1720–1820 CE; 3.3 m) and an average of 170 data points per year for the entire 20‐m of core (1–1820 CE). The maximum data points collected in 1 year for this record is 1,187 in 1818 CE This research expands on previous studies that validate LA‐ICP‐MS technique (Bohleber et al., 2018; Haines et al., 2016; Mayewski et al., 2013; More et al., 2017; Sneed et al., 2015; Spaulding et al., 2017). Applying this novel technique to alpine ice cores allowed us to develop the longest continuous summer Saharan dust record and provide detailed interpretations of past atmospheric conditions on subannual to storm‐scale event scales.