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date: 21 November 2019

Alpine Ice Cores as Climate and Environmental Archives

Summary and Keywords

The European Alps feature a unique situation with the densest network of long-term instrumental climate observations and anthropogenic emission sources located in the immediate vicinity of glaciers suitable for ice core studies. To archive atmospheric changes in an undisturbed sequence of firn and ice layers, ice core drilling sites require temperatures low enough to minimize meltwater percolation. In the Alps, this implies a restriction to the highest summit glaciers of comparatively small horizontal and vertical extension (i.e., with typical ice thickness not much exceeding 100 m). As a result, Alpine ice cores offer either high-resolution or long-term records, depending on the net snow accumulation regime of the drilling site. High-accumulation Alpine ice cores have been used with great success to study the anthropogenic influence on aerosol-related atmospheric impurities over the last 100 years or so. However, respective long-term reconstructions (i.e., substantially exceeding the instrumental era) from low-accumulation sites remain comparatively sparse. Accordingly, deciphering Alpine ice cores as long-term climate records deserves special emphasis. Certain conditions must exist for Alpine ice cores to serve as climate archives, and this is important in particular regarding the challenges and achievements that have significance for ice cores from other mountain areas: (a) a reliable chronology is the fundamental prerequisite for interpreting any ice core proxy time series. Advances in radiometric ice dating and annual layer counting offer the tools to crucially increase dating precision in the preinstrumental era. (b) Glacier flow effects and spatio-seasonal snow deposition variability challenge linking the ice core proxy signals to the respective atmospheric variability (e.g., of temperature, mineral dust, and impurity concentrations). Here, assistance comes from combining multiple ice cores from one site and from complementary meteorological, glaciological, and geophysical surveys. (c) As Alpine ice cores continue to advance their contribution to Holocene climate science, exploring the link to instrumental, historical, and other natural climate archives gains increasing importance.

Keywords: European Alps, Alpine ice cores, summit glaciers, ice core dating, paleoclimate, Colle Gnifetti

Introduction

Natural climate archives can provide us with fundamental insights into the earth’s climate system, in particular regarding assessing the anthropogenic impact in relation to the natural climate variability. Among these archives, cold1 (nontemperate) glaciers store a unique range of past environmental signals, since they comprise both past precipitation (as snow and ice) and atmospheric air (as enclosed bubbles). This information can be retrieved by drilling and analyzing ice cores. Ice cores drilled at polar ice sheets have made invaluable contributions to the quantification of past changes in temperature, atmospheric circulation patterns, and biogeochemical cycles, up to more than 800,000 years ago (e.g., Dahl-Jensen et al., 2013; EPICA Community Members, 2004; Legrand & Mayewski, 1997). However, ice core records from the polar regions are not necessarily representative for mid and low latitudes, especially for short-lived atmospheric constituents such as aerosol-related impurities and the stable water isotopic signature of precipitation. At least for the Holocene period, important complementary information can come from ice cores drilled at cold (nontemperate) glaciers and ice caps of high mountain ranges in nonpolar areas. Respective investigations have been performed in various mountain ranges over the world, including central Asia (e.g., Grigholm et al., 2016; Aizen et al., 2006), the Himalayas (e.g., Kang et al., 2002; Thompson & Yao, 2000), the Andes (e.g., Eichler et al., 2017; Thompson et al., 2013), some selected sites in the tropics (e.g., Thompson et al., 2002, 2003), and the European Alps.

In the Alps, the extensive ice coring efforts started in the 1970s result in a unique array of drilling sites at cold but also at polythermal (partially temperate) (Gabrielli et al., 2016) and temperate sites (Pavlova et al., 2015; Stichler, Baker, Oerter, & Trimborn, 1982). Ice cores drilled to bedrock at cold glaciers include Colle Gnifetti (CG; Oeschger, Schotterer, Stauffer, Haeberli, & Röthlisberger, 1978; Wagenbach, Bohleber, & Preunkert, 2012) and Col del Lys (CDL) in the Monte Rosa region (Smiraglia, Maggi, Novo, Rossi, & Johnston, 2000), Col du Dôme (CDD), Mont Blanc (Preunkert, Wagenbach, Legrand, & Vincent, 2000), as well as Fiescherhorn (FH), Bernese Alps (Schwikowski, Brütsch, et al., 1999). Alpine ice cores2 offer a broad range of climate-related signals (“proxies”), including stable water isotopes and various inorganic and organic impurity species (related to deposition via aerosols and as short-lived trace gases). The respective ice core records constitute an archive regarding pre-industrial levels, natural variability, and the anthropogenic impact. However, the limited glacier thickness (typically not much exceeding 100 m) requires making a trade-off between high temporal resolution and long-term ice core records, offered by sites with high and low net snow accumulation, respectively. Among the cold sites, only CG has a net snow accumulation low enough to, in spite of the limited glacier depth, provide for records substantially exceeding the instrumental period at a reasonable time resolution (Bohleber, Erhardt, et al., 2018; Jenk et al., 2009). In addition to the many ice core drillings, Europe also offers the densest network of long-term instrumental climate observations (Auer et al., 2007; Böhm et al., 2010) and a nexus of climate reconstructions from other natural archives (Trachsel et al., 2012).

The success of any mountain ice core study targeting climate-related records fundamentally depends on the glacio-meteorological settings of the drilling site. Ice core studies at mid-latitudes differ fundamentally from their counterparts at polar ice sheets due to their proximity to continental sources and their comparatively complex small-scale geometry. The latter leads to distinct gradients in glacier geometry, ice flow, and net snow accumulation, complicating the age scale determination as well as the interpretation of the ice core proxy signals in terms of past atmospheric change. Early on, it was recognized that ice core studies performed in the Alps are particularly affected by these challenges (Wagenbach, 1989). Subsequently, it was shown how the glacio-meteorological settings need to be considered all the way from site selection to ice core record interpretation (Wagenbach, 1992).

Within this framework, this article revisits the circumstances allowing Alpine ice cores to work as climate archives and discusses the resulting challenges in retrieving information from this archive. A detailed overview on the manifold proxy time series derived from Alpine ice cores has already been presented elsewhere (see, e.g., Preunkert & Legrand, 2013; Schwikowski, 2004). This article includes the latest advances made in mastering the special set of problems of studying Alpine ice core as climate archives, such as ice core dating and atmospheric signal identification. In order to present the matter in some adequate detail, the focus is confined to lessons learned at well-explored cold drilling sites and mainly comparatively short-lived chemical and stable water isotopic ice core proxy species (as opposed to long-lived species, e.g., some trace gases).

In spite of the geographical confinement to the Alps, the special array of ice cores and dense network of instrumental as well as other proxy records of this region make it a unique benchmark: The glaciological properties of many other cold non-polar drill sites are not substantially different from Alpine sites regarding geometry, energy-, and mass balance. As a result, the Alpine findings have broader significance for deciphering paleoclimatic records from non-polar ice cores.

Alpine Ice Cores as Climate and Environmental ArchivesClick to view larger

Figure 1. Well-explored drilling sites for climate-related ice core studies in the European Alps. Shown as blue dots are cold sites in the Western Alps, within the Mont Blanc and Monte Rosa range, as well as at FH. Also shown is the polythermal (partially temperate) site at Ortles (O) mountain in the Eastern Alps. Elevation is color-coded using the digital elevation model of Jarvis, Reuter, and Nelson (2008).

Prerequisites of Alpine Glaciers to Serve as Ice Core Drilling Sites

An ideal drilling site archives past precipitation as a quasi-continuous sequence of snow, firn, and ice layers, thereby facilitating the preservation of atmospheric signals in a datable ice core record. The suitability of a glacier to reliably record atmospheric signals in this respect is determined by local glacio-meteorological and topographical conditions. First and foremost, this concerns the influence of meltwater percolation. Since meltwater disturbs the chemical and isotopic signals stored in the sequence of layers (Eichler, Schwikowski, & Gäggeler, 2001; Schotterer, Stichler, & Ginot, 2004), meltwater percolation needs to be absent or minimal, which is only guaranteed in cold firn, more specifically within the so-called recrystallization, recrystallization-infiltration, and cold infiltration zones. In these zones, meltwater percolation is absent or restricted to less than one annual snow layer, respectively (Shumskii, 1964).

Regarding site selection in the Alps, high elevation is the primary factor to ensure the required cold firn temperature regime (Fig. 1). Slope, aspect (determining insolation), and net snow accumulation of the site also play an important role (Suter, Laternser, Haeberli, Frauenfelder, & Hoelzle, 2001). At high elevation, for instance, flat firn saddles and mountain tops with a low accumulation rate and effective turbulent energy exchange generally favor cold firn conditions and reduced meltwater infiltration. Due to the latent heat released in refreezing, firn temperatures are very sensitive to meltwater input. Accordingly, gradients of firn temperature are considerably higher with respect to altitude than those of air temperature (–1.2°C/100 m vs. –0.65°C/100 m, respectively; Haeberli & Funk, 1991). Direct measurements and modeling reveal that, for the European Alps, cold-firn and -ice areas can be found mostly on high-altitude glaciers above 3400 m asl and 4100 m asl for northerly and southerly aspect, respectively (Suter et al., 2001). Based on these considerations, only comparatively few glaciers in the Alps can be expected to provide conditions suitable for ice core programs targeting the retrieval of environmental records (Fig. 2). Almost all of the available drilling sites have been well explored and have already been used for multiple ice core studies (Garzonio et al., 2018).

Alpine Ice Cores as Climate and Environmental ArchivesClick to view larger

Figure 2. Illustration of the limited availability of cold sites suitable for ice core studies in the Alps. Shown in blue is the elevation distribution of glaciers in the Alps using data of the World Glacier Inventory (National Snow and Ice Data Center, 2016). The red line indicates the elevation dependency of the mean annual firn temperature (MAFT) obtained by Haeberli and Funk (1991) for summit/crest/saddle situations (cf. Haeberli & Alean, 1985). Additional mean firn temperature measurements of FH, CDD, and CG are shown as red dots (Gilbert & Vincent, 2013; Hoelzle, Darms, Lüthi, & Suter, 2011; Schwerzmann et al., 2006).

Development of Ice Coring Programs at Cold Alpine Glaciers

Pilot Drillings at Monte Rosa and Mont Blanc

In spite of a long tradition of glaciological research in the Alps, ice core drillings initially focused on the polar ice sheets. This meant an obvious gap of knowledge regarding climatic records and the investigation of a potential anthropogenic impact on the local atmosphere (Wagenbach, 1989). To a large extent this gap was the result of Alpine glaciers suspected to be generally temperate and thus unsuitable for paleoclimate-related ice core studies. Eventually, it was shown that areas with cold firn and ice conditions do exist in the Alps, especially at high altitudes (Haeberli, 1976). In 1976, CG (Monte Rosa, 4450 m asl) was chosen as the target of the first Alpine ice core study dedicated to the reconstruction of atmospheric changes (Oeschger et al., 1978). This ice core study provided the first age estimate and revealed a distinct change in snow chemistry over the 20th century (Gäggeler, Von Gunten, Rössler, Oeschger, & Schotterer, 1983; Wagenbach, Münnich, Schotterer, & Oeschger, 1988). Glaciological reconnaissance studies at CG investigated glacier geometry, surface accumulation, and englacial temperature distribution (Alean, Haeberli, & Schädler, 1983; Haeberli & Alean, 1985; Haeberli & Funk, 1991; Haeberli, Schmid, & Wagenbach, 1988; Lüthi & Funk, 2001). The first drilling to reach bedrock at CG was accomplished in 1982 (Stauffer & Schotterer, 1985).

In the early 1970s, shallow firn cores were recovered from the Mont Blanc summit area. These first firn cores were primarily targeting the investigation of surface accumulation and firn temperature (Lliboutry, Briat, Creseveur, & Pourchet, 1976). In 1980 an ice core was also drilled at CDD (4250 m asl) near the Mont Blanc summit (Jouzel, Legrand, Pinglot, & Pourchet, 1984) followed by extended ice core drilling efforts started in 1994. Two cores were recovered from a near-identical position, almost reaching bedrock around 140 m (Preunkert et al., 2000; Vincent, Vallon, Pinglot, Funk, & Reynaud, 1997). Acting to some extent as the high accumulation sister site of CG, the CDD ice core program mainly focused on revealing snow chemistry changes at high temporal resolution. Likewise as at CG, detailed glaciological investigations on glacier geometry, ice flow, and temperature depth profiles were carried out (Suter et al., 2001; Vincent et al., 1997, 2007).

Multicore Arrays

In addition to the position of the twin cores at CDD, drilling to bedrock was achieved later in the eastern saddle flank and within the summit of Dôme du Goûter, located upstream of the twin cores. Shallow coring and glaciological and geophysical surveying was also further intensified (Gilbert, Gagliardini, Vincent, & Wagnon, 2014; Gilbert & Vincent, 2013). At CG, three additional cores on approximately a common flow line from the north-facing flank toward the saddle were obtained in 1995 as part of the European projects ALPTRAC and ALPCLIM (Wagenbach et al., 1997). Around this time, a dedicated study investigating the firn-air transfer of chemical aerosol species at CG was also carried out (Preunkert & Wagenbach, 1998). Drilling activities were again intensified at the beginning of the 21st century, with additional deep cores recovered at CG: Near the saddle point of CG, two twin cores were drilled closely together in 2003 (Gabrieli et al., 2011) and re-drilled in 2015. Two additional cores were drilled on a separate flow line from the north-facing flank toward the eastern ice cliff in 2005 and 2013 (Bohleber, Erhardt, et al., 2018). Numerous glaciological field and modeling studies were carried out at CG, often in combination with the drilling activities. These efforts included geodetic surveys, ground penetrating radar, borehole temperature, and borehole closure analyses. Extensive investigations were performed on modeling of glacier flow and englacial temperature measurements and ice core synchronization (Eisen, Nixdorf, Keck, & Wagenbach, 2003; Haeberli & Funk, 1991; Hoelzle et al., 2011; Konrad, Bohleber, Wagenbach, Vincent, & Eisen, 2013; Lüthi & Funk, 2000). Figure 3 shows the multicore arrays established at CDD and CG.

Alpine Ice Cores as Climate and Environmental ArchivesClick to view larger

Figure 3. Multicore arrays at CDD and CG, shown in (a) and (b), respectively. Note the generally similar extension and topography of the sites, as well as their distinctly different saddle orientation with respect to the main wind direction. Whereas snow is shaded from wind at parts of CDD, it is highly exposed to wind erosion at CG. This leads to highly different net accumulation rates, in spite of similar annual precipitation values. Drilling sites are shown as circles, with full circles depicting locations where multiple cores have been drilled at one site. Flowlines are indicated with dashed white lines. Note the shading from insolation at the north-facing flank of CG.

Unlocking Additional Drilling Sites in the Alps

Additional cold-based high Alpine glaciers were explored for their suitability as drilling sites, thus improving the geographical coverage of the Alpine ice core records. The first deep cores were recovered at FH glacier (3900 m asl, Bernese Alps) between 1986 and 1989 (Schotterer, Fröhlich, Gäggeler, Sandjordj, & Stichler, 1997; Schwikowski, Brütsch, et al., 1999). Complemented by glaciological investigations, another core was drilled 150 m to bedrock in 2002 (Schwikowski, 2004; Schwerzmann et al., 2006). The FH site has high net accumulation but, more importantly, gains particular relevance due to its location in the Northern Alps as well as its close proximity to the Jungfraujoch research station.

Additional high accumulation ice cores from the South-Western Swiss Alps were obtained at two locations in the Monte Rosa massif, at upper Grenzgletscher (GG; 4200 m asl; Eichler et al., 2000) and CDL (Smiraglia et al., 2000). Until the early 2000s, this array of drilling sites left an obvious gap in the Eastern Alps. The discovery of the Oetztal ice man at Tisenjoch (3210 m asl), dated to be older than 5,000 years (Baroni & Orombelli, 1996; Kutschera & Müller, 2003), clearly showed that, under certain conditions, old ice can also be preserved at comparatively lower altitudes and in the Eastern Alps. Some selected sites with cold firn temperatures were also predicted in the Eastern Alps by models (Suter et al., 2001). Haeberli, Frauenfelder, Kääb, and Wagner (2004) initiated more systematic investigations of cold ice at comparatively lower altitudes. Although at least partially affected by temperate near-surface conditions, ice cores were drilled at cold-based summit glaciers at Piz Murtel (3340 m asl, Eastern Swiss Alps; May, 2009), Piz Zupo (3850 m asl, Eastern Swiss Alps; Sodemann, Palmer, Schwierz, Schwikowski, & Wernli, 2006), and the Ortles summit range (3860 m asl, South Tyrolian Alps; Gabrielli et al., 2016). A detailed discussion of the special settings of these (partially) temperate drilling sites remains outside of the scope of this article.

Dating Alpine Ice Cores: Tools and Challenges

Establishing a reliable age scale is one of the fundamental challenges when attempting the interpretation of any ice core data set as a time series of past atmospheric variability. Ice core dating can be accomplished by various techniques covering different age ranges. The applicability of an individual technique is closely related to the glacio-meteorological conditions of the drilling site, however. This means that a direct transfer of dating techniques established for polar ice sheets to Alpine ice cores has its limits.

Polar ice sheets have much greater horizontal than vertical extension (as an order of magnitude, a length/thickness ratio of 1000:1). In contrast, Alpine summit glaciers have smaller scales of comparable extension in horizontal and vertical direction (typical length/thickness ratio of 10:1 or even less). With typical horizontal flow velocities at the surface of around 1 m/year, the characteristic time scales of 50 to 250 years correspond to nominal trajectories already close to the length of the upstream catchment area at Alpine sites. This implies the need for a rapid decrease in horizontal flow velocities with depth at cold Alpine sites, accompanied by shear-induced layer thinning. Additional important peculiarities regarding glacier flow include

  1. 1. A substantial portion of the glacier depth is made up of firn. Firn densification yields a significant contribution to the decrease of vertical velocity with depth.

  2. 2. An englacial temperature profile is characterized by seasonal variations affecting roughly the upper 15 m, with only a small temperature gradient in the lower ice part. This means that, at cold sites, the ice is frozen to bedrock, resulting in high rates of deformation and layer thinning in the lowermost ice parts.

  3. 3. There are significant longitudinal gradients in glacier geometry and surface velocities. With vertical and horizontal extension of comparable order of magnitude, glacier flow at Alpine sites is complex and strongly influenced by glacier geometry.

Firn densification and flow-induced deformation causes rapid thinning of annual layers. As a result of these complex glaciological settings, the age-depth relation at Alpine sites generally becomes non-linear at shallow depths. A reliable ice core dating can only be achieved by a combination of dating techniques providing consistent age information.

Stratigraphical Methods

Counting annual layers is a well-established technique to obtain relative age information and thus is widely used to date ice cores whenever applicable. The annual cycle present in ice core proxy data sets is the basis for identifying and counting preserved annual layers. The stable water isotope ratios (delta O-18 and delta D), as well as short-lived impurity species, are commonly used for this purpose. In the case of the stable water isotopes, the annual signal stems from seasonal variations in the local atmospheric temperature during precipitation events. The summer/winter contrast in temperature at high Alpine drilling sites (extending from -30°C to close to 0°C in high winter and summer, respectively) is reflected in a quasi-harmonic isotopic change with a large seasonal amplitude (e.g., exceeding a summer/winter contrast between –10 and –20 permil in δ‎18O, respectively). However, the stable water isotope signal is also strongly affected by postdepositional biases, such as meltwater percolation and diffusion of water vapor in the firn column (Schotterer et al., 2004). While the isotope seasonality can remain preserved at high accumulation drilling sites, the effect of wind erosion hampers the interpretation of the isotope signal on an interannual scale at low accumulation sites (Bohleber, Wagenbach, Schöner, & Böhm, 2013). Aerosol-related impurities provide a better preserved annual signal in this case. The aerosol annual cycle primarily stems from the summer/winter contrast in atmospheric mixing (see later discussion). The aerosol seasonality is less harmonic but better represented by a box-type function with sharply increased concentrations (with respect to the winter half year) approximately during April to September. Mean summer/winter ratios for nitrate, sulfate, and calcium are close to 4 but reach approximately 14 in the case of ammonium (Preunkert et al., 2000), making ammonium the primary choice for annual layer counting. Means to estimate general counting uncertainties are well established from annual layer counting in Greenland ice (Rasmussen et al., 2006). At low accumulation Alpine sites, missing years due to annual snowfall fully eroded from the surface can occur. In this case an incomplete stratigraphy constitutes a potential systematic bias toward underestimating age. Absolute age constraints are crucial to quantify this bias. Known large deposition events of Saharan dust can be detected (primarily via their enhancement in calcium) and used as reference horizons in this respect (Haeberli, Schotterer, Wagenbach, Schwitter, & Bortenschlager, 1983; Preunkert et al., 2000). As outlined later, the use of basic ice chemistry profiles to identify volcanic eruptions as age markers is only of very limited use at Alpine sites. As a consequence, reference horizons are limited to the 20th century only.

Radiometric Methods

Radiometric dating methods provide important age information to constrain annual layer counting. 3H and 137Cs data sets constitute especially important reference horizons (Eichler et al., 2000; Schotterer et al., 1977; Vincent et al., 1997). During atmospheric nuclear bomb tests, 3H and 137Cs both entered the stratosphere. Their profiles reveal a distinct maximum corresponding to the maximum of radioactive fallout in the northern hemisphere (in particular around the years 1963, 1954). For 3H, which is part of the hydrological cycle, a clear annual cycle is found in a high accumulation ice core from FH (Schwikowski, Brütsch, et al., 1999). Due to its half-life of 22.3 years, 210Pb offers radiometric ice core dating roughly within the last 100 years (Gäggeler et al., 1983). 210Pb is part of the 238U decay family, of continental origin and thus transported to Alpine drilling sites via vertical mixing. The use of 210Pb as a dating tool is based on the assumption of (a) a constant input of 210Pb on the multiannual scale and (b) the absence of postdepositional alterations. Due to the vertical transport, assumption (a) may not hold on decadal to centennial time scales, however (Eichler et al., 2000).

Flow Modeling

Flow modeling provides important assistance in ice core interpretation, first and foremost for calculation of trajectories (cf. upstream effects) and model-based age-depth relations. Based on early two-dimensional approaches using parallel-sided ice slab approximations (Haeberli et al., 1988), models were further refined to account for firn rheology and three-dimensional (3D) glacier flow using finite-element techniques (Gagliardini & Meyssonnier, 1997; Lüthi & Funk, 2000). Sophisticated state-of-the-art studies utilize 3D full Stokes and fully thermo-mechanical coupled models employing firn rheology (Gilbert et al., 2014) and anisotropic ice flow (Licciulli, 2018). Using the two-dimensional ice slab approximation and adding spatially variable topography and accumulation rates results in flow models of intermediate complexity. Such models have been employed successfully at Alpine sites to derive a preliminary dating (Vincent et al., 1997) and spatial age interpolation (Konrad et al., 2013). Notably, for flow modeling the glacier is generally assumed to be in steady state, which may not always be the case with respect to the targeted long time interval.

Establishing Age Scales of High Accumulation Drilling Sites

At drilling sites with high net snow accumulation, annual layer counting is the main dating tool and can be performed reliably even to depths comparatively close to bedrock. Counting the distinct annual signals in the stable water isotope and tritium profiles in the FH ice core was used to date the time period 1988–1946 with an uncertainty of only one year (Schwikowski, Brütsch, et al., 1999). Based on a dedicated combination of annual layer counting in ammonium and stable water isotopes, reference horizons, the 210Pb technique, and simple kinematic ice flow considerations, the ice core from a high accumulation site at GG was dated with a maximum uncertainty of two years at 1937 (Eichler et al., 2000).

At CDD, annual layers were counted along the ammonium profile to a depth of 114 m (total core length 126 m). Corroborated by reference horizons of Saharan dust and 137Cs, the resulting age-depth scale had an uncertainty of five years at 114 m, or ca. 60 years in age (Preunkert et al., 2000), and showed how to refine the preliminary dating based on flow modeling (Vincent et al., 1997). Importantly, radiometric data may also reveal potential stratigraphic disturbances: Legrand, Preunkert, Wagenbach, Cachier, and Puxbaum (2013) found the 3H bomb maximum to be absent in an ice core drilled at CDD in 2004. A vanishing annual signal in the ammonium profile between 65.5 and 68 m w.e.3 was taken as additional evidence that the continuity of the chronology in this core is challenged over a localized section covering an approximately 15-year period. The annual signal clearly recovers again below 68 m w.e. (Legrand et al., 2013).

Alpine Ice Cores as Climate and Environmental ArchivesClick to view larger

Figure 4. Overview on state-of-the-art dating of a low accumulation ice core from CG. The two major dating tools are annual layer counting and micro-radiocarbon (see text). The black lines illustrates a typical age-depth relation constrained by absolute horizons (red dots) such as Saharan dust events (dust), atmospheric nuclear bomb tests (3H) and, for deeper sections, further age constraint by micro-radiocarbon (14C, red dots with uncertainty bars). Note the logarithmic scale used for age in years before present (i.e., drilling date).

The Special Challenge of Dating a Low Accumulation Alpine Ice Core

Figure 4 presents an overview of the state-of-the-art methods employed for dating low accumulation ice cores. At the low accumulation site CG, initial flow modeling already suggested the possibility of retrieving ice core records encompassing the last millennium (Haeberli et al., 1988). However, establishing a long-term age scale remained a challenge. The complex 3D bed geometry may be strongly different from ice surface topography and results in complex ice flow and layer thinning. Evidently, the application of annual layer counting requires (a) no systematic occurrence of annual layers being fully eroded by wind, that is, more specifically, (b) at least partial preservation of the summer/winter contrast in the proxy signal, and (c) a sufficiently high resolution to detect the annual signal. Due to the rapid layer thinning with depth, the latter issue fundamentally limits the application of annual layer counting at CG at cm-scale depth resolution, at least for time periods exceeding a few hundred years (Bohleber, Erhardt, et al., 2018). To constrain annual layer counting beyond the 20th century, the use of age markers such as known volcanic eruptions is desirable. However, the unambiguous identification of volcanic eruptions based on basic ice chemistry profiles like sulfate can provide only limited assistance at Alpine ice cores (different from polar sites). A main challenge consists in the relatively weak signals of volcanic sulfate or volcanic acidity being embedded into the relatively large variability of sulfate (mainly from gypsum) and (acidity consuming) carbonate, both connected to Saharan dust input. A future remedy may come from sophisticated analyses of relatively volatile trace elements (Kellerhals et al., 2010), the detection of tephra markers (Luongo et al., 2018), and sulphur isotopic analysis (Baroni, Savarino, Cole-Dai,Rai, & Thiemens, 2008).

As a result of these challenges, the interpretation of the pre-industrial period in the CG ice core records faced a typical dating uncertainty of more than 10 years (Schwikowski et al., 2004; Wagenbach, Preunkert, Schäfer, Jung, & Tomadin, 1996). The maximum age of the ice at CG, and the maximum span of a useful climatic record, remained largely unknown until the early 2000s.

The Breakthrough of the Radiocarbon Dating Technique

Radiocarbon (14C) is an indispensable ice core dating tool for non-polar ice core sites spanning an age range exceeding 1,000 years before the present. High Alpine ice samples provide only very small concentrations of organic material (in the microgram range). Since finding macroscopic organic objects (tree parts, insects) is extremely rare, microscopic organic impurities incorporated into the ice are commonly used. The microscopic organic material is distinguished by size into dissolved organic carbon (DOC) and particulate organic carbon (POC). The concentrations of DOC are within the range of approximately 50 to 100 μ‎gC/kg for high Alpine ice samples (May, 2009; May et al., 2013). For high altitude, low accumulation glaciers, DOC has been demonstrated as biased by in situ production of 14C (Hoffmann, 2016; May, 2009), thus hampering its immediate dating application. POC concentrations are a factor of 2 to 10 smaller than DOC, in the range of 10 to 50 μ‎gC/kg (Jenk et al., 2009; Steier et al., 2006), making its analysis a major technical challenge requiring rigorous decontamination and low system blanks (Hoffmann et al., 2018; Uglietti et al., 2016). A main assumption for the dating via POC 14C is to take the age of the dated organic material as representative for the age of the encompassing ice. However, the incorporation of aged material can lead to systematic age biases, which has been shown to be a particular risk for the case of Saharan dust intakes (Hoffmann et al., 2018; Jenk et al., 2009). For further details regarding the 14C ice core dating method, see Hoffmann et al. (2018) and Uglietti et al. (2016).

Dating the Deep Layers at Low Accumulation Colle Gnifetti

The advent of the micro-radiocarbon technique introduced a major advance in dating CG ice cores, with seminal contributions made by Jenk et al. (2009) and May (2009), both indicating the possibility of Pleistocene ice conserved in the bottommost layers. By means of PO14C dating, the radiocarbon technique revealed the first reliable age markers on CG ice cores for the time period exceeding a few hundred years before the present (Jenk et al., 2009). With annual layer counting limited to a few hundred years, however, the 14C age constraints require an interpolation method in order to derive a continuous age-depth relation. A two-parameter model is widely used for this purpose (see Uglietti et al., 2016). The so-called 2p-model was first applied for dating constraints at the Dunde ice cap, assuming a known ice thickness and (constant) average accumulation, leaving the thinning parameter p for adjustment (Thompson et al., 1990). The underlying ice flow considerations were developed for a polar ice divide, considering two types of strain rate functions, class A (laminar-like) and class B (thin-skin) (Bolzan, 1985). Notably, class A and B can be associated with the flow considerations utilizing the parallel-sided ice slab and the 2p-model, respectively. Both functions yield identical strain rate distributions in case of p = 1. With increasing values of p, class B functions have ice deformation confined to an increasingly thin surface layer above a large lower portion of stagnant ice. While substantial near-surface deformation is likely represented in alpine slope glaciers, in particular due to firn densification, a large fraction of deep stagnant ice appears less realistic (Lüthi & Funk, 2000). Accordingly, it comes as no surprise that values of p close to 1 produce a better fit of the class B analytical age scale to the observed age distribution (e.g., p = 0.87 +/– 0.05 reported by Jenk et al., 2009).

The simple flow considerations typically assume constant accumulation rate and thinning and a constant and precisely known ice thickness, which remains a general shortcoming with respect to Alpine drilling sites. Taking this into account, however, this type of interpolation between 14C constraints provided the first reliable chronology for the lower core sections at CG. This chronology suggested that Alpine climate records covering the entire Holocene could be retrieved from CG ice cores (Jenk et al., 2009). In concert with 14C age constraints, novel ultra-high resolution impurity analysis (better than 120 µm) paved the way for extending annual layer counting to highly thinned layers in the CG ice cores, thus avoiding the need for interpolation. By this means, a fully annual layer counted age scale covered for the first time the entire last millennium, with an estimated maximum uncertainty of 72 years at 1000ce (Bohleber, Erhardt, et al., 2018).

As already noted by Jenk et al. (2009), the finding of a continuous age-depth relation in the deep core parts is not a priori to be expected (e.g., as strong shear could potentially decouple the deformation of the basal ice frozen to bed from its adjacent top layer, which would be reflected in a hiatus in the age-depth relation). In fact, the 14C profile obtained by Hoffmann et al. (2018) for a core located on CG’s north-facing slope (with significant bedrock inclination, cf. the saddle location of the core investigated by Jenk et al., 2009) revealed a localized discontinuity in 14C ages, thus indicating the need for further investigations regarding the age-depth continuity in deep CG core sections.

Deciphering Atmospheric Signals in the Alpine Ice Core Climate Archive

The exposed, small-scale locations of Alpine drilling sites entail important consequences not only with respect to ice flow and age distribution but, equally important, regarding the recording of atmospheric signals (Wagenbach, 1992).

The recorded variability in an ice core proxy signal is a combination of the changes in the respective emission source(s), advection by atmospheric circulation, snow deposition/preservation, and potentially further post-depositional change. This general statement holds for any ice core, including polar sites. However, Alpine drilling sites show some key characteristics in this respect, primarily their proximity to continental emission sources and snow sampling characteristics.

On the polar ice sheets, production of trace substances is virtually absent. Accordingly, the impurity content of the air at ground level is primarily controlled by long-range air mass transport and downward mixing through the local boundary layer. Alpine glaciers, however, are located close to sources of various trace substances. In this case, the atmospheric impurity load at high Alpine sites is primarily a result of the local vertical mixing intensity, which is strongest during the summer season. Vice versa, in the winter seasons with stratified atmospheric conditions prevailing, the high Alpine sites become decoupled from the local boundary layer and are primarily influenced by the free troposphere, with highly clean air conditions. The pronounced seasonal difference in vertical mixing creates a distinct annual cycle, most prominently for aerosol-related impurity species of continental origin (Lugauer et al., 1998). For some species, dry deposition and the local transport of material from exposed rocks and soils in the ultimate vicinity of the glacier can also play a role.

As a consequence, glaciochemical records from high Alpine sites can represent a broad range in scales, from large-scale tropospheric background conditions to regional and local influences. This means that the spatial representativity of the ice core records needs to be carefully assessed, in particular in presence of systematic snow sampling biases (e.g., summer vs. winter precipitation).

The Important Role of Snow Preservation

A typical annual precipitation rate of 2 m w.e./year in the high Alpine realm (Efthymiadis et al., 2006), which is considerably higher than on the polar ice sheets, creates the opportunity for ice core investigations at very high temporal resolution. However, achieving high temporal resolution requires the majority of the annual snowfall to be preserved in the record. Due to the limited ice thickness, a high resolution record with large annual layers can only cover a comparatively short period, not much exceeding the last 100 years or so (e.g., Wagenbach et al., 2012).

Wind erosion is a common phenomenon at the exposed summit locations of Alpine ice core drilling sites. The effect of wind scouring may greatly vary among and especially within sites. Essentially no Alpine drilling site provides a truly closed system with respect to precipitation. The magnitude of the systematic snow loss depends first and foremost on the relative orientation of the glacier surface topography and main wind direction. If shading from wind is provided by topography, a surplus of snow accumulation with respect to the annual precipitation rate is also possible (Fig. 3; Preunkert et al., 2000). The strong effect of wind redistribution hampers establishing a link between layer thickness and precipitation rate (different from some polar cores) and introduces a fundamental bias to any estimate of the mean aerosol flux density (Wagenbach, 1992).

To further illustrate the decisive influence of snow preservation, in particular at low accumulation sites, consider a freshly deposited snow layer at CG. The effect of wind erosion is counteracted by snow consolidation (driven by insolation and temperature) and ice-lens formation (Schotterer, Haeberli, Good, Oeschger, & Röthlisberger, 1981). Accordingly, the course of wind strength, temperature, and insolation until the next snowfall determines the chance for the eventual preservation of this layer and its impurity signal. While this effect is essentially impossible to reconstruct for single snowfall events, there are also important systematic tendencies in snow preservation. Due to higher insolation and warmer temperatures, surface snow consolidates faster during summer. Hence winter precipitation has a comparatively higher erosion probability. As a result, interannual to long-term means become significantly influenced by the variable amount of winter snow preserved (Fig. 5). Notably, this snow sampling effect can introduce an apparent coupling between otherwise unrelated proxy-species (Wagenbach, 1992). For any atmospheric signal with an annual cycle, snow preservation introduces both temporal and spatial signal variabilities recorded in the ice cores that are not directly linked to their commonly assumed climatological meaning. It is thus important to consider the following snow preservation influences in the ice core proxy interpretation.

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Figure 5. Illustration of the snow preservation influence on the recording of atmospheric signals as long-term averages in Alpine ice cores. For this purpose the example of the stable water isotope ratio (δ‎18O) is shown, using data of Bohleber et al. (2013) and Schöner et al. (2002). The seasonality of δ‎18O results primarily from respective variations in local atmospheric temperature (in blue). High net accumulation drilling sites at CDL, FH, CDD, and GG sample, on average, the entire annual snowfall. Their respective mean values (adjusted to the reference elevation of CG, red dots) are close to the annual average in precipitation. In contrast, the summer-biased snow preservation at low accumulation CG results in a systematic shift toward the summer levels. The snow preservation effect differs between drilling sites on the northern flank and the saddle, thus leading to so-called upstream-effects by ice flow (see text).

Temporal Variability Introduces Depositional Noise

On a year-to-year basis, a variable amount of winter snow preservation produces interannual changes in the recorded ice core proxies that are not directly associated with any changes in the atmospheric signal. This effect is commonly referred to as “depositional noise.” It can be shown with simple conceptual models that, apart from interannual changes in net accumulation, the amplitude of the seasonality in the atmospheric signal dominates the magnitude of the depositional noise (Fisher & Koerner, 1994; Wagenbach et al., 2012). The depositional noise can substantially mask the imprint of an atmospheric signal to be reconstructed from the ice core data, depending on (a) the duration of the signal (short vs. long-term variability) and (b) the relation between noise and signal strength. Short-term events such as individual intakes of Saharan dust may not be recorded reliably; this is also true at high accumulation sites (Preunkert et al., 2000). Moreover, the depositional noise effect is comparatively minor in the case of the strong impurity increase over the 20th century due to anthropogenic emissions (e.g., sulfate and nitrate; Wagenbach et al., 2012). In this case, models simulating emission and meteorological changes show interannual and trend variabilities consistent with Alpine ice cores, in particular in the example of sulfate (Fagerli et al., 2007). On the other hand, the relatively weak long-term trends in the stable isotope signal or the pre-industrial aerosol load are inflicted to a much greater degree (Fig. 7). Here, the availability of a multicore array provides an important tool for tackling the identification of an atmospheric signal. The atmospheric signal variability is uniform across the glacier surface and hence results in a common signal among multiple cores from different locations. In contrast, the stochastic influence of snow preservation is spatially highly variable, thus resulting in an individual noise imprint at each location. By means of dedicated time series comparison, a shared imprint can be identified. For instance, this common signal investigation was performed for the stable isotope time series of multiple CG cores. To verify the atmospheric origin of the common signal, the latter was compared to a specifically adjusted instrumental temperature data set (reflecting on average the bias toward the summer season at CG). Regarding multidecadal trends (Fig. 6), the shared isotope signal was found concurrent with the instrumental data and thus interpreted as reflecting changes in atmospheric temperature (Bohleber et al., 2013).

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Figure 6. A composite stable isotope record (delta O-18) record of multiple CG ice cores (blue line) in comparison with instrumental temperature data (red line). The instrumental temperature data was specially adjusted for altitude, precipitation, and snow deposition in order to closely reflect the snow sampling biases seen by the ice cores. Data are shown as deviations from the 2000–1901 mean. Source: Bohleber et al. (2013).

Spatial Variability Introduces Upstream Effects

The spatial variability of the mean net snow accumulation is linked to surface gradients in shading from wind and/or insolation (cf. Fig. 3). If the ratio of summer to winter snow preserved varies systematically upstream of a borehole, a bias to the long-term signal measured in the ice core is to be expected as a result of ice flow. An illustration of this effect is included for the CG cores in Figure 5: If the amount of winter snow preserved decreases upstream of a drilling site, the respective long-term mean in the ice core will be biased toward higher (summer) concentrations. Accordingly, if multiple deep cores along a flow line are available, a systematic concentration difference will be observed in the respective long-term averages. Evidently, this difference is not a climatic signal, however, and is referred to as the “upstream effect.”

Upstream effects can be evaluated through a shallow ice core series along a flow line or through the clear dissection into winter- and summer-related levels. The latter is typically only a possibility at high accumulation sites. We take here as an illustration the case of CDD, where a precise evaluation of the upstream effect has been conducted (Preunkert et al., 2000; Preunkert, Legrand, & Wagenbach, 2001). By separating winter versus summer levels in the ammonium record, the ratio of winter to summer net accumulation was calculated for each year. This revealed a decrease from a ratio of close to 1 at the surface to about 0.5 at 100 m depth. Using the respective depth-dependent fraction of summer versus winter snow and the known average summer and winter impurity levels, the depth-dependent concentration bias can be corrected. The results show a bias of about 20% for nitrate, sulfate, and calcium and 30% for ammonium, again depending on the magnitude of their seasonality (Preunkert et al., 2000).

At CG, investigations of the spatial variability of seasonal signals in the upper snow layers suggest that the upstream effect in deep ice core layers could be as large as 100% (Preunkert et al., 2000). Since an unambiguous dissection of summer and winter layers is hampered due to the low net accumulation of CG, this site still awaits a dedicated assessment of the upstream effect (Bohleber, Erhardt, et al., 2018). Future efforts in this direction need to utilize shallow core data in concert with sophisticated flow modeling for a reliable identification of the catchment area (Licciulli, 2018).

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Figure 7. Illustration of different magnitudes in long-term trends shown for aerosol-related impurities (sulfate, left, adapted from Preunkert et al., 2001) and stable water isotopes (right, adapted from Bohleber et al., 2013). Trends are shown relative to their 1930–1910 averages (period indicated by red lines). The 20th-century increase in emissions reflects in a more than fivefold increase in sulfate recorded at CDD. However, the increase of atmospheric temperature over the 20th century (more than 1°C) leaves a weak signal of only less than 20% increase in the stable water isotope ratio (e.g., at CG). Consequently, the latter is much more susceptible to depositional noise (see text).

Alpine Climate Significance of Stable Isotope and Impurity Records

The comparison of multiple cores from a single site has proven to be a successful approach to constraining snow sampling related artifacts. However, the spatial significance of any findings from a single drilling site is not self-evident considering the seasonal differences in air advection and different climatological settings with respect to sites north or south of the main Alpine chain. Here, much can be gained from a comparison of ice core records obtained at two drilling sites differing in their glaciological settings, as well as the comparison with long-term instrumental climate series.

Two-Site Comparisons

Detailed comparisons between high accumulation CDD and low accumulation CG records have been presented in Preunkert et al. (2000) and Wagenbach et al. (2012). The subseasonal CDD records are dissected into summer and winter records in order to consider separately the summer half-year records for comparison to the CG ice cores. Mean summer concentrations of the most recent sections of the CDD core agree with the respective values found at CG, particularly for ammonium, nitrate, sulfate, and calcium. Similar results were obtained for nitrate and sulfate when attempting a comparison of winter layers identified in shallow core sections at CG (Preunkert et al., 2000). The agreement between sulphate and nitrate also holds when comparing the general shape of the 20th century increasing trends (cf. Fig. 7). This includes agreement in the absolute values of the smoothed trends between both sites (Wagenbach et al., 2012). Consequently, this two-site comparison suggests that these two Alpine drilling sites underlie similar atmospheric conditions and implies that, at least for aerosol-related species like sulphate and nitrate, summer signals representative at least on a regional scale (in the range of 100 km) are reliably archived.

Regarding two-site comparison of stable water isotope records, cores from high net accumulation sites FH and GG revealed (a) a high degree of intrasite correspondence exhibited by the two cores drilled about 100 m apart at FH and (b) the need to account specifically for the different precipitation regimes found at the northern and southern side of the Alpine chain for FH and GG, respectively (Mariani et al., 2014). In the Monte Rosa range, a 25-year period was compared for a CG low accumulation core with a core drilled nearby at the high accumulation site CDL. The CG time series clearly lacked any distinct winter-related isotope minima and thus any sign of a seasonal signal. Instead, the CG isotope data was found in good agreement with the summer maxima levels of the CDL isotope time series, thereby forming close to an upper envelope signal (Wagenbach et al., 2012). Regarding the 20th-century multidecadal trends, summer isotope levels of CDD were compared to a stacked record of multiple CG isotope time series but revealed only fair agreement regarding the strong overall increasing trend and much weaker intersite compatibility on the decadal scale (Wagenbach et al., 2012). In view of the local depositional noise particularly affecting the stable isotope data, using a stacked record of multiple cores at CDD for the comparison may also make a potential multidecadal agreement more clear. Since the overall isotope variability is related to atmospheric temperature, the comparison with instrumental data provides more conclusive evidence of the spatial representativity in this case.

Comparison with Instrumental Climate Data Sets

Investigations of recent and historical climatological data sets of the Alpine region can reveal important climatological settings of the sites, such as the typical seasonality in precipitation and air temperature, potential long-term trends of the vertical air mass exchange, and the spatial representativeness of local temperature changes (Auer et al., 2007; Brunetti et al., 2009). Dedicated studies have combined multiple cores with instrumental data sets to assess the relationship between (precipitation weighted) instrumental temperature and the stable water isotope signal recorded in Alpine ice cores for seasonal and annual scale at high net accumulation sites (Mariani et al., 2014) and for multidecadal trends at low accumulation CG (Bohleber et al., 2013). The dense network of instrumental data in central Europe allowed demonstration of the generally high spatial correlation of high elevation Alpine air-temperature signals with central European lowland sites, for both the summer and winter periods (Auer et al., 2007; Schöner, Auer, Böhm, Keck, & Wagenbach, 2002). Temperature inversion weather patterns during winter decouple lowland areas from high-altitude sites in winter. Consequently, the correlation between high- and low-altitude temperatures is generally higher in summer than in winter (Schöner et al., 2002). Figure 8 provides an illustration of this effect, showing the seasonal difference in spatial correlation of temperature data sets, correlating monthly averages of weather station measurements at CG (Arpa Piemonte) and 2 m temperature reanalysis data set of ERA-Interim (Dee et al., 2011). Following these considerations, it is not surprising that the isotope-based temperature reconstructions from the high Alpine sites have been shown to feature an at least regional-scale representativity (Bohleber et al., 2013; Mariani et al., 2014).

The availability of dense instrumental data (precipitation and temperature) for the high Alpine region also provides unique means to assess the isotope/temperature relationship recorded in Alpine ice cores (Brönnimann, Mariani, Schwikowski, Auchmann, & Eichler, 2013; Keck, 2001). Of particular importance with respect to long-term temperature reconstructions from the CG isotope time series is the so far insufficiently understood high isotope/temperature relationship, typically about threefold higher than what is expected for European temporal trends in precipitation (Bohleber et al., 2013). Evidence from preliminary investigations suggests that the seasonal bias in snow sampling and the confinement to multidecadal trends are both connected to the resulting enhanced isotope/temperature sensitivity (Bohleber, Erhardt, et al., 2018). The enigmatic isotope/temperature sensitivity still awaits a detailed investigation, which should ideally combine regional climate–isotope modeling and a dedicated constraint of all post-depositional effects such as snow preservation, upstream effects and isotope diffusion.

Another enigmatic signal found in the stable water isotope profiles of the low accumulation ice cores at CG and Dôme du Gouter (Mont Blanc) is an extreme basal isotope anomaly, featuring a distinct depletion by several permil over the last few meters above bedrock. On the one hand, this common signal could suggest a climatic origin, and apparent similarities exist to the isotope decrease observed in Greenland cores at the Last Glacial/Holocene transition. On the other hand, however, this basal isotope depletion has been found also at other non-polar ice core drilling sites, including small cold-based summit ice caps (Bohleber, Hoffman, et al., 2018; Keck, 2001; Lorrain & Haeberli, 1990). At these sites, the basal isotope phenomenon occurs within ice layers of different ages (yet consistently younger than the Last Glacial/Holocene transition) and is more likely explained as a glaciological artifact rather than a climatic signal (Jenk et al., 2009; Keck, 2001). A detailed discussion of the basal isotope anomaly is presented in Wagenbach et al. (2012).

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Figure 8. Spatial correlation in 2 m temperature data sets of CG weather station data (Arpa Piemonte) and ERA-Interim time series, using monthly averages for the time period 2003–2015. Shown here for the summer (JJA) and winter (DJF) season, in (a) and (b), respectively. Note the higher spatial correlation expressed during summer (see text). Images from Climate Reanalyzer, Climate Change Institute, University of Maine.

Alpine Climate and Environmental Change Recorded in Ice Cores

Since the launch of the first Alpine ice coring programs in the 1970s, numerous ice cores have been investigated to reveal an extensive picture of the Alpine glaciochemistry, with complementary work on natural aerosols like mineral dust, pre-industrial aerosol changes, and temperature proxy records. As a result, we have obtained comprehensive data of the anthropogenic impact on the high Alpine snow chemistry (Figure 9). Dedicated overviews on the Alpine glaciochemistry of the industrial period (including comprehensive references) can be found elsewhere (Preunkert & Legrand, 2013; Schwikowski, 2004). Here, the main characteristics generally observed in Alpine ice cores are highlighted before turning to the pre-industrial period.

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Figure 9. Concentration trends shown as about five-year averages for nitrate, ammonium, lead, and black carbon in ice cores from CG. Also shown is ex-sulphate, representing sulphate corrected for the sea-salt and mineral dust contribution. Data from Lavanchy, Gäggeler, Schotterer, Schwikowski, and Baltensperger (1999) and Schwikowski et al. (2004). Figure reproduced from Schwikowski (2006) with kind permission by M. Schwikowski.

The Anthropogenic Impact and the Industrial Period

Alpine ice core studies provide clear evidence for an anthropogenic perturbation of the snow chemistry at high-elevation glaciers, extending over more than the 20th century. In particular, Alpine ice cores revealed the increase in aerosol-related atmospheric species comprising the major ions, sulphate, nitrate, and ammonium (Döscher, Gäggeler, Schotterer, & Schwikowski, 1996; Preunkert et al., 2001; Preunkert et al., 2003; Schwikowski, Döscher, et al., 1999). Other ice core based studies at Alpine sites have provided complementary investigations of organic species (Legrand et al., 2003; Legrand et al., 2007), heavy metals (Barbante et al., 2004; Schwikowski et al., 2004; Velde et al., 2000), and anthropogenic radionuclides (Gabrieli et al., 2011). These studies revealed certain outstanding periods characterized by distinct temporal changes: No significant long-term trends for sulfate, nitrate, ammonium, and heavy metals are present typically within ca. 1750–1850. The period of 1850–1950 shows the onset of a systematic increasing trend for these species around ca. 1860. Initial relatively weak increase rates intensify, with maximum growth rates within the time interval 1950–1970. The years following the 1970s feature a strong decrease in sulfate and lead, thus creating a distinct maximum in the 1970s. On the other hand, trends in nitrate and ammonium continued to increase after the 1970s.

Importantly, the total impurity inventory of Alpine ice cores comprises a substantial fraction of mineral aerosols due to the long-range advection of Saharan dust to the Alps (Preunkert & Legrand 2013). While individual dust events typically only last less than a few days (Schwikowski, Seibert, Baltensperger, & Gaggeler, 1995; Sodemann et al., 2006), they occur regularly and frequently, mainly during late spring, summer, and early fall (Prodi & Fea, 1979). An individual Saharan dust deposition event can be distinguished from background and other pollution events in the ice core data sets based on its chemical footprint (essentially using calcium and an alkalinity measure) as well as the insoluble particle size distribution (Schwikowski et al., 1995; Wagenbach & Geis, 1989; Wagenbach et al., 1996). Prominent large dust events known for the 20th century have been identified by their outstanding imprint in several Alpine ice cores, in particular marking the years of 1977, 1947, 1936–1937, and 1901 (Eichler et al., 2000; Vincent et al., 1997; Wagenbach & Geis, 1989). With respect to snow chemistry, the large amount of carbonates in Saharan dust produce enhanced alkalinity and calcium concentration as well as an increased level of several other ions (Preunkert & Legrand, 2013). As a result, the high alpine snow is consistently at a mean alkaline level until changing to an acidic background level around 1950, primarily due to the large input of acidic species like nitrate and sulfate (Wagenbach et al., 1988).

Regarding the temperature-related proxy records, in particular the stable water isotopes, the multidecadal isotope variability of the CG ice cores follows the strong 20th-century temperature changes rather faithfully. At the same time, the correspondence to the long-term temperature variability of the early instrumental era (back to 1760) appears less clear. This may be in part explained by the comparably weak long-term trends in the pre-1900 period, uncertainty in the early instrumental data, and also fundamental difficulties in the isotope/temperature link (Bohleber et al., 2013).

The Pre-Industrial Period and the Context of Other Alpine Climate Archives

The precise chronologies of the high net accumulation sites FH and GG only extend to the 20th century (Eichler et al., 2000; Schwikowski, Brütsch, et al., 1999). At CDD, the pre-industrial period (i.e., substantially extending before 1850) is confined to the deepest ice layers, thus highly affected by vertical strain and systematic upstream flow effects. A detailed examination of the pre-industrial period at CDD remains challenging (Preunkert et al., 2000, 2003). Accordingly, studies focusing on the pre-industrial period remain mostly focused on ice cores from low accumulation CG.

From their broad set of glaciochemical data, CG ice cores offer an intriguing set of complementary information to the nexus of Alpine climate proxies, already comprising tree ring indices (Corona et al., 2010), historical evidences (Meier, Rutishauser, Pfister, Wanner, & Luterbacher, 2007), speleothems (Spötl, Mangini, Bums, Frank, & Pavuza, 2004), Alpine lake sediments (Heiri, Lotter, Hausmann, & Kienast, 2003), glacier length changes (Joerin, Nicolussi, Fischer, Stocker, & Schlüchter, 2008), and multiproxy reconstructions (Luterbacher et al., 2016). Of special interest here is using the unique ice core archive, for at least the Common Era, to (a) obtain temperature- and circulation-related records of the high Alpine realm and (b) extend the study of anthropogenic imprints, in particular if associated with respective climatic and historical data.

As an example, with respect to (a), several studies focused on the CG glaciochemical records (especially calcium) to obtain a long record of Saharan dust events. Because Saharan dust advection to the Alps is coupled to certain types of synoptic weather patterns (Prodi & Fea, 1979), such records promise information on past atmospheric circulation changes. Since individual dust events are generally not an Alpine-wide phenomenon, and since a single ice core record may be plagued by snow preservation, detecting the frequency of occurrence in dust events deserves special attention. An early study inspected the period back to 1600 to find no evidence of long-term changes in the dust layer frequency but an increase in mean calcium levels within the 20th century concurrent with the stable water isotope trends (Wagenbach et al., 1996). The calcium-isotope covariability was further explored for a potential connection to atmospheric temperature, detecting evidence of known paleoclimate phenomena like the Little Ice Age cooling and the Medieval Warm Period. The latter period coincides with a distinct increased occurrence of dust events for the period 1200–1000 indicating potential changes in circulation or desert dust source strength (Bohleber, Erhardt, et al., 2018). Using sophisticated analyses of mineral dust particles and elemental black carbon, another study investigated mineral dust levels, biomass, and fossil fuel combustion over the last millennium. At multiannual to multidecadal resolution, several periods of increased dust levels were detected and inspected with respect to past circulation, alongside changes in biomass burning activities for the Little Ice Age period (Thevenon, Anselmetti, Bernasconi, & Schwikowski, 2009). The ice core records of black carbon provide an important tool to investigate the history of natural biomass and anthropogenic fossil fuel burning and are of special significance regarding a potential link to Alpine glacier retreat (Painter et al., 2013; Sigl et al., 2018). Regarding further proxy developments in connection to dust transport, the analysis of pollen in non-polar ice cores gains importance in view of a detailed investigation of past vegetation dynamics and atmospheric transport, as well as regarding support for annual layer identification and dating applications (Brugger et al., 2018; Festi et al., 2017).

Regarding (b), the long-term investigation of anthropogenic imprints on the CG snow chemistry, the study of heavy metals has arguably received the most attention so far—detailed findings have been summarized in a dedicated overview (Gabrieli & Barbante, 2014). This concerns in particular lead, as it has already been demonstrated that emissions from human activities (such as metal production) can be recognized in polar ice cores (Hong, Candelone, Patterson, & Boutron, 1994) and other European climate archives (Shotyk et al., 1998). Pioneering analyses of heavy metals in CG ice cores were conducted covering the time period since the 17th century (Barbante et al., 2004; Schwikowski et al., 2004). Lead and lead isotope records show a clear rise in lead concentrations since the 17th century, in line with historic emission scenarios. The record shows a decline in lead concentrations starting only recently since about 1975, coinciding with the mandatory reduction in emission of leaded gasoline (Schwikowski et al., 2004). The lead record was further extended ultimately beyond the last millennium, enabling further comparison with historical data (Gabrieli & Barbante, 2014). Exploring the potential of ultra-high resolution lead analysis covering historical periods of special interest, a distinct minimum in lead concentration was found in a CG ice core to coincide with the economic regression in Europe following the Black Death pandemic around 1350 (More et al., 2017).

Future Perspectives

With respect to the numerous studies covering the industrial period and various sophisticated glaciochemical parameters in Alpine ice cores, respective long-term investigations of the CG ice cores are comparatively sparse. However, the development of the micro-radiocarbon dating and high-resolution annual layer counting provide important aids to lift the barrier imposed by dating uncertainties. These breakthroughs make longer time scales accessible for interpretation and, at the same time, create an increasing demand for an adequate process understanding, in particular (a) to constrain upstream effects for the deeper ice core layers, (b) to assess radiometric evidence of age discontinuities, and (c) to determine the integrity of the lowermost core sections comprising a clear anomaly in the stable water isotope signature (Wagenbach et al., 2012). Glaciological and ice core technological innovations will need to go hand-in-hand to achieve this goal. In this context, it is encouraging to see new deep ice core drillings and glaciological surveys conducted since 2013, led by several groups, further continuing more than four decades of ice coring investigations at CG.

Regarding the extension of ice coring programs beyond the highest summits of the Western Alps, the paradigm of immediately useful climate signals being confined to cold (non-temperate) drilling sites has been challenged and the existence of old/cold ice at lower elevations verified (Gabrielli et al., 2012; Haeberli et al., 2004). This has sparked new efforts to explore cold-based summit glaciers at comparatively lower elevations and to retrieve still untapped ice core records that are, however, under immediate pressure by ongoing warming conditions. As a particular example in this respect, the Ortles ice core may promise long-term ice core climate records complementary to the CG drilling site (Gabrielli et al., 2016). This is a good illustration of the mutual transfer of knowledge between well-explored cold and new (partially) temperate drilling sites. On the one hand, established tools and process understanding from the cold sites are readily available for ice core dating and time series interpretation. On the other hand, the state of the lower elevation drilling sites in the 21st century is of interest with respect to the future of the cold sites under persistent warming conditions. As pointed out by Gabrielli et al. (2012), the Ortles drilling site is undergoing change from a cold to a temperate state. Moreover, the Ortles drilling site experienced a very recent change in the ice dynamics with enhanced ice flow near the base that is likely connected to the ongoing thermal changes of the site (Gabrielli et al., 2012). Worth noting in this context, ice temperature profiles at both the Mont Blanc and Monte Rosa areas reveal a systematic warming trend over several decades (Gilbert & Vincent, 2013; Hoelzle et al., 2011). Although the englacial temperature at both drilling sites at CDD and CG is still far below the melting point, the role of infiltrating meltwater for the glacier energy balance and the significant temperature dependence of the ice viscosity must be recognized. If the warming trend persists, the increasing meltwater influence can endanger the old ice archive at CG (Hoelzle et al., 2011) and potentially elsewhere (Vincent et al., 2007).

Conclusion

Considering the complex settings of the small-scale drilling sites in the Alps, what is the significance of Alpine ice core studies? A key product of Alpine ice cores has been the study of environmental conditions covering past time periods without direct atmospheric measurements. With respect to retrieving climate-related records, significant innovations have been achieved, in particular regarding atmospheric signal identification and dating precision. In comparison to other remote mountain areas, a long-term ice coring program can be sustained in the Alps at substantial yet comparatively lower logistical cost. Almost all suitable cold drilling sites have been exploited in the Alps, resulting in a high spatial density of ice cores drilled at sites with different glacio-meteorological properties. Multiple cores have even been drilled at some locations. Here, the comparison of time series from multiple cores provides an important tool to assess (a) the upstream-variability along a flow line and (b) to what extent the extracted proxy variability is affected by local depositional noise masking an atmospheric signal (presumably leaving a common imprint on all cores). Having reliable ice core age scales is a basic requirement for achieving intercore synchronization. Precise knowledge of the age-depth relation calls for a dedicated combination of dating tools, in particular the use of 14C for deep layers at CG. In view of the seasonal differences in air advection (intense vertical mixing confined to summer) and climatological properties of the Alps (e.g., differences with respect to sites north or south of the main Alpine chain), the spatial representativity of any findings from a single drilling site must be assessed. While records from low accumulation sites may feature a distinct summer bias (similar to the bias toward the growing season in tree rings), records extracted from high accumulation sites may offer a more complete seasonality. Hence, a key approach is using multiple ice core records from sites differing in their glaciological settings for an intersite comparison, as well as the comparison with long-term instrumental climate series.

Based on the high spatial density of ice cores drilled at multiple sites with different glaciological settings, the availability of long-term instrumental climate series, and intense glaciological surveys, the Alps provide a worldwide unique framework for investigating non-polar ice cores as climate archives. Over the course of more than four decades, advances have been made with broader significance for this field, in particular regarding evaluating dating reliability, assessing spatiotemporal significance, and regarding the identification of atmospheric signals. Situated in directed proximity to emission sources, the manifold glaciochemical analysis of high accumulation Alpine ice cores provides a comprehensive view of the anthropogenic impact on the high Alpine snow chemistry. Not least due to advances in dating the low accumulation ice core records from CG, studying the anthropogenic emission history over extended time scales, including the last two millennia, has become possible. Further deciphering temperature- and circulation-related information in the climate archive of Alpine ice cores promises to remedy their present underrepresentation in multiproxy reconstructions of the European paleoclimate history.

Acknowledgments

The author would like to thank Susanne Preunkert for valuable discussions and feedback and Andrea Fischer for her suggestions and help with figure production. The author would also like to acknowledge editor Ingeborg Auer and an anonymous referee for their helpful comments and suggestions for improving the article.

Further Reading

Preunkert, S., & Legrand, M. (2013). Towards a quasi-complete reconstruction of past atmospheric aerosol load and composition (organic and inorganic) over Europe since 1920 inferred from Alpine ice cores. Climate of the Past, 9(4), 1403–1416.Find this resource:

Schwikowski, M. (2004). Reconstruction of European air pollution from Alpine ice cores. In L. D. Cecil, J. R. Green, & L. G. Thompson (Eds.), Earth paleoenvironments: Records preserved in mid-and low-latitude glaciers (pp. 95–119). Dordrecht, The Netherlands: Springer.Find this resource:

Wagenbach, D., Bohleber, P., & Preunkert, S. (2012). Cold, Alpine ice bodies revisited: What may we learn from their impurity and isotope content?Geografiska Annaler: Series A, Physical Geography, 94(2), 245–263.Find this resource:

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Notes:

(1.) Cold glaciers: featuring englacial temperatures below the pressure melting point, as opposed to temperate glaciers with ice temperatures at the pressure melting point.

(2.) The term “Alpine ice core” is used here to refer to an ice core drilled in the European Alps, cf. the term “alpine” sometimes being used to generally refer to any mountain drilling site.

(3.) Water equivalent depths (m w.e.) denote the thickness of a respective layer when compressed to the density of water. This is to account for the different densities of snow, firn, and ice.