Melt-Induced Fractionation of Major Ions and Trace Elements in An

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RESEARCH ARTICLE Melt‐Induced Fractionation of Major Ions and Trace 10.1029/2019JF005026 Elements in an Alpine Snowpack Key Points: Sven E. Avak1,2,3 , Jürg C. Trachsel4,5, Jacinta Edebeli1,5, Sabina Brütsch1, • Impurity profiles in the seasonal 1 4 1,2,3 snowpack at Weissfluhjoch, Swiss Thorsten Bartels‐Rausch , Martin Schneebeli , Margit Schwikowski , and Alps, suggest a well‐preserved Anja Eichler1,3 atmospheric composition in winter • Melting during warm periods causes 1Laboratory of Environmental Chemistry, Paul Scherrer Institute, Villigen, Switzerland, 2Department of Chemistry and preferential elution of selected major Biochemistry, University of Bern, Bern, Switzerland, 3Oeschger Centre for Climate Change Research, University of Bern, ions and trace elements from the 4 ‐ 5 snowpack Bern, Switzerland, WSL Institute for Snow and Avalanche Research SLF, Davos Dorf, Switzerland, Department of • Ammonium, the rare‐earth Environmental Systems Science, ETH Zurich, Zürich, Switzerland elements, and low concentrated trace elements tend to be most resistant to meltwater‐induced Abstract Understanding the impact of melting on the preservation of atmospheric compounds in relocation high‐Alpine snow and glacier ice is crucial for future reconstruction of past atmospheric conditions. However, detailed studies investigating melt‐related changes of such proxy information are rare. Here we present a series of five snow pit profiles of 6 major ions and 34 trace elements at Weissfluhjoch, Correspondence to: Switzerland, collected between January and June 2017. Atmospheric composition was preserved during the A. Eichler, [email protected] cold season, while melting toward the summer resulted in preferential loss of certain species from the snowpack or enrichment at the base of the snowpack. Increasing mobilization of major ions with meltwater + − + − 2+ 2− (NH4

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Virkkunen et al., 2007; Wang et al., 2018; Zong‐Xing et al., 2015). Preferential elution of certain ions relative to others has been observed, being strongly dependent on the respective geographical location of the snow- 2− + pack. These studies further indicate that SO4 is generally significantly depleted with melting, while NH4 appears to be preserved despite meltwater percolation. In the majority of these studies available in the literature, either melting had strongly affected the MI records making the initial conditions before melting unknown or a direct comparison of concentration records before and after melting to quantify the degree of depletion has not been performed. In contrast to MIs, the effect of melting on the fate of TEs in snow has still not been well characterized. To the best of our knowl- edge, only a few studies have addressed the influence of melting on TE concentration records in snow pits. Wong et al. (2013) artificially infiltrated meltwater into Greenland snow pits to investigate melt effects on 12 different TE records. They observed that mineral dust particle‐bound TEs remained immobile during meltwater percolation resulting in the preservation of their seasonal chemical signal. On the contrary, Zhongqin et al. (2007) reported from the analyses of snow‐firn pits taken in the Central Asian Tien Shan Mountains that meltwater percolation during the summer may have eluted the five investigated TEs (Al, Cd, Fe, Pb, and Zn) from the snow‐firn pack. The TE records from the analysis of a meltwater‐affected firn part of an Alpine ice core indicated that meltwater percolation led to preferential loss of certain TEs (Avak et al., 2018). Water‐insoluble TEs and low‐abundant water‐soluble TEs remained largely immobile with meltwater. Since this study proposed a geographical site specificity for the preferential elution of TEs (Avak et al., 2018), it remains unclear how representative these findings are for the Alpine region. The high‐Alpine snow field site at Weissfluhjoch (WFJ), Switzerland, is well suited for snowpack studies. Numerous studies have been conducted at this site focusing on snow characterization, snow mechanics, snow metamorphism, and the development of measurement methods since 1936 (Marty & Meister, 2012). 2− − So far, studies investigating the chemical impurities in the snowpack at WFJ focused on SO4 ,NO3 , Cl, + + + 2+ 2+ K ,Na ,NH4 ,Ca , and Mg . Baltensperger et al. (1993) compared consecutive measurements of surface snow sampled from January to March 1988 to that from a snow pit taken at the end of March at WFJ. The snow pit samples were found to be representative of precipitation deposition during the winter. Snow pit sampling, sample handling procedures, and chemical analysis of MIs were further refined by Schwikowski et al. (1997). In addition, the seasonal variability in deposition and the different emission sources of impurities were reflected in the vertical distribution of MIs in the snowpack (Schwikowski et al., 1997). Here we present the first study monitoring the postdepositional fate of an extensive set of environmentally relevant atmospheric impurities in the high‐Alpine snowpack at WFJ. We monitored 6 MIs and 34 TEs during an entire winter/spring season and investigated their behavior during melting of the snowpack in early summer. Five snow pits sampled from January to June 2017 allowed us to capture both the initial, undis- turbed records of atmospheric compounds in the snowpack and the records after melting had occurred during the warmer season. Impurity profiles reflecting dry conditions (without significant melting) were compared with profiles reflecting wet conditions to systematically investigate the impact of melting on the preservation of atmospheric tracers in high‐Alpine snow.

2. Materials and Methods 2.1. Study Site and Meteorological Setting Five snow pit samplings were conducted at the high‐Alpine snow ﬁeld site WFJ of the WSL‐Institute for Snow and Avalanche Research, Eastern Switzerland (2,536 m above sea level, 46°49′47″N 9°48′33″E) at reg- ular time intervals in the winter, spring, and early summer seasons in 2017. Sampling dates were 25 January, 22 February, 21 March, 17 April, and 1 June. Earlier snow sampling studies suggest uniform spatial snow deposition and insigniﬁcant perturbations of the snow stratigraphy due to strong winds at this site (Baltensperger et al., 1993; Schwikowski et al., 1997). Samples were obtained from the snow pits either 1 day before or after the weekly density measurements with 3‐cm resolution, which were part of an extensive snowpack monitoring program of the WSL‐Institute for Snow and Avalanche Research during the winter season 2016/2017 (Calonne et al., 2016). The snow heights during the sampling were 87 cm (25 January), 126 cm (22 February), 185 cm (21 March), 166 cm (17 April), and 83 cm (1 June; Figure 1). Snow surface (via an infrared radiometer) and 2‐m air temperatures (by an automated weather station) indicated that

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dry conditions without significant melting prevailed for the first three sampling dates on 25 January, 22 February, and 21 March (Figure 1). Only 6 days with a mean 2‐m air temperature above 0 °C occurred during the period 1 January until 21 March (1.3 ± 0.8 °C on average). Partial melting was identifiable in the snow pit on the fourth sampling date on 17 April due to 13 days between 21 March and 17 April with mean 2‐m air temperatures above 0 °C (1.8 ± 1.3 °C on average). The snowpack was entirely wet on the fifth sampling date on 1 June due to another 25 days between 17 April and 1 June with mean 2‐m air temperatures above 0 °C (4.8 ± 3.4 °C on average). 2.2. Snow Pit Sampling As snow is particularly sensitive to trace levels of contamination due to its low impurity content, precautions were taken during sampling. Sterile clean room overalls (Tyvek® IsoClean®, DuPont, Wilmington, DE, USA), particulate respirator face masks (3M, Maplewood, MN, USA), and ultra- Figure 1. The 2‐m air and snow surface temperatures and daily precipita- clean plastic gloves (Semadeni, Ostermundigen, Switzerland) were worn fl fi tion rate and snowpack heights at the Weiss uhjoch eld site, Swiss Alps, during the sampling. All tools were carefully rinsed with ultrapure water during the winter season of 2016/2017. Snow pit samplings were conducted Ω both in the cold season, where dry conditions without significant melting (18 M cm quality, arium® pro, Sartorius, Göttingen, Germany) prior to prevailed, and in the warm season, where severe melting of the snowpack use. Snow pits were sampled with a vertical resolution of 6 cm down to occurred. the bottom of the snowpack by pushing a custom‐built rectangular (15 × 24 cm) sampler made from polycarbonate into the pit wall. To allow for sufficient sample volume, the snow was filled into 50‐ml polypropylene vials (Sarstedt, Nümbrecht, Germany) by pushing them twice with the opening facing downward into the snow. Separate vials were used for MI, WSI, and TE analyses. Polypropylene vials were precleaned five times with ultrapure water for MI and WSI samples. For TE samples, the tubes were precleaned five times with ultrapure water (18 MΩ cm

quality, Milli‐Q® Element, Merck Millipore, Burlington, MA, USA) plus once with 0.2 M HNO3 prepared TM from ultrapure HNO3 (Optima , Fisher Chemical, Loughborough, UK). Samples for TE analysis were taken from the snow at the front part of the sampler to prevent possible cross contamination of the vials used

for MI sampling with HNO3. 2.3. Major ion, water stable isotope, and trace element analysis A total of 324 samples for MI, WSI, and TE analyses was kept frozen at −20 °C until analysis at the Paul Scherrer Institute (Villigen, Switzerland). + + 2+ − − 2− MIs (Na ,NH4 ,Ca ,Cl ,NO3 , and SO4 ) present in the snow pit samples were analyzed after melting at room temperature using ion chromatography (IC; 850 Professional IC equipped with a 872 Extension Module Liquid Handling and a 858 Professional Sample Processor auto sampler, Metrohm, Herisau,

Switzerland). Cations were separated using a Metrosep C4 column (Metrohm) and 2.8 mM HNO3 as eluent at a ﬂow rate of 1 ml/min. Anions were separated using a Metrosep A Supp 10 column (Metrohm) and were

eluted stepwise using ﬁrst, a 1.5 mM Na2CO3/0.3 mM NaHCO3 (1:1 mixture) eluent, and then an 8 mM Na2CO3/1.7 mM NaHCO3 (1:1 mixture) eluent at a ﬂow rate of 0.9 ml/min. Possible instrumental drifts were monitored by measuring an in‐house standard after every twentieth sample. The precision of the method was ~5%. WSI samples were melted at room temperature, and 1 ml aliquots were analyzed for δD and δ18O using a wavelength‐scanned cavity ring down spectrometer (WS‐CRDS, L2130‐i Analyzer, Picarro, Santa Clara, CA, USA). Samples were injected into the vaporizer (A0211, Picarro, Santa Clara, CA, USA) using a PAL HTC‐xt autosampler (LEAP Technologies, Carrboro, NC, USA). Three in‐house standards were measured after every tenth sample for calibration and to monitor instrumental drifts. The measurement uncertainty was <0.1‰ for δ18O and <0.5‰ for δD.

Snow pit samples were melted at room temperature, acidified with concentrated ultrapure HNO3 to 0.2 M (1% v/v), and analyzed 3–4 hr after acidification following the same procedure as described in Avak et al. (2018) using discrete inductively coupled plasma sector field mass spectrometry (Element 2, Thermo Fisher Scientific, Bremen, Germany). Low (LR, R = 300) or medium resolution (MR, R = 4,000) data were

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acquired for Ag (LR), Al (MR), Ba (LR), Bi (LR), Ca (MR), Cd (LR), Ce (LR), Co (MR), Cs (LR), Cu (LR and MR), Eu (LR), Fe (MR), La (LR), Li (LR and MR), Mg (MR), Mn (MR), Mo (LR), Na (MR), Nd (LR), Ni (LR and MR), Pb (LR), Pr (LR), Rb (LR), Sb (LR), Sc (MR), Sm (LR), Sr (LR), Tl (LR), Th (LR), U (LR), V (MR), W (LR), Yb (LR), Zn (LR and MR), and Zr (LR). If both LR and MR resolution data were available, MR data were used for further data evaluation. Quantiﬁcation of intensity values was performed by internal and external calibrations using a Rh‐standard and seven liquid standards covering the typical concentration range of TEs in natural snow and ice samples, respectively. Linear regressions of the calibration curves consistently revealed correlation coefﬁcients >0.999.

2.4. Data Evaluation IC raw data were processed using the MagIC Net 3.2 software (Metrohm, Herisau, Switzerland). Sample concentrations were not blank‐corrected as the concentrations of the blank, determined by analyzing ultrapure water, were below the detection limit (DL). Sample concentrations below the DL were substituted with half the value of the DL. On average, 10% of the measurement values were below the DL. Inductively coupled plasma mass spectrometry raw data were evaluated following the method described by Knüsel et al. (2003). Concentrations were blank‐corrected by subtracting a measurement blank consisting of four measurements of

ultrapure 0.2 M HNO3. The instrumental DL was defined as 3σ of the measurement blank, and concentrations below the DL were substituted with half of the value of the DL. 109Ag was excluded from the data set for further evaluation and discussion as concentrations of all samples were below the DL. Total depths of the snow profiles were converted to water equivalents (w. fi + − 2+ fi Figure 2. Concentration pro les of NH4 ,Cl , and Ca of the ve snow eq.) by multiplying with the respective density. As 17 April had the largest pits taken at the Weissfluhjoch field site during winter/spring (black/blue snow depth, the profiles of 25 January, 22 February, 21 March, and 1 June curves) and early summer (red curve). For comparison, the corresponding 18 fi ‐ δ O records are shown. The gray dashed line indicates the bottom of the were aligned relative to the pro le of 17 April, with 0 cm w.eq. being the snowpack. Profile depths were aligned to the one of 17 April, and 0‐cm w.eq. surface on this day and assuming the same bottom depth for all profiles. reflects the surface of the snowpack on this day. The surface sample of the All five profiles cover the depth interval 45‐ to 65‐cm w.eq. The profiles snow pit from 1 June is shown with a dashed line (see text for details). The of 22 February, 21 March, 17 April, and 1 June cover 36‐ to 65‐cm w.eq., shaded area indicates the depth interval (45‐ to 65‐cm w.eq.) used for cal- the profiles of 22 February, 21 March, and 17 April cover 30‐ to 65‐cm culation of the concentration ratio and determining an elution sequence. w. fi ‐ ‐ eq. = water equivalents. w.eq., and the pro les of 21 March and 17 April cover 5 to 65 cm w.eq. The sample at the base of each snow pit was omitted from all chemical profiles to exclude a possible influence of the soil at WFJ.

3. Results and Discussion 3.1. Major ions 3.1.1. Comparison of the Five MI Concentration Profiles + − 2+ 18 The chemical profiles of selected MIs (NH4 ,Cl , and Ca ) and the δ O records in the five snow pits are shown in Figure 2. The general MI patterns of the four profiles, reflecting the winter and spring periods (January–April), show a strong correspondence in the respective overlapping parts. Apart from possible changes caused by melting (17 April), slight concentration differences in the profiles can be attributed to the spatial variability of impurities within the snowpack as the locations of the individual snow pit samplings were several meters (up to 20 m) apart. The resemblance between the four winter and spring snow pits is also visible for the corresponding δ18O profiles which support the depth assignment of the different snow pits. + − 2+ The chemical profiles of NH4 ,Cl , and Ca are exemplary for the different emission sources and transport characteristics of MIs in Alpine snow and ice. The depths 0‐ to 30‐cm w.eq. (21 March and 17 April) indicate

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Table 1 precipitation occurring in spring after 22 February, while 30‐ to fi a Concentration Ratio, Classi cation , Mean Concentration of the Dry Snow Pits (25 65‐cm w.eq. represent winter precipitation (Figure 1). The NH + January, 22 February, and 21 March), and Detection Limits of Major Ions and 4 Trace Elements Investigated in This Study springtime concentrations are roughly three times higher than win- + tertime concentrations. NH4 detected in snow pits and ice cores Concentration Mean concentration Detection from European high‐Alpine sites is nowadays generally of anthropo- ratio/classification (ng/L) limit (ng/L) + genic origin (Döscher et al., 1996; Gabrieli et al., 2011). NH4 also Na+ 0.62 ± 0.45 17,000 ± 2,840 500 + shows a pronounced maximum toward summer due to enhanced NH4 0.77 ± 0.27 26,300 ± 4,580 500 2+ emissions from agricultural activities and stronger convection Ca 0.37 ± 0.18 71,500 ± 11,400 10,000 − + Cl 0.64 ± 0.43 34,000 ± 4,980 1,000 (Baltensperger et al., 1997). Higher concentrations of NH4 in the − NO 0.37 ± 0.11 249,000 ± 21,200 1,000 springtime snowfall compared to wintertime snowfall at WFJ were 3 − 2 2+ − SO4 0.36 ± 0.10 57,300 ± 5,330 5,000 also observed by Schwikowski et al. (1997). Ca and Cl , unlike Al i.b.s. 4,870 ± 792 115 + NH4 , have higher concentrations in the wintertime snow than in Ba 0.39 ± 0.26/group 2 262 ± 67 3.5 2+ − Bi 0.50 ± 0.28/group 2 4.5 ± 0.91 0.01 spring. At Alpine sites, Ca and Cl are indicative of the input from Ca 0.46 ± 0.24/group 2 110,000 ± 21,400 498 mineral dust (Bohleber et al., 2018; Schwikowski et al., 1995; Cd 0.55 ± 0.23/group 2 1.9 ± 0.26 0.27 Wagenbach et al., 1996) and sea salt aerosols (Eichler et al., 2004), Ce 1.5 ± 1.1/group 1 14 ± 2.9 0.03 respectively. A very pronounced peak in the Ca2+ and Na+ profiles Co i.b.s. 24 ± 4.5 0.22 between 50‐ and 60‐cm w.eq. (Figure 2) can be most likely attributed Cs i.b.s. 2.1 ± 0.33 0.07 Cu i.b.s. 48 ± 5.6 2.7 to a Saharan dust event. This is corroborated by back trajectory ana- Eu 1.1 ± 0.79/group 1 0.51 ± 0.10 0.13 lysis for the beginning of January 2017 and measurements by the Fe i.b.s. 5,990 ± 1,180 245 Swiss Federal Office of Meteorology and Climatology at the La 1.5 ± 1.1/group 1 6.1 ± 1.2 0.03 Jungfraujoch high‐Alpine research station on 5 January 2017 Li i.b.s. 6.1 ± 0.90 3.8 (MeteoSwiss, 2014). Mg i.b.s. 37,000 ± 6,600 30 Mn i.b.s. 612 ± 110 0.69 After several weeks with temperatures above 0 °C, the snowpack was Mo 1.1 ± 0.59/group 1 3.1 ± 0.46 1.8 very wet from meltwater (and rainwater) at the beginning of the sum- Na 0.50 ± 0.32/group 2 15,900 ± 2,620 1,280 mer (1 June; Figure 1). The uppermost (surface) sample of the snow Nd 1.2 ± 0.92/group 1 7.7 ± 1.7 0.01 fl Ni i.b.s. 101 ± 22 9 pit from 1 June (red dotted line in Figure 2) is most likely in uenced Pb 0.84 ± 0.36/group 1 180 ± 15 2.7 by residual snow impurities after surface melting and wet or dry Pr 1.1 ± 0.84/group 1 1.8 ± 0.38 0.01 deposition between 17 April and 1 June (Figure 1) and was thus not Rb i.b.s. 15 ± 2.1 0.54 taken into consideration. The δ18O profile of 1 June reveals a strong Sb 0.88 ± 0.36/group 1 9.1 ± 1.2 0.13 smoothing compared to the previous profiles, indicative of strong Sc 1.5 ± 1.4/group 1 1.8 ± 0.41 0.17 ‐ Sm 1.2 ± 0.88/group 1 2.0 ± 0.46 0.02 melting (Thompson et al., 1993). However, a signature between 45 Sr 0.37 ± 0.21/group 2 270 ± 55 3.4 and 65‐cm w.eq. resembling that from the previous profiles is still Th 0.45 ± 0.30/group 2 1.4 ± 0.25 0.03 visible. This indicates that at this depth, meltwater percolation was Tl i.b.s. 0.39 ± 0.04 0.01 concentrated to the porous space in the snowpack and primarily per- U 1.5 ± 0.99/group 1 2.3 ± 0.64 0.01 colated along ice surfaces, keeping the ice matrix in principle pre- V i.b.s. 19 ± 2.5 0.37 fi fi W 1.2 ± 0.69/group 1 0.54 ± 0.07 0.18 served. The MI pro les of 1 June compared to those of the rst + Yb i.b.s. 0.63 ± 0.13 0.02 three sampling dates are almost unaltered (NH4 ), slightly depleted Zn 0.47 ± 0.36/group 2 2,780 ± 463 28 + − 2+ − 2− (Na and Cl ), or strongly depleted (Ca ,NO3 , and SO4 ), most Zr 0.13 ± 0.08/group 2 5.4 ± 0.75 0.26 likely due to different elution behavior of the investigated ions with aTrace elements were classified depending on whether the profile of the snow meltwater (Eichler et al., 2001). fl pit of 1 June showed a strong enrichment from the soil at Weiss uhjoch below 3.1.2. Preferential Elution of MIs: Elution Sequence 55‐cm water equivalent (influenced by soil [i.b.s.]), an unbiased profile pattern compared to the dry condition profiles (group 1), or a heavily depleted concen- and Discussion tration profile (group 2). To quantify and compare the apparent preferential elution of MIs, a

concentration ratio cwet/cdry for the overlapping depth (45‐ to 65‐cm w.eq.) was calculated for each MI (Table 1). cwet corresponds to the integrated MI concentration for the wet profile of 1 June, whereas cdry represents the mean of the integrated concentration profiles during dry (no significant melting) periods (25 January, 22 February, and 21 March). The profiles on 17 April were not included as the snowpack was neither completely dry nor very wet. Corresponding to the concentration + − + − 2+ 2− + ratio, an elution sequence was established: NH4

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Elution sequences of MIs were also determined through laboratory studies (Cragin et al., 1996; Tranter et al., 1992; Tsiouris et al., 1985). We recently performed an elution experiment where, for the first time, homo- genous impurity‐doped artificial snow was exposed to well‐defined snow metamorphism conditions prior to leaching with 0 °C ultrapure water. This experiment was conducted to determine enrichment differences of MIs between ice crystal interiors and surfaces after metamorphism (Trachsel et al., 2017). Snow undergoes drastic structural transformation cycles during metamorphism (Pinzer et al., 2012), which may result in significant impurity redistribution. Our elution sequence at the WFJ agrees well with the findings of the laboratory‐based elution experiment + (Figure 3). MIs having a higher solubility in ice such as NH4 and Cl (Feibelman, 2007; Hobbs, 1974) showed less mobility most likely due to incorporation into the crystal interior during snow metamorphism. They can be incorporated either by substituting water molecules located on lattice sites of the ice crystal (Zaromb & Brill, 1956) or by occupying interstitial spaces of the crystal structure (Petrenko & Whitworth, 2002). 2+ 2− On the other hand, Ca and SO4 were enriched on ice crystal surfaces with progressing snow metamorphism (Figure 3); explaining their avail- ability for mobilization with meltwater. This is supported by previous location studies of salts in ice. Mulvaney et al. (1988) used a combination + + 2+ − Figure 3. Time evolution of relative enrichment of Na ,NH4 ,Ca ,Cl , ‐ ‐ 2− of scanning electron microscopy and energy dispersive X ray microanaly- and SO4 concentrations at ice crystal surfaces determined by leaching artificial snow, homogeneously doped with known concentrations of MIs, sis to show that H2SO4 concentrations at triple junctions are several orders and exposed for different time periods to a temperature gradient mimicking of magnitude higher compared to grain interiors in polar ice from snow metamorphism, with 0 °C ultrapure water (adapted from Trachsel Antarctica. Similar observations for H2SO4 and HNO3 using Raman spec- et al., 2017). troscopy were reported by Fukazawa et al. (1998). Accumulation of

MgSO4 at grain boundaries in ice from the Greenland Ice Sheet Project (GISP) ice core 2 was revealed by low‐vacuum scanning electron microscopy‐energy‐dispersive X‐ray − (Baker et al., 2003). The varying elution behavior of MIs observed in the snow pit of 1 June with NO3 , 2+ 2− Ca , and SO4 being most heavily depleted can therefore be explained by their microscopic locations on ice surfaces, exposing them to relocation during melting. Our ﬁndings show that the atmospheric composition of MIs is well preserved in the snowpack at WFJ during the cold season. Melting during the warm season leads to preferential leaching of MIs depending on their microscopic location either on the ice surface or in the ice interior. The loss of MIs is particularly signiﬁcant 2+ 2− + + for Ca and SO4 . In contrast, the strong persistence of NH4 emphasizes that NH4 can still serve as environmental tracer for the interpretation of snow pit and ice core records affected by melting.

3.2. Trace elements 3.2.1. TE Concentration Profiles Concentrations of Co, Fe, Ce, Sb, Ca, and Sr in the snow pits on 25 January, 22 February, 21 March, 17 April, and 1 June are exemplarily shown in Figure 4. The general pattern of the profiles of the first four snow pits (January to April) agrees well in the overlapping depths. As for the MI profiles, misalignment in peaks can be attributed to small‐scale spatial variability of impurities in the snowpack. The reproducibility of chemical profiles with ultratrace level concentrations (e.g., Ce and Sb) from different sampling campaigns demon- strates that significant contamination during the snow pit samplings did not occur. Inductively coupled plasma mass spectrometry measurements of Ca and Na concentration profiles are in good agreement with the records of Ca2+ and Na+ concentrations obtained by IC (Figures 2 and 4). The uppermost (surface) sample of the snow pit from 1 June revealed highly elevated concentrations for many TEs (red dotted line in Figure 4). This is most likely due to residual snow impurities after surface melting and TEs that reached the snowpack by wet or dry deposition between 17 April and 1 June (Figure 1). For this reason, the uppermost sample of the snow pit from 1 June was not taken into consideration for the following discussion. The 34 TEs can be either of geogenic origin or emitted by anthropogenic sources. At high‐Alpine sites, Al, Ba, Bi, Ca, Cs, Fe, Li, Mg, Mn, Na, Rb, Sr, Th, Tl, U, W, Zr, and the rare‐earth elements (Ce, Eu, La, Nd, Pr, Sc,

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Sm, and Yb) are mainly deposited with mineral dust (Gabrieli et al., 2011; Gabrielli et al., 2008), whereas Ag, Cd, Co, Cu, Mo, Ni, Pb, Sb, V, and Zn in Alpine snow and ice are characteristic of anthropogenic atmospheric pollution (Barbante et al., 2004; Gabrieli et al., 2011; Gabrielli et al., 2008; Schwikowski et al., 2004; van de Velde et al., 1999, 2000). The chemical profiles of Ca, Ce, Fe, and Sr (Figure 4) are representative of TEs originat- ing from mineral dust particles and reveal an enrichment in the depths reflecting winter precipitation (30‐ to 65‐cm w.eq.), where dry conditions prevailed (25 January, 22 February, and 21 March). These TEs show a major peak between 50‐ and 60‐cm w.eq. due to a Saharan dust event beginning of January 2017 (see section 3.1.1). As explained above for + NH4 , TEs indicative of anthropogenic influence such as Sb (Figure 4) are enriched in depths reflecting spring precipitation deposited after 22 February (0‐ to 30‐cm w.eq.). This is due to the typical seasonality of con- vective air mass transport from the planetary boundary layer to high‐ Alpine sites occurring in spring and summer (Baltensperger et al., 1997). Based on the concentration profiles on 1 June, the 34 investigated TEs were divided into three groups (Table 1). Group 1 (Ce, Eu, La, Mo, Nd, Pb, Pr, Sb, Sc, Sm, U, and W) is characterized by an almost unchanged profile pattern between 45‐ and 65‐cm w.eq. compared to the dry condition profiles (e.g., Ce and Sb, Figure 4). This indicates that group 1 TEs were not affected by melting. Group 2 TEs (Ba, Bi, Ca, Cd, Na, Sr, Th, Zn, and Zr) were strongly depleted on 1 June compared to winter and springtime records (as shown for Ca and Sr in Figure 4). Group 3 TEs (Al, Co, Cs, Cu, Fe, Li, Mg, Mn, Ni, Rb, Tl, V, and Yb) exhibit a strong enrichment below 55‐cm w.eq. close to the soil (see, e.g., Co and Fe, Figure 4). The bottom of the snowpack on 1 June was very wet, probably enriched with impurities not of atmospheric origin, but present in the soil at WFJ. We therefore attribute the strong increase in concentration below 55‐cm w.eq. to the contact with soil and denote this group as “influenced by soil.” Group 3 TEs were therefore not included in further evaluation. 3.2.2. Different Preservation of TEs Under Melt Conditions A quantitative classification of TE fractionation during melting of the snowpack (groups 1 and 2) was performed by calculating a concentration

ratio cwet/cdry for each TE in the overlapping depths (see above, Table 1). The highest concentration ratio was obtained for La, indicative of its well preserved concentration record on 1 June, whereas the concentration profile of Zr is most severely affected by melting, having the lowest concentration ratio. Concentration ratios of TEs classified as group 1 are close to 1 Figure 4. Concentration profiles of Co, Fe, Ce, Sb, Ca, and Sr of the five and range between 1.5 ± 1.1 (La) and 0.84 ± 0.36 (Pb). Concentration fl fi snow pits taken at the Weiss uhjoch eld site between winter/spring ratios of group 2 TEs are significantly below 1 and range between (black/blue curves) and early summer (red curve). The gray dashed line 0.55 ± 0.23 (Cd) and 0.13 ± 0.08 (Zr). Arranging the concentration ratios indicates the bottom of the snowpack. Profile depths were aligned to the one of 17 April, and 0‐cm water equivalent reflects the surface of the snowpack according to size and plotting this rank against the ratio indicates that on this day. The surface sample of the snow pit from 1 June is shown with a each TE belonging to groups 1 and 2 was differently preserved in the dashed line (see text for details). The shaded area indicates the depth inter- snowpack during melting (Figure 5). val (45‐ to 65‐cm water equivalent) used for calculation of the concentration ratio. Trace elements were classified as “influenced by soil,” or allocated to The preservation of group 1 TEs with observed concentration ratios >0.7 group 1 or 2 (retained or depleted concentration profile, according to the (Ce, Eu, La, Mo, Nd, Pb, Pr, Sb, Sc, Sm, U, and W) is in excellent agreement pattern of 1 June). with findings from another Alpine site (Avak et al., 2018, Figure 5). At this ~180‐km distant site, meltwater percolation led to postdepositional distur- bance of a 16‐m firn section of a high‐Alpine ice core from the upper Grenzgletscher (Monte Rosa massif, southern Swiss Alps, 4,200 m above sea level, 45°55′28″N 7°52′3″E). Preferential elution of TEs led to significant concentration depletion in the records of Ba, Ca, Cd, Co, Mg, Mn, Na, Ni, Sr, and Zn, whereas

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the seasonality in the Al, Bi, Ce, Cs, Cu, Eu, Fe, La, Li, Mo, Nd, Pb, Pr, Rb, Sb, Sc, Sm, Tl, Th, U, V, W, Yb, and Zr records was well preserved (Avak et al., 2018). The different behavior of TEs during meltwater percolation was related to their varying water solubility and location at the microscopic scale. TEs mainly present in water‐insoluble mineral dust particles at the upper Grenzgletscher site (Al, Fe, Zr, and the rare‐earth elements) were enriched on ﬁrn grain surfaces and mostly preserved, since their insolubility in water resulted in immobility with meltwater. TEs likely present in water‐soluble compounds (Ba, Bi, Ca, Cd, Co, Cs, Cu, Li, Mg, Mn, Mo, Na, Ni, Pb, Rb, Sb, Sr, Tl, Th, U, V, W, and Zn; Avak et al., 2018; Birmili et al., 2006; Greaves et al., 1994; Hsu et al., 2010; T. Li et al., 2015) revealed variable mobility with meltwater presumably due to the different microscopic location of the ions in the ice structure. For those rather water‐soluble TEs, the concentration at the upper Grenzgletscher site was found to be primarily decisive determining either incorporation into the ice interior during snow metamorphism or segregation to ice surfaces because of exceeded solubility limits in ice. In correspondence with the upper Grenzgletscher study and the work reported by Wong et al. (2013), also at WFJ, records of investigated TEs present as water‐insoluble mineral dust particles (rare‐earth elements:

Figure 5. Rank of preservation plotted against the concentration ratio cwet/ Ce, Eu, La, Nd, Pr, Sc, and Sm) are well preserved most probably due ‐ ‐ cdry in the overlapping part (45 to 65 cm water equivalent depth) for each to their immobility with meltwater. Contrariwise, Zhongqin et al. fi fi trace element classi ed into group 1 (retained concentration pro le) or 2 (2007) observed a depletion of mineral dust‐bond elements, such as Fe (depleted concentration profile). c corresponds to the integrated trace wet and Al at Urumqi glacier 1 (eastern Tien Shan) in summer and relate this element concentration of the wet profile (1 June), whereas cdry represents the mean of the integrated concentration profiles during dry conditions (25 to meltwater relocation. However, it cannot be excluded that the low con- January, 22 February, and 21 March). Symbols in bold indicate a similar centrations in summer observed for all investigated elements are due to behavior as observed for high‐Alpine firn affected by meltwater percolation dilution effects during the wet season. (Avak et al., 2018). Circle sizes represent the mean concentrations in the dry snow pits where no melting occurred. There is also strong agreement in the behavior of rather water‐soluble TEs between upper Grenzgletscher and WFJ, showing a dependence on the concentration level. While TEs present in ultratrace amounts tend to be preserved, more abundant TEs were preferentially eluted (Figure 5). The only three TEs revealing a different behavior between the two sites are Bi, Th, and Zr, which still show a preservation of the seasonality at upper Grenzgletscher, but significant depletion at WFJ. One explanation could be a higher proportion of the water‐soluble fraction of Bi, Th, and Zr deposited during winter at WFJ, favoring higher meltwater mobility. Another possible reason is dif- fering concentrations. However, Th and Zr concentrations are not significantly different between the two sites. Only the higher Bi concentrations at WFJ could explain stronger melt‐induced relocation. These findings in the preservation of TEs corroborate our previous hypothesis (Avak et al., 2018) that for ice cores and snow pits from Alpine areas, representing central European atmospheric aerosol composition, and affected by partial melting, Ce, Eu, La, Mo, Nd, Pb, Pr, Sb, Sc, Sm, U, and W may still be used as robust environmental proxies. However, chemical profiles of Ba, Ca, Cd, Na, Zn, and Sr are prone to be depleted by melting. Records of Bi, Th, and Zr behaved differently at the two Alpine sites. The potential of TEs classified as influenced by soil here and previously found to be preserved (Al, Cs, Cu, Fe, Li, Rb, Tl, V, and Yb) or depleted (Co, Mg, Mn, and Ni) in firn (Avak et al., 2018) could not be further investigated at the WFJ site because of the influence of the soil in the presence of meltwater.

4. Conclusions Melting processes induced by climate warming can significantly alter concentration records of atmospheric pollutants deposited in high‐altitude glacier ice and snowpacks. Understanding the postdepositional behavior of atmospheric trace species is essential for reconstructing past environmental conditions from these natural archives. The analysis of five snow pits sampled during the winter/spring season of 2017 at the high‐Alpine site WJF, Switzerland, is presented here. Comparison of impurity profiles representing dry and wet snow conditions allowed to systematically investigate the impact of melting on the preservation

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of atmospheric tracers in high‐Alpine snow. While the atmospheric composition was found to be well preserved during the winter, melting in the spring and early summer caused a preferential loss of certain MIs and TEs. We propose that the elution behavior of MIs is depending on their microscopic location, which is most likely defined by redistribution processes occurring during snow metamorphism. Among the six 2+ − + + − 2− + investigated MIs (Ca ,Cl ,Na ,NH4 ,NO3 , and SO4 ), NH4 was retained the strongest, probably 2+ 2− due to its preferred location in the ice interior. Ca and SO4 concentrations were significantly depleted; this can be explained by their predominant occurrence on ice crystal surfaces. Variable mobility was also observed for TEs, with Ce, Eu, La, Mo, Nd, Pb, Pr, Sb, Sc, Sm, U, and W being the best‐preserved elements. In tendency, high concentrations of TEs were found to favor relocation with meltwater, while low abundant TEs were preferably retained. The MIs and 18 out of 21 investigated TEs revealed a consistent behavior with meltwater percolation at two Alpine sites, Weissfluhjoch and upper Grenzgletscher, which are 180 km apart. This indicates that the observed species dependent preservation from melting snow and ice is particularly valid for the Alpine region, reflecting central European atmospheric aerosol composition. Based on the observations at the + two Alpine sites, we propose that NH4 , TEs deposited as water‐insoluble particles, and TEs of water‐soluble particles occurring in low concentrations may still be applicable as environmental proxies in snow pits and ice cores affected by melting. Corroborating the existence of meltwater‐resistant proxies is particularly relevant as many high‐mountain glaciers worldwide, which provide avenues for assessing natural and anthropogenic impact on the atmosphere, are increasingly affected by melting.

Acknowledgments References The authors would like to thank Philipp Baumann and Matthias Jaggi (both Avak, S. E., Schwikowski, M., & Eichler, A. (2018). Impact and implications of meltwater percolation on trace element records observed in ‐ – from SLF) for support during the a high Alpine ice core. Journal of Glaciology, 64(248), 877 886. https://doi.org/10.1017/jog.2018.74 ‐ – – sampling campaigns of 22 February and Baker, I., Cullen, D., & Iliescu, D. (2003). The microstructural location of impurities in ice. Canadian Journal of Physics, 81(1 2), 1 2), 1 9. ‐ 1 June, respectively. The authors also https://doi.org/10.1139/p03 030 acknowledge Franziska Aemisegger Baltensperger, U., Gäggeler, H. W., Jost, D. T., Lugauer, M., Schwikowski, M., Weingartner, E., & Seibert, P. (1997). Aerosol climatology at ‐ – (Institute for Atmospheric and Climate the high alpine site Jungfraujoch, Switzerland. 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