Article pubs.acs.org/est
Polychlorinated Biphenyls in a Temperate Alpine Glacier: 1. Effect of Percolating Meltwater on their Distribution in Glacier Ice Pavlina Aneva Pavlova,†,‡,§,∥ Theo Manuel Jenk,‡,§ Peter Schmid,† Christian Bogdal,⊥,# Christine Steinlin,⊥ and Margit Schwikowski*,‡,§,∥ Empa, Swiss Federal Laboratories for Materials Testing and Research, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland PSI, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland § Oeschger Centre for Climate Change Research, University of Berne, Falkenplatz 16, 3012 Bern, Switzerland ∥ Department of Chemistry and Biochemistry, University of Berne, Freiestrasse 3, 3012 Bern, Switzerland ⊥ Institute for Chemical and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zürich, Switzerland # Agroscope, Institute for Sustainability Sciences ISS, 8046 Zürich, Switzerland † ‡
ABSTRACT: In Alpine regions, glaciers act as environmental archives and can accumulate significant amounts of atmospherically derived pollutants. Due to the current climate-warming-induced accelerated melting, these pollutants are being released at correspondingly higher rates. To examine the effect of melting on the redistribution of legacy pollutants in Alpine glaciers, we analyzed polychlorinated biphenyls in an ice core from the temperate Silvretta glacier, located in eastern Switzerland. This glacier is affected by surface melting in summer. As a result, liquid water percolates down and particles are enriched in the current annual surface layer. Dating the ice core was a challenge because meltwater percolation also affects the traditionally used parameters. Instead, we counted annual layers of particulate black carbon in the ice core, adding the years with negative glacier mass balance, that is, years with melting and subsequent loss of the entire annual snow accumulation. The analyzed samples cover the time period 1930−2011. The concentration of indicator PCBs (iPCBs) in the Silvretta ice core follows the emission history, peaking in the 1970s (2.5 ng/L). High PCB values in the 1990s and 1930s are attributed to meltwater-induced relocation within the glacier. The total iPCB load at the Silvretta ice core site is 5 ng/cm2. A significant amount of the total PCB burden in the Silvretta glacier has been released to the environment.
1. INTRODUCTION Polychlorinated biphenyls (PCBs) represent a group of anthropogenic organic substances that were widely used in the 1960s and 1970s, resulting in unintentional emissions into the environment. Due to their hazardous properties, the application of PCBs was restricted in the 1970s until the 1990s and PCBs were banned worldwide by the Stockholm Convention on Persistent Organic Pollutants in 2004.1 Because PCBs are very persistent and volatile enough to be efficiently transported in the atmosphere, they have been globally distributed by long-range atmospheric transport. Also today, decades after the first regulation, PCBs are still found and even enriched in remote polar and mountain areas, far away from their initial emission sources.2 In high Alpine regions, glaciers act as environmental archives of past atmospheric composition and can accumulate significant amounts of atmospherically derived pollutants. Previous studies have taken advantage of these properties to determine the inventory of hydrophobic organic contaminants such as PCBs.3,4 Due to the current climate-warming-induced accelerated melting, these pollutants are being released at correspondingly higher rates.5−7 This implies even at greater © 2015 American Chemical Society
extent for temperate glaciers, the most abundant type of glaciers in the Alps. They are characterized by ice at the pressure melting temperature throughout, except for a surface layer, which undergoes seasonal freezing. Up to 9% of the water is liquid and present within the ice matrix in veins between the grain boundaries.8 However, the physical processes involved in the storage, transport within the temperate glacier, and release with meltwater of hydrophobic organic contaminants are not well understood. Surface melting on the glacier can lead to a fractionation of organic contaminants in the ice core. Different partitioning behavior of chemicals between ice, liquid water, particles, and pore air results in spatial redistribution of pollutants. Compounds dissolved in the percolating water can be transported through the porous firn to deeper layers, where meltwater might refreeze or be discharged from the ice. In contrast to percolating water, the snowpack acts as a filter for particles, resulting in coagulation9 and accumulation at the Received: Revised: Accepted: Published: 14085
July 8, 2015 October 19, 2015 November 4, 2015 December 3, 2015 DOI: 10.1021/acs.est.5b03303 Environ. Sci. Technol. 2015, 49, 14085−14091
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Environmental Science & Technology
Figure 1. Map of the Silvretta glacier with the ice core drilling site indicated by a red star. The grid represents 1 km (© 2014 swisstopo JD100043). (Bottom left) Overview map of Switzerland showing the location of the study site.
Silvretta ice core consists of alternating layers of superimposed ice and normal glacier ice containing air bubbles. The superimposed ice layers are formed by refreezing of percolated meltwater in deeper parts of the glacier. They are free of air bubbles and accordingly have the density of water ice which is higher than that of porous firn and normal glacier ice. In the case of the Silvretta ice core these layers are up to 1 m thick, containing visible particle deposits. After drilling, the 70 cm long core segments with a diameter of 7 cm were individually packed in polyethylene tubes and stored below −1 °C in insulating boxes covered by fresh snow on the glacier. The ice core was additionally cooled with dry ice during the transport to the Paul Scherrer Institut, where it was stored in a cold room at −20 °C. Each segment was photographed on a light table and stratigraphic features like dark particles or bubble free layers were carefully documented. Subsequently, samples were prepared by dissecting the ice core segments using an electrical band saw, equipped with a stainless steel blade and a polyvinylidene fluoride (PVDF) covered table as described by Eichler et al.15 For this study, the uppermost 61 m (approximately 49 m water equivalent (weq)) of the ice core were analyzed. The outer part of the ice core was used for trace species, which are less susceptive to contamination (e.g., radionuclides). In the inner part, concentrations of black carbon (BC) and PCBs were determined. In addition, the volume of the PCBs samples was measured to derive the density of each segment, which was then used to convert ice core sampling depth in meter to meter weq. To establish the age-depth relationship of the Silvretta ice core we used a combination of detecting a reference horizon, nuclear dating, and annual layer counting as outlined in the following. As reference horizon, the radioactive fallout of tritium (3H) from atmospheric thermonuclear weapon tests was selected, which peaked in the year 1963, exceeding the background levels
surface in summer, which leads to enrichment of particle-bound chemicals. Obtaining a reliable age-depth relationship for ice cores from temperate glaciers represents a particular challenge because meltwater percolation affects the seasonality of traditional proxies used for dating, such as water-soluble ions or the stable isotopes of water.10 In an interdisciplinary research project, we investigate deposition, incorporation, transport, and release characteristics of PCBs in temperate Alpine glaciers.3,11,12 In the part presented here, we analyzed dissolved and particleassociated PCBs in dated layers from an ice core from the temperate Silvretta glacier in the Swiss Alps. The goal was to examine the effect of PCB loss and their enrichment in particle layers and to quantify the effect of melting on storage and redistribution of PCBs in temperate glaciers, compared to cold glaciers (not affected by melting). In an accompanying work we present a detailed study on the importance of the chemical processes using a chemical fate model, which is validated by the data presented here.11
2. STUDY SITE, METHODS, AND CHRONOLOGY OF THE SILVRETTA ICE CORE Silvretta glacier is located in the eastern Swiss Alps. It has a surface area of around 3 km2 and expands between 2500 and 3000 m above sea level (a.s.l.; Figure 1). Silvretta is a temperate glacier with ice temperatures near the pressure melting point from 10 m below the surface down to the bedrock and is strongly affected by melting, resulting in a significant amount of liquid percolating meltwater within the solid ice matrix.13 In April 2011, a 101 m long surface-to-bedrock ice core was extracted from the accumulation area of Silvretta glacier (46°50′47″ N, 10°05′19″ E, 2927 m a.s.l.). We used a combined electromechanical-thermal drill system, whereby the first 12 m were drilled with the electromechanical drill.14 The 14086
DOI: 10.1021/acs.est.5b03303 Environ. Sci. Technol. 2015, 49, 14085−14091
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Environmental Science & Technology of 3H by at least a factor of 1000. 3H has a half-life of 12.3 years and the 1963 horizon is commonly used for ice core dating.16 Compared to other fallout constituents like 137Cs or 239Pu, 3H is present in the water molecule and is thus part of the ice matrix. Therefore, the 3H activity signal is assumed to be less affected by melting compared to water-soluble tracers, which are easily removed with percolating meltwater.17 3H was determined in the Silvretta ice samples using liquid scintillation counting (TriCarb 2770 SLL/BGO, Packard SA). The naturally occurring radioactive isotope 210Pb is often applied for nuclear dating, as it is constantly deposited in particle-bound form on the glacier surface through wet and dry deposition. 210Pb has a half-life of 22.3 years and allows dating back about 100 years.18 The 210Pb activity was indirectly determined from the α-decay of its granddaughter nuclide 210 Po, electrolytically deposited on Ag plates, at an energy of 5.3 MeV using an α-spectrometer (Enertec Schlumberger 7164) with PIPS detectors. Seasonally varying parameters normally used in ice cores to detect and count annual layers, such as ionic species (e.g., NH4+) and stable isotope ratios (e.g., δ18O),19 do not show seasonal variations in the Silvretta core, because of removal or relocation by meltwater. Instead, we used the concentration of black carbon (BC) which is present in particulate form and therefore less susceptible to meltwater influence. BC was analyzed with 10 cm resolution in the ice core using a Single Particle Soot Photometer (SP2, Droplet Measurement Technology, Inc.).20 To validate the dating of the Silvretta ice core, we used glacier local mass balance (LMB, Figure 2) data for the location of the drilling site, calibrated using a mass balance model of Silvretta glacier covering the period 1915− 2011.13,21 For the analysis of PCBs in ice samples, we applied the method presented in our earlier studies.3,22 Briefly, the analytes were extracted by partitioning from a water sample into the polydimethylsiloxane (PDMS) coating of an open tubular fused silica capillary, followed by analysis of PCBs by gas chromatography (HRGC Mega 2 series, Fisons Instruments) coupled to electron ionization high-resolution mass spectrometry (GC/EI-HRMS; MAT 95, Thermo Finnigan MAT). A silver membrane filter with a pore size of 0.45 μm was installed after the capillary to collect the particles, which entered the capillary (i.e., smaller than 0.32 mm) but were not retained in it. Thus, we operationally defined particle and dissolved fractions in our data, but we are aware that the finest dispersed fraction in the record may be retained in the capillary (i.e., measured with the dissolved fraction). We quantified the PCB congeners 28, 52, 101, 138, 153, and 180, referred to thereafter as iPCBs. For the quantification of the analytes, the internal standard method was used, which enabled the quantification of loss of iPCBs during extraction and handling. The use of the internal standard method accounts for losses of analytes during sample preparation and is therefore indispensable for the quantification of trace analytes at concentrations present in glacier ice samples. With the use of isotope labeled internal standards added to the sample before the analytical procedure (also referred to as the isotope dilution method), calculated concentrations are directly corrected for recovery. The obtained recoveries of 5 (PCB 180) to 25% (PCB 28) are mainly due to adsorption loss depending on different sorptivity of the analytes and are in accordance with previous results.22
Figure 2. Net annual accumulation of snow on the Silvretta glacier at the drilling site in m weq: (top blue axis) accumulation; (blue bars) positive, (red bars) negative,13 (bottom black axis), (black line) average annual air temperature at the ice core site according to Steinlin et al.11
Further, we calculated instrumental blanks based on the performance of our equipment.3 The data presented herein are not blank-corrected. To examine the effect of melting on the redistribution of organic pollutants in glaciers, we compared the concentration profiles of iPCBs in the Silvretta ice core to two other Alpine ice cores. The first one is from the cold Fiescherhorn glacier (3900 m a.s.l.),23 which is not affected by melting, and can be interpreted as a reference for atmospheric input of PCBs in Alpine glaciers.3 The second one is from the Grenzgletscher (4200 m a.s.l.), where the stratigraphy is well preserved except for one melt event.24 Samples cover the period between 1941 and 1993 and were analyzed using the same method as described above. Depending on the availability and the condition of the ice, the sample resolution in the ice cores is between 1 and 5 years.
3. RESULTS Ice Core Chronology. The age−depth relationship for the Silvretta ice core based on complementary dating methods is presented in Figure 3. All depths are provided in meters weq, calculated using the density profile presented in the Supporting Information of Steinlin et al.11 The 210Pb age-depth relationship was derived from the exponential fit through the 210Pb activity data as a function of depth in m weq using the least-squares method and the law of radioactive decay for 210Pb, suggesting an age of 97 ± 20 years (yrs) at 48 m weq.18 Compared to cold glaciers, the 210Pb variability is higher in the Silvretta glacier, 14087
DOI: 10.1021/acs.est.5b03303 Environ. Sci. Technol. 2015, 49, 14085−14091
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Environmental Science & Technology which can likely be attributed to a redistribution of 210Pb and 210 Pb-containing particles with meltwater and enrichment in layers with high particle content. Elevated 3H activities detected in the Silvretta ice core spread over 6 m weq, between 25 and 31 m weq. The maximum at 28.9 m weq was attributed to the year 1963 (Figure 3). BC concentrations vary between 0.5 and 200 μg/L, with some layers showing exceptionally high values of up to 1000 μg/L (Figure 4). We interpreted every peak in BC concentration as an annual layer, and added missing years indicated by a negative LMB at the according depths (17 years in total, because in some years, net loss was more than one annual layer, see Figure 2). The resulting age from this annual layer counting is 79 ± 11 yrs (1932 AD) at 48 m weq, it is constrained with the 1963 horizon and used as the relevant age scale in the following. The LMB data suggests slightly younger ages with 71 yrs at 48 m weq. Overall, there is a good agreement between the independent dating results considering the uncertainties.
Figure 4. Concentration of black carbon (BC) in the Silvretta ice core.
Figure 3. Dating of the Silvretta ice core. (left axis) 210Pb activity with 1σ measurement uncertainties as a function of depth (gray dots). Exponential fit through the 210Pb data using the least-squares method (black line and equation). (right axis) Age fit with 1σ and 2σ uncertainty bands (black line and gray shaded area). The uncertainty at the surface is 0 as it equals the day of drilling. Annual layer counting (ALC) based on BC concentration adjusted for missing years as defined by a negative local mass balance (red dot with corresponding counting errors, which result from equivocal peaks). 3H activity peak in the 1963 horizon (green ×). Local mass balance age-depth relationship (blue line).
Figure 5. Concentration records of iPCBs (red bars) and BC (black bars, values up to 200 μg/L) in the Silvretta ice core. Data gaps correspond to years with a negative LMB.
1970s and overall representing the expected time trend of iPCBs in ice cores in the Alps (Figure 6).
PCB Records. Figure 2 represents the annual net snow accumulation and the corresponding temperature at the Silvretta ice core site. Although temperature is not the only parameter effecting glacier melt, extensive mass losses occurred during warm periods (especially the 1950s and the most recent period after the 1990s). The concentration records of iPCB and BC in the ice core from Silvretta glacier for the time period 1932−2010 can only be gathered in years with positive net mass balance (Figure 5). The BC concentrations are averaged over each ice core segment. The concentrations of iPCB vary between 0.16 and 2.7 ng/L and peak in the 1970s. The late 1990s and early 1940s are other periods of high pollutant loads. BC concentrations preserve the variations from 1 and 200 μg/ L, with an average of 14.9 μg/L (8.3 for the period 1930− 1980). To examine the effect of melting on the redistribution of iPCBs in glacier ice, we analyzed an additional, well studied ice core from Grenzgletscher.19,24 The concentration of iPCBs in this ice core varies between 0.5 and 3 ng/L, peaking in the
4. DISCUSSION Dating Uncertainty. We used a combination of independent dating methods, each with its characteristic uncertainty. Nuclear dating based on 210Pb is affected by melt processes and missing annual layers, resulting in increased uncertainty. Nevertheless it still allows retrieving an approximate time scale. The 1963 3H horizon is well-defined and could be used to constrain the dating by ALC. Because the ALC is only constrained by this single tie-point the uncertainty increases with depth. As shown in Figure 3, the age of the ice obtained at a depth of 48 m weq is 79 ± 11 years, which is between the ages obtained by the 210Pb and LMB data. Because of the reasons discussed in the following, the chronology based on adjusted ALC was used. LMB is probably underestimating the age supported by the fact that it assigns the 1963 horizon to 1966 only. Also some uncertainty in the LMB data is 14088
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Figure 6. Concentration records of iPCBs in the three ice cores with a map showing their locations: (A) Fiescherhorn glacier (no melting); (B) Grenzgletscher (melting in the period of 1985−1989, marked with gray shaded area); and (C) Silvretta glacier (temperate, melting most of the year); (red) dissolved and (black) particulate phases; missing values represent years with a negative mass balance.
Table 1. Inventories of iPCBs in the Ice Cores from the Temperate Silvretta Glacier and the Cold Grenzgletscher and Fiescherhorn Glaciers ice core
average iPCBs dissolved (ng/L)
average iPCBs particulate (ng/L)
snow accumulation rate (m weq)
time period covered by ice core (years)
total burden (ng/cm2)
Silvretta Grenzgletscher Fiescherhorn
0.8 1.1 1.9
0.2 0.3 0.2
0.9 2.7 1.7
1940−2010 1942−1993 1940−2002
5 19 18
This suggests that the removal by percolating meltwater is less important for BC than for PCBs, except if there was a strong vertical gradient of BC concentration in the atmosphere, because the Colle Gnifetti glacier is about 1500 m higher than the Silvretta glacier. This is in agreement with the observations of Steinlin et al.11 Between 1985 and 1992, 47% of the particles, compared to 77% of the water, is washed out. In addition, high concentrations of iPCBs were detected deeper in the core, corresponding to the 1930s, a period of no atmospheric input of pollutants. These samples are not characterized by enriched particulate PCB load, or high BC concentrations, but increased firn/ice density. We assume that liquid water containing dissolved iPCBs originating from melting in the 1950s (Figure 2) might have been fixed by freezing in the layer of the late 1930s.11 Total PCB Inventory. In Figure 6, we present ice core concentration profiles of iPCBs from the glaciers Grenzgletscher (Figure 6B), Silvretta (Figure 6C), and Fiescherhorn (Figure 6A), the latter not affected by melting.3 The concentration trends in the Silvretta and Grenzgletscher ice cores correspond to the record from Fiescherhorn glacier and are in agreement with the PCB emission history.25 There are some fluctuations of the concentration, which originate from year-to-year variations in atmospheric transport3 or postdepositional processes and are typical for ice core records. In the Grenzgletscher ice core, the segment between 11 and 24 m weq depth, corresponding to the years 1985−1989, is strongly affected by melting, as shown by the water-soluble ion records.24 Analogically to Silvretta, in this segment, we observe partial washout of PCBs. Two further distinct samples originating from 1961 to 1962 and from 1949 to 1950 show remarkable high pollutant loads, which might also be a result of relocation and refreezing of percolating chemicals. A comparison of the partitioning of iPCBs between the dissolved and particulate phase in the ice reveals a relatively constant distribution in the Fiescherhorn ice core compared to the
considered because snow accumulation was not measured directly at the drilling site. The data used was thus derived from interpolation within the gridded model. The dating by 210Pb results in slightly older ages but has a relatively large uncertainty due to melt-related processes and most importantly it is not capable to account for missing years. Enrichment Layers of Particle-Bound and Refrozen Dissolved PCBs. The maximum of PCB concentrations observed in the 1970s in the Silvretta ice core coincides with the peak of emissions.25 The period 1940−1970 is also characterized with the highest load of heavy PCBs (PCB 138, PCB 153, PCB 180) in the particulate phase. The heavier PCBs represent 30% of the total iPCBs and are present to 27% in the particulate phase. In contrast, high PCBs values in the ice observed for the late 1990s and late 1930s cannot be explained with emissions of PCBs, but rather result from transport processes in the ice. Whereas for the 1990s event high BC concentrations were observed (Figure 5), for the 1930s events heavy PCBs were predominantly present in the soluble phase. Probably, heavy PCBs are bound to very fine suspended particles that with our analytical setup are included in the dissolved phase (operationally defined dissolved phase). We assume that particles are enriched at the surface of the glacier during melt events, which were characteristic for this time period (Figure 2). Such particle-rich layers are covered by fresh snow and, thus, are buried in the ice. As a consequence of further meltwater percolation, PCBs are also bound to these particle-rich layers.26 Thus, enrichment layers of particle-bound iPCBs are formed, indicated by higher loads of particle-bound PCBs in the deeper part of the record (60% of the total particle-bound load of iPCBs present in the period of 1940− 1970) and in layers underlying negative LMB (Figure 6C). The average BC concentration in the Silvretta ice core for the period of 1930−1980 is 8.3 μg/L, comparable to the average for the same period from another high-alpine glacier, which is not affected by melting (Colle Gnifetti 1930−1980, 6.5 μg/L27). 14089
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(3) Pavlova, P. A.; Schmid, P.; Bogdal, C.; Steinlin, C.; Jenk, T. M.; Schwikowski, M. Polychlorinated Biphenyls in Glaciers. 1. Deposition History from an Alpine Ice Core. Environ. Sci. Technol. 2014, 48, 7842−7848. (4) Garmash, O.; Hermanson, M. H.; Isaksson, E.; Schwikowski, M.; Divine, D.; Teixeira, C.; Muir, D. C. G. Deposition history of polychlorinated biphenyls to the Lomonosovfonna glacier, Svalbard: A 209 congener analysis. Environ. Sci. Technol. 2013, 47, 12064−12072. (5) Bogdal, C.; Schmid, P.; Zennegg, M.; Anselmetti, F. S.; Scheringer, M.; Hungerbühler, K. Blast from the past: melting glaciers as a relevant source for persistent organic pollutants. Environ. Sci. Technol. 2009, 43, 8173−8177. (6) Blais, J. M.; Schindler, D. W.; Muir, D. C. G.; Sharp, M.; Donald, D.; Lafrenière, M.; Braekevelt, E.; Strachan, W. M. J. Melting glaciers: A major source of persistent organochlorines to subalpine Bow Lake in Banff National Park, Canada. Ambio 2001, 30, 410−415. (7) Villa, S.; Negrelli, C.; Finizio, A.; Flora, O.; Vighi, M. Organochlorine compounds in ice melt water from Italian Alpine rivers. Ecotoxicol. Environ. Saf. 2006, 63, 84−90. (8) Lliboutry, L. Temperate ice permeability, stability of water veins and percolation of internal meltwater. J. Glaciol. 1996, 42, 201−211. (9) Herbert, B. M. J.; Villa, S.; Halsall, C. J. Chemical interactions with snow: Understanding the behavior and fate of semi-volatile organic compounds in snow. Ecotoxicol. Environ. Saf. 2006, 63, 3−16. (10) Nakazawa, F. Application of pollen analysis to dating of ice cores from lower-latitude glaciers. J. Geophys. Res. 2004, 109, 168−170. (11) Steinlin, C.; Bogdal, C.; Pavlova, P. A.; Schwikowski, M.; Scheringer, M.; Schmid, P.; Hungerbühler, K.; Polychlorinated Biphenyls in a Temperate Alpine Glacier: 2. Model Results of Chemical Fate Processes. Environ. Sci. Technol. in press, 10.1021/ acs.est.5b03304. (12) Steinlin, C.; Bogdal, C.; Scheringer, M.; Pavlova, P. A.; Schwikowski, M.; Schmid, P.; Hungerbühler, K. Polychlorinated Biphenyls in Glaciers. 2. Model Results of Deposition and Incorporation Processes. Environ. Sci. Technol. 2014, 48 (14), 7849− 7857, DOI: 10.1021/es501793h. (13) Huss, M.; Bauder, A. 20Th-Century Climate Change Inferred From Four Long-Term Point Observations of Seasonal Mass Balance. Ann. Glaciol. 2009, 50, 207−214. (14) Schwikowski, M.; Jenk, T. M.; Stampfli, D.; Stampfli, F. A new thermal drilling system for high-altitude or temperate glaciers. Ann. Glaciol. 2014, 55 (68), 131−136. (15) Eichler, A.; Schwikowski, M.; Gäggeler, H. W. An Alpine icecore record of anthropogenic HF and HCl emissions. Geophys. Res. Lett. 2000, 27, 3225−3228. (16) Olivier, S.; Bajo, S.; Fifield, L. K.; Gäggeler, H. W.; Papina, T.; Santschi, P. H.; Schotterer, U.; Schwikowski, M.; Wacker, L. Plutonium from global fallout recorded in an ice core from the Belukha Glacier, Siberian Altai. Environ. Sci. Technol. 2004, 38, 6507− 6512. (17) Van Der Wel, L. G.; Streurman, H. J.; Isaksson, E.; Helsen, M. M.; Van De Wal, R. S. W.; Martma, T.; Pohjola, V. a.; Moore, J. C.; Meijer, H. a. J. Using high-resolution tritium profiles to quantify the effects of melt on two Spitsbergen ice cores. J. Glaciol. 2011, 57, 1087−1097. (18) Gäggeler, H.; von Gunten, H. R.; Rössler, E.; Oeschger, H.; Schotterer, U. 210Pb-Dating of cold alpine firn/ice cores from Colle Gnifetti, Switzerland. J. Glaciol. 1983, 29, 165−177. (19) Eichler, A.; Schwikowski, M.; Gäggeler, H. W.; Furrer, V.; Synal, H. A.; Beer, J.; Saurer, M.; Funk, M. Glaciochemical dating of an ice core from upper Grenzgletscher (4200 m a.s.l.). J. Glaciol. 2000, 46, 507−515. (20) Wendl, I. a.; Menking, J. a.; Färber, R.; Gysel, M.; Kaspari, S. D.; Laborde, M. J. G.; Schwikowski, M. Optimized method for black carbon analysis in ice and snow using the Single Particle Soot Photometer. Atmos. Meas. Technol. 2014, 7, 2667−2681. (21) Huss, M.; Bauder, A.; Funk, M.; Hock, R. Determination of the seasonal mass balance of four Alpine glaciers since 1865. J. Geophys. Res. 2008, 113, F01015.
increased importance of the particulate fraction with depth in both Silvretta and Grenzletscher ice cores. This effect is attributed to enrichment of particle-bound pollutants as a result of melting. The total iPCB inventories in ng/cm2 at the drilling sites of Silvretta, Fiescherhorn, and Grenzgletscher were calculated by integrating the annual deposition fluxes over the entire depth (Table 1). Since the three glaciers are located relatively close together (distance
