Research Post-17th-Century Changes of European Lead Emissions Recorded in High-Altitude Alpine Snow and Ice M A R G I T S C H W I K O W S K I , * ,† C A R L O B A R B A N T E , ‡,§ T H O M A S D O E R I N G , †,| H E I N Z W . G A E G G E L E R , †,| C L A U D E B O U T R O N , ⊥,∇ U L R I C H S C H O T T E R E R , †,O L E O T O B L E R , † KATJA VAN DE VELDE,⊥ C H R I S T O P H E F E R R A R I , ⊥,# GIULIO COZZI,‡ KEVIN ROSMAN,[ AND P A O L O C E S C O N ‡,§ Paul Scherrer Institut, CH-5232 Villigen-PSI, Switzerland, Department of Environmental Sciences, University of Venice, Ca’ Foscari, I-30123 Venice, Italy, Institute for the Dynamics of Environmental ProcessessCNR, University of Venice, Ca’ Foscari, I-30123 Venice, Italy, Departement fu ¨r Chemie u. Biochemie, Universita¨t Bern, Freiestrasse 3, CH-3012 Bern, Switzerland, Laboratoire de Glaciologie et Ge´ophysique de l’Environnement du CNRS, 54 rue Molie`re, Boıˆte Postale 96, 38402 Saint Martin d’He`res, France, Observatoire des Sciences de l’Univers et Unite´ de Formation et de Recherche de Physique, Universite´ Joseph Fourier de Grenoble (Institut Universitaire de France), Boıˆte Postale 68, 38041 Grenoble, France, Physikalisches Institut, Universita¨t Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland, Institut des Sciences et Techniques de Grenoble, Universite´ Joseph Fourier de Grenoble, 28 avenue Benoıˆt Frachon, Boıˆte Postale 53, 38041 Grenoble, France, and Department of Applied Physics, Curtin University of Technology, WA 6845 Perth, Australia
Lead concentrations and lead isotope ratios were analyzed in two firn/ice cores covering the period from 1650 to 1994, which were obtained from the 4450 m high glacier saddle Colle Gnifetti located in the Monte Rosa massif at the Swiss-Italian border. This study presents the first glaciochemical time series with annual resolution, spanning several centuries of lead concentrations and lead isotopic compositions in precipitation in Europe. Lead concentrations in firn dated from the 1970s are ∼25 times higher than in ice dated from the 17th century, confirming the massive rise in lead pollution in Europe during the last few centuries. A decline of the lead concentration is then observed * Corresponding author phone: +41 56 310 4110; fax: +41 56 310 4435; e-mail:
[email protected]. † Paul Scherrer Institut. ‡ Department of Environmental Sciences, University of Venice. § Institute for the Dynamics of Environmental ProcessessCNR, University of Venice. | Departement fu ¨ r Chemie u. Biochemie, Universita¨t Bern. ⊥ Laboratoire de Glaciologie et Ge ´ ophysique de l’Environnement du CNRS. ∇ Observatoire des Sciences de l’Univers et Unite ´ de Formation et de Recherche de Physique, Universite´ Joseph Fourier de Grenoble. O Physikalisches Institut, Universita ¨ t Bern. # Institut des Sciences et Techniques de Grenoble, Universite ´ Joseph Fourier de Grenoble. [ Curtin University of Technology. 10.1021/es034715o CCC: $27.50 Published on Web 01/07/2004
2004 American Chemical Society
during the last two decades, i.e., from 1975 to 1994. The lead isotope ratio 206Pb/207Pb decreased from about 1.18 in the 17th and 18th centuries to about 1.12 in the 1970s. These variations are in good agreement with available information on variations in anthropogenic lead emissions from West European countries, especially from the use of lead additives in gasoline.
1. Introduction The large Greenland and Antarctic ice caps have provided unique historical records of the changing occurrence of lead in the remote areas of both hemispheres during the last two centuries; see, e.g., ref 1. The Greenland lead data show evidence of considerable contamination of the Northern Hemisphere, starting as early as Roman times (500 B.C. to 300 A.D.) (2), and dramatically increasing from the Industrial Revolution (about 1850) to the 1970s (3-5), before decreasing during recent decades (5, 6) as a consequence of the reduced use of lead additives in gasoline (7). The Antarctic data confirm the global character of lead pollution (8, 9), and show that the remote Antarctic continent was already contaminated for lead as early as the end of the 19th century (10). Further information on the origin of this pollution was obtained by isotopic fingerprinting (10-13). Contrasting this, there is a lack of glaciochemical data concerning historical changes of lead in the proximity of lead pollution sources, such as the populated areas in Europe. These data may be obtained from high-altitude glaciers in the Alps. To date, the only published study examines changes of lead concentration and lead isotope ratios in a firn/ice core drilled near the summit of Mont Blanc in the FrenchItalian Alps (14). However, this record was only dated with good precision for the past ∼60 years. We present here a comprehensive data set for lead concentrations and lead isotope composition in high-altitude Alpine firn (compacted snow) and ice, which was dated from the 17th century to the mid 1990s. The data were obtained from the analysis of two firn/ice cores, which were drilled at the high-altitude glacier saddle Colle Gnifetti (4450 m above sea level (asl)) near the summit of the Monte Rosa at the Swiss-Italian border, using inductively coupled plasma sector field mass spectrometry (ICP-SFMS), thermal ionization mass spectrometry (TIMS), and graphite furnace atomic absorption spectroscopy (GFAAS). The data are discussed in the context of available information on past changes in lead emissions in the nearby European countries.
2. Experimental Section 2.1. Ice Archive Characteristics. The samples analyzed in this study originated from two cores: a 109 m firn/ice core drilled at the Colle Gnifetti in 1982 (15) (only the uppermost 83 m was examined in this study) and a 25 m firn core drilled at the same site in 1995 (4450 m asl, 45°53′33′′N, 7°51′5′′E, Figure 1). The Colle Gnifetti is the uppermost part of the accumulation area of Grenzgletscher, forming a glacier saddle between Zumsteinspitze and Punta Gnifetti at 4400-4550 m asl in the Monte Rosa massif. The Monte Rosa itself, located at the Swiss-Italian border, is the second highest mountain in Western Europe (4634 m asl). The drilling site is located near the boundary between the infiltration-recrystallization and the cold infiltration zones of the glacier (16, 17), and measured ice temperatures never exceed the range between VOL. 38, NO. 4, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. (a, top) Map of Europe showing the position of the Colle Gnifetti/Monte Rosa (triangle) and the area of the Alps (gray area). (b, bottom) Detailed map of the Monte Rosa with the Colle Gnifetti, showing the location of the drilling site (circle). A square represents 1 km2. Reprinted with permission from swisstopo (BA035552), 31.7.2003. -9 and -14 °C (18). The firn-ice transition is between 33 and 35 m depth. Meltwater produced occasionally by intense solar radiation refreezes in the centimeters below the surface and does not percolate through the annual layers. Thus, the chemical stratigraphy is preserved, which is a prerequisite for the interpretation of glaciochemical records. However, previous analysis of the stable oxygen isotopes of water (δ18O) in the firn/ice core showed that preferentially parts of the winter snow cover are removed by wind erosion (19). Indeed, the obtained net accumulation rate ranges between 0.2 and 1.1 m water equivalent per year (m weq y-1) (18). This is significantly lower than the accumulation rate observed on Grenzgletscher, 300 m below the Colle Gnifetti (2.7 m weq y-1 (20)), and the precipitation rate measured at the nearest weather station in the Alps (Col du Grand St Bernard, 2469 m asl, 2.4 m weq y-1 (21)). The advantage of the low net accumulation is that, for the given glacier thickness of 124 m at the drilling site, a much larger time period can be accessed compared to high accumulation sites. Due to the strong seasonality of trace species concentrations in snow at high-alpine sites, with concentration peaks in summer (see below), anthropogenic emissions are predominantly reflected in the summer snow layer. Thus, the winter snow erosion has little effect on the overall deposition record of anthropogenic species, as indicated by the high correlation 958
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observed between estimated SO2 emissions in Europe and the sulfate deposition record deduced from a Colle Gnifetti ice core (22). Dating of the firn/ice core was performed by counting annual layers of the seasonally varying concentrations of ammonium (23), calcium, and sulfate. Further, stratigraphic markers were used, such as the Saharan dust events of the years 1901-02, 1936-37, and 1977 (24) (identified by yellow layers and high calcium concentrations), 1963 fallout from atmospheric nuclear tests (identified by high tritium activity concentrations), and volcanic eruptions (Katmai, 1912; Krakatau, 1883; Tambora, 1815; Laki, 1783; identified by high sulfate concentrations and high sulfate-to-calcium ratios). The resulting time scale was corroborated by nuclear dating using 210Pb (from 1900 to 1980), and by three-dimensional glacier flow modeling (23, 25). The firn core was dated by annual layer counting and was aligned to the firn/ice core using the 1977 Saharan dust event and the 1963 tritium peak, which were observed in both cores. Altogether, both cores cover a period of 344 years, from 1650 to 1994. The dating uncertainty is estimated to be (2 years for the period 18831994, (5 years for the period 1760-1882 (22), and (10 years prior to 1760. Paleo records obtained from the ice core by chemical analysis of ionic and carbonaceous species revealed a pronounced concentration increase for nitrate and sulfate (25), ammonium (23), and black carbon (26) from preindustrial to modern times, reflecting emissions from human activities. These results suggest that this archive is ideally suited for the study of lead pollution history. 2.2. Sample Preparation. From the 109 m firn/ice core, 455 sections with lengths varying from 10 to 35 cm were selected, since the sample had to be sufficiently large for the decontamination procedure. Cutting of the core sections was performed before dating. Thus, a core section can be formed by firn/ice from two different years. Continuous analysis of trace elements was not possible for the entire ice core due to previous sampling for the analyses of various other species. In the upper 40 m of the core the selected sections cover 79% of the core and in the remaining part 56%. From the firn core, 26 sections of about 40 cm length were analyzed within the first 13 m, covering the time period 1973-1994. A total of 430 samples from the 109 m firn/ice core were prepared and analyzed at the Paul Scherrer Institut (PSI), and 25 samples were prepared at the Laboratoire de Glaciologie et Ge´ophysique de l’Environnement in Grenoble (LGGE) and analyzed at the University of Venice (UnV), at LGGE and partly at the Curtin University of Technology. The 26 samples from the 25 m firn core were prepared and analyzed at the University of Venice. Since the outside of the cores was suspected to be significantly contaminated for lead, each core section was mechanically decontaminated to remove about 3 cm of the outer material and reveal the pristine inner core (27). The decontaminated central parts of the core sections were then transferred into high-density (PSI) or lowdensity (LGGE) polyethylene containers, acidified with nitric acid (quality Ultrex II, Baker (PSI) or NIST ultrapure (LGGE)) to 0.1 mol L-1, and melted at room temperature. The acidity of 0.1 mol L-1 was tested to be sufficient to dissolve lead quantitatively even if the sample contained mineral dust, which is often the case for Alpine ice. Sample handling was always performed under class 100 clean room conditions in the cold room (-20 °C) as well as in the laboratory (28). All materials used for decontamination and sample storage (stainless steel scalpel, containers, etc.) were carefully cleaned with 0.1 mol L-1 nitric acid. 2.3. Lead and Aluminum Analysis. Lead concentrations were determined by ICP-SFMS using Finnigan MAT ELEMENT 1 instruments, either at PSI (29) or at UnV (28, 30). Analysis was performed under class 100 clean room condi-
FIGURE 2. Lead concentrations determined in the 481 ice core sections against depth in the glacier. Depth is given in m weq to account for the different densities of firn and ice. Zero represents the glacier surface in 1982, whereas the part from 0 to 8.3 m weq corresponds to firn deposited between 1982 and 1995. Concentrations are plotted on a logarithmic scale. Open circles represent sections from the 109 m firn/ice core prepared and analyzed at PSI. Closed triangles and tilted squares represent sections from the same core, but prepared at LGGE and analyzed at UnV (triangles) and at LGGE (tilted squares). Closed squares represent sections from the 25 m firn core prepared and analyzed at UnV. tions. The detection limit (3σ of the blank) was 3 pg g-1 and was due to the blank and not to the sensitivity of the instruments. The precision of the analysis expressed as relative standard deviation is 10% (28, 29). Parts of the samples were also analyzed by TIMS at the Curtin University of Technology (31) and by GFAAS at LGGE. Comparisons of three methods were conducted using a few samples and confirmed the overall consistency of the data set. In addition, lead isotopic ratios 206Pb/207Pb and 206Pb/208Pb were determined in most sections by either ICP-SFMS or TIMS. The relative standard error of the 206Pb/207Pb and 206Pb/208Pb ratio determination by ICP-SFMS is 0.14% for samples with lead concentrations >1000 pg g-1, and 0.24% for samples with lead concentrations 2500 pg p-1) amounts to more than 60% of the total lead. Natural contributions from sources such as rock and soil dust were always minor, as the comparison with the concentrations of the earth crust tracer aluminum shows (Figure 6). Al is used as a conservative mineral dust tracer since its concentration is not significantly influenced by anthropogenic emissions as illustrated by the absence of a trend in the paleo record; see also ref 22. Nevertheless, Al concentrations varied significantly from one 5- or 10-year average to the other. High values can be attributed to major Saharan dust events, for example, the historically documented events in 1901-02, 1936-37, 1946-47, and 1977, resulting in the most prominent aluminum peaks in the 20th century. Thus, the major input of mineral dust to this glacier
TABLE 1. Lead Emissions to the Atmosphere Estimated for Selected European Countries from 1955 to 1995 (Listed in Alphabetical Order) (42, 49-52) lead emissions to the atmosphere (1000 kg y-1)
Austria Beneluxa France Germany Italy Spain Switzerland total a
1955
1960
1965
1970
1975
1979
1985
1900 5000 7400 10200 2400 1700 411 29011
2800 6000 9300 14800 3900 2100 716 39616
2700 7200 13000 15000 7900 2700 1220 49720
3400 8300 16800 19800 13200 4700 1530 67730
2600 6900 18900 16400 13900 6300 1290 66290
1900 6700 10500 11400 9400 5500 1300 46700
400 2600b 7300 3600 4800 2400 502 21602
Belgium, Luxemburg, The Netherlands.
b
1990
1995
300
50
4000 1300 1900 3300 292 11092
900 300 1500 900 90 3740
Referred to 1987.
TABLE 2. Lead Emission Ratios 1970/1960 and 1979/1970 for Various European Countries (42, 49-52) as Well as the Corresponding Concentration Ratios from the Colle Gnifetti Firn/Ice Corea
Austria Benelux France Germany-FRGb Germany-GDRc
ratio 1970/1960
ratio 1979/1970
1.2 (6) 1.4 (14) 1.8 (24) 1.2 (30) 2.5 (4)
0.6 (4) 0.8 (13) 0.6 (24) 0.6 (23) 0.8 (4)
Italy Spain Switzerland Colle Gnifetti ice core
a The percentage contribution of the nation to the total emissions is given in parentheses. Republic.
b
ratio 1970/1960
ratio 1979/1970
3.4 (15) 2.2 (6) 2.2 (1) 1.3
0.7 (20) 1.2 (10) 0.9 (2) 0.8
Federal Republic of Germany. c German Democratic
record, EFs are generally below 10 from 1650 to 1850 and reach a maximum of more than 500 in the 1960s (Figure 6).
FIGURE 6. Paleo record of Al concentrations (thin line), given as 5-year (period 1892-1982) and 10-year averages (period 1760-1890). The 1972-1977 Al average amounts to 353 ng g-1, due to the major Saharan dust event in 1977. Solid bars represent enrichment factors calculated using the composition of the upper continental crust (48). site is due to long-range transport, whereas local sources are of minor importance. This is consistent with an estimation based on size distributions of insoluble particulate matter in melted ice samples, attributing two-thirds of the mineral dust deposited at the Colle Gnifetti to Saharan dust (46). A similar conclusion was drawn from a 30-year deposition record of aluminum and calcium from Col du Doˆme, Mont Blanc, French Alps (47). Enrichment factors (EFs) were calculated using the conventional equation EF ) [Pb/Al]sample/ [Pb/Al]reference, with the composition of upper continental crust as reference (48). As a result of the strong trend in the lead
3.3. Lead Emission Source Areas. To identify emission source areas for lead deposited on the Colle Gnifetti, the timing and shape of the maximum in the late 1960s and early 1970s was compared to lead emission estimates of different West European countries. The only data we could find are for the years 1955-1995 (Table 1), and were compiled from refs 42 and 49-52. They show that the total lead emissions to the atmosphere of the considered countries peaked around 1970, i.e., at the time when lead concentrations at Colle Gnifetti were maximal. As already discussed above, snow accumulated at the Colle Gnifetti is dominated by summer precipitation. This is the period of the year during which the transfer of pollutants emitted regionally to high altitudes is not prevented by low-altitude thermal inversions, contrary to what happens frequently in winter. Thus, lead in Colle Gnifetti firn and ice is likely to mainly originate from countries located within 700-1000 km around the Alps, rather than from distant sources. This was nicely demonstrated by a study comparing the summer sulfate deposition record at the Col du Doˆme in the Mont Blanc area with SO2 emission trends. A high correlation was observed with the sum of the SO2 emissions from France, West Germany, Belgium, Spain, Italy, and Switzerland, whereas SO2 emissions from the United Kingdom showed a different trend (37). Lead emission ratios for the years 1970/1960 and 1979/1970 were calculated for various European countries and compared to the corresponding lead concentration ratios from the Colle Gnifetti ice core (Table 2). For this purpose the 5-year averages 19681973, 1958-1963, 1978-1982, and 1968-1973 were used. The increase in lead concentration in the ice by a factor of 1.3 between 1960 and 1970 agrees rather well with the 1970/ 1960 lead emission ratios in Austria, Benelux, France, and Germany-FRG. In contrast, 1970/1960 lead emission ratios of Germany-GDR, Switzerland, Italy, and Spain were larger, indicating a steeper increase. We therefore assume that the glacier record represents lead emissions from Austria, Benelux, France, and Germany-FRG, instead of being exclusively influenced by lead emissions of the countries where VOL. 38, NO. 4, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. 206Pb/207Pb isotopic composition determined in 417 ice core sections against depth in the glacier. A total of 13 values with lead concentrations below 30 pg g-1 were removed from the data set because of the influence of the blank on the isotopic composition (29). Error bars indicate the relative standard error of the 206Pb/207Pb ratio (29). Open triangles show values obtained by TIMS (uncertainty at 95% confidence interval smaller than triangle size). the glacier is located (Switzerland and Italy). A similar observation was also made for other trace species such as sulfate (22) and black carbon (26) and was attributed to the main air mass transport direction in Europe (north-west). The 1979/1970 lead emission ratios of all the considered countries (Table 2) are relatively similar and are comparable to the concentration decrease ratio 0.8 observed in the ice core. This suggests a similar reduction of lead emissions in the considered countries without any specific pattern. 3.4. Lead Isotopic Composition. Figure 7 presents the lead isotopic record 206Pb/207Pb for the period 1650-1982. The temporal trend of the isotopic composition is characterized by a constant level of about 1.18 for the period from 1650 to the 1880s, followed by a gradual decrease to a value of about 1.16 in the 1950s. A steeper decrease to a ratio of 1.12 is then observed for the time period 1950-1975. Afterward, a slight recovery to more radiogenic values (higher 206 Pb/207Pb) seems to occur. 206Pb/207Pb ratios show a so far unexplained higher variability during preindustrial times (1650-1880). The analytical uncertainty in determining 206Pb/ 207 Pb ratios by ICP-SFMS is larger in this period, since lead concentrations are lower. The relative standard error of the 206 Pb/207Pb ratio is 0.14% for samples with lead concentrations >1000 pg g-1, and 0.24% for samples with lead concentrations
