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Lead and lead isotopes in agricultural soils of Europe – The continental perspective
Applied Geochemistry 27 (2012) 532–542
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Lead and lead isotopes in agricultural soils of Europe – The continental perspective Clemens Reimann a,⇑, Belinda Flem a, Karl Fabian a, Manfred Birke b, Anna Ladenberger c, Philippe Négrel d, Alecos Demetriades e, Jurian Hoogewerff f, The GEMAS Project Team 1 a
Geological Survey of Norway (NGU), P.O. Box 6315 Sluppen, N-7491 Trondheim, Norway Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Postfach 510153, D-30631 Hannover, Germany c Geological Survey of Sweden (SGU), Box 670, S-751 28 Uppsala, Sweden d BRGM, Service Métrologie Monitoring Analyse, 3 Avenue Claude Guillemin, BP 6009, 45060 Orléans Cedex 2, France e Institute of Geology and Mineral Exploration, Entrance C, Olympic Village, Acharnae, Athens GR-13677, Greece f Oritain Global Ltd., 8 Pacific Street, Dunedin 9010, New Zealand b
a r t i c l e
i n f o
Article history: Received 8 November 2011 Accepted 20 December 2011 Available online 27 December 2011 Editorial handling by R. Fuge
a b s t r a c t Lead isotopes are widely used for age dating, for tracking sources of melts, sediments, Pb products, food and animals and for studying atmospheric Pb contamination. For the first time, a map of a Pb isotope landscape at the continental-scale is presented. Agricultural soil samples (Ap-horizon, 0–20 cm) collected at an average density of 1 site/2500 km2 were analysed for Pb concentration and Pb isotopes (206Pb, 207Pb, 208 Pb). Lead concentrations vary from 1.6 to 1309 mg/kg, with a median of 16 mg/kg. Isotopic ratios of 206 Pb/207Pb range from 1.116 to 1.727 with a median of 1.202. The new data define the soil geochemical Pb background for European agricultural soil, providing crucial information for geological, environmental and forensic sciences, public health, environmental policy and mineral exploration. The European continental-scale patterns of Pb concentrations and Pb isotopes show a high variability dominated by geology and influenced by climate. Lead concentration anomalies mark most of the known mineralised areas throughout Europe. Some local Pb anomalies have a distinct anthropogenic origin. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Lead has been mined and used by humans for several thousand years. The accumulated total world Pb production since ancient times is estimated to be 300 Mt (based on Nriagu, 1998 and updated to 2010 according to current world mine production figures). Estimates of the anthropogenic Pb fraction in the environment range from <10% (Strauss, 1978; Kownacka et al., 1990) to >90% (Nriagu, 1979). Detailed studies on environmental samples (e.g., peat bogs, ice cores, sediment cores) has suggested major Pb contamination of the northern hemisphere since ancient times ⇑ Corresponding author. E-mail address: [email protected] (C. Reimann). S. Albanese, M. Andersson, A. Arnoldussen, R. Baritz, M.J. Batista, A. Bel-lan, D. Cicchella, E. Dinelli, B. De Vivo, W. De Vos, M. Duris, A. Dusza-Dobek, O.A. Eggen, M. Eklund, V. Ernstsen, P. Filzmoser, T.E. Finne, D. Flight, S. Forrester, M. Fuchs, U. Fugedi, A. Gilucis, M. Gosar, V. Gregorauskiene, A. Gulan, J. Halamic´, E. Haslinger, P. Hayoz, G. Hobiger, R. Hoffmann, H. Hrvatovic, S. Husnjak, L. Janik, C.C. Johnson, G. Jordan, J. Kirby, J. Kivisilla, V. Klos, F. Krone, P. Kwecko, L. Kuti, A. Lima, J. Locutura, P. Lucivjansky, D. Mackovych, B.I. Malyuk, R. Maquil, M. McLaughlin, R.G. Meuli, N. Miosic, G. Mol, P. O’Connor, K. Oorts, R.T. Ottesen, A. Pasieczna, V. Petersell, S. Pfleiderer, M. Ponˇavicˇ, C. Prazeres, U. Rauch, I. Salpeteur, A. Schedl, A. Scheib, I. Schoeters, P. Sefcik, E. Sellersjö, F. Skopljak, I. Slaninka, A. Šorša, R. Srvkota, T. Stafilov, T. Tarvainen, V. Trendavilov, P. Valera, V. Verougstraete, D. Vidojevic´, A.M. Zissimos, Z. Zomeni. 1
0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.12.012
(Komárek et al., 2008; Bindler, 2011). The European Commission has identified diffuse contamination as one of the eight major threats to soil quality (European Commission, 2006). Because Pb deposits have characteristic Pb isotope compositions (for data on the economically important deposits see: Sangster et al., 2000), Pb concentration and Pb-isotope ratios in combination may in some cases be used as a fingerprint to trace Pb to its source. Natural Pb comprises four stable isotopes (natural abundance in brackets): 204Pb (1.4%), 206Pb (24.1%), 207Pb (22.1%) and 208Pb (52.4%) in varying proportions, uniquely defined by the three ratios 206Pb/207Pb, 208Pb/206Pb, 206Pb/204Pb. The reliable measurement of the isotope 204Pb needs much care and specialised instrumentation and is time consuming. The ratios between the more abundant isotopes 206Pb, 207Pb and 208Pb can be easily determined by inductively coupled plasma mass spectrometry. In environmental sciences the 206Pb/207Pb isotope ratio is commonly used to suggest Pb contamination of different compartments of the environment at the local to global scale (for a recent review on the use of Pb isotopes in environmental sciences see Komárek et al., 2008). Data presented in Sangster et al. (2000) demonstrate that the 206Pb/207Pb isotope ratio of the most important Pb deposits varies between 0.98 and 1.41. One major producer, Broken Hill in Australia, is characterised by a very low 206Pb/207Pb isotope ratio of 1.04, while the largest Pb deposits in the world, the Mississippi
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Valley type deposits in the United States, show a characteristically high 206Pb/207Pb isotope ratio of 1.4. It thus would be easy to determine the origin of Pb in a sample, if one would have to decide whether Pb from Broken Hill or the Mississippi Valley was the original source. In natural samples, determining the origin of Pb is more complicated and there are certain requirements for the successful use of Pb isotopes in environmental studies. For example, the isotope ratios of all sources of Pb that contribute to the Pb concentration of a sample must be known. This will rarely be the case, when soils or plant materials are collected in nature. Furthermore, and even more importantly, it is required that the isotopic composition of Pb is not significantly affected by physico-chemical fractionation processes in natural systems (Komárek et al., 2008), e.g., during soil formation and weathering. To be able to proceed with using Pb isotopes in forensic and environmental studies on a reliable foundation, it is important to know the Pb background concentration and isotopic compositions on a global or at least continental scale. However, in spite of its importance, not even the continentalscale distribution of Pb and Pb isotopes combined has ever been documented for any geochemical sample material. As the result of a united effort of the European Geological Surveys, backed by the European metals industry, here the first geochemical reference maps of Pb concentrations combined with Pb isotope ratios at the European scale are provided, against which the impact of contamination can be realistically assessed. The maps localise the major anomalies of Pb in agricultural soils and help identify the processes that generate the observed distribution patterns.
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2. The survey area Fig. 1 shows a simplified geological map of Europe, including the main features discussed in this paper. Further maps covering topography and land use of Europe can be found in almost any world atlas. A number of maps covering different themes at about the scale of the GEMAS project (topography, geology, tectonics, fault and fracture zones, distribution of different rock types, distribution of the main sedimentary basins, precipitation and population density) can be found in Reimann and Birke (2010). For Europe, an excellent source of land use information is the CORINE land use map of Europe (GLC 2000 database, 2003). A detailed geological map of Europe is provided by Asch (2003), concise descriptions of the geology of Europe can be found in Blundell et al. (1992) and McCann (2008). The soil atlas of Europe provides a wealth of information about the soils of Europe, but also contains maps of average precipitation, temperature, land use, population density, extent of the last glaciation and soil texture (Jones et al., 2005).
3. Material and methods 3.1. Sampling and sample preparation Sampling took place from summer 2008 to early spring 2009. In total, 2211 samples of agricultural soil (Ap-horizon, 0–20 cm) were collected at an average sampling density of 1 site per 2500 km2 (grid of 50 50 km) across Europe. A field handbook describes
Fig. 1. Generalised geological map of Europe. For a detailed geological map of Europe see Asch (2003) –).
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the sampling procedure in detail (EGS, 2008). All samples were shipped to a central laboratory for preparation (air-drying and sieving to <2 mm using nylon screens). 3.2. Analyses
Table 1 Statistical parameters of Pb concentration (in mg/kg) and isotope ratios in European agricultural soil (Ap horizon, 0–20 cm).
Pb 206
Pb/207Pb Pb/208Pb 208 Pb/206Pb 207
Lead concentrations were determined following an aqua regia extraction using an inductively coupled plasma mass spectrometer (ICP-MS) using tight external quality control procedures (Reimann et al., 2009a). All Pb isotope ratio measurements were carried out on a sector field inductively coupled plasma mass spectrometer (SF-ICP-MS; ELEMENT 1, Finnigan MAT, Bremen, Germany). A sample weight of 0.5 g was carefully stirred with 8 mL 7 N HNO3 in PTFE vessels on a Vortex Genie shaker before the samples were extracted under N2 pressure at 250 °C in an ultraclave (Milestone). During heating the pressure increases from the start pressure of 50 bar to approximately 120 bar. After cooling the acid extracts were filtered through folded Whatman filters. All samples were diluted to Pb concentrations below 100 lg/L and a HNO3-concentration of 5% (v/v) prior to SF-ICP-MS analysis. The common Pb isotope standard NIST SRM 981 was used to correct for instrumental mass discrimination. The instrumental uncertainty in each
Minimum
Median
Maximum
1.6 1.116 0.287 1.477
15.7 1.202 0.403 2.067
1309 1.727 0.417 2.702
measurement was between 0.003 and 0.0002 on the ratio. The certified reference material Mess-3 (NRC, National Research Council Canada) was prepared in the same way as the samples and used as a day-to-day control standard of both the SF-ICP-MS analysis and the digestion. From these data, the reproducibility of the Pb isotope ratio was estimated at 0.11% for 208Pb/207Pb and 0.10% for 206Pb/207Pb. 3.3. Data analysis and mapping Geochemical data are compositional (closed) data, element concentrations reported in wt.% or mg/kg sum up to a constant and are
Fig. 2. Map of the Pb concentration in European agricultural soil (Ap horizon, 0–20 cm, <2 mm fraction, aqua regia analyses). Numbered anomalies as detailed in Table 2.
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thus not free to vary. The information value of such data generally lies in the ratios between the variables (Aitchinson, 1986; Filzmoser et al., 2009). Filzmoser et al. (2009) discuss problems and possibilities of univariate data analyses for compositional data. The
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solutions they suggested for the univariate case (bivariate or multivariate results are not provided here) are used throughout this paper. In terms of calculating statistical parameters, such as mean or standard deviation, it is important to note that compositional
Table 2 Explanation of the Pb concentration anomalies as indicated in Fig. 2. Nr.
GEMAS Pb-concentration anomalies – Supposed Primary Reason
Possible other sources at site/remarks
2 10 11 23 40 41 49
Cities Bergen Amsterdam London Paris Rome Naples Lisbon
Sample taken within settled area Industry, contaminated floodplain sediments? Industry Industry Volcanic rocks with elevated Pb-background, industry Industry, Vesuvius and volcanic rocks Hardly visible at scale of map, industry
15 16 18 19 21 24 26
Ore deposits and districts Vassbo, Sweden Bergslagen district, Sweden Ireland, Zn–Pb–Cd deposits in Carboniferous rocks Peak district/S. Peninne, UK Central Wales mining field, Conway valley mineralisation SW England mining field Rhenish slate mountains, Aachen–Stolberg and Benzberg ore district in Germany; La Calamine, Moresnet district in Belgium Northern anomaly: southern contact of Ramberg granite; southern anomaly: Thüringer Wald, Germany Ore Mountains (Erzgebirge), Germany Kutna Hora Silesian-Cracowian Zn–Pb-district Slovakia, Banska Stiavnica Freihung ore district, Germany France Innsbruck area, Austria
27 31
Gurktaler Alps Pb–Zn occurences France, Poitou ridge
33 34
Cantabrian basin (small Pb occurences) Massif Central, Cevenoles district, France
35 36 37 38 39 42 44 47 53
S. Croatia, Pb mineralisation in Triassic carbonates Sumadija, central Serbia Lece-Haldiki, Serbia border to FYROM Italy, Colline Metaliferre Italy, northern Latium Sardinia, Iglesiente-Sulcis District Guajaraz Pb deposit, Toledo, Spain Iberian pyrite belt Linares – La Carolina district, Spain
6 17 30
Contamination Midland valley, Scotland Ostrava, Czech Republic Lombardy, Northern Italy
32 50 51 52
S of Oviedo, Spain – highway? Sines, Portugal Nordenham lead smelter near Bremen Pribram, Pb smelter
Mineralisation, granites Coal mines, industrialised area Downstream Gorno and other Pb–Zn deposits but also a heavily urbanised and industrialised area Coal mines, industrialised area Local anomaly Hardly visible at scale of map Ore deposits, mining
5 20 25 28 29 43 46 48
Geology Island of Mull, Scotland Bohemian Massif, Czech Republic Vosges, France Hungary – border to Austria Southern Slovenia Catalonian coastal range granites Central Spain, Pedochres Batholith Northern Portugal
Evolved acid igneous rocks Granites, some mining Granites, small Pb–Zn district in Southern Vosges (Lembach) Tertiary volcanics(?) Karst, elevated Pb background in soils Leucogranites, Pb–F–Ba vein-type mineralisation; granites on the French site Contains Pb–Zn lodes Granites, minor vein-type mineralisation
4 22 45
Unexplained NW Scotland France N of Ciudad Real, Spain
Mineralisation (remote location) or local contamination No mineralisation known, no smelter, no major city but note proximity to Verdun (WW 1?) Mn mineralisation documented, limestones, probably elevated Pb background values
1 3 7 8 9 12 13 14
Mining Mining Undeveloped Mining, stretching into industrial and current urban areas Mining, smelting, streches into urban areas and Cardiff coalfield Mining, smelting, stretches into urban areas Mining, smelting, metals industry Pb mineralisation following Hercynian faults; mining Mining, smelting, metals industry Mining Mining, industry, urban areas Mining, industry Mining Ba, F, Pb, Zn veins in cover rocks near the Permo-Triassic unconformity Mining Schwaz/Brixlegg, Tyrol, Karwendelgebirge (Lafatsch, Tyrol); in southern part of the anomaly – mining in Obernberg/Brenner (Tyrol) and Schneeberg area (Italy); coincides with Salzburg–Innsbruck–Brenner highway Small scale mining Small Pb–Zn mine but also smelters (Melle, Alloue, Cherchonnes). Disseminated Pb in Jurassic arkosic sandstones. Industry? Three major abandoned mines (Les Malines, Largentière, St Sebastine d’Aigrefeuille). Total Pb extracted > 500 Kt metal. Undeveloped, elevated natural background Mining (Rudnik, Babe) Mining (Lece, Trepca) Mining Volcanic rocks with elevated Pb-background, small scale mining Mining Mining, granites Mining Mining (more than 100 mines), smelting
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Table 3 Statistical parameters of Pb concentration (in mg/kg) and isotope ratios in northern and southern European agricultural soil (Ap horizon, 0–20 cm). Northern Europe
Pb 206
Pb/207Pb Pb/208Pb 208 Pb/206Pb 207
Southern Europe
Minimum
Median
Maximum
Minimum
Median
Maximum
1.6 1.143 0.287 1.477
9.6 1.258 0.397 2.017
52 1.727 0.414 2.702
2.1 1.116 0.379 1.781
20 1.195 0.404 2.075
1309 1.434 0.417 2.251
Fig. 3. Map of the major Pb–Zn deposits of Europe (updated after De Vos et al., 2006).
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Fig. 4. Map of the
206
537
Pb/207Pb isotope ratio in European agricultural soils.
data do not plot in the Euclidian space, but rather on the Aitchison simplex. Thus, only order statistics are used to present the data (note that percentiles will remain unchanged under log-transformation but not under a log-ratio-transformation). For producing the colour surface maps kriging was used to convert the values from the irregularly distributed sampling sites to a regular grid. Successful kriging is based on a careful variogram analysis. Class boundaries used for the colour maps are based on percentiles. Figs. 7 and 8 were produced in Wolfram Mathematica 8.
4. Results and discussion Lead concentrations (aqua regia extraction) in the uppermost 20 cm of agricultural soils (Ap-horizon) from western Europe range from 1.6 to 1309 mg/kg, with a median of 16 mg/kg (Table 1, Fig. 2). The major Pb anomalies in the Pb-concentration map are numbered and an explanation for the most likely source of any of the Pb anomalies is provided in Table 2. The map (Fig. 2) reveals a prominent, almost straight boundary in Pb concentrations between northern and southern Europe. Table 3 demonstrates that northern European soil Pb concentrations (median 10 mg/kg) are lower by a factor of 2 compared to southern Europe (median 20 mg/kg). When comparing the geochemical map with the geological map (Fig. 1) the boundary coincides remarkably well with the maximum extent of the last glaciation, and occurs near the Trans-European Suture Zone, one of the main tectonic borders in Europe (Grad et al., 1999; Janik et al., 2005). Because the same pattern is known from deep (C-horizon) soils (Salminen et al., 2005; De Vos et al., 2006), the Pb concentration in the surface soil Ap-
horizon at the European scale reflects the natural background variation. In addition to the difference in the source rocks, the large-scale increase in Pb concentration from north to south also reflects the natural difference between the young, often coarsegrained and less weathered soils in northern Europe and the older, finer grained and more weathered soils of southern Europe. At a more local scale the majority of Pb anomalies on the map coincide with known Pb mineral belts or deposits (Fig. 3, Table 2 – e.g., Peak district, Central Wales Mining Field, Ore Mountains, Rhenish Slate Mountains, Iberian Pyrite Belt). Because most of the known deposits have been mined, local industrial Pb-sources often developed in the same areas. At the resolution of a continental-scale map (Fig. 2) it is thus difficult to quantify the natural versus the anthropogenic origin of the Pb causing these anomalies. Such quantification will require geochemical mapping at a much more detailed scale (several samples per km2), typical for local studies, e.g., urban geochemical investigations (Johnson et al., 2011) or mineral exploration projects (Reimann et al., 2009c, 2010; Cohen et al., 2011). The data presented here show anomalies indicating contamination around several cities. For example London, Paris, Naples, Rome and Lisbon are all marked by Pb concentration anomalies in Fig. 2. However, even here it is not easy to pinpoint their exact origin: traffic or industry, or, in the case of Naples and Rome, geology (presence of volcanic rocks with elevated Pb-concentrations). It must also be noted that the majority of European cities are not marked by a Pb anomaly at the scale of this survey. The map also indicates several anomalies where neither a Pb deposit nor contamination are a likely Pb source. These anomalies are directly related to geology, for example the Island of Mull (evolved acid igneous rocks), Bavarian Forest (granites), southern
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Fig. 5. Map of the
207
Pb/208Pb isotope ratio in European agricultural soils.
Slovenia (karst), central Spain (Pedochres Batholith) and several granitic intrusions in France (see Fig. 1, Table 2). For all local anomalies, the Pb concentrations decrease on a scale <100 km distance from source towards the regional geochemical background (Reimann and Garrett, 2005). Although theoretically independent of Pb concentration, the continental scale distribution of the 206Pb/207Pb isotope ratios in European agricultural soils in Fig. 4 closely resembles a mirror image of the Pb concentration pattern (Fig. 2). The 206Pb/207Pb isotope ratios range from 1.116 to 1.727, and the median value for European agricultural soils is 1.202 (Table 1). The map of the 206 Pb/207Pb isotope ratios (Fig. 4) is dominated by high ratios in the old, granitic terrains of northern Europe (Fennoscandian Shield) and predominantly low ratios in southern Europe. Increased concentrations of the isotope 206Pb in soils developed on Archaean and Palaeoproterozoic granitic rocks are related to the radioactive decay of 238U on a geological timescale. Higher values of the 206Pb/207Pb isotope ratio, therefore, occur north-east of the Trans-European Suture Zone but also in those regions of southern Europe where younger granitic bedrocks, rich in 238U, bias the Pb isotopes towards a higher ratio (e.g., Northern Portugal/Spain within the Iberian Variscides, central Spain, Massif Central, Bohemian Massif, Ore Mountains). Also the Scandinavian Caledonides clearly show lower 206Pb/207Pb isotope ratios than the older bedrocks forming the remainder of Scandinavia. Anomalously low isotope ratios are often observed in coastal areas (especially well visible in Norway, Denmark and Scotland – Fig. 4). They coincide with wet and/or cold climatic zones, where special conditions for weathering and evolution of soils are suspected to influence the
Pb isotopic system (Reimann et al., 2008a, 2009b, 2011). Although no samples were taken directly within any major city the use of Broken Hill Pb (206Pb/207Pb = 1.04, Sangster et al., 2000) in European leaded gasoline (e.g., Novak et al., 2003) will still be the most likely source of decreased isotope ratios in local areas with dense traffic (e.g., London, Paris). In both cases the small size of the negative 206Pb/207Pb anomaly pattern in the map (Fig. 4) related to these cities demonstrates how local the influence remains at the European scale. The location of the majority of the large European cities is not even indicated by unusually low 206Pb/207Pb isotope ratios, though many case studies demonstrate massive Pb contamination within the city borders of almost any investigated urban area (see examples in Johnson et al., 2011). This demonstrates the truly local scale of the anthropogenic impact. It is also not possible to establish a relationship between traffic density and the patterns observed in the map (Fig. 4). The influence of Pb derived from fertilizers should, in contrast, increase the value of the isotope ratio (Négrel and Roy, 2002) but again remains invisible at the scale of this survey. Agricultural soils are prone to wind erosion and thus the natural variation of Pb isotopes in these soils will influence the isotopic composition observed in the atmosphere (Négrel and Roy, 2002). Fig. 5 shows the spatial distribution of the 207Pb/208Pb isotope ratio in the European agricultural soils. Again a pattern emerges that clearly reflects the ice-age boundary. In general the 207 Pb/208Pb isotope ratios are lower in northern Europe than in the south. Great Britain and Ireland, The Netherlands and parts of Northern Germany and Belgium as well as the Alps show increased 207 Pb/208Pb isotope ratios. High 207Pb/208Pb ratios are a characteristic
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Fig. 6. Binary plot of 206Pb/207Pb versus 208Pb/206Pb in European agricultural soils. The isotopic composition of Broken Hill and Mississippi Valley type ore is shown in addition (data from Sangster et al., 2000).
of many European Pb ores (see data in Sangster et al., 2000). A number of well localised high values in the 207Pb/208Pb isotope ratio also occur in Norway and Sweden, marking more or less the line of Laisvall type Pb–Zn deposits in Fig. 3. However, in general the soils developed on the Precambrian igneous rocks in Finland are characterised by especially low 207Pb/208Pb isotope ratios. A common approach to use Pb isotopes for source identification is to use cross-plots of the isotope ratios, e.g. 206Pb/207Pb versus 208 Pb/207Pb, or 206Pb/207Pb versus 208Pb/206Pb (Fig. 6). These diagrams are intended to reveal trends from, for example, the upper crust composition (206Pb/207Pb = 1.20) towards end-members characteristic for ‘‘pollution’’, e.g. the Pb isotope composition of the Broken Hill deposit in Australia as ‘‘typical for the European
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leaded gasoline signal’’ (e.g., Novak et al., 2003). Such trends are then interpreted as a proof of contamination. However, due to the use of dependent variables (the same isotope occurs in the ratios on both axes, e.g., 206Pb/207Pb versus 208Pb/207Pb or 206 Pb/207Pb versus 208Pb/206Pb), these diagrams are mathematically flawed because intrinsic correlations must be expected when two ratios that share one or more variables are compared (Kenney, 1982; Ellam, 2010; Lenahan et al., 2011). The interpretation of these cross-plots is, therefore, much less straight-forward or easy than usually presented in the environmental literature. Fig. 6 demonstrates that the diagram, when all the European soil samples are included, could be used to argue for almost anything: the input of Broken Hill Pb as well as the input of Mississippi Valley Pb and even the existence of extra-terrestrial Pb in the agricultural soils of Europe because many soil samples show isotope ratios that cannot be traced to any Pb deposit world-wide. To derive at a conclusion about the source of the Pb additional information, for example the spatial distribution of the samples on a map, is needed. According to Ellam (2010) independent diagrams can be plotted only when the isotope 204Pb is measured, i.e. usually only when thermal ionisation mass spectrometry (TIMS) or multi-collector SF-ICP-MS measurements are available and all four stable isotopes are involved in the plot. Most environmental data presented in the literature, however, are obtained by either high resolution (HR) ICP-MS or even quadrupole (Q) ICP-MS analysis and 204Pb is not measured. Moreover, even a plot of 206Pb/207Pb versus 208Pb/204Pb is not completely independent because the fractional contents of all four stable Pb isotopes add up to 100%. In Fig. 7 the three Pb isotopes 206, 207 and 208 are plotted in a ternary diagram as suggested in Reimann et al. (2008b) to solve this dependence problem. On this diagram the colour of the symbol changes continuously with location from dark blue (north) to red (south). In addition to all soil samples, the 50 most important European Pb deposits, as well as the two often used ‘‘Pb end
Fig. 7. Ternary plot of the relative proportions of the three Pb isotopes 206Pb, 207Pb and 208Pb in European agricultural soils (Y). The isotopic composition of 50 European Pb deposits and of Broken Hill and Mississippi Valley type ore (from Sangster et al., 2000) is also provided (circles). Symbol colours change with location from north (blue) to south (red).
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members’’ Broken Hill and Mississippi Valley (Sangster et al., 2000) are shown as circles and similarly colour-coded according to location (the non-European deposits Broken Hill and Mississippi Valley in black). A first important observation is that the soil samples cover a larger range of isotopic compositions than the ore deposits. This demonstrates that the natural variation of Pb isotopes in soils is wider than the variation observed for Pb deposits. Broken Hill Pb falls far outside the range covered by the European agricultural soils. The majority of European Pb deposits and the majority of the soil samples plot within a limited field in the ternary diagram of 24% < 206Pb(%) < 28%, 20% < 207Pb(%) < 22%, 52% < 208Pb(%) < 54%. Because the distribution patterns of Pb concentrations and isotopic ratios in Figs. 2, 4 and 5 closely follow climatic and geological units, they provide no evidence that atmospheric-Pb contamination is the prevailing source of Pb in European agricultural soils. In this case one would expect to see anthropogenic activity or recent paths of wind transport dominating the Pb distribution patterns. The absence of such patterns in the maps for agricultural soil does not exclude the existence of Pb-contamination as recorded by special media like peat bogs (e.g., Kylander et al., 2010; Bindler, 2011), but questions the continental scale conclusions drawn from these studies. In order to identify and characterise contamination via atmospheric transport local-scale, detailed studies in the surroundings of likely sources are required. Ideally, the source emissions should have a well defined Pb-isotope ratio, deviating from the local background. Such studies exist, and they have repeatedly demonstrated that the atmospheric Pb contamination signal disappears in the background variation at a distance of metres to some kilometres from the source (e.g., Ault et al., 1970; Ettler et al., 2006; Hou et al., 2006; Klaminder et al., 2008). Figs. 4 and 5 imply that at the continental scale the large isotopic variability of the geological background masks any additional signal from atmospheric-Pb contamination. However, combining the data of Pb concentration with the isotopic composition makes it possible to isolate an isotopic composition of the excess Pb that generates local variations in concentration. The idea is that local sites with low Pb concentration should be less atmospherically contaminated than neighbouring sites with high Pb concentration. The isotopic composition of the excess Pb, therefore, should clearly reflect the atmospheric contamination signal. The calculation proceeds in the following way: For each site S a cell C with a radius of 400 km around S is considered. Within C there are sites Smin with minimal Pbmin(S) = Pb(Smin), and Smax with maximal Pb concentration Pbmax(S) = Pb(Smax). The excess Pb Pbex at S is the difference be-
Fig. 8. Isotopic composition of calculated ‘‘excess Pb’’ in the Ap samples (+) plotted against ‘‘location’’ (Longitude + Latitude). The isotopic composition of 50 European Pb deposits (Sangster et al., 2000) in relation to their location in Europe is also provided (filled squares). The geographical location of a few outliers that plot outside the isotopic range shown in this diagram is provided in the form of black circles.
tween Pbmax(S) and Pbmin(S). Using the isotopic data at Smin and Smax one can separately calculate the concentrations 206 Pbex = 206Pb(Smax) 206Pb(Smin), as well as 207Pbex and 208Pbex. The ratio 206Pbex/207Pbex varies for south-western Europe (longitude + latitude < 70°) between 1.16 and 1.22 (Fig. 8). These values can be related to the typical low 206Pb/207Pb ratios of SW-European Pb ores which provide a potential natural source of local Pb-anomalies in soils. They, therefore, do not prove predominant atmospheric contamination. Further towards the NE the 206 Pbex/207Pbex variation increases mainly due to the occurrence of very high 206Pbex/207Pbex ratios, again consistent with the occurrence of very high 206Pb/207Pb ratios in some NE-European Pb ores, but also with the general geology (old granitic bedrocks with high 206 Pb/207Pb ratios). The outstanding break between low and very high variation in this diagram marks exactly the location of the Trans-European Suture Zone. For some sites in NE-Europe 206 Pbex/207Pbex deviates towards very low isotope ratios (Fig. 8). Such deviations would usually be interpreted as a predominant atmospheric contamination by Broken Hill Pb with its characteristically low 206Pb/207Pb ratio (e.g., Rosman et al., 1994; Renberg et al., 2002; Novak et al., 2003; Kylander et al., 2010), especially when plotted in a traditional binary diagram of isotope ratios against a proposed hypothetical source as shown in Fig. 6. However, here also this interpretation is not conclusive, because 206 Pbex/207Pbex in these soils again most likely simply reflects a relatively low 206Pb/207Pb isotope ratio of the local ore deposits or local deviations in the bedrock. Fig. 8 shows that deposits with such low isotope ratios occur in NE-Europe. A second argument, against predominant atmospheric contamination at these sites, is that traffic density is much lower here than in most other locations throughout Europe. Another potential influence on the Pb system at these northern locations that needs consideration is simply a higher content of organic material in some of the soils (Reimann et al., 2008a, 2009b).
5. Conclusions Soil formation through weathering of bedrock and sediments is an extremely slow process, and soils must be considered an essential and non-renewable resource. Knowledge and extrapolation of threats to soils, e.g., changes in element concentrations and the reasons thereof, are of primary importance for society. In this context, the addition of isotope tracing at the continental scale provides important supplementary information about the origin and behaviour of elements in soils. The pattern dominating the Pb maps presented here is the southern limit of the last glaciation, almost coincident with the tectonic border between the old Precambrian craton of north-eastern Europe and the younger Palaeozoic platform of western Europe. The Pb isotope map also depicts the border between the Caledonides and the Baltic Shield. Evidently, a close relationship of the Pb concentrations and isotopic ratios with geology is preserved in the agricultural soils at the European scale. This proves that the majority of Pb in European agricultural soils is at present still of natural origin. The atmospheric Pb contamination of the northern hemisphere as demonstrated using ice cores (Rosman et al., 1994), lake sediments (Renberg et al., 2002) or peat bogs (Kylander et al., 2010) contributes little to the total Pb inventory of European agricultural soils. Therefore, the quantity of diffuse atmospheric Pb contamination at the continental scale does not at this time really represent a threat to agricultural soils and humanity at large, while local pollution certainly does. Attention clearly needs to be directed towards local contamination sources and the toxic potential of
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atmospheric Pb, rather than to the quantity of regional or panEuropean scale atmospheric Pb transport. The maps presented here establish the natural geochemical background for both Pb concentration and Pb isotope ratios in European agricultural soils in aqua regia/HNO3-extracts and demonstrate the existence of two major continental-scale natural background regimes for northern and southern Europe, each in turn substantially influenced by local-scale internal variability. An important conclusion, for any interpretation of local Pb isotope studies, is that they have to demonstrate that a clearly defined and well known source of Pb contamination contrasts to the variations in the regional geochemical background. Using global average values, as representative for a certain source (e.g., the average upper crust for ‘‘geogenic Pb’’), is simply not sufficient. Availability of the data All results from the GEMAS project will be published in the form of a book in 2013. All data will accompany that book in the form of excel files on an attached CD-ROM. Until release of the book, the data will be made available to third parties via a request to the first author if the Geological Surveys of Europe and the International Lead Association – Europe (ILA) agree on a suggested use of the data. Acknowledgements The GEMAS project is a cooperation project of the EuroGeoSurveys Geochemistry Expert Group with a number of outside organisations (e.g., Alterra in The Netherlands, the Norwegian Forest and Landscape Institute, several Ministries of the Environment and University Departments of Geosciences in a number of European countries, CSIRO Land and Water in Adelaide, Australia) and Eurometaux. Sampling was covered by the participating organisations in their home countries, the analytical work was co-financed by the following organisations: Eurometaux, Cobalt Development Institute (CDI), European Copper Institute (ECI), Nickel Institute, Europe, European Precious Metals Federation (EPMF), International Antimony Association (i2a), International Manganese Institute (IMnI), International Molybdenum Association (IMoA), ITRI Ltd. (on behalf of the REACH Tin Metal Consortium), International Zinc Association (IZA), International Lead Association-Europe (ILAEurope), European Borates Association (EBA), the (REACH) Vanadium Consortium (VC) and the (REACH) Selenium and Tellurium Consortium. Important discussions with Arnold Arnoldussen, Peter Englmaier and Friedrich Koller are acknowledged. Lead isotope analyses were carried out as an internal research project at the Geological Survey of Norway. The GEMAS project is managed by the Geological Survey of Norway with financial support from EuroGeoSurveys. The Directors of the European Geological Surveys and the additional participating organisations are thanked for making sampling of almost all of Europe, on a tight time schedule, possible. The authors thank the two reviewers for a thorough and insightful review of the manuscript, resulting in many improvements. References Aitchinson, J., 1986. The Statistical Analysis of Compositional Data. Chapman and Hall, London, UK. Asch, K. (Ed.), 2003. The 1:5 Million International Geological Map of Europe and Adjacent Areas: Development and Implementation of a GIS-enabled Concept; Geologisches Jahrbuch; SA 3. BGR, Hannover, Schweitzerbart (Stuttgart). Ault, W.U., Senechal, R.G., Erlebach, W.E., 1970. Isotopic composition as a natural tracer of lead in the environment. Environ. Sci. Technol. 4, 305–317. Bindler, R., 2011. Contaminated lead environments of man: reviewing the lead isotopic evidence in sediments, peat, and soils for the temporal and spatial patterns of atmospheric lead pollution in Sweden. Environ. Geochem. Health 33, 311–329.
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Investigating the sources of the labile fraction in sediments from silicate-drained rocks using trace elements, and strontium and lead isotopes. Sci. Total Environ. 298, 163–181. Novak, M., Emmanuel, S., Vile, M.A., Yigal, E., Veron, A., Paces, T., Kelman, W., Vanecek, M., Stepanova, M., Brizova, E., Hovorka, J., 2003. Origin of lead in eight central European peat bogs determined from isotope ratios, strength and operation times of regional pollution sources. Environ. Sci. Technol. 37, 437– 445. Nriagu, J.O., 1979. Global inventory of natural and anthropogenic emissions of trace metals to the atmosphere. Nature 279, 409–411. Nriagu, J.O., 1998. Tales told in lead. Science 281, 1622–1623. Reimann, C., Birke, M. (Eds.), 2010. Geochemistry of European Bottled Water. Borntraeger Science Publishers, Stuttgart. Reimann, C., Garrett, R.G., 2005. Geochemical background: concept and reality. Sci. Total Environ. 350, 12–27. 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Lead and lead isotopes in agricultural soils of Europe – The continental perspective Clemens Reimann a,⇑, Belinda Flem a, Karl Fabian a, Manfred Birke b, Anna Ladenberger c, Philippe Négrel d, Alecos Demetriades e, Jurian Hoogewerff f, The GEMAS Project Team 1 a
Geological Survey of Norway (NGU), P.O. Box 6315 Sluppen, N-7491 Trondheim, Norway Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Postfach 510153, D-30631 Hannover, Germany c Geological Survey of Sweden (SGU), Box 670, S-751 28 Uppsala, Sweden d BRGM, Service Métrologie Monitoring Analyse, 3 Avenue Claude Guillemin, BP 6009, 45060 Orléans Cedex 2, France e Institute of Geology and Mineral Exploration, Entrance C, Olympic Village, Acharnae, Athens GR-13677, Greece f Oritain Global Ltd., 8 Pacific Street, Dunedin 9010, New Zealand b
a r t i c l e
i n f o
Article history: Received 8 November 2011 Accepted 20 December 2011 Available online 27 December 2011 Editorial handling by R. Fuge
a b s t r a c t Lead isotopes are widely used for age dating, for tracking sources of melts, sediments, Pb products, food and animals and for studying atmospheric Pb contamination. For the first time, a map of a Pb isotope landscape at the continental-scale is presented. Agricultural soil samples (Ap-horizon, 0–20 cm) collected at an average density of 1 site/2500 km2 were analysed for Pb concentration and Pb isotopes (206Pb, 207Pb, 208 Pb). Lead concentrations vary from 1.6 to 1309 mg/kg, with a median of 16 mg/kg. Isotopic ratios of 206 Pb/207Pb range from 1.116 to 1.727 with a median of 1.202. The new data define the soil geochemical Pb background for European agricultural soil, providing crucial information for geological, environmental and forensic sciences, public health, environmental policy and mineral exploration. The European continental-scale patterns of Pb concentrations and Pb isotopes show a high variability dominated by geology and influenced by climate. Lead concentration anomalies mark most of the known mineralised areas throughout Europe. Some local Pb anomalies have a distinct anthropogenic origin. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Lead has been mined and used by humans for several thousand years. The accumulated total world Pb production since ancient times is estimated to be 300 Mt (based on Nriagu, 1998 and updated to 2010 according to current world mine production figures). Estimates of the anthropogenic Pb fraction in the environment range from <10% (Strauss, 1978; Kownacka et al., 1990) to >90% (Nriagu, 1979). Detailed studies on environmental samples (e.g., peat bogs, ice cores, sediment cores) has suggested major Pb contamination of the northern hemisphere since ancient times ⇑ Corresponding author. E-mail address: [email protected] (C. Reimann). S. Albanese, M. Andersson, A. Arnoldussen, R. Baritz, M.J. Batista, A. Bel-lan, D. Cicchella, E. Dinelli, B. De Vivo, W. De Vos, M. Duris, A. Dusza-Dobek, O.A. Eggen, M. Eklund, V. Ernstsen, P. Filzmoser, T.E. Finne, D. Flight, S. Forrester, M. Fuchs, U. Fugedi, A. Gilucis, M. Gosar, V. Gregorauskiene, A. Gulan, J. Halamic´, E. Haslinger, P. Hayoz, G. Hobiger, R. Hoffmann, H. Hrvatovic, S. Husnjak, L. Janik, C.C. Johnson, G. Jordan, J. Kirby, J. Kivisilla, V. Klos, F. Krone, P. Kwecko, L. Kuti, A. Lima, J. Locutura, P. Lucivjansky, D. Mackovych, B.I. Malyuk, R. Maquil, M. McLaughlin, R.G. Meuli, N. Miosic, G. Mol, P. O’Connor, K. Oorts, R.T. Ottesen, A. Pasieczna, V. Petersell, S. Pfleiderer, M. Ponˇavicˇ, C. Prazeres, U. Rauch, I. Salpeteur, A. Schedl, A. Scheib, I. Schoeters, P. Sefcik, E. Sellersjö, F. Skopljak, I. Slaninka, A. Šorša, R. Srvkota, T. Stafilov, T. Tarvainen, V. Trendavilov, P. Valera, V. Verougstraete, D. Vidojevic´, A.M. Zissimos, Z. Zomeni. 1
0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.12.012
(Komárek et al., 2008; Bindler, 2011). The European Commission has identified diffuse contamination as one of the eight major threats to soil quality (European Commission, 2006). Because Pb deposits have characteristic Pb isotope compositions (for data on the economically important deposits see: Sangster et al., 2000), Pb concentration and Pb-isotope ratios in combination may in some cases be used as a fingerprint to trace Pb to its source. Natural Pb comprises four stable isotopes (natural abundance in brackets): 204Pb (1.4%), 206Pb (24.1%), 207Pb (22.1%) and 208Pb (52.4%) in varying proportions, uniquely defined by the three ratios 206Pb/207Pb, 208Pb/206Pb, 206Pb/204Pb. The reliable measurement of the isotope 204Pb needs much care and specialised instrumentation and is time consuming. The ratios between the more abundant isotopes 206Pb, 207Pb and 208Pb can be easily determined by inductively coupled plasma mass spectrometry. In environmental sciences the 206Pb/207Pb isotope ratio is commonly used to suggest Pb contamination of different compartments of the environment at the local to global scale (for a recent review on the use of Pb isotopes in environmental sciences see Komárek et al., 2008). Data presented in Sangster et al. (2000) demonstrate that the 206Pb/207Pb isotope ratio of the most important Pb deposits varies between 0.98 and 1.41. One major producer, Broken Hill in Australia, is characterised by a very low 206Pb/207Pb isotope ratio of 1.04, while the largest Pb deposits in the world, the Mississippi
C. Reimann et al. / Applied Geochemistry 27 (2012) 532–542
Valley type deposits in the United States, show a characteristically high 206Pb/207Pb isotope ratio of 1.4. It thus would be easy to determine the origin of Pb in a sample, if one would have to decide whether Pb from Broken Hill or the Mississippi Valley was the original source. In natural samples, determining the origin of Pb is more complicated and there are certain requirements for the successful use of Pb isotopes in environmental studies. For example, the isotope ratios of all sources of Pb that contribute to the Pb concentration of a sample must be known. This will rarely be the case, when soils or plant materials are collected in nature. Furthermore, and even more importantly, it is required that the isotopic composition of Pb is not significantly affected by physico-chemical fractionation processes in natural systems (Komárek et al., 2008), e.g., during soil formation and weathering. To be able to proceed with using Pb isotopes in forensic and environmental studies on a reliable foundation, it is important to know the Pb background concentration and isotopic compositions on a global or at least continental scale. However, in spite of its importance, not even the continentalscale distribution of Pb and Pb isotopes combined has ever been documented for any geochemical sample material. As the result of a united effort of the European Geological Surveys, backed by the European metals industry, here the first geochemical reference maps of Pb concentrations combined with Pb isotope ratios at the European scale are provided, against which the impact of contamination can be realistically assessed. The maps localise the major anomalies of Pb in agricultural soils and help identify the processes that generate the observed distribution patterns.
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2. The survey area Fig. 1 shows a simplified geological map of Europe, including the main features discussed in this paper. Further maps covering topography and land use of Europe can be found in almost any world atlas. A number of maps covering different themes at about the scale of the GEMAS project (topography, geology, tectonics, fault and fracture zones, distribution of different rock types, distribution of the main sedimentary basins, precipitation and population density) can be found in Reimann and Birke (2010). For Europe, an excellent source of land use information is the CORINE land use map of Europe (GLC 2000 database, 2003). A detailed geological map of Europe is provided by Asch (2003), concise descriptions of the geology of Europe can be found in Blundell et al. (1992) and McCann (2008). The soil atlas of Europe provides a wealth of information about the soils of Europe, but also contains maps of average precipitation, temperature, land use, population density, extent of the last glaciation and soil texture (Jones et al., 2005).
3. Material and methods 3.1. Sampling and sample preparation Sampling took place from summer 2008 to early spring 2009. In total, 2211 samples of agricultural soil (Ap-horizon, 0–20 cm) were collected at an average sampling density of 1 site per 2500 km2 (grid of 50 50 km) across Europe. A field handbook describes
Fig. 1. Generalised geological map of Europe. For a detailed geological map of Europe see Asch (2003) –
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the sampling procedure in detail (EGS, 2008). All samples were shipped to a central laboratory for preparation (air-drying and sieving to <2 mm using nylon screens). 3.2. Analyses
Table 1 Statistical parameters of Pb concentration (in mg/kg) and isotope ratios in European agricultural soil (Ap horizon, 0–20 cm).
Pb 206
Pb/207Pb Pb/208Pb 208 Pb/206Pb 207
Lead concentrations were determined following an aqua regia extraction using an inductively coupled plasma mass spectrometer (ICP-MS) using tight external quality control procedures (Reimann et al., 2009a). All Pb isotope ratio measurements were carried out on a sector field inductively coupled plasma mass spectrometer (SF-ICP-MS; ELEMENT 1, Finnigan MAT, Bremen, Germany). A sample weight of 0.5 g was carefully stirred with 8 mL 7 N HNO3 in PTFE vessels on a Vortex Genie shaker before the samples were extracted under N2 pressure at 250 °C in an ultraclave (Milestone). During heating the pressure increases from the start pressure of 50 bar to approximately 120 bar. After cooling the acid extracts were filtered through folded Whatman filters. All samples were diluted to Pb concentrations below 100 lg/L and a HNO3-concentration of 5% (v/v) prior to SF-ICP-MS analysis. The common Pb isotope standard NIST SRM 981 was used to correct for instrumental mass discrimination. The instrumental uncertainty in each
Minimum
Median
Maximum
1.6 1.116 0.287 1.477
15.7 1.202 0.403 2.067
1309 1.727 0.417 2.702
measurement was between 0.003 and 0.0002 on the ratio. The certified reference material Mess-3 (NRC, National Research Council Canada) was prepared in the same way as the samples and used as a day-to-day control standard of both the SF-ICP-MS analysis and the digestion. From these data, the reproducibility of the Pb isotope ratio was estimated at 0.11% for 208Pb/207Pb and 0.10% for 206Pb/207Pb. 3.3. Data analysis and mapping Geochemical data are compositional (closed) data, element concentrations reported in wt.% or mg/kg sum up to a constant and are
Fig. 2. Map of the Pb concentration in European agricultural soil (Ap horizon, 0–20 cm, <2 mm fraction, aqua regia analyses). Numbered anomalies as detailed in Table 2.
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thus not free to vary. The information value of such data generally lies in the ratios between the variables (Aitchinson, 1986; Filzmoser et al., 2009). Filzmoser et al. (2009) discuss problems and possibilities of univariate data analyses for compositional data. The
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solutions they suggested for the univariate case (bivariate or multivariate results are not provided here) are used throughout this paper. In terms of calculating statistical parameters, such as mean or standard deviation, it is important to note that compositional
Table 2 Explanation of the Pb concentration anomalies as indicated in Fig. 2. Nr.
GEMAS Pb-concentration anomalies – Supposed Primary Reason
Possible other sources at site/remarks
2 10 11 23 40 41 49
Cities Bergen Amsterdam London Paris Rome Naples Lisbon
Sample taken within settled area Industry, contaminated floodplain sediments? Industry Industry Volcanic rocks with elevated Pb-background, industry Industry, Vesuvius and volcanic rocks Hardly visible at scale of map, industry
15 16 18 19 21 24 26
Ore deposits and districts Vassbo, Sweden Bergslagen district, Sweden Ireland, Zn–Pb–Cd deposits in Carboniferous rocks Peak district/S. Peninne, UK Central Wales mining field, Conway valley mineralisation SW England mining field Rhenish slate mountains, Aachen–Stolberg and Benzberg ore district in Germany; La Calamine, Moresnet district in Belgium Northern anomaly: southern contact of Ramberg granite; southern anomaly: Thüringer Wald, Germany Ore Mountains (Erzgebirge), Germany Kutna Hora Silesian-Cracowian Zn–Pb-district Slovakia, Banska Stiavnica Freihung ore district, Germany France Innsbruck area, Austria
27 31
Gurktaler Alps Pb–Zn occurences France, Poitou ridge
33 34
Cantabrian basin (small Pb occurences) Massif Central, Cevenoles district, France
35 36 37 38 39 42 44 47 53
S. Croatia, Pb mineralisation in Triassic carbonates Sumadija, central Serbia Lece-Haldiki, Serbia border to FYROM Italy, Colline Metaliferre Italy, northern Latium Sardinia, Iglesiente-Sulcis District Guajaraz Pb deposit, Toledo, Spain Iberian pyrite belt Linares – La Carolina district, Spain
6 17 30
Contamination Midland valley, Scotland Ostrava, Czech Republic Lombardy, Northern Italy
32 50 51 52
S of Oviedo, Spain – highway? Sines, Portugal Nordenham lead smelter near Bremen Pribram, Pb smelter
Mineralisation, granites Coal mines, industrialised area Downstream Gorno and other Pb–Zn deposits but also a heavily urbanised and industrialised area Coal mines, industrialised area Local anomaly Hardly visible at scale of map Ore deposits, mining
5 20 25 28 29 43 46 48
Geology Island of Mull, Scotland Bohemian Massif, Czech Republic Vosges, France Hungary – border to Austria Southern Slovenia Catalonian coastal range granites Central Spain, Pedochres Batholith Northern Portugal
Evolved acid igneous rocks Granites, some mining Granites, small Pb–Zn district in Southern Vosges (Lembach) Tertiary volcanics(?) Karst, elevated Pb background in soils Leucogranites, Pb–F–Ba vein-type mineralisation; granites on the French site Contains Pb–Zn lodes Granites, minor vein-type mineralisation
4 22 45
Unexplained NW Scotland France N of Ciudad Real, Spain
Mineralisation (remote location) or local contamination No mineralisation known, no smelter, no major city but note proximity to Verdun (WW 1?) Mn mineralisation documented, limestones, probably elevated Pb background values
1 3 7 8 9 12 13 14
Mining Mining Undeveloped Mining, stretching into industrial and current urban areas Mining, smelting, streches into urban areas and Cardiff coalfield Mining, smelting, stretches into urban areas Mining, smelting, metals industry Pb mineralisation following Hercynian faults; mining Mining, smelting, metals industry Mining Mining, industry, urban areas Mining, industry Mining Ba, F, Pb, Zn veins in cover rocks near the Permo-Triassic unconformity Mining Schwaz/Brixlegg, Tyrol, Karwendelgebirge (Lafatsch, Tyrol); in southern part of the anomaly – mining in Obernberg/Brenner (Tyrol) and Schneeberg area (Italy); coincides with Salzburg–Innsbruck–Brenner highway Small scale mining Small Pb–Zn mine but also smelters (Melle, Alloue, Cherchonnes). Disseminated Pb in Jurassic arkosic sandstones. Industry? Three major abandoned mines (Les Malines, Largentière, St Sebastine d’Aigrefeuille). Total Pb extracted > 500 Kt metal. Undeveloped, elevated natural background Mining (Rudnik, Babe) Mining (Lece, Trepca) Mining Volcanic rocks with elevated Pb-background, small scale mining Mining Mining, granites Mining Mining (more than 100 mines), smelting
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Table 3 Statistical parameters of Pb concentration (in mg/kg) and isotope ratios in northern and southern European agricultural soil (Ap horizon, 0–20 cm). Northern Europe
Pb 206
Pb/207Pb Pb/208Pb 208 Pb/206Pb 207
Southern Europe
Minimum
Median
Maximum
Minimum
Median
Maximum
1.6 1.143 0.287 1.477
9.6 1.258 0.397 2.017
52 1.727 0.414 2.702
2.1 1.116 0.379 1.781
20 1.195 0.404 2.075
1309 1.434 0.417 2.251
Fig. 3. Map of the major Pb–Zn deposits of Europe (updated after De Vos et al., 2006).
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Fig. 4. Map of the
206
537
Pb/207Pb isotope ratio in European agricultural soils.
data do not plot in the Euclidian space, but rather on the Aitchison simplex. Thus, only order statistics are used to present the data (note that percentiles will remain unchanged under log-transformation but not under a log-ratio-transformation). For producing the colour surface maps kriging was used to convert the values from the irregularly distributed sampling sites to a regular grid. Successful kriging is based on a careful variogram analysis. Class boundaries used for the colour maps are based on percentiles. Figs. 7 and 8 were produced in Wolfram Mathematica 8.
4. Results and discussion Lead concentrations (aqua regia extraction) in the uppermost 20 cm of agricultural soils (Ap-horizon) from western Europe range from 1.6 to 1309 mg/kg, with a median of 16 mg/kg (Table 1, Fig. 2). The major Pb anomalies in the Pb-concentration map are numbered and an explanation for the most likely source of any of the Pb anomalies is provided in Table 2. The map (Fig. 2) reveals a prominent, almost straight boundary in Pb concentrations between northern and southern Europe. Table 3 demonstrates that northern European soil Pb concentrations (median 10 mg/kg) are lower by a factor of 2 compared to southern Europe (median 20 mg/kg). When comparing the geochemical map with the geological map (Fig. 1) the boundary coincides remarkably well with the maximum extent of the last glaciation, and occurs near the Trans-European Suture Zone, one of the main tectonic borders in Europe (Grad et al., 1999; Janik et al., 2005). Because the same pattern is known from deep (C-horizon) soils (Salminen et al., 2005; De Vos et al., 2006), the Pb concentration in the surface soil Ap-
horizon at the European scale reflects the natural background variation. In addition to the difference in the source rocks, the large-scale increase in Pb concentration from north to south also reflects the natural difference between the young, often coarsegrained and less weathered soils in northern Europe and the older, finer grained and more weathered soils of southern Europe. At a more local scale the majority of Pb anomalies on the map coincide with known Pb mineral belts or deposits (Fig. 3, Table 2 – e.g., Peak district, Central Wales Mining Field, Ore Mountains, Rhenish Slate Mountains, Iberian Pyrite Belt). Because most of the known deposits have been mined, local industrial Pb-sources often developed in the same areas. At the resolution of a continental-scale map (Fig. 2) it is thus difficult to quantify the natural versus the anthropogenic origin of the Pb causing these anomalies. Such quantification will require geochemical mapping at a much more detailed scale (several samples per km2), typical for local studies, e.g., urban geochemical investigations (Johnson et al., 2011) or mineral exploration projects (Reimann et al., 2009c, 2010; Cohen et al., 2011). The data presented here show anomalies indicating contamination around several cities. For example London, Paris, Naples, Rome and Lisbon are all marked by Pb concentration anomalies in Fig. 2. However, even here it is not easy to pinpoint their exact origin: traffic or industry, or, in the case of Naples and Rome, geology (presence of volcanic rocks with elevated Pb-concentrations). It must also be noted that the majority of European cities are not marked by a Pb anomaly at the scale of this survey. The map also indicates several anomalies where neither a Pb deposit nor contamination are a likely Pb source. These anomalies are directly related to geology, for example the Island of Mull (evolved acid igneous rocks), Bavarian Forest (granites), southern
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Fig. 5. Map of the
207
Pb/208Pb isotope ratio in European agricultural soils.
Slovenia (karst), central Spain (Pedochres Batholith) and several granitic intrusions in France (see Fig. 1, Table 2). For all local anomalies, the Pb concentrations decrease on a scale <100 km distance from source towards the regional geochemical background (Reimann and Garrett, 2005). Although theoretically independent of Pb concentration, the continental scale distribution of the 206Pb/207Pb isotope ratios in European agricultural soils in Fig. 4 closely resembles a mirror image of the Pb concentration pattern (Fig. 2). The 206Pb/207Pb isotope ratios range from 1.116 to 1.727, and the median value for European agricultural soils is 1.202 (Table 1). The map of the 206 Pb/207Pb isotope ratios (Fig. 4) is dominated by high ratios in the old, granitic terrains of northern Europe (Fennoscandian Shield) and predominantly low ratios in southern Europe. Increased concentrations of the isotope 206Pb in soils developed on Archaean and Palaeoproterozoic granitic rocks are related to the radioactive decay of 238U on a geological timescale. Higher values of the 206Pb/207Pb isotope ratio, therefore, occur north-east of the Trans-European Suture Zone but also in those regions of southern Europe where younger granitic bedrocks, rich in 238U, bias the Pb isotopes towards a higher ratio (e.g., Northern Portugal/Spain within the Iberian Variscides, central Spain, Massif Central, Bohemian Massif, Ore Mountains). Also the Scandinavian Caledonides clearly show lower 206Pb/207Pb isotope ratios than the older bedrocks forming the remainder of Scandinavia. Anomalously low isotope ratios are often observed in coastal areas (especially well visible in Norway, Denmark and Scotland – Fig. 4). They coincide with wet and/or cold climatic zones, where special conditions for weathering and evolution of soils are suspected to influence the
Pb isotopic system (Reimann et al., 2008a, 2009b, 2011). Although no samples were taken directly within any major city the use of Broken Hill Pb (206Pb/207Pb = 1.04, Sangster et al., 2000) in European leaded gasoline (e.g., Novak et al., 2003) will still be the most likely source of decreased isotope ratios in local areas with dense traffic (e.g., London, Paris). In both cases the small size of the negative 206Pb/207Pb anomaly pattern in the map (Fig. 4) related to these cities demonstrates how local the influence remains at the European scale. The location of the majority of the large European cities is not even indicated by unusually low 206Pb/207Pb isotope ratios, though many case studies demonstrate massive Pb contamination within the city borders of almost any investigated urban area (see examples in Johnson et al., 2011). This demonstrates the truly local scale of the anthropogenic impact. It is also not possible to establish a relationship between traffic density and the patterns observed in the map (Fig. 4). The influence of Pb derived from fertilizers should, in contrast, increase the value of the isotope ratio (Négrel and Roy, 2002) but again remains invisible at the scale of this survey. Agricultural soils are prone to wind erosion and thus the natural variation of Pb isotopes in these soils will influence the isotopic composition observed in the atmosphere (Négrel and Roy, 2002). Fig. 5 shows the spatial distribution of the 207Pb/208Pb isotope ratio in the European agricultural soils. Again a pattern emerges that clearly reflects the ice-age boundary. In general the 207 Pb/208Pb isotope ratios are lower in northern Europe than in the south. Great Britain and Ireland, The Netherlands and parts of Northern Germany and Belgium as well as the Alps show increased 207 Pb/208Pb isotope ratios. High 207Pb/208Pb ratios are a characteristic
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Fig. 6. Binary plot of 206Pb/207Pb versus 208Pb/206Pb in European agricultural soils. The isotopic composition of Broken Hill and Mississippi Valley type ore is shown in addition (data from Sangster et al., 2000).
of many European Pb ores (see data in Sangster et al., 2000). A number of well localised high values in the 207Pb/208Pb isotope ratio also occur in Norway and Sweden, marking more or less the line of Laisvall type Pb–Zn deposits in Fig. 3. However, in general the soils developed on the Precambrian igneous rocks in Finland are characterised by especially low 207Pb/208Pb isotope ratios. A common approach to use Pb isotopes for source identification is to use cross-plots of the isotope ratios, e.g. 206Pb/207Pb versus 208 Pb/207Pb, or 206Pb/207Pb versus 208Pb/206Pb (Fig. 6). These diagrams are intended to reveal trends from, for example, the upper crust composition (206Pb/207Pb = 1.20) towards end-members characteristic for ‘‘pollution’’, e.g. the Pb isotope composition of the Broken Hill deposit in Australia as ‘‘typical for the European
539
leaded gasoline signal’’ (e.g., Novak et al., 2003). Such trends are then interpreted as a proof of contamination. However, due to the use of dependent variables (the same isotope occurs in the ratios on both axes, e.g., 206Pb/207Pb versus 208Pb/207Pb or 206 Pb/207Pb versus 208Pb/206Pb), these diagrams are mathematically flawed because intrinsic correlations must be expected when two ratios that share one or more variables are compared (Kenney, 1982; Ellam, 2010; Lenahan et al., 2011). The interpretation of these cross-plots is, therefore, much less straight-forward or easy than usually presented in the environmental literature. Fig. 6 demonstrates that the diagram, when all the European soil samples are included, could be used to argue for almost anything: the input of Broken Hill Pb as well as the input of Mississippi Valley Pb and even the existence of extra-terrestrial Pb in the agricultural soils of Europe because many soil samples show isotope ratios that cannot be traced to any Pb deposit world-wide. To derive at a conclusion about the source of the Pb additional information, for example the spatial distribution of the samples on a map, is needed. According to Ellam (2010) independent diagrams can be plotted only when the isotope 204Pb is measured, i.e. usually only when thermal ionisation mass spectrometry (TIMS) or multi-collector SF-ICP-MS measurements are available and all four stable isotopes are involved in the plot. Most environmental data presented in the literature, however, are obtained by either high resolution (HR) ICP-MS or even quadrupole (Q) ICP-MS analysis and 204Pb is not measured. Moreover, even a plot of 206Pb/207Pb versus 208Pb/204Pb is not completely independent because the fractional contents of all four stable Pb isotopes add up to 100%. In Fig. 7 the three Pb isotopes 206, 207 and 208 are plotted in a ternary diagram as suggested in Reimann et al. (2008b) to solve this dependence problem. On this diagram the colour of the symbol changes continuously with location from dark blue (north) to red (south). In addition to all soil samples, the 50 most important European Pb deposits, as well as the two often used ‘‘Pb end
Fig. 7. Ternary plot of the relative proportions of the three Pb isotopes 206Pb, 207Pb and 208Pb in European agricultural soils (Y). The isotopic composition of 50 European Pb deposits and of Broken Hill and Mississippi Valley type ore (from Sangster et al., 2000) is also provided (circles). Symbol colours change with location from north (blue) to south (red).
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members’’ Broken Hill and Mississippi Valley (Sangster et al., 2000) are shown as circles and similarly colour-coded according to location (the non-European deposits Broken Hill and Mississippi Valley in black). A first important observation is that the soil samples cover a larger range of isotopic compositions than the ore deposits. This demonstrates that the natural variation of Pb isotopes in soils is wider than the variation observed for Pb deposits. Broken Hill Pb falls far outside the range covered by the European agricultural soils. The majority of European Pb deposits and the majority of the soil samples plot within a limited field in the ternary diagram of 24% < 206Pb(%) < 28%, 20% < 207Pb(%) < 22%, 52% < 208Pb(%) < 54%. Because the distribution patterns of Pb concentrations and isotopic ratios in Figs. 2, 4 and 5 closely follow climatic and geological units, they provide no evidence that atmospheric-Pb contamination is the prevailing source of Pb in European agricultural soils. In this case one would expect to see anthropogenic activity or recent paths of wind transport dominating the Pb distribution patterns. The absence of such patterns in the maps for agricultural soil does not exclude the existence of Pb-contamination as recorded by special media like peat bogs (e.g., Kylander et al., 2010; Bindler, 2011), but questions the continental scale conclusions drawn from these studies. In order to identify and characterise contamination via atmospheric transport local-scale, detailed studies in the surroundings of likely sources are required. Ideally, the source emissions should have a well defined Pb-isotope ratio, deviating from the local background. Such studies exist, and they have repeatedly demonstrated that the atmospheric Pb contamination signal disappears in the background variation at a distance of metres to some kilometres from the source (e.g., Ault et al., 1970; Ettler et al., 2006; Hou et al., 2006; Klaminder et al., 2008). Figs. 4 and 5 imply that at the continental scale the large isotopic variability of the geological background masks any additional signal from atmospheric-Pb contamination. However, combining the data of Pb concentration with the isotopic composition makes it possible to isolate an isotopic composition of the excess Pb that generates local variations in concentration. The idea is that local sites with low Pb concentration should be less atmospherically contaminated than neighbouring sites with high Pb concentration. The isotopic composition of the excess Pb, therefore, should clearly reflect the atmospheric contamination signal. The calculation proceeds in the following way: For each site S a cell C with a radius of 400 km around S is considered. Within C there are sites Smin with minimal Pbmin(S) = Pb(Smin), and Smax with maximal Pb concentration Pbmax(S) = Pb(Smax). The excess Pb Pbex at S is the difference be-
Fig. 8. Isotopic composition of calculated ‘‘excess Pb’’ in the Ap samples (+) plotted against ‘‘location’’ (Longitude + Latitude). The isotopic composition of 50 European Pb deposits (Sangster et al., 2000) in relation to their location in Europe is also provided (filled squares). The geographical location of a few outliers that plot outside the isotopic range shown in this diagram is provided in the form of black circles.
tween Pbmax(S) and Pbmin(S). Using the isotopic data at Smin and Smax one can separately calculate the concentrations 206 Pbex = 206Pb(Smax) 206Pb(Smin), as well as 207Pbex and 208Pbex. The ratio 206Pbex/207Pbex varies for south-western Europe (longitude + latitude < 70°) between 1.16 and 1.22 (Fig. 8). These values can be related to the typical low 206Pb/207Pb ratios of SW-European Pb ores which provide a potential natural source of local Pb-anomalies in soils. They, therefore, do not prove predominant atmospheric contamination. Further towards the NE the 206 Pbex/207Pbex variation increases mainly due to the occurrence of very high 206Pbex/207Pbex ratios, again consistent with the occurrence of very high 206Pb/207Pb ratios in some NE-European Pb ores, but also with the general geology (old granitic bedrocks with high 206 Pb/207Pb ratios). The outstanding break between low and very high variation in this diagram marks exactly the location of the Trans-European Suture Zone. For some sites in NE-Europe 206 Pbex/207Pbex deviates towards very low isotope ratios (Fig. 8). Such deviations would usually be interpreted as a predominant atmospheric contamination by Broken Hill Pb with its characteristically low 206Pb/207Pb ratio (e.g., Rosman et al., 1994; Renberg et al., 2002; Novak et al., 2003; Kylander et al., 2010), especially when plotted in a traditional binary diagram of isotope ratios against a proposed hypothetical source as shown in Fig. 6. However, here also this interpretation is not conclusive, because 206 Pbex/207Pbex in these soils again most likely simply reflects a relatively low 206Pb/207Pb isotope ratio of the local ore deposits or local deviations in the bedrock. Fig. 8 shows that deposits with such low isotope ratios occur in NE-Europe. A second argument, against predominant atmospheric contamination at these sites, is that traffic density is much lower here than in most other locations throughout Europe. Another potential influence on the Pb system at these northern locations that needs consideration is simply a higher content of organic material in some of the soils (Reimann et al., 2008a, 2009b).
5. Conclusions Soil formation through weathering of bedrock and sediments is an extremely slow process, and soils must be considered an essential and non-renewable resource. Knowledge and extrapolation of threats to soils, e.g., changes in element concentrations and the reasons thereof, are of primary importance for society. In this context, the addition of isotope tracing at the continental scale provides important supplementary information about the origin and behaviour of elements in soils. The pattern dominating the Pb maps presented here is the southern limit of the last glaciation, almost coincident with the tectonic border between the old Precambrian craton of north-eastern Europe and the younger Palaeozoic platform of western Europe. The Pb isotope map also depicts the border between the Caledonides and the Baltic Shield. Evidently, a close relationship of the Pb concentrations and isotopic ratios with geology is preserved in the agricultural soils at the European scale. This proves that the majority of Pb in European agricultural soils is at present still of natural origin. The atmospheric Pb contamination of the northern hemisphere as demonstrated using ice cores (Rosman et al., 1994), lake sediments (Renberg et al., 2002) or peat bogs (Kylander et al., 2010) contributes little to the total Pb inventory of European agricultural soils. Therefore, the quantity of diffuse atmospheric Pb contamination at the continental scale does not at this time really represent a threat to agricultural soils and humanity at large, while local pollution certainly does. Attention clearly needs to be directed towards local contamination sources and the toxic potential of
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atmospheric Pb, rather than to the quantity of regional or panEuropean scale atmospheric Pb transport. The maps presented here establish the natural geochemical background for both Pb concentration and Pb isotope ratios in European agricultural soils in aqua regia/HNO3-extracts and demonstrate the existence of two major continental-scale natural background regimes for northern and southern Europe, each in turn substantially influenced by local-scale internal variability. An important conclusion, for any interpretation of local Pb isotope studies, is that they have to demonstrate that a clearly defined and well known source of Pb contamination contrasts to the variations in the regional geochemical background. Using global average values, as representative for a certain source (e.g., the average upper crust for ‘‘geogenic Pb’’), is simply not sufficient. Availability of the data All results from the GEMAS project will be published in the form of a book in 2013. All data will accompany that book in the form of excel files on an attached CD-ROM. Until release of the book, the data will be made available to third parties via a request to the first author if the Geological Surveys of Europe and the International Lead Association – Europe (ILA) agree on a suggested use of the data. Acknowledgements The GEMAS project is a cooperation project of the EuroGeoSurveys Geochemistry Expert Group with a number of outside organisations (e.g., Alterra in The Netherlands, the Norwegian Forest and Landscape Institute, several Ministries of the Environment and University Departments of Geosciences in a number of European countries, CSIRO Land and Water in Adelaide, Australia) and Eurometaux. Sampling was covered by the participating organisations in their home countries, the analytical work was co-financed by the following organisations: Eurometaux, Cobalt Development Institute (CDI), European Copper Institute (ECI), Nickel Institute, Europe, European Precious Metals Federation (EPMF), International Antimony Association (i2a), International Manganese Institute (IMnI), International Molybdenum Association (IMoA), ITRI Ltd. (on behalf of the REACH Tin Metal Consortium), International Zinc Association (IZA), International Lead Association-Europe (ILAEurope), European Borates Association (EBA), the (REACH) Vanadium Consortium (VC) and the (REACH) Selenium and Tellurium Consortium. Important discussions with Arnold Arnoldussen, Peter Englmaier and Friedrich Koller are acknowledged. Lead isotope analyses were carried out as an internal research project at the Geological Survey of Norway. The GEMAS project is managed by the Geological Survey of Norway with financial support from EuroGeoSurveys. The Directors of the European Geological Surveys and the additional participating organisations are thanked for making sampling of almost all of Europe, on a tight time schedule, possible. The authors thank the two reviewers for a thorough and insightful review of the manuscript, resulting in many improvements. References Aitchinson, J., 1986. The Statistical Analysis of Compositional Data. Chapman and Hall, London, UK. Asch, K. (Ed.), 2003. The 1:5 Million International Geological Map of Europe and Adjacent Areas: Development and Implementation of a GIS-enabled Concept; Geologisches Jahrbuch; SA 3. BGR, Hannover, Schweitzerbart (Stuttgart). Ault, W.U., Senechal, R.G., Erlebach, W.E., 1970. Isotopic composition as a natural tracer of lead in the environment. Environ. Sci. Technol. 4, 305–317. Bindler, R., 2011. Contaminated lead environments of man: reviewing the lead isotopic evidence in sediments, peat, and soils for the temporal and spatial patterns of atmospheric lead pollution in Sweden. Environ. Geochem. Health 33, 311–329.
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Rosman, K.J.R., Chisholm, W., Boutron, C.F., Candelone, J.P., Hong, S., 1994. Isotopic evidence to account for changes in the concentration of lead in Greenland snow between 1960 and 1988. Geochim. Cosmochim. Acta 58, 3265–3269. Salminen, R. (Chief-Editor), Batista, M.J., Bidovec, M. Demetriades, A., De Vivo. B., De Vos, W., Duris, M., Gilucis, A., Gregorauskiene, V., Halamic, J., Heitzmann, P., Lima, A., Jordan, G., Klaver, G., Klein, P., Lis, J., Locutura, J., Marsina, K., Mazreku, A., O’Connor, P.J., Olsson, S.Å., Ottesen, R.-T., Petersell, V., Plant, J.A., Reeder, S., Salpeteur, I., Sandström, H., Siewers, U., Steenfelt, A., Tarvainen, T., 2005. Geochemical Atlas of Europe. Part 1 – Background Information, Methodology and Maps. Geological Survey of Finland, Espoo, Finland.