Science of the Total Environment 321 (2004) 105–113
A comparison of lead isotope ratios in the bark pockets and annual rings of two beech trees collected in Derbyshire and South Yorkshire, UK David J. Bellisa,*, Kenichi Satakea, Cameron W. McLeodb a National Institute for Environmental Studies, Onogawa 16-2, Tsukuba, Ibaraki 305-5806, Japan Centre for Analytical Sciences, Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield S3 7HF, UK
b
Received 29 March 2003; received in revised form 28 July 2003; accepted 1 August 2003
Abstract ICP-MS analysis of the bark pockets and annual rings of two beech (Fagus sylvatica L.) trees collected from Longshaw, Derbyshire and Swinton, South Yorkshire in the UK recorded differences in the 206 Pby207 Pb isotope ratio. In the Longshaw sample, the 206Pby207 Pb isotope ratio of the bark pockets (f1914–1998, 78–260 mg gy1 Pb) declined from approximately 1.16 to 1.12, whilst the annual rings (1899–1998, 0.2–2.5 mg gy1 Pb) had a 206Pby 207 Pb ratio of approximately 1.18. In the Swinton sample, the bark pockets (f1919–1998, 7–78 mg gy1 Pb) declined from 1.15 to 1.11 and the annual rings (1899–1998, 0.2–0.5 mg gy1 Pb) from 1.18 to 1.15. The data implied that the bark pockets accumulated lead directly from the atmosphere through wet and dry deposition, whilst the annual rings accumulated lead from the soil via the roots. The bark pockets recorded a relative decline in the accumulation of lead from indigenous sources, such as lead smelting and coal combustion (1.17–1.19), and increase in imported sources such as the smelting of Australian ores (1.04) and leaded petrol usage (1.06–1.09). In contrast, the annual rings at Longshaw recorded ratios typical of indigenous lead, whilst the annual rings in Swinton recorded a relatively small decrease in 206Pby207 Pb reflecting leaded petrol usage. The decline in 206 Pby207 Pb of the bark pockets was consistent with the historical decline in 206 Pby207 Pb of atmospheric lead recorded in peat, lake sediments and archival herbage at other UK locations. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Atmospheric deposition; Dendrochemistry; historical change; Lead isotopes; ICP-MS
1. Introduction Stable lead isotope ratios provide useful tracer of lead emission and accumulation in the environ*Corresponding author. Tel.: q81-29-850-2447; fax: q8129-856-7170. E-mail address:
[email protected] (D.J. Bellis).
ment (Ault et al., 1970). In particular, historical changes in the sources of atmospheric lead deposition may be resolved through the analysis of stable lead isotopes, most typically 206Pby 207Pb, in various natural materials including ice cores, peat bogs, lake sediments and archival herbage collections (Weiss et al., 1999; Farmer et al., 2002).
0048-9697/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2003.08.030
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Similarly, a number of studies have described the measurement of lead isotopes in the annual rings ˚ (Aberg et al., 1999; Watmough et al., 1999; Watmough and Hutchinson, 2002) or bark pockets (Bellis et al., 2002, 2002a) of trees. The principal advantage of trees is their widespread geographical distribution, including urban and industrial locations, rural areas and forests, where the other materials are often absent. There are considerable problems, however, with the interpretation of annual ring data as accumulation occurs principally from the soil and there is strong potential for redistribution processes within the trunk (Lepp, 1975; Cutter and Guyette, 1993; Hagemeyer, 1993; ¨ Hagemeyer and Schafer, 1995; Watmough et al., 1999). In this regard, the use of tree bark pockets (bark located within the tree trunk) is attractive as bark accumulates lead directly from the atmosphere through wet and dry deposition prior to its inclusion by the trunk (Satake et al., 1996). In the UK, there is clear distinction between indigenous sources of lead with relatively high 206 Pby 207Pb ratios, including lead ores, coals and natural lead in soils (1.17–1.19) (Sugden et al., 1993; Farmer et al., 1999), and imported sources of lead with relatively low 206Pby 207Pb ratio, including Australian lead ores (1.04) and leaded petrol (1.06–1.09) (Gulson et al., 1994; Monna et al., 1997; Farmer et al., 2000). Industrial sources, waste incineration and secondary smelting may contribute lead with intermediate 206Pby 207Pb ratios (1.14–1.16) (Monna et al., 1997), as these activities incorporate both indigenous and imported sources. As a result, substantial geographical variation in 206Pby 207Pb has been recorded in aerosols ¨ in the UK (1.101–1.124, Bollhofer et al., 1999). Archival grass in southern England (Bacon et al., 1996), and peat, lake sediments and archival moss samples in Scotland (Farmer et al., 1996, 1997, 2002) have indicated a general decline in 206 Pby 207Pb, from approximately 1.17 in the 1800s to 1.11 to 1.13 in the 1980s, indicating a shift from indigenous to imported lead sources. The onset of the reduction occurred from approximately 1890 in England and approximately 1910 in Scotland, prior to the introduction of leaded gasoline, showing the influence of Australian ore smelting. In the UK as a whole, lead production from UK
ores was exceeded by lead imports from 1870 (Burt, 1984). Leaded petrol usage dominated the decline in 206Pby 207Pb from approximately 1940, due to increases in road traffic. Increases in 206Pby 207 Pb have been observed since approximately 1985 in archival grass and moss (Bacon et al., 1996; Farmer et al., 2002) following reductions in maximum permissible lead concentrations in petrol in 1986 and the subsequent use of lead-free petrol. Leaded petrol was removed from sale in the UK in 2000. 206Pby 207Pb in tree bark collected from different locations in the UK varied from 1.10 to 1.17 (Bellis et al., 2001), however, highlighting the need for more widely available monitors. At present, there has been no direct comparison of lead isotope ratios in the bark pockets and annual rings of the same trees. In this paper, we present data from the ICP-MS analysis of bark pockets and annual rings of two beech trees collected in Derbyshire and South Yorkshire in northern England, UK. Current and historical sources of atmospheric lead include lead mining and smelting in Derbyshire (Burt, 1984), coal mining combustion in South Yorkshire (Hill, 2001), and leaded petrol usage. 2. Experimental Trunk sections of beech (Fagus sylvatica L.) trees containing bark pockets were collected in 1998 from the Longshaw Estate near Hathersage in Derbyshire and near Swinton in South Yorkshire, northern England, UK (Bellis et al., 2002). Radial sections of 1–2 cm in thickness were prepared using band saws and frozen for storage. The bark pocket samples were dated by counting the annual rings, as described by Bellis et al. (2002). The Longshaw sample provided a discontinuous series of bark pockets, enclosed between opposing lobes of the trunk (Fig. 1), dating from approximately 1914 to 1998, which were sampled at intervals of 1 cm. The Swinton sample contained a continuous sequence of bark pockets enclosed at the junction of two branches dating from approximately 1919 to 1998, sampled at intervals of 2 cm (Bellis et al., 2002). The thickness of the bark pocket samples was 1–2 mm. The annual rings were sampled by preparing a 1=1-cm2 strip and
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corrections were determined using a solution of the standard reference material 982 (equal atom lead). Lead concentration measurements were calibrated by analysis of standard lead solutions (SPEX). ‘In situ’ analysis of the Longshaw bark pockets was conducted by laser ablation ICP-MS (LSX2000, Cetac Technologies; HP4500, Agilent Technologies, UK). Two-centimetre thickness radial slices of the bark pockets were prepared into cubes 3=3 cm2 in area. The laser was operated as a continuous (20 Hz) scan travelling at 40 mm sy1, monitoring mass 208 (1 point) for 0.1 s per measurement, as part of a multi-element analysis. The laser was initiated in the annual rings and directed to scan across the bark pocket in the tangential plane, before termination in the annual rings of the opposing flank. 3. Results and discussion 3.1. Overview of
Fig. 1. Trunk section of beech tree collected in Longshaw, Derbyshire, UK containing bark pockets.
separating 5-year increments up to 100 years (1899–1998). One hundred to two-hundred milligrams (dry weight) of the samples were digested by high-purity nitric acid using double vessel Teflon bombs at 140 8C for 4 h (as described by Bellis et al., 2002a). The digest was diluted by high-purity water (Millipore 18V). Identical methods were employed to digest 2cm2 bark samples collected from five trees, and five aliquots of a sample of the surface soil (-5 cm) collected from the sample sites in February 2003. Five 10-mg portions of Derbyshire galena minerals, purchased from the Peak District Mining Museum in Matlock Bath, Derbyshire were also digested. Lead concentration and 206Pby 207Pb were measured by ICP-MS (HP4500 Yokogawa, Japan), monitoring masses 206, 207 and 208 for 1, 1 and 0.5 s, respectively with 10 replicates. Mass bias
206
Pby 207Pb
Lead concentrations and 206Pby 207Pb isotope ratios determined in the samples are given in Tables 1–3. At Longshaw, Derbyshire, the 206Pby 207 Pb isotope ratio of the bark pockets reduced from approximately 1.16 to 1.12 (78–260 mg gy1 Pb), whilst the annual rings recorded a 206Pby 207 Pb ratio of 1.18 (0.2–2.5 mg gy1 Pb). At Swinton, South Yorkshire, the bark pockets declined from 1.15 to 1.11 (7–78 mg gy1 Pb) and the annual rings from 1.18 to 1.15 (0.2–0.5 mg gy1 Pb). Bark collected in 2003 (five samples) recorded a mean 206Pby 207Pb ratio of 1.121"0.002 (10"7 mg gy1 Pb) in Longshaw and 1.113"0.002 (42"22 mg gy1 Pb) in Swinton. The surface soil (five samples) at the two sites recorded mean 206Pby 207Pb ratios of 1.152"0.001 (141"21 mg gy1 Pb) and 1.157"0.002 (80"12 mg gy1 Pb), respectively. Galena minerals from Derbyshire recorded a mean 206Pby 207Pb ratio of 1.180"0.002. 3.2. Comparison of 206Pby 207Pb in the bark pockets and annual rings The bark pockets at both sites had significantly lower 206Pby 207Pb, and substantially higher lead
D.J. Bellis et al. / Science of the Total Environment 321 (2004) 105–113
108 Table 1 Lead concentration and
206
Pby207Pb in the bark pockets of beech in Longshaw, Derbyshire and Swinton, South Yorkshire, UK
Bark pockets yeara
Longshaw Pb mg gy1
206
s
1998 1998–1994 1994–1979 1974–1964 1950–1945 1941–1937 1933–1927 1927–1918 1918–1910
184 183 78 123 85 79 173 260 102
1.122 1.119 1.130 1.146 1.153 1.151 1.154 1.157 1.160
0.002 0.002 0.003 0.003 0.006 0.002 0.003 0.003 0.004
a
Pby 207Pb
Bark pockets yeara
Swinton Pb mg gy1
206
s
1998 1998–86 1986–82 1982–79 1979–73 1973–65 1965–51 1951–43 1943–35 1935–28 1928–22 1922–19 -1919
46 68 58 51 45 74 78 19 16 9 12 8 7
1.113 1.112 1.119 1.121 1.124 1.126 1.132 1.141 1.146 1.143 1.142 1.139 1.153
0.001 0.002 0.003 0.002 0.002 0.002 0.002 0.002 0.002 0.003 0.003 0.004 0.003
Pby 207Pb
Dates refer to the nearest annual rings intersecting the bark pocket, adjusted for the bark thickness (Bellis et al., 2002).
contents, than the annual rings (Fig. 2), implying that the bark pockets accumulated lead from the atmosphere whilst the annual rings accumulated lead from the soil. Bark accumulates lead directly from the atmosphere through wet and dry deposi¨ tion (Hammp and Holl, 1974), principally of Table 2 Lead concentration and Annual rings year 1998–1994 1993–1989 1988–1984 1983–1979 1978–1974 1973–1969 1968–1964 1963–1959 1958–1954 1953–1949 1948–1944 1943–1939 1938–1934 1933–1929 1928–1924 1923–1919 1918–1914 1913–1909 1908–1904 1903–1899
206
Pby207Pb in the annual rings of beech in Longshaw, Derbyshire and Swinton, South Yorkshire, UK
Longshaw Pb mg g 0.5 0.2 0.4 0.4 0.6 1.1 0.9 1.6 2.1 1.9 1.7 1.8 2.1 1.9 2.2 2.5 1.9 1.4 1.5 1.4
particles. Annual rings accumulate lead predominantly from soil (Lepp, 1975), via root uptake of the soil solution. Accumulation is thus restricted to soluble lead, either naturally occurring or deposited from the atmosphere either recently or in past. 206 Pby 207Pb in the bark pockets was lower than
y1
206
Pby
1.179 1.181 1.184 1.188 1.181 1.175 1.175 1.184 1.178 1.183 1.181 1.183 1.181 1.176 1.183 1.184 1.184 1.182 1.184 1.183
207
Pb
s 0.008 0.009 0.009 0.006 0.005 0.006 0.005 0.004 0.004 0.005 0.004 0.007 0.003 0.006 0.003 0.004 0.005 0.005 0.003 0.005
Annual rings year 1998–1994 1993–1989 1988–1984 1983–1979 1978–1974 1973–1969 1968–1964 1963–1959 1958–1954 1953–1949 1948–1944 1943–1939 1938–1934 1933–1929 1928–1924 1923–1919 1918–1914 1913–1909 1908–1904 1903–1899
Swinton Pb mg gy1
206
Pby 207Pb
s
0.4 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.3 0.2 0.3 0.4 0.3 0.3 0.2 0.2 0.2 0.3 0.2
1.156 1.160 1.164 1.148 1.149 1.168 1.163 1.160 1.163 1.169 1.177 1.180 1.181 1.177 1.177 1.179 1.180 1.190 1.179 1.179
0.009 0.007 0.006 0.007 0.005 0.009 0.005 0.008 0.008 0.010 0.006 0.005 0.006 0.006 0.007 0.012 0.010 0.010 0.008 0.009
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Table 3 Lead concentration and 206Pby207 Pb in the bark of beech and soil in Longshaw, Derbyshire and Swinton, South Yorkshire, UK, and in Derbyshire galena minerals Longshaw Pb mg g
y1
Swinton 206
Pby
207
Pb
s
Pb mg gy1
206
Pby 207Pb
s
Bark 2003 1 2 3 4 5 Mean s
5 8 10 4 21 10 7
1.122 1.118 1.121 1.122 1.122 1.121 0.002
0.007 0.009 0.006 0.010 0.006
1 2 3 4 5 Mean s
79 19 35 36 41 42 22
1.113 1.116 1.112 1.112 1.111 1.113 0.002
0.005 0.006 0.002 0.003 0.004
Soil 2003 1 2 3 4 5 Mean s
154 131 146 109 162 141 21
1.153 1.152 1.150 1.151 1.152 1.152 0.001
0.004 0.005 0.004 0.003 0.002
1 2 3 4 5 Mean s
66 72 88 97 77 80 12
1.156 1.159 1.155 1.154 1.157 1.157 0.002
0.004 0.004 0.005 0.004 0.002
1.183 1.180 1.179 1.180 1.179 1.180 0.002
0.004 0.003 0.004 0.004 0.004
Derbyshire galena 1 2 3 4 5 Mean s
n.d. n.d. n.d. n.d. n.d.
n.d., not determined.
typical indigenous 206Pby 207Pb (1.17–1.19), and decreased over time, indicating accumulation of imported lead from Australian ore smelting (1.04) and subsequently leaded petrol usage (1.06–1.09) (Bellis et al., 2002). In contrast, 206Pby 207Pb in the annual rings was typical of indigenous lead, though lower 206Pby 207Pb was observed from approximately 1940 in Swinton indicating some accumulation of imported lead, mostly likely due to leaded petrol usage. Peat bogs near the Longshaw site have indicated that lead deposition in the 18th and 19th Centuries was higher than that in the 20th Century due to lead mining and smelting of local ores (Lee and Tallis, 1973; Livett et al., 1979). Peat near the Swinton site indicated lead deposition prior to the 20th Century from coal mining and combustion (Gilbertson et al., 1997). Our analysis of Derby-
shire galena minerals indicated that local lead mining and smelting would contribute 206Pby 207Pb of 1.18, whilst UK coals have 206Pby 207Pb of 1.17–1.19 (Sugden et al., 1993; Farmer et al., 1999). It is likely that the soils at the two sites were contaminated with indigenous lead prior to the 20th Century. Accumulation of this lead would account for the observed 206Pby 207Pb ratios of the annual rings. The surface soils, however, had 206Pby 207Pb of 1.152"0.001 (141"21 mg gy1 Pb) and 1.157"0.002 (80"12 mg gy1 Pb) in Longshaw and Swinton, respectively, lower than the natural ratio of UK soils and of indigenous pollution sources, indicating atmospheric deposition of imported lead to soil. In the case of Swinton, the surface soil and outermost annual rings had similar ratios. The lack of a decline in 206Pby 207Pb in the
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Pb ratios of the annual rings were not equal to the 206Pby 207Pb of the atmosphere, although the Swinton rings did indicate the atmospheric deposition of imported lead due to leaded petrol usage from approximately 1940. Our conclusion that lead accumulation occurred principally from the soil, and that the 206Pby 207Pb does not equal to 206Pby 207 Pb of the atmosphere, is consistent with that of Watmough and Hutchinson (2002), who recorded a decline in 206Pby 207Pb from 1.17 prior to the 1930s to 1.16 from 1975 to 1985 in sycamore rings in north-east England. It is possible that annual rings may accumulate lead following diffusion through the bark, but we consider this route unlikely, given that lead is essentially a surface deposit on the bark and its concentration decreases exponentially with depth ¨ (Hammp and Holl, 1974; Satake et al., 1996; Schulz et al., 1999). Watmough and Hutchinson (2003) applied solutions containing 207Pb to bark and recorded bark concentrations up to 50 mg gy1 but only 50 ng gy1 in the annual rings, concluding transport through the bark is a relatively minor route compared to soil uptake. Lead in solution is likely to be more mobile than particulate lead. Laser ablation ICP-MS analysis (Fig. 3) showed sharp peaks in lead concentration at the contact point of the two bark surfaces forming the bark pocket. The majority of lead was located within the outer 0.5 mm of the bark (outer bark) with relatively low concentrations in the inner bark. The relative concentration in the annual rings was similar to that in the inner bark. Fig. 2. Comparison of 206Pby207Pb ("s, analytical precision ns10) in the bark pockets (full circles) and annual rings (open circles) of beech in Longshaw, Derbyshire and Swinton, South Yorkshire, UK.
Longshaw annual rings, as observed in Swinton, suggests either that lead was accumulated from deeper in the soil profile, as the more recent deposition of imported lead may be restricted to the surface layers; or that there was transport of lead from older annual rings, which have high lead concentrations, to the younger rings (Table 2). It is clear from the above discussion that the 206Pby
3.3. Historical change of pockets
206
Pby 207Pb of the bark
The decline in 206Pby 207Pb recorded by the bark pockets is consistent with the decline recorded in peat, lake sediments and archival herbage in the UK (Bacon et al., 1996; Farmer et al., 2002). For example, the decadal average of 206Pby 207Pb of archival grass in southern England and moss in Scotland from 1910 to 1919 was 1.161"0.003 and 1.172"0.003, respectively, and declined to a minimum of 1.113"0.018 and 1.120"0.018, respectively, from 1980 to 1989. 206Pby 207Pb in the bark pockets prior to approximately 1940 was
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Fig. 3. Relative distribution of lead within a bark pocket (Longshaw 1944–1948) measured by laser ablation ICP-MS.
lower than typical indigenous lead (1.17–1.19), indicating accumulation of lead resulting from the smelting of Australian ores (1.04). The decline in 206 Pby 207Pb was greatest after approximately 1940, showing the accumulation of lead due to leaded petrol usage (1.06–1.09). 206 Pby 207Pb was lower at Swinton than in Longshaw. The Longshaw site is likely to have a higher contribution of indigenous lead due to continuing mining and smelting, and reworking of ores for phosphorus whilst the Swinton site is closer to urban areas, which could result in a greater contribution of leaded petrol. Comparison of these sites is somewhat limited as only one bark pocket sequence was analysed at each site, but it was notable that 206Pby 207Pb of the soil samples was lower in Longshaw than that in Swinton. Assuming leaded petrol was the dominant source of imported lead from approximately 1940, and by applying mass balance equations using values of 1.08 for leaded petrol (UK mean 1.076"0.011, Farmer et al., 2000) and 1.18 for indigenous lead, it was estimated that the maximum contribution of petrol
lead to the bark was 60% in Longshaw and 70% in Swinton. The archival moss in Scotland (Farmer et al., 2002) increased from approximately 1.120"0.018 in 1985 to 1.151"0.009 in 2000, whilst the bark pocket ratios continued to decline. The bark pocket data during the 1990s is, however, in agreement with the ‘British’ aerosols (1995–1996) reported by Flament et al. (2002) of 1.122"0.038. The ratios are towards the lower end of the ranges of ¨ aerosols in the UK reported by Bollhofer et al. (1999) (1.101–1.124). Beech bark, however, has a slow rate of shedding and grows principally by expansion, and is thus exposed to the atmosphere for a number of years prior to inclusion within trunk. It is possible that the lead isotope ratio is a product of both previous and more recent deposits. Five bark samples collected in February 2003 had a mean 206Pby 207Pb ratio of 1.121"0.002 (10"7 mg gy1 Pb) in Longshaw and 1.113"0.002 (33"9 mg gy1 Pb) in Swinton, almost identical to those recorded in 1998. Given that leaded petrol was removed from sale in the UK on 1st January
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2000, the data indicate that lead is retained by trunk beech bark for at least 2 years and possibly longer, unless there is resuspension of petrol lead. Tree species such as Scots pine (Pinus sylvestris), which sheds bark on a 1–2 year timescale (Schulz et al., 1999), may not be subject to such effects.
(approximately half stem diameter) than the tree sampled in 1998. Bark pockets collected in Japan, however, showed coincident historical trends in arsenic concentration and copper production (Bellis et al., in press), indicating that bark pockets can provide effective records of the level of atmospheric deposition.
3.4. Lead concentrations 4. Conclusions The concentration of lead in the bark pockets was substantially higher than that in the annual rings (Tables 1 and 2). Concentrations in both the bark pockets and the annual rings in Longshaw were higher than that in Swinton, suggesting a higher level of lead pollution. Unlike lead isotope ratios, measured lead concentrations in bark can vary significantly (Table 3). The Longshaw bark pockets recorded high concentrations from approximately 1920 and an overall rise from approximately 1940, but the data were variable. This may in part result from the small size of the discontinuous series of bark pockets and the difficulty incurred in their physical removal. The lead concentration of the annual rings also peaked from approximately 1920, showing an overall decline thereafter. As described above, however, the annual rings did not reflect lead of low 206Pby 207Pb ratio in the atmosphere. The Swinton bark pockets recorded an overall 10-fold increase in lead concentration and a peak from approximately 1960 and the annual rings recorded a 2-fold increase in concentration. The increase in concentration of lead from approximately 1940 in the bark pockets is attributable to leaded petrol increase and the growth in road traffic. There has, however, been a large decrease in lead emission in the UK as a whole in recent decades. There is a possibility from this and the lead isotope ratio data that lead is retained by beech bark over a number of years. In this case, deposition may be cumulative. The lead concentrations recorded in bark at the two sites in 2003 were lower than that in 1998 (10"7 mg gy1 Pb in Longshaw and 42"22 mg gy1 Pb in Swinton), suggesting removal of lead from the bark is occurring, but there can be variation in lead contents between neighbouring trees. In the case of Longshaw, the trees sampled in 2003 were younger
The data showed that the bark pockets accumulated a greater concentration of lead with lower 206 Pby 207Pb than the annual rings, implying that the bark pockets accumulated lead directly from the atmosphere and the annual rings predominantly accumulated lead from the soil. The bark pockets indicated a relative decline in indigenous sources of lead pollution such as lead smelting and coal combustion, and an increase in imported sources such as the smelting of Australian ores and leaded petrol usage. Lead isotopic analysis of tree bark pockets thus appears as a useful indicator of the source of atmospheric lead deposition, but it is necessary to be mindful that slowly shed barks such as beech may retain lead over a considerable period of time. The annual rings were not effective monitors of atmospheric lead at the sample sites, due to accumulation of soil lead. Acknowledgments The authors would like to thank A. Cox and R. Ma at the University of Sheffield, UK, in particular for support of the laser ablation measurements. Also K. Kawakami, T. Noguchi and K. Takada for technical support in Japan. References ˚ Aberg G, Pacyna JM, Stray H, Skjelkvale ˚ BL. The origin of atmospheric lead in Oslo, Norway, studied with the use of isotopic ratios. Atmos Environ 1999;33:3335 –3344. Ault WU, Senechal RG, Erlebach WE. Isotopic composition as a natural tracer of lead in the environment. Environ Sci Technol 1970;4:305 –313. Bacon JR, Jones KC, McGrath SP, Johnstone AE. Isotopic character of lead deposited from the atmosphere at a grassland site in the United Kingdom since 1860. Environ Sci Technol 1996;30:2511 –2518.
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