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Isotopic composition of epiphytic lichens as a tracer of the sources of atmospheric lead emissions in southern Québec, Canada
Geochimica et Cosmochimica Acta, Vol. 59, No. 21, pp.4427-4433, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/95 $9.50 + .oO
Pergamon
0016-7037( 95)00302-9
Isotopic composition of epiphytic lichens as a tracer of the sources of atmospheric emissions in southern Qkbec, Canada JEAN CARIGNAN GEOTOP,
Universitk
and
CLEMENT
lead
GARI~PY
du QuCbec A Monn&l, C.P. 8888, Succ. Centreville, Montrkal H3C 3P8, Canada
(Received
November
22, 1994; accepted
in revisedform
July 24, 1995)
Abstract-Lead isotopic data are reported for epiphytic lichens, vegetation samples, and lacustrine sediments collected in the boreal forest of Quebec between 47” and 55”N, and along the St. Lawrence Valley between 45” to 48”N. Lichens located up to 500 km north of Noranda (48”N) record a significant input of anthropogenic Pb emitted to the atmosphere from smelting activities. This input is not apparent beyond 53”N where only the isotopic signal typical of Canadian aerosols is recorded. Lichens along the St. Lawrence Valley show evidences for a dominant input from U.S. sources. The lead isotopic composition of lichens allow quantitative monitoring of the sources of atmospheric Pb. However, their slow metabolism and their unknown age detract from recording the Pb signal on short and precise timescales. Spruce needles have isotopic compositions undistinguishable from that of lichens; this reflects integration of the atmospheric Pb signal over a comparable time span, a result confirmed by the lead isotopic record in lacustrine sediments. Vegetation samples such as spruce bark, spruce wood, and deciduous tree leaves are more radiogenic than lichens from the same site. This may reflect mixing of radiogenic Pb metabolized from soil solutions through the root system with atmospheric Pb. 1. INTRODUCTION
(D&uelle and Lallemant, 1983). The processes by which epiphytic lichens intercept allogenic atmospheric matter include wet deposition during rainout, washout, snow, fog, dew, and mist events, and dry deposition of particles during inter-storm periods (e.g., Knops et al., 1991). Nutrient, metal, and particle uptake likely occurs across the entire lichen surface because, in contrast to vascular plant leaves, lichens lack cuticules and stomata. The principal accumulation mechanisms are via extracellular complexing of metals by ion-exchange processes, by particle entrapment and by direct uptake of gases (e.g., Galun and Ronen, 1988; Nash, 1990, Knops et al., 1991; Schwartzman et al., 1991). To our knowledge, the composition of stable lead isotopes accumulated in lichen has never been used to assess long-range atmospheric transport of anthropogenic Pb. The isotopic composition of Pb is not measurably fractionated by biological and industrial usage (Ault et al., 1970) and, when the isotopic composition of potential endmembers are different and accurately defined, the sources of atmospheric Pb and their relative contributions can, therefore, be identified. This study reports Pb concentrations and lead isotope data for lichens sampled close to the St. Lawrence Valley between 45” and 48”N (sites #l to 5; Fig. 1) and along a north-south transect between 47” and 55”N (sites #6-13; Fig. 1) in the boreal forest region of the province of Quebet. At three sites (#6, 9, and 11) along the latter transect, other vegetation samples such as spruce needles, spruce bark, deciduous tree leaves, as well as lacustrine sediments were also analyzed for their lead isotopic composition to compare with the lichen data. Variations in the lead isotopic composition of lichens in the St. Lawrence Valley are related to varying contributions from U.S. and Canadian sources. Lichens in the boreal forest record the presence of a significant Pb component emitted from smelting activities
With the advent of the industrial era, large amounts of Pb and other heavy metals have been released to the atmosphere from activities such as metal refining, incineration of refuse and coal, wood, and alkyl-leaded petrol combustion, (e.g., Patterson and Settle, 1987; Nriagu and Pacyna, 1988; and references therein). Records showing increasing additions of Pb to the environment are preserved in lacustrine and marine sediments (e.g., Shirahata et al., 1980; Ouellet and Jones, 1983; V&on et al., 1987; Hamelin et al., 1990; Graney et al., 1995; Gobeil et al., 1995; Lucotte et al., 1995), in corals from the Atlantic and Pacific Oceans (Shen and Boyle, 1987), and in snow and ice of polar regions (e.g., Murozomi et al., 1969; Boutron et al., 1991, 1994; Rosman et al., 1993, 1994). For the past century, lichens have been extensively used as natural indicators of air quality around urban areas and pollution sources (e.g., D&uelle and Lallemant, 1983). The term indicator refers to the ability of an organism to point out the presence/absence and the high/low level of a polluting component of the atmosphere (e.g., Sloof and Wolterbeek, 1991). Where an organism provides quantitative informations about air pollution, the term monitor has been applied. The use of lichens, and particularly of epiphytic lichens, as biomonitors of trace-element air pollution is now widely acknowledged (e.g., Lawrey and Hale, 1988; Rope and Pearson, 1990; Sloof and Wolterbeek, 1991; Lawrey, 1993; and references therein). Lichens around industrial sites, for example, can take up hundreds to thousands pg. g -’ of heavy metals (e.g., Nash, 1990). Epiphytic lichens derive moisture and nutrients exclusively from air and are efficient biomonitors because of their ( 1) perennating photosynthetic activity; (2) slow metabolism; (3) high and nonselective capacity to remove matter from the atmosphere; and (4) lack of powers to fight against pollution 4421
4428
J. Carignan and C. GariCpy
riods where the air stream persisted from the north crossing the Noranda mining and refining area (Fig. l), the aerosols recorded unradiogenic *“‘Pb/*“‘Pb values ( 1.04 to 1.14) well below the Canadian marker value of 1.15. Considering that Noranda is by far the largest single emitter of industrial Pb in Canada and that the abundances of In, As, Pb, Zn, and Se varied with the 2”hPb/2”7Pb ratios (Sturges and Barrie, 1989), a contribution from the northern smelters (2% of the total atmospheric Pb at Dorset in 1984 and 9% in 1986) was deemed likely. 2. MATERIALS AND METHODS
FIG. 1. Location map of the sampled sites. Site numbers refer to Table 1.
in northwestern Quebec, from the point source.
reaching
1.1. Lead Isotopic Composition Emissions
distances
up to 500 km
of Anthropogenic
Atmospheric Pb is a complex mixture of multiple and widespread sources which may considerably vary with time and yield fluctuating isotopic signatures (e.g., Hamelin et al., 1990; Graney et al., 1995). However, direct measurements of aerosols in northeastern North America by Sturges and Barrie ( 1987) showed a clear cut and fairly constant isotopic difference over periods of several months between 1982 and 1986: the 206Pb/207Pb ratio in eastern U.S. averaged 1.213 ? 0.008 (1982-85) and 1.221 + 0.009 in U.S. Midwest (1986), whereas it was 1.15 ? 0.01 (1982-86) in south central Canada (Sturges and Barrie, 1987). The more radiogenic character of the U.S. aerosols is due to the use of Mississippi Valley Pb in additives of gasoline since the mid-1960s. The phasing out of leaded gasoline resulted in a change of the U.S. *osPb/207Pb signatures from 1.21- 1.23 in the mid-1980s to 1.19- 1.20 in the period 1988- 1990, as shown by recent studies of the tropospheric Pb input over the western North Atlantic (V&on et al., 1992, 1993) and measured 206Pb/2”7Pb ratios in U.S. gasoline and aerosols (Rosman et al., 1994). Legal et al. ( 1989) showed that the surface waters of the Great Lakes, collected in August 1987, indeed vary within the marker values determined by Sturges and Barrie ( 1987) for Canadian and American industrial Pb. Similarly, Graney et al. ( 1995) reported 2osPb/207Pb ratios in the range of 1.181.20 for the anthropogenic Pb component accumulated in sediments of some of the Great Lakes since the mid 1970s. Sturges and Barrie (1989) were also able to apportion atmospheric aerosols collected at the rural site of Dorset (Fig. 1) in the fall of 1984 and the spring of 1986 amongst its Canadian and American sources. However, throughout a few pe-
The vegetation samples were collected between the summers of 1990 and of 1994 at sites, in most cases uninhabited lake areas, remote from industries and major or secondary roads. The epiphytic lichens used for analysis were a foliose species, Pam&a suZca& at sites #l-2 and fruticose specimens of Evemia mesomorphaand/or (Isnea spp. at sites #3- 13. The lichens were collected on evergreen and/or birch trees, and were all taken near the tip of tree branches less than a few millimeters in diameter. This was done to obtain lichens that were exposed to the atmospheric signal for a time period as short as possible. In the case of spruce or fir trees one could clearly determine, from the lichen position relative to the annual shoots, that the samples were < 1 to 3 years old, at the most. Samples were taken with pre-cleaned gloves and plastic tools where necessary, and transferred to cleaned plastic vials or sealed-plastic bags. In the laboratory, the samples were immediately freeze-dried and stored in hermetic vessels. Lichens, spruce needles, and tree leaves were separated from their substratum using nylon tweezers. Between 10 and 100 mg of vegetation samples were directly transferred to Teflon capsules and digested in nqua regia. Lead separation was done using anion-exchange chromatography (Manhhs et al., 1980). Organic-rich sediments were also digested in aqua regia and any residual detrital material was discarded. Lead concentrations were determined by isotope dilution on a solid aliquot of the samples, using a”“Pb spike calibrated against the NIST SRM 981 Pb standard. All isotopic measurements were made using Faraday cage detectors operated either in multi-collection or single peal-switching modes. A mass fractionation correction of 0.09% amu -’ was applied to all ratios. Repeated measurements of the NIST SRM 981 Pb standard yielded uncertainties ( 2omran) 1ess than 0.1, 0.2, 0.3, and 0.4% for the *06Pbl’07Pb, *NPb/2WPb, z”7Pb/z”Pb, and 2osPb/2MPb ratios, respectively. Total Pb blanks during this study were between 50 and 100 pg and negligible.
3. RESULTS
AND DISCUSSION
The Pb concentration and isotopic composition of all lichens analyzed are listed in Table 1. At site #3 two different samples of the lichen Evemia mesomorpha collected a few hundred meters from each other yielded identical lead isotopic compositions, within errors. Such was the case at site #5, except that Usnea spp. yielded a Pb content of 7.1 ppm that is significantly higher than that of 2.7 measured in Evemia m. These
two lichens
have
a very similar
habitat,
hanging
from
in their efficiency of trapping atmospheric Pb. However at sites #6 and 11, specimens of Evemia m. collected several hundreds of meters apart yielded variations in their *06Pb/*04Pb and 206Pb/207Pb ratios that are larger than typical reproducibility (Table 1) This may indicate that the specimens differ in age and did not monitor the very same atmospheric signal. This may also reflect the presence of distinct sources of aerosol Pb that were not bio-accumulated in the same proportions because of differences in lichen exposure, tree cover, etc. Nevtree branches,
and it may
indicate
that they
differ
Detection Table I. lsotooiccomoositio"
2"8& ""Pb
"*pb ""Pb
I-Lx Brome Y4-Ps-I IX.554
15.h53
3X.22Y
l.lX5
IX.55
2-Rougemont 93.Ps-I IX.475
15.h27
3X.104
l.lX2
2X.74
3-LliSigllall %‘-Em-l RI-Em-2
15.h54 15.h31
3X.2YX 3X.235
l.lX7 I.lXX
".d ".d.
15.606
3X.125
l.IXO
2.13
IX.423 IX.3Yh
15.587 15.hO7
3x.03') 3x.112
1.1x2 1.17')
2.70 7.10
O-Lx Lapurte I)O-EI~-I lh.Xlh WE"-2 lh.h25 WE"-3 16.YI0
15.255 15.212 15.22X
3h.5h3 3h.412 3h.535
I.102 I.093 I.110
".d ".d. ".d.
7-Lamothe Y4-Em-l
15.h36
14.Y70
35.3X0
l.04h
33.21
X-Matagami Y4-U-l
17.lhY
15.302
3h.YlX
I.122
4.13
9-Evans Y4-Em-l
'7.570
15.410
37.291
I.140
h.X5
IO-Opinaca Y4-Em-l
17.5Xx
15.4Oh
37.273
I.142
IO.14
2'fiPb Ll!4%
lX.57Y IX.574
4-Rivikre
Creche
Y4-U-l
IX.407
S-Lx
[Pb] ppm'
Decoiene
Y4-Em-l Y4-U-l
I I-L(:-2 'A-Em-1 17.7Xx 17.x07 WE"-2 ')(I-Em-3 17.h53
15.440 15.443 15.407
37.4Xh 37.450 37.304
I.152 I.153 I.145
".d. ".d. ".d.
IZ-Duncan Y4-Em-l 17.77X
15.45X
37.517
I.150
4.07
1%Koury W-Em-I
15.442
37.411
I.147
l.YY
17.713
Q
0
boreal
the intra-site
forest
inter-site differences
variations
at these two localities
4(X'
from Noranda
hot'
xcn,
(km)
FIG. 2. (a) Plot of the zosPb/z”7Pb and (b) the ‘“Pb/zWPb of lichens from sites #6- 13 as a function of the distance from Noranda, measured as the crow flies. Also shown for reference are the isotopic compositions of Canadian and U.S. aerosols for the period 1984-86 (Sturges and Bark, 1987) and fields representative of Cu sulfides (Deloule et al., 1989) and galena (Franklin et al., 1983) from ore deposits and showings within the late Archean Abitibi greenstone belt.
in the
lo-20 times smaller than the 1), which must reflect regional in the sources of atmospheric Pb. of Qukbec
variability
200
distance
1 Sample number prefixindicatesyearofcollection. Em = EvetnirI m~.somorphr~: Ps = Pmmliu sulcatu; u = u.\,wu spp. I.2. 3.... = differentsamples from the same site. 2 Concentrations ilreexpressed on 4 dry-weight basis. n.d.= not determined.
ertheless,
4429
Pb in lichens 1.25
of lichens.
""Pb ",MPb -
Site# S;unple'
of anthropogenic
remain
(Table
3.1. The 47-55”N Transect The lichens from sites #6- 13 have Pb contents in the range of 2-33 ppm (Table 1), with the highest value found closest to the town of Noranda, the site of a large Cu smelter operation, and the lowest values furthest north of it. The lichen from the site closest to Noranda (Lamothe, #7) has very unradiogenie lead isotopic compositions: 206Pb/2”Pb = 15 .64., 208Pb/ 204Pb = 33.2; 206Pb/207Pb = 1.046 (Table 1). The latter is much lower than the Canadian and U.S. marker values for aerosol Pb, and can only represent Pb derived from a Precambrian reservoir with low U/Pb and Th/Pb ratios. Figure 2a shows the 2MPb/zo7Pb ratios of all lichens from site #6 to 13 plotted against their distance from the town of Noranda. Also shown on Fig. 2a are the marker values for Canadian and U.S. aerosol Pb for the period 1984-1986 and the range of 207Pb/206Pb values determined for copper sulfides, mainly chalcopyrite, from a variety of localities situated within the late Archean Abitibi greenstone belt of the Superior Province. The Abitibi Cu sulfides are slightly more radiogenic than galena ores, which is interpreted to reflect in situ growth of some radiogenic Pb from the decay of U and Th either incorporated in the sulfides or present as micro-inclusions
within them (Deloule et al., 1989). Nevertheless, the bulk U/ Pb and Th/Pb ratios of the Cu sulfides must have remained very low to explain their unradiogenic signatures compared to that of common rock-forming silicates. The 2osPb/207Pb value reported for a sample of Noranda smelter dust (Sturges and Barrie, 1989) is within the range shown for Cu sulfides in Fig. 2a. Figure 2a shows that sites closer to Noranda, whether located south (#6) or north (#7-10) of it, have 2MPb/207Pb values much lower than that of Canadian aerosols. The iso-
FIG. 3. Plot of 20*Pb/zobPbvs. zo6Pb/*07Pbshowing the compositions of lichens from sites #l-S, located in the southern St. Lawrence Valley of Quebec. The field for Canadian atmospheric Pb was drawn using the values calculated in text, whereas that of U.S. aerosols is from Rosman et al. (1994).
J. Carignan and C. Garitpy
4430
topic composition of lichens increases away from Noranda to reach values undistinguishable from that of Canadian aerosols at sites #I 1 - 13 which are located more than 600 km from the smelter. The mean 2”6Pb/207Pb of the five lichen samples from sites #l l-13 is 1.150 + 0.003 (20,,,,) and there are no evidence for any input of more radiogenic U.S. Pb. Lichens have high Pb contents (Table 1) and, in contrast to ice or aerosol samples, their full isotopic spectrum can be determined very accurately, with negligible blank corrections. All measured Pb ratios in lichens (Table 1) from sites #6- 13 display identical trends to that of Fig. 2a. An example is given in Fig. 2b using the 2”8Pb/2MPb values: lichens at sites closer to Noranda record the presence of an Archean Pb component and the values level off at distance greater than 600 km from Noranda. If our interpretation that lichens from sites #I I - 13 only trapped average Canadian aerosol Pb is correct, mean marker values for the full lead isotopic spectrum can be derived from the five analyzed lichen samples at these sites. These values are: 2osPb/204Pb = 17.75 & 0.06,
higher and lower than the mean value determined above for average Canadian aerosol Pb. Actually, lichens from these sites plot closer to values determined for U.S. aerosols over the past decade (Fig. 3), suggesting that they accumulated Pb derived for the most part (-60%) from U.S. sources. This situation is analogous to the observations made in 1984 and 1986 at Dorset (Fig. 1) by Sturges and Barrie (1989), albeit direct measurements of aerosols at this locality displayed: ( 1) important seasonal isotopic variations, which depend on the prevailing wind trajectories; (2) a small, but significant contribution from smelting activities; and (3) a lower contribution from eastern U.S. sources. Lichens can only yield a signal averaged over their life span and the results obtained for sites #l-5 do not preclude some input from smelting activities in the northwestern Quebec, in which case the contribution of 60% from U.S. sources must be regarded as a minimum value. In any case, the lead isotopic results obtained on lichens confirm the suggestion of Ouellet and Jones ( 1983) that the Midwest-Great Lakes region is a major source of aerosol Pb deposited along the corridor of the St. Lawrence Valley in southem Quebec.
207Pb/204Pb = 15.44 t 0.02, 208Pb/204Pb = 37.43 t- 0.08, 206Pb/2”7Pb = 1.150 ? 0.003, 208Pb/2ffiPb = 2.109 t 0.004. A mixing calculation using for endmembers the mean composition of Canadian aerosol Pb and the compositional range of Abitibi Cu sulfides indicates that 50-90% of the atmospheric Pb at site #7, the closest to Noranda, may derive from smelting activities. This percentage is in the range of 50 to 5% for sites #6 to #8 and <2% for sites #I9 and #lo. These figures depend on which values are chosen for the isotopic composition of the smelter endmember. However, noting that Noranda is a custom smelter (Sturges and Barrie, 1989), any feedstock from Proterozoic or Phanerozoic Cu deposits should increase the figures calculated above. If this is the case, results from site #7 place an upper limit of - 1.05 for the mean 2osPb/2”7Pb value of the Noranda emission over the past few years. 3.2. The St. Lawrence Valley Ouellet and Jones ( 1983) suggested that the Midwest-Great Lakes region is one of the major possible sources for the origin of the high heavy metal contents of several remote lakes located in the southern part of the Precambrian Shield, north of the St. Lawrence River. This is because southern Quebec is generally crossed by cyclonic disturbances that move northeastward along the St. Lawrence Valley in summer just as well as in winter (Proulx et al., 1987). These disturbances are submitted to orographic phenomena when they encounter some of the higher topographic features of the Precambrian Shield, giving rise to elevated precipitations (Ouellet and Jones, 1983). Figure 3 shows the lead isotopic results for lichens from five sites located close to the St. Lawrence Valley (#I -2) and in the southern part of the Precambrian Shield (#3-5). These lichens have 2ohPb/2D7Pb and 20xPb1206Pb ratios respectively
3.3. Tree Samples and Lake Sediments At the sites of Lac Laporte (#6, Fig. 1) and LG-2 (#1 1), the lead isotopic composition of several types of vegetation samples such as black spruce needles, spruce bark, leaves of birch, and alder trees have been analyzed for comparison with the lichen results. Surface sediments from the deepest part of remote lakes at the Lac Laporte (#6), Evans (#/9), and LG-2 (#l 1) sites were also analyzed. These consisted of the topmost, -2 cm thick layer which represents lo-20 years of accumulation (Lucotte et al., 1995). The data are listed in Table 2 and shown in conventional Pb-Pb diagrams in Fig. 4. The lead isotopic data for Lac Laporte, the site that was studied in greater detail, display several features (Fig. 4a). Each category of vegetation samples yielded very consistent results. Lichens and spruce needles have similar unradiogenic lead isotopic compositions. The spruce bark samples are more radiogenic but their mean 2”6Pb/2”7Pb ( 1.133 -C 0.006) remains below that of Canadian aerosols, in contrast to that of deciduous tree leaves ( 1.16) and surface sediments ( 1.18). A bulk sample of spruce wood, representing several years of growth, yielded the most radiogenic composition (Fig. 4a). All results for Lac Laporte are well correlated in both *“‘Pb/ *O“Pbvs. 2”Pb/2”Pb ( r2 = 0.987; Fig. 4a) and 2”xPb/204Pb vs. 2osPb/2”4Pb (r* = 0.995; Fig. 4c) diagrams. The data for the LG-2 site display features similar to those of Lac Laporte: consistent results for each category of samples; comparable lichen and spruce needle data; more radiogenie spruce barks; very radiogenic surface sediments; and well correlated linear trends in Figs. 4b ( r2 = 0.960) and 4c (T’ = 0.975) which overlap those of Lac Laporte. The pathways for the accumulation of heavy metals in trees are diverse (e.g., Robitaille, 1981). Atmospheric heavy metals are directly deposited on both leaves and bark via wet and dry precipitations which also reach the soil and mix with ambient soil solutions. According to Robitaille ( 198 I), the greatest contribution of heavy metals to the xylem (the woody element) may come from the root system. Leaf and bark met-
Detection
of anthropogenic
Table 2. lsotoptc composition of needles. bark. wood. leave\ and sediments.
C-Lac Lapurte ‘HI-SN-I lh.717 YO-SN-2 I h.Y 15 YO-SB-I 17.371 YO-SB-2 17.52X Y0-SB-3 17.430 YO-SW-1*1X.63 Yo-AL-I 17.YX2 ‘HI-AL-2 lX.03h Yil-AL-3 lX.Oh3 YCBL-I IX.135 YCBL-2 17.Y5Y YCBL-3 lX.OXI YO-SS-I 1X.372 YO-C-2 1X.3X3 ‘)-Evans IX.79 YO-ss* Si(I-DS. 22.3X I I-L(;-2 YO-SN-I 17.550 ‘)(I-SN-2 17.515 YO-SN-3 17.76X YO-SN-4 l7.4Xh YO-SN-5 17.hO5 YO-SB-I IX.037 Yll-SB-2 17.Y72 VII-SB-3 IX.23Y YO-SS-I lY.001 ‘~(I-SS-2 lY.02h
IS.213 15.253 15.363 l5.4lh 15.410 15.67 15.468 15.495 15.4Xx 15.535 15.51X 15.531 15.552 15.557
30.459 37.04’) 37.255 37.201 3x.37 37.724 37.74X 37.X4X 3X.(127 37.h.12 37.760 3X.055 3X.(lhl
I .OYU l.lOY I.131 I.137 I.131 I.lY l.lh2 I.164 l.lhh l.lh7 I.157 l.lh4 l.IXl l.lX2
15.64 16.24
3x.53 42.32
I .20 1.3X
l5.42Y 15.372 15.425 15.353 15.473 15.52X 15.477 15.5hh lS.hhY 15.h7l
37.31X 37.171 37.3x4 37.087
1.13x 1.13’) I.152 I.139 1.13x l.lhl I.160 I.173 I.213 I.214
3h.hhl
37.h35
37.771 37.h4h 3X.027 3X.hl4 3X.h5Y
Pb in lichens
4431
At the two sites where they were analyzed, all spruce needle samples yielded lead isotopic compositions comparable to those of lichens (Fig. 4) implying that the needles essentially derived their Pb from the atmosphere. This is consistent with the results of Nilsson ( 1972) who reported a net increase of Pb in aging spruce needles. The life expectancy of a spruce needle may be greater than 15 years, however air pollution, combined with environmental particularly SO2 burden, stresses such as frost and frost dessication accelerate needle death (e.g., Pfanz et al., 1994). The analyzed needles were populations of mixed ages taken from small spruce branches in the exterior part of the trees. Thus they included needles with ages between less than one year and potentially up to 15 years. However, the probability for older needles to remain on the tree branches is smaller than for young ones, due to normal ageing, environmental stresses, and pollution. In addition spruce needles may export some of their Pb to the phloem (Robitaille, 198 1) but keep accumulating heavy metals as they age (Nilsson, 1972), which indicates that the export flux must be smaller than the overall accumulation flux. The combination of needle loss with ageing, Pb export from
1
’ Sample number prefix indicates year of collection. I, 2, 3,... = different samples from the same site. SN: spruce needle; SB: spruce bark; SW: spruce wood; AL: alder leaf; BL: birLh leaf; SS: wrface sediment (O-2 cm): DS: pre-indusuial sediment; * = mean compositions.
A
15.7 :
Laporte Lake
??
15.6 [_ 15.5 15.4
als may be exported via the phloem into the xylem, and some of the xylem metal may be translocated to the bark (Robitaille, 1981). Such pathways of accumulation would easily explain the distribution of the data points in Fig. 4 at both sites, reflecting simple mixture of only two endmembers consisting of anthropogenic Pb and a natural, radiogenic component from the soil. Table 2 lists the average lead isotopic compositions of surface (O-2 cm) and pre-industrial ( > 10 cm depth) sediments from core 90-27 collected at the Evans site (#9). These sediments have mean Pb contents of 50 and 12 ppm, respectively (Chaire de Recherche en Environnement, 1992). Assuming that the difference, 38 ppm, is the contribution of atmospheric Pb to the surface sediments, a mixing calculation indicates that the atmospheric endmember should have *06Pb/2”Pb, 208Pb/2@‘Pb,and 206Pb/207Pb ratios of 17.66, 37.33, and 1.143, respectively. This is very close to the composition of a lichen at this site (*06Pb/204Pb = 17.57; *“*Pb/204Pb = 37.29; and 2osPb/207Pb = 1.140) showing that lichen and surface sediments record comparable atmospheric Pb signal. As is the case at Lac Laporte and LG-2, the surface sediments of Evans Lake have a bulk 206Pb/207Pb composition ( 1.20) nowhere near that of the atmospheric component. This is because the sediments in the deepest part of many lakes in the Canadian Shield are largely made up of organic matter with little detrital components ( Lucotte et al., 1995 ) . The very radiogenic composition of this organic component, with 206Pb/2WPb ratios as high as 22.4, likely reflects a predominant input of labile radiogenic Pb preferentially derived from soil organic matter and/or rock dissolution (e.g., Graney et al., 1995).
0
sediments
15.2 c-
15.7
B
LG.2
lh.5
17.0
15.6 a” $
15.5
“3 a
15.4
E
15.3
. . . c3 .. 0:
0
15.2 3Y.O 3x.5 B e “. % w
f
3x.0 37.5 37.0 36.5 3h.U
I
L
17.5
IX.0
IX.5
IV.0
IY.5
2t16pb,204pb FIG. 4. *07Pb/2WPbvs. 206Pb/*wPbdiagrams showing the Pb-isotopic results for (a) lichens, spruce needles, spruce barks, spruce wood, birch, and alder leaves and surface sediments at the Lac Laporte site (47”N) and for (b) lichens, spruce needles, spruce barks, and surface sediments at the LG-2 site (53”N); (c) ‘08Pb/zWPb vs. *06Pb/2”Pb diagram showing all results obtained at the Lac Laporte (open symbols) and LG-2 (filled symbols) sites.
J. Carignan and C. Gariepy
4432
the needles and yearly accumulation of new Pb imply that the mean age of the isotopic signal recorded by spruce needles is biased towards the first half of their life cycle and is likely less than seven years. Lichens must thus reflect the mean isotopic composition of atmospheric Pb integrated over a time span comparable to that of the needles, except if the composition of atmospheric Pb did not change significantly at these sites over a longer period of time. Quite clearly, one should sample needles from the youngest twigs to verify if the mean lead isotopic signals of lichens and needles remain comparable on a shorter timescale. Spruce barks have been exposed to Pb fallouts for a longer duration than lichens and their more radiogenic signatures could indicate that changes in the composition of atmospheric Pb have occurred through time. This cannot be completely ruled out but the data for barks and wood are consistent with the pathways for heavy metal transportation in trees (e.g., Robitaille, 1981) Although the soils are likely contaminated by the atmospheric precipitations, they likely still contain a significant component of natural radiogenic Pb as inferred from the composition of sediments at sites #6 and 11. The very radiogenic signature of spruce wood from site #6 (Fig. 4a,c) is indeed compatible with the view that the greatest contribution of Pb to the xylem is from the root system (Robitaille, 198 1) . Similarly, if a small proportion of the natural Pb is translocated from the xylem to the bark (Robitaille, 1981) and mixed with atmospheric Pb, the more radiogenic signatures of barks, compared to lichens, are readily explained. In contrast to spruce needles, the leaves of alder and birch trees collected at site #6 are very radiogenic which may be interpreted in several ways that will require further studies to be fully understood. This may, for example, reflect that the isotopic composition of atmospheric Pb has changed recently, which can be verified through comparison with spruce needles growing over the same time span. Alternatively, it may show that Pb aerosols from the Noranda smelter are barely available during the season when the leaves were present on the trees, due for example to prevailing northward wind trajectories in the summer months. Comparing the isotopic composition of Pb accumulated in snow and deciduous tree leaves from the same sites shall help elucidate potential seasonal variations. A third possibility may be that, in contrast to spruce needles, the shape of alder and birch leaves makes them inefficient air filters and/or that these tree species contain significant proportions of metabolic Pb derived from soil solutions; this can be unraveled through determinations of the Pb content and the isotopic compositions of leaves and sap. 4. CONCLUSIONS Epiphytic lichens located up to 500 km north of Noranda record a significant input of anthropogenic Pb emitted to the atmosphere from smelting activities. The Noranda input is not apparent north of 53”N where lichens only record the typical Canadian aerosol Pb signal. Lichens collected along the St. Lawrence Valley between 45” and 48”N show isotopic evidences for a dominant Pb input from U.S. sources. Lichens are highly efficient biomonitors allowing, when the isotopic composition of the atmospheric Pb sources are known, quan-
titative assessments of the various inputs. However, their slow metabolism and their unknown age detract from recording the Pb atmospheric signal on short and precise timescales. At two sites in the boreal forest region of Quebec, spruce needles have isotopic compositions undistinguishable from that of lichens. This reflects integration of the mean atmospheric Pb signal over a comparable time span, a result that is confirmed by the lead isotopic record in lacustrine sediments. Deciduous tree leaves are significantly more radiogenic than lichens which may reflect seasonal variations of the Pb signal and/or significant inputs of Pb metabolized from soil solutions through the root system. The latter scenario is supported by the very radiogenic composition of spruce wood. Acknowledgmenrs-Supported
by grants from the Atmospheric Environment Service of Environment Canada, Hydro-Qu&ec and NSERC-Canada. R. Canuel, B. Caron, P. Ferland, C. Hillaire-Marcel, C. Innocent, R. Loupiac, M. Lucotte, and F. Seimbille are warmly thanked for their technical assistance, comments, discussions, and scientific inputs. Helpful comments were provided by A. V&on and two anonymous reviewers.
Editorial handling: K. Mezger
REFERENCES Ault W. A., Senechal R. G., and Erlebach W. E. (1970) Isotopic composition as a natural tracer of lead in the environment. Environ. Sci. Tech. 4, 305-313. Boutron C. F., Gorlach U., Candelone J. P., Bolshov M. A., and Delmas R. J. ( 199 1) Decrease in anthropogenic lead, cadmium and zinc in Greenland snows since the late 1970s. Nature 353, 153-156. Boutron C. F., Candelone J. P., and Hong S. ( 1994) Past and recent changes in the large-scale tropospheric cycles of lead and other heavy metals as documented in Antarctic and Greenland snow and ice: A review. Geochim. Cosmochim. Acta S&3217-3225. Chaire de Recherche en Environnement ( 1992) Sources et devenir du mercure dam les reservoirs hydrotlectriques. Annual Report 1991-1992, UQAM, 3-3-6. Deloule E., Gariepy C., and Dupre B. (1989) Metallogenesis of the Abitibi greenstone belt of Canada: a contribution from the analysis of trace lead in sulfide minerals. Canadian J. Earth Sci. 26,25292540. Dtruelle S. and Lallemant R. ( 1983) Les lichens &wins de la pollution. Librairie Vuibext. Flegal A. R., Nriagu J. O., Niemeyer S., and Coales K. H. (1989) Isotopic tracers of lead contamination in the Great Lakes. Nature 339,455-458. Franklin J. M., Roscoe S. M., Loveridge W. D., and Sangster D. F. ( 1983) Lead isotope studies in Southern and Superior provinces: Canada. Geol. Surv. Canada Pap. 351. Galun M. and Ronen R. ( 1988) Interaction of lichens and pollutants. In Handbook oflichenology, Vol. III (ed. M. Galun), pp. 55-72, CRC Press. Gobeil C., Johnson W. K., Macdonald R. W., and Wong C. S. ( 1995) Sources and burden of lead in St. Lawrence estuary sediments: isotopic evidence. Environ. Sci. Techn. 28, 193-201. Graney J. R., Halliday A. N., Keeler G. J., Nriagu J. O., Robbins J. A., and Norton S. A. ( 1995) Isotopic record of lead pollution in lake sediments from the northeastern United States. Geochim. Cosmochim. Acta 59, 1715-1728. Hamelin B., Grousset F., and Sholkovitz E. R. (1990) Pb isotopes in surficial pelagic sediments from the North Atlantic. Geochim. Cosmochim. Acta 54, 37-47. Knops J. M. H., Nash T. H., Boucher V. L., and Schlesinger W. H. ( 1991) Mineral cycling and epiphytic lichens: implications at the ecosystem level. Lichenologist 23, 309-321.
Detection
of anthropogenic
Lawrey J. D. ( 1993) Lichens as monitors of pollutant elements at permanent sites in Maryland and Virginia. Bryologist 96, 339341. Lawrey J. D. and Hale M. E. ( 1988) Lichen evidence for changes in atmospheric pollution in Shenandoah National Park, Virginia. Bryologist 91, 21-23. Lucotte M., Mucci A., Hillaire-Marcel C., Pichet P., and Grondin A. ( 1995) Global anthropogenic mercury contamination in remote lakes of Northern Quebec (Canada). Water Air Soil Poll. 80,467476. Murozomi M., Chow T. J., and Patterson C. C. (1969) Chemical concentrations of pollutant lead aerosols, terrestrial dusts and seasalts in Greenland and Antarctic snow strata. Geochim. Cosmochim. Acta 33, 1247-1294. Nash T. H., III ( 1990) Metal tolerance in Lichens. In Heavy metal tolerance in Plants: Evolutionary aspects (ed. A. J. Shaw), pp. 119- 152. CRC Press. Nilsson I. ( 1972) Accumulation of metals in spruce needles and needle litter. Gikos 23, 132-136. Nriagu J. 0. and Pacyna J. M. ( 1988) Quantitative assessment of worldwide contamination of air, water and soils by trace metals. Nature 333, 134- 139. Ouellet M., and Jones H. G. ( 1983) Paleolimnological evidence for the long-range transport of acidic pollutants and heavy metals into the Province of Quebec, eastern Canada. Canadian J. Earth Sci. 20,23-36. Patterson C. C. and Settle D. M. ( 1987) Review of data on eolian fluxes of industrial and natural lead to the lands and seas in remote region on a global scale. Mar. Chem. 22, 137- 162. Pfanz H. et al. ( 1994) Life expectancy of spruce needles under extremely high air pollution stress: performance of trees in the Ore Mountains. Trees 8, 213-222. Proubi H., Jacques G., Lamothe A. M., and Lytynski J. (1987) Climatologie du Quebec M&idional. kfinist. Environ. Quebec Rept. 65. Robitaille G. ( 1981) Heavy-metal accumulation in the annual rings of balsam fir Abies Balsamea (L.) Environ. Poll. Ser. B 2, 193-202. Rope S. K. and Pearson L. C. ( 1990) Lichens as air pollution biomonitors in a semiarid environment in Idaho. Bryologist 93,50-61.
Pb in lichens
4433
Rosman K. J. R., Chilsom W., Boutron C. F., Candelone J. P., and Gorlach U. (1993) Isotopic evidence for the source of lead in Greenland snows since the late 1960s. Nature 362, 333-335. Rosman K. J. R.. Chilsom W., Boutron C. F.. Candelone J. P.. and 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. Schwartzman D. W., Stieff L., Kasim M., Kombe E., Aung S., Atekwana E., Johnson J., and Schwartzman K. ( 199 1) An ion-exchange model of lead-2 10 and lead uptake in a foliose lichen: application to quantitative monitoring of airborne lead fallout. Sci. Total. Environ. 100, 319-336. Shen C. T. and Boyle E. A. (1987) Lead in corals: reconstruction of historical fluxes to the surface ocean. Earth Planet. Sci. I&t. 82, 289-304. Shirahata H., Elias R. W., and Patterson C. C. ( 1980) Chronological variations in concentrations and isotopic compositions of anthropogenic atmospheric lead in sediments of a remote subalpine pond. Geochim. Cosmochim. Acta 44, 149- 162. Sloof J. E. and Wolterbeek H. T. ( 1991) National trace-element air pollution monitoring survey using epiphytic lichens. Lichenologist 23, 139-165. Sturges W. T. and Barrie L. A. (1987) Lead 206/207 isotope ratios in the atmosphere of North America as tracer of U.S. and Canadian emissions. Nature 329, 144 146. Sturges W. T. and Barrie L. A. ( 1989) The use of stable lead 206/ 207 isotope ratios and elemental composition to discriminate the origins of lead in aerosols at a rural site in Eastern Canada. Atmos. Environ. 28, 1645- 1657. V&on A., Lambert C. E., Isley I., Linet P., and Grousset F. ( 1987) Evidence of recent lead pollution in deep north-east Atlantic sediments. Nature 326, 278-281. V&on A., Church T. M., Patterson C. C., Erel Y., and Merrill J. T. ( 1992) Continental origin and industrial sources of trace metals in the northwest Atlantic troposphere. J. Atmos. Chem. 14,339-35 1. V&on A., Church T. M., Flegal A. R., Patterson C. C., and Erel Y. (1993) Response of lead cycling in the surface Sargasso Sea to changes in tropospheric input. J. Geophys. Res. 98, 18,26918,276.
Pergamon
0016-7037( 95)00302-9
Isotopic composition of epiphytic lichens as a tracer of the sources of atmospheric emissions in southern Qkbec, Canada JEAN CARIGNAN GEOTOP,
Universitk
and
CLEMENT
lead
GARI~PY
du QuCbec A Monn&l, C.P. 8888, Succ. Centreville, Montrkal H3C 3P8, Canada
(Received
November
22, 1994; accepted
in revisedform
July 24, 1995)
Abstract-Lead isotopic data are reported for epiphytic lichens, vegetation samples, and lacustrine sediments collected in the boreal forest of Quebec between 47” and 55”N, and along the St. Lawrence Valley between 45” to 48”N. Lichens located up to 500 km north of Noranda (48”N) record a significant input of anthropogenic Pb emitted to the atmosphere from smelting activities. This input is not apparent beyond 53”N where only the isotopic signal typical of Canadian aerosols is recorded. Lichens along the St. Lawrence Valley show evidences for a dominant input from U.S. sources. The lead isotopic composition of lichens allow quantitative monitoring of the sources of atmospheric Pb. However, their slow metabolism and their unknown age detract from recording the Pb signal on short and precise timescales. Spruce needles have isotopic compositions undistinguishable from that of lichens; this reflects integration of the atmospheric Pb signal over a comparable time span, a result confirmed by the lead isotopic record in lacustrine sediments. Vegetation samples such as spruce bark, spruce wood, and deciduous tree leaves are more radiogenic than lichens from the same site. This may reflect mixing of radiogenic Pb metabolized from soil solutions through the root system with atmospheric Pb. 1. INTRODUCTION
(D&uelle and Lallemant, 1983). The processes by which epiphytic lichens intercept allogenic atmospheric matter include wet deposition during rainout, washout, snow, fog, dew, and mist events, and dry deposition of particles during inter-storm periods (e.g., Knops et al., 1991). Nutrient, metal, and particle uptake likely occurs across the entire lichen surface because, in contrast to vascular plant leaves, lichens lack cuticules and stomata. The principal accumulation mechanisms are via extracellular complexing of metals by ion-exchange processes, by particle entrapment and by direct uptake of gases (e.g., Galun and Ronen, 1988; Nash, 1990, Knops et al., 1991; Schwartzman et al., 1991). To our knowledge, the composition of stable lead isotopes accumulated in lichen has never been used to assess long-range atmospheric transport of anthropogenic Pb. The isotopic composition of Pb is not measurably fractionated by biological and industrial usage (Ault et al., 1970) and, when the isotopic composition of potential endmembers are different and accurately defined, the sources of atmospheric Pb and their relative contributions can, therefore, be identified. This study reports Pb concentrations and lead isotope data for lichens sampled close to the St. Lawrence Valley between 45” and 48”N (sites #l to 5; Fig. 1) and along a north-south transect between 47” and 55”N (sites #6-13; Fig. 1) in the boreal forest region of the province of Quebet. At three sites (#6, 9, and 11) along the latter transect, other vegetation samples such as spruce needles, spruce bark, deciduous tree leaves, as well as lacustrine sediments were also analyzed for their lead isotopic composition to compare with the lichen data. Variations in the lead isotopic composition of lichens in the St. Lawrence Valley are related to varying contributions from U.S. and Canadian sources. Lichens in the boreal forest record the presence of a significant Pb component emitted from smelting activities
With the advent of the industrial era, large amounts of Pb and other heavy metals have been released to the atmosphere from activities such as metal refining, incineration of refuse and coal, wood, and alkyl-leaded petrol combustion, (e.g., Patterson and Settle, 1987; Nriagu and Pacyna, 1988; and references therein). Records showing increasing additions of Pb to the environment are preserved in lacustrine and marine sediments (e.g., Shirahata et al., 1980; Ouellet and Jones, 1983; V&on et al., 1987; Hamelin et al., 1990; Graney et al., 1995; Gobeil et al., 1995; Lucotte et al., 1995), in corals from the Atlantic and Pacific Oceans (Shen and Boyle, 1987), and in snow and ice of polar regions (e.g., Murozomi et al., 1969; Boutron et al., 1991, 1994; Rosman et al., 1993, 1994). For the past century, lichens have been extensively used as natural indicators of air quality around urban areas and pollution sources (e.g., D&uelle and Lallemant, 1983). The term indicator refers to the ability of an organism to point out the presence/absence and the high/low level of a polluting component of the atmosphere (e.g., Sloof and Wolterbeek, 1991). Where an organism provides quantitative informations about air pollution, the term monitor has been applied. The use of lichens, and particularly of epiphytic lichens, as biomonitors of trace-element air pollution is now widely acknowledged (e.g., Lawrey and Hale, 1988; Rope and Pearson, 1990; Sloof and Wolterbeek, 1991; Lawrey, 1993; and references therein). Lichens around industrial sites, for example, can take up hundreds to thousands pg. g -’ of heavy metals (e.g., Nash, 1990). Epiphytic lichens derive moisture and nutrients exclusively from air and are efficient biomonitors because of their ( 1) perennating photosynthetic activity; (2) slow metabolism; (3) high and nonselective capacity to remove matter from the atmosphere; and (4) lack of powers to fight against pollution 4421
4428
J. Carignan and C. GariCpy
riods where the air stream persisted from the north crossing the Noranda mining and refining area (Fig. l), the aerosols recorded unradiogenic *“‘Pb/*“‘Pb values ( 1.04 to 1.14) well below the Canadian marker value of 1.15. Considering that Noranda is by far the largest single emitter of industrial Pb in Canada and that the abundances of In, As, Pb, Zn, and Se varied with the 2”hPb/2”7Pb ratios (Sturges and Barrie, 1989), a contribution from the northern smelters (2% of the total atmospheric Pb at Dorset in 1984 and 9% in 1986) was deemed likely. 2. MATERIALS AND METHODS
FIG. 1. Location map of the sampled sites. Site numbers refer to Table 1.
in northwestern Quebec, from the point source.
reaching
1.1. Lead Isotopic Composition Emissions
distances
up to 500 km
of Anthropogenic
Atmospheric Pb is a complex mixture of multiple and widespread sources which may considerably vary with time and yield fluctuating isotopic signatures (e.g., Hamelin et al., 1990; Graney et al., 1995). However, direct measurements of aerosols in northeastern North America by Sturges and Barrie ( 1987) showed a clear cut and fairly constant isotopic difference over periods of several months between 1982 and 1986: the 206Pb/207Pb ratio in eastern U.S. averaged 1.213 ? 0.008 (1982-85) and 1.221 + 0.009 in U.S. Midwest (1986), whereas it was 1.15 ? 0.01 (1982-86) in south central Canada (Sturges and Barrie, 1987). The more radiogenic character of the U.S. aerosols is due to the use of Mississippi Valley Pb in additives of gasoline since the mid-1960s. The phasing out of leaded gasoline resulted in a change of the U.S. *osPb/207Pb signatures from 1.21- 1.23 in the mid-1980s to 1.19- 1.20 in the period 1988- 1990, as shown by recent studies of the tropospheric Pb input over the western North Atlantic (V&on et al., 1992, 1993) and measured 206Pb/2”7Pb ratios in U.S. gasoline and aerosols (Rosman et al., 1994). Legal et al. ( 1989) showed that the surface waters of the Great Lakes, collected in August 1987, indeed vary within the marker values determined by Sturges and Barrie ( 1987) for Canadian and American industrial Pb. Similarly, Graney et al. ( 1995) reported 2osPb/207Pb ratios in the range of 1.181.20 for the anthropogenic Pb component accumulated in sediments of some of the Great Lakes since the mid 1970s. Sturges and Barrie (1989) were also able to apportion atmospheric aerosols collected at the rural site of Dorset (Fig. 1) in the fall of 1984 and the spring of 1986 amongst its Canadian and American sources. However, throughout a few pe-
The vegetation samples were collected between the summers of 1990 and of 1994 at sites, in most cases uninhabited lake areas, remote from industries and major or secondary roads. The epiphytic lichens used for analysis were a foliose species, Pam&a suZca& at sites #l-2 and fruticose specimens of Evemia mesomorphaand/or (Isnea spp. at sites #3- 13. The lichens were collected on evergreen and/or birch trees, and were all taken near the tip of tree branches less than a few millimeters in diameter. This was done to obtain lichens that were exposed to the atmospheric signal for a time period as short as possible. In the case of spruce or fir trees one could clearly determine, from the lichen position relative to the annual shoots, that the samples were < 1 to 3 years old, at the most. Samples were taken with pre-cleaned gloves and plastic tools where necessary, and transferred to cleaned plastic vials or sealed-plastic bags. In the laboratory, the samples were immediately freeze-dried and stored in hermetic vessels. Lichens, spruce needles, and tree leaves were separated from their substratum using nylon tweezers. Between 10 and 100 mg of vegetation samples were directly transferred to Teflon capsules and digested in nqua regia. Lead separation was done using anion-exchange chromatography (Manhhs et al., 1980). Organic-rich sediments were also digested in aqua regia and any residual detrital material was discarded. Lead concentrations were determined by isotope dilution on a solid aliquot of the samples, using a”“Pb spike calibrated against the NIST SRM 981 Pb standard. All isotopic measurements were made using Faraday cage detectors operated either in multi-collection or single peal-switching modes. A mass fractionation correction of 0.09% amu -’ was applied to all ratios. Repeated measurements of the NIST SRM 981 Pb standard yielded uncertainties ( 2omran) 1ess than 0.1, 0.2, 0.3, and 0.4% for the *06Pbl’07Pb, *NPb/2WPb, z”7Pb/z”Pb, and 2osPb/2MPb ratios, respectively. Total Pb blanks during this study were between 50 and 100 pg and negligible.
3. RESULTS
AND DISCUSSION
The Pb concentration and isotopic composition of all lichens analyzed are listed in Table 1. At site #3 two different samples of the lichen Evemia mesomorpha collected a few hundred meters from each other yielded identical lead isotopic compositions, within errors. Such was the case at site #5, except that Usnea spp. yielded a Pb content of 7.1 ppm that is significantly higher than that of 2.7 measured in Evemia m. These
two lichens
have
a very similar
habitat,
hanging
from
in their efficiency of trapping atmospheric Pb. However at sites #6 and 11, specimens of Evemia m. collected several hundreds of meters apart yielded variations in their *06Pb/*04Pb and 206Pb/207Pb ratios that are larger than typical reproducibility (Table 1) This may indicate that the specimens differ in age and did not monitor the very same atmospheric signal. This may also reflect the presence of distinct sources of aerosol Pb that were not bio-accumulated in the same proportions because of differences in lichen exposure, tree cover, etc. Nevtree branches,
and it may
indicate
that they
differ
Detection Table I. lsotooiccomoositio"
2"8& ""Pb
"*pb ""Pb
I-Lx Brome Y4-Ps-I IX.554
15.h53
3X.22Y
l.lX5
IX.55
2-Rougemont 93.Ps-I IX.475
15.h27
3X.104
l.lX2
2X.74
3-LliSigllall %‘-Em-l RI-Em-2
15.h54 15.h31
3X.2YX 3X.235
l.lX7 I.lXX
".d ".d.
15.606
3X.125
l.IXO
2.13
IX.423 IX.3Yh
15.587 15.hO7
3x.03') 3x.112
1.1x2 1.17')
2.70 7.10
O-Lx Lapurte I)O-EI~-I lh.Xlh WE"-2 lh.h25 WE"-3 16.YI0
15.255 15.212 15.22X
3h.5h3 3h.412 3h.535
I.102 I.093 I.110
".d ".d. ".d.
7-Lamothe Y4-Em-l
15.h36
14.Y70
35.3X0
l.04h
33.21
X-Matagami Y4-U-l
17.lhY
15.302
3h.YlX
I.122
4.13
9-Evans Y4-Em-l
'7.570
15.410
37.291
I.140
h.X5
IO-Opinaca Y4-Em-l
17.5Xx
15.4Oh
37.273
I.142
IO.14
2'fiPb Ll!4%
lX.57Y IX.574
4-Rivikre
Creche
Y4-U-l
IX.407
S-Lx
[Pb] ppm'
Decoiene
Y4-Em-l Y4-U-l
I I-L(:-2 'A-Em-1 17.7Xx 17.x07 WE"-2 ')(I-Em-3 17.h53
15.440 15.443 15.407
37.4Xh 37.450 37.304
I.152 I.153 I.145
".d. ".d. ".d.
IZ-Duncan Y4-Em-l 17.77X
15.45X
37.517
I.150
4.07
1%Koury W-Em-I
15.442
37.411
I.147
l.YY
17.713
Q
0
boreal
the intra-site
forest
inter-site differences
variations
at these two localities
4(X'
from Noranda
hot'
xcn,
(km)
FIG. 2. (a) Plot of the zosPb/z”7Pb and (b) the ‘“Pb/zWPb of lichens from sites #6- 13 as a function of the distance from Noranda, measured as the crow flies. Also shown for reference are the isotopic compositions of Canadian and U.S. aerosols for the period 1984-86 (Sturges and Bark, 1987) and fields representative of Cu sulfides (Deloule et al., 1989) and galena (Franklin et al., 1983) from ore deposits and showings within the late Archean Abitibi greenstone belt.
in the
lo-20 times smaller than the 1), which must reflect regional in the sources of atmospheric Pb. of Qukbec
variability
200
distance
1 Sample number prefixindicatesyearofcollection. Em = EvetnirI m~.somorphr~: Ps = Pmmliu sulcatu; u = u.\,wu spp. I.2. 3.... = differentsamples from the same site. 2 Concentrations ilreexpressed on 4 dry-weight basis. n.d.= not determined.
ertheless,
4429
Pb in lichens 1.25
of lichens.
""Pb ",MPb -
Site# S;unple'
of anthropogenic
remain
(Table
3.1. The 47-55”N Transect The lichens from sites #6- 13 have Pb contents in the range of 2-33 ppm (Table 1), with the highest value found closest to the town of Noranda, the site of a large Cu smelter operation, and the lowest values furthest north of it. The lichen from the site closest to Noranda (Lamothe, #7) has very unradiogenie lead isotopic compositions: 206Pb/2”Pb = 15 .64., 208Pb/ 204Pb = 33.2; 206Pb/207Pb = 1.046 (Table 1). The latter is much lower than the Canadian and U.S. marker values for aerosol Pb, and can only represent Pb derived from a Precambrian reservoir with low U/Pb and Th/Pb ratios. Figure 2a shows the 2MPb/zo7Pb ratios of all lichens from site #6 to 13 plotted against their distance from the town of Noranda. Also shown on Fig. 2a are the marker values for Canadian and U.S. aerosol Pb for the period 1984-1986 and the range of 207Pb/206Pb values determined for copper sulfides, mainly chalcopyrite, from a variety of localities situated within the late Archean Abitibi greenstone belt of the Superior Province. The Abitibi Cu sulfides are slightly more radiogenic than galena ores, which is interpreted to reflect in situ growth of some radiogenic Pb from the decay of U and Th either incorporated in the sulfides or present as micro-inclusions
within them (Deloule et al., 1989). Nevertheless, the bulk U/ Pb and Th/Pb ratios of the Cu sulfides must have remained very low to explain their unradiogenic signatures compared to that of common rock-forming silicates. The 2osPb/207Pb value reported for a sample of Noranda smelter dust (Sturges and Barrie, 1989) is within the range shown for Cu sulfides in Fig. 2a. Figure 2a shows that sites closer to Noranda, whether located south (#6) or north (#7-10) of it, have 2MPb/207Pb values much lower than that of Canadian aerosols. The iso-
FIG. 3. Plot of 20*Pb/zobPbvs. zo6Pb/*07Pbshowing the compositions of lichens from sites #l-S, located in the southern St. Lawrence Valley of Quebec. The field for Canadian atmospheric Pb was drawn using the values calculated in text, whereas that of U.S. aerosols is from Rosman et al. (1994).
J. Carignan and C. Garitpy
4430
topic composition of lichens increases away from Noranda to reach values undistinguishable from that of Canadian aerosols at sites #I 1 - 13 which are located more than 600 km from the smelter. The mean 2”6Pb/207Pb of the five lichen samples from sites #l l-13 is 1.150 + 0.003 (20,,,,) and there are no evidence for any input of more radiogenic U.S. Pb. Lichens have high Pb contents (Table 1) and, in contrast to ice or aerosol samples, their full isotopic spectrum can be determined very accurately, with negligible blank corrections. All measured Pb ratios in lichens (Table 1) from sites #6- 13 display identical trends to that of Fig. 2a. An example is given in Fig. 2b using the 2”8Pb/2MPb values: lichens at sites closer to Noranda record the presence of an Archean Pb component and the values level off at distance greater than 600 km from Noranda. If our interpretation that lichens from sites #I I - 13 only trapped average Canadian aerosol Pb is correct, mean marker values for the full lead isotopic spectrum can be derived from the five analyzed lichen samples at these sites. These values are: 2osPb/204Pb = 17.75 & 0.06,
higher and lower than the mean value determined above for average Canadian aerosol Pb. Actually, lichens from these sites plot closer to values determined for U.S. aerosols over the past decade (Fig. 3), suggesting that they accumulated Pb derived for the most part (-60%) from U.S. sources. This situation is analogous to the observations made in 1984 and 1986 at Dorset (Fig. 1) by Sturges and Barrie (1989), albeit direct measurements of aerosols at this locality displayed: ( 1) important seasonal isotopic variations, which depend on the prevailing wind trajectories; (2) a small, but significant contribution from smelting activities; and (3) a lower contribution from eastern U.S. sources. Lichens can only yield a signal averaged over their life span and the results obtained for sites #l-5 do not preclude some input from smelting activities in the northwestern Quebec, in which case the contribution of 60% from U.S. sources must be regarded as a minimum value. In any case, the lead isotopic results obtained on lichens confirm the suggestion of Ouellet and Jones ( 1983) that the Midwest-Great Lakes region is a major source of aerosol Pb deposited along the corridor of the St. Lawrence Valley in southem Quebec.
207Pb/204Pb = 15.44 t 0.02, 208Pb/204Pb = 37.43 t- 0.08, 206Pb/2”7Pb = 1.150 ? 0.003, 208Pb/2ffiPb = 2.109 t 0.004. A mixing calculation using for endmembers the mean composition of Canadian aerosol Pb and the compositional range of Abitibi Cu sulfides indicates that 50-90% of the atmospheric Pb at site #7, the closest to Noranda, may derive from smelting activities. This percentage is in the range of 50 to 5% for sites #6 to #8 and <2% for sites #I9 and #lo. These figures depend on which values are chosen for the isotopic composition of the smelter endmember. However, noting that Noranda is a custom smelter (Sturges and Barrie, 1989), any feedstock from Proterozoic or Phanerozoic Cu deposits should increase the figures calculated above. If this is the case, results from site #7 place an upper limit of - 1.05 for the mean 2osPb/2”7Pb value of the Noranda emission over the past few years. 3.2. The St. Lawrence Valley Ouellet and Jones ( 1983) suggested that the Midwest-Great Lakes region is one of the major possible sources for the origin of the high heavy metal contents of several remote lakes located in the southern part of the Precambrian Shield, north of the St. Lawrence River. This is because southern Quebec is generally crossed by cyclonic disturbances that move northeastward along the St. Lawrence Valley in summer just as well as in winter (Proulx et al., 1987). These disturbances are submitted to orographic phenomena when they encounter some of the higher topographic features of the Precambrian Shield, giving rise to elevated precipitations (Ouellet and Jones, 1983). Figure 3 shows the lead isotopic results for lichens from five sites located close to the St. Lawrence Valley (#I -2) and in the southern part of the Precambrian Shield (#3-5). These lichens have 2ohPb/2D7Pb and 20xPb1206Pb ratios respectively
3.3. Tree Samples and Lake Sediments At the sites of Lac Laporte (#6, Fig. 1) and LG-2 (#1 1), the lead isotopic composition of several types of vegetation samples such as black spruce needles, spruce bark, leaves of birch, and alder trees have been analyzed for comparison with the lichen results. Surface sediments from the deepest part of remote lakes at the Lac Laporte (#6), Evans (#/9), and LG-2 (#l 1) sites were also analyzed. These consisted of the topmost, -2 cm thick layer which represents lo-20 years of accumulation (Lucotte et al., 1995). The data are listed in Table 2 and shown in conventional Pb-Pb diagrams in Fig. 4. The lead isotopic data for Lac Laporte, the site that was studied in greater detail, display several features (Fig. 4a). Each category of vegetation samples yielded very consistent results. Lichens and spruce needles have similar unradiogenic lead isotopic compositions. The spruce bark samples are more radiogenic but their mean 2”6Pb/2”7Pb ( 1.133 -C 0.006) remains below that of Canadian aerosols, in contrast to that of deciduous tree leaves ( 1.16) and surface sediments ( 1.18). A bulk sample of spruce wood, representing several years of growth, yielded the most radiogenic composition (Fig. 4a). All results for Lac Laporte are well correlated in both *“‘Pb/ *O“Pbvs. 2”Pb/2”Pb ( r2 = 0.987; Fig. 4a) and 2”xPb/204Pb vs. 2osPb/2”4Pb (r* = 0.995; Fig. 4c) diagrams. The data for the LG-2 site display features similar to those of Lac Laporte: consistent results for each category of samples; comparable lichen and spruce needle data; more radiogenie spruce barks; very radiogenic surface sediments; and well correlated linear trends in Figs. 4b ( r2 = 0.960) and 4c (T’ = 0.975) which overlap those of Lac Laporte. The pathways for the accumulation of heavy metals in trees are diverse (e.g., Robitaille, 1981). Atmospheric heavy metals are directly deposited on both leaves and bark via wet and dry precipitations which also reach the soil and mix with ambient soil solutions. According to Robitaille ( 198 I), the greatest contribution of heavy metals to the xylem (the woody element) may come from the root system. Leaf and bark met-
Detection
of anthropogenic
Table 2. lsotoptc composition of needles. bark. wood. leave\ and sediments.
C-Lac Lapurte ‘HI-SN-I lh.717 YO-SN-2 I h.Y 15 YO-SB-I 17.371 YO-SB-2 17.52X Y0-SB-3 17.430 YO-SW-1*1X.63 Yo-AL-I 17.YX2 ‘HI-AL-2 lX.03h Yil-AL-3 lX.Oh3 YCBL-I IX.135 YCBL-2 17.Y5Y YCBL-3 lX.OXI YO-SS-I 1X.372 YO-C-2 1X.3X3 ‘)-Evans IX.79 YO-ss* Si(I-DS. 22.3X I I-L(;-2 YO-SN-I 17.550 ‘)(I-SN-2 17.515 YO-SN-3 17.76X YO-SN-4 l7.4Xh YO-SN-5 17.hO5 YO-SB-I IX.037 Yll-SB-2 17.Y72 VII-SB-3 IX.23Y YO-SS-I lY.001 ‘~(I-SS-2 lY.02h
IS.213 15.253 15.363 l5.4lh 15.410 15.67 15.468 15.495 15.4Xx 15.535 15.51X 15.531 15.552 15.557
30.459 37.04’) 37.255 37.201 3x.37 37.724 37.74X 37.X4X 3X.(127 37.h.12 37.760 3X.055 3X.(lhl
I .OYU l.lOY I.131 I.137 I.131 I.lY l.lh2 I.164 l.lhh l.lh7 I.157 l.lh4 l.IXl l.lX2
15.64 16.24
3x.53 42.32
I .20 1.3X
l5.42Y 15.372 15.425 15.353 15.473 15.52X 15.477 15.5hh lS.hhY 15.h7l
37.31X 37.171 37.3x4 37.087
1.13x 1.13’) I.152 I.139 1.13x l.lhl I.160 I.173 I.213 I.214
3h.hhl
37.h35
37.771 37.h4h 3X.027 3X.hl4 3X.h5Y
Pb in lichens
4431
At the two sites where they were analyzed, all spruce needle samples yielded lead isotopic compositions comparable to those of lichens (Fig. 4) implying that the needles essentially derived their Pb from the atmosphere. This is consistent with the results of Nilsson ( 1972) who reported a net increase of Pb in aging spruce needles. The life expectancy of a spruce needle may be greater than 15 years, however air pollution, combined with environmental particularly SO2 burden, stresses such as frost and frost dessication accelerate needle death (e.g., Pfanz et al., 1994). The analyzed needles were populations of mixed ages taken from small spruce branches in the exterior part of the trees. Thus they included needles with ages between less than one year and potentially up to 15 years. However, the probability for older needles to remain on the tree branches is smaller than for young ones, due to normal ageing, environmental stresses, and pollution. In addition spruce needles may export some of their Pb to the phloem (Robitaille, 198 1) but keep accumulating heavy metals as they age (Nilsson, 1972), which indicates that the export flux must be smaller than the overall accumulation flux. The combination of needle loss with ageing, Pb export from
1
’ Sample number prefix indicates year of collection. I, 2, 3,... = different samples from the same site. SN: spruce needle; SB: spruce bark; SW: spruce wood; AL: alder leaf; BL: birLh leaf; SS: wrface sediment (O-2 cm): DS: pre-indusuial sediment; * = mean compositions.
A
15.7 :
Laporte Lake
??
15.6 [_ 15.5 15.4
als may be exported via the phloem into the xylem, and some of the xylem metal may be translocated to the bark (Robitaille, 1981). Such pathways of accumulation would easily explain the distribution of the data points in Fig. 4 at both sites, reflecting simple mixture of only two endmembers consisting of anthropogenic Pb and a natural, radiogenic component from the soil. Table 2 lists the average lead isotopic compositions of surface (O-2 cm) and pre-industrial ( > 10 cm depth) sediments from core 90-27 collected at the Evans site (#9). These sediments have mean Pb contents of 50 and 12 ppm, respectively (Chaire de Recherche en Environnement, 1992). Assuming that the difference, 38 ppm, is the contribution of atmospheric Pb to the surface sediments, a mixing calculation indicates that the atmospheric endmember should have *06Pb/2”Pb, 208Pb/2@‘Pb,and 206Pb/207Pb ratios of 17.66, 37.33, and 1.143, respectively. This is very close to the composition of a lichen at this site (*06Pb/204Pb = 17.57; *“*Pb/204Pb = 37.29; and 2osPb/207Pb = 1.140) showing that lichen and surface sediments record comparable atmospheric Pb signal. As is the case at Lac Laporte and LG-2, the surface sediments of Evans Lake have a bulk 206Pb/207Pb composition ( 1.20) nowhere near that of the atmospheric component. This is because the sediments in the deepest part of many lakes in the Canadian Shield are largely made up of organic matter with little detrital components ( Lucotte et al., 1995 ) . The very radiogenic composition of this organic component, with 206Pb/2WPb ratios as high as 22.4, likely reflects a predominant input of labile radiogenic Pb preferentially derived from soil organic matter and/or rock dissolution (e.g., Graney et al., 1995).
0
sediments
15.2 c-
15.7
B
LG.2
lh.5
17.0
15.6 a” $
15.5
“3 a
15.4
E
15.3
. . . c3 .. 0:
0
15.2 3Y.O 3x.5 B e “. % w
f
3x.0 37.5 37.0 36.5 3h.U
I
L
17.5
IX.0
IX.5
IV.0
IY.5
2t16pb,204pb FIG. 4. *07Pb/2WPbvs. 206Pb/*wPbdiagrams showing the Pb-isotopic results for (a) lichens, spruce needles, spruce barks, spruce wood, birch, and alder leaves and surface sediments at the Lac Laporte site (47”N) and for (b) lichens, spruce needles, spruce barks, and surface sediments at the LG-2 site (53”N); (c) ‘08Pb/zWPb vs. *06Pb/2”Pb diagram showing all results obtained at the Lac Laporte (open symbols) and LG-2 (filled symbols) sites.
J. Carignan and C. Gariepy
4432
the needles and yearly accumulation of new Pb imply that the mean age of the isotopic signal recorded by spruce needles is biased towards the first half of their life cycle and is likely less than seven years. Lichens must thus reflect the mean isotopic composition of atmospheric Pb integrated over a time span comparable to that of the needles, except if the composition of atmospheric Pb did not change significantly at these sites over a longer period of time. Quite clearly, one should sample needles from the youngest twigs to verify if the mean lead isotopic signals of lichens and needles remain comparable on a shorter timescale. Spruce barks have been exposed to Pb fallouts for a longer duration than lichens and their more radiogenic signatures could indicate that changes in the composition of atmospheric Pb have occurred through time. This cannot be completely ruled out but the data for barks and wood are consistent with the pathways for heavy metal transportation in trees (e.g., Robitaille, 1981) Although the soils are likely contaminated by the atmospheric precipitations, they likely still contain a significant component of natural radiogenic Pb as inferred from the composition of sediments at sites #6 and 11. The very radiogenic signature of spruce wood from site #6 (Fig. 4a,c) is indeed compatible with the view that the greatest contribution of Pb to the xylem is from the root system (Robitaille, 198 1) . Similarly, if a small proportion of the natural Pb is translocated from the xylem to the bark (Robitaille, 1981) and mixed with atmospheric Pb, the more radiogenic signatures of barks, compared to lichens, are readily explained. In contrast to spruce needles, the leaves of alder and birch trees collected at site #6 are very radiogenic which may be interpreted in several ways that will require further studies to be fully understood. This may, for example, reflect that the isotopic composition of atmospheric Pb has changed recently, which can be verified through comparison with spruce needles growing over the same time span. Alternatively, it may show that Pb aerosols from the Noranda smelter are barely available during the season when the leaves were present on the trees, due for example to prevailing northward wind trajectories in the summer months. Comparing the isotopic composition of Pb accumulated in snow and deciduous tree leaves from the same sites shall help elucidate potential seasonal variations. A third possibility may be that, in contrast to spruce needles, the shape of alder and birch leaves makes them inefficient air filters and/or that these tree species contain significant proportions of metabolic Pb derived from soil solutions; this can be unraveled through determinations of the Pb content and the isotopic compositions of leaves and sap. 4. CONCLUSIONS Epiphytic lichens located up to 500 km north of Noranda record a significant input of anthropogenic Pb emitted to the atmosphere from smelting activities. The Noranda input is not apparent north of 53”N where lichens only record the typical Canadian aerosol Pb signal. Lichens collected along the St. Lawrence Valley between 45” and 48”N show isotopic evidences for a dominant Pb input from U.S. sources. Lichens are highly efficient biomonitors allowing, when the isotopic composition of the atmospheric Pb sources are known, quan-
titative assessments of the various inputs. However, their slow metabolism and their unknown age detract from recording the Pb atmospheric signal on short and precise timescales. At two sites in the boreal forest region of Quebec, spruce needles have isotopic compositions undistinguishable from that of lichens. This reflects integration of the mean atmospheric Pb signal over a comparable time span, a result that is confirmed by the lead isotopic record in lacustrine sediments. Deciduous tree leaves are significantly more radiogenic than lichens which may reflect seasonal variations of the Pb signal and/or significant inputs of Pb metabolized from soil solutions through the root system. The latter scenario is supported by the very radiogenic composition of spruce wood. Acknowledgmenrs-Supported
by grants from the Atmospheric Environment Service of Environment Canada, Hydro-Qu&ec and NSERC-Canada. R. Canuel, B. Caron, P. Ferland, C. Hillaire-Marcel, C. Innocent, R. Loupiac, M. Lucotte, and F. Seimbille are warmly thanked for their technical assistance, comments, discussions, and scientific inputs. Helpful comments were provided by A. V&on and two anonymous reviewers.
Editorial handling: K. Mezger
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