Millennial-Scale Records of Atmospheric Mercury ... - ACS Publications

changes in net atmospheric mercury deposition in Switzerland for the past 2-3 millennia. ... rates and Hgex, an indicator of significant anthropogenic...
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Environ. Sci. Technol. 2003, 37, 235-244

Millennial-Scale Records of Atmospheric Mercury Deposition Obtained from Ombrotrophic and Minerotrophic Peatlands in the Swiss Jura Mountains FIONA ROOS-BARRACLOUGH† AND W I L L I A M S H O T Y K * ,‡ Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, CH-3012 Bern, Switzerland, and Institute of Environmental Geochemistry, University of Heidelberg, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany

Peat cores from two bogs were used to reconstruct changes in net atmospheric mercury deposition in Switzerland for the past 2-3 millennia. The two records were compared to assess the reliability of peat cores as archives of atmospheric mercury deposition. Net mercury accumulation rates and Hgex, an indicator of significant anthropogenic mercury contamination, were calculated for both cores. Both records showed stable background values (0.5-1.9 and 1.03.0 µg of Hg m-2 yr-1). In both profiles, mercury accumulation rates began to increase during the 12th century, and Hgex appeared during the 14th century. The late 19th and early 20th centuries have been studied in detail. The profiles match well with the history of local and global mercury emissions. The magnitude of increase from the pre-anthropogenic to anthropogenic period was also very similar in both records. Although the two sites are botanically very similar and lie only 3.5 km apart, accumulation rates at TGE were generally higher than those at EGR. This indicates that, although such records can be used to determine the chronologies of and relative changes in rates of atmospheric mercury deposition, differences in rates of mercury accumulation rates between sites do not necessarily indicate differences in deposition rates of mercury from the atmosphere.

Introduction Mercury (Hg) has attracted attention as a dangerous environmental pollutant because of its toxicity and its ability to bioaccumulate in the food chain [chiefly in its most toxic form, methylmercury (1)]. Also, the long atmospheric residence time of elemental Hg (6 month-2 yr), which makes up ca. 98% of total atmospheric Hg (2, 3), allows the longrange atmospheric transport of Hg to regions far from industrial sources. High levels of Hg have been found in biota in nonindustrial regions such as the Arctic and the Faeroe Islands (4-7), and indications of neurotoxicological effects in human populations in these regions have been attributed to the high concentrations of methylmercury in foods such as whale meat (8, 9). However, it is not yet clear to what * Corresponding author telephone: +49 (6221) 54 4801; fax: +49 (6221) 54 4803; e-mail: [email protected]. † University of Bern. ‡ University of Heidelberg. 10.1021/es0201496 CCC: $25.00 Published on Web 12/14/2002

 2003 American Chemical Society

extent these high levels are due to anthropogenic sources (5, 10, 11). To determine whether the high levels of Hg in nonindustrial environments are primarily due to natural or anthropogenic sources, it is useful to study archives of atmospheric Hg deposition. Natural archives such as lake and marine sediments, ice cores, and peat cores have all been used to provide information about past rates of atmospheric Hg deposition (12-16). These natural rates can be compared to present levels, which then allows the proportion of anthropogenic Hg to be calculated. Also, by studying profiles from longterm archives, we can observe the effects of natural processes such as climate change and volcanic emissions on atmospheric Hg deposition rates (17, 18). This knowledge can be helpful in the interpretation of modern data. Another interesting question that could potentially be solved by studying archives is whether there are natural geographical gradients present in atmospheric Hg deposition rates. For instance, it has been suggested that cold climatic conditions can increase net atmospheric Hg deposition rates (12, 17, 19). It has also been suggested that sea-salt aerosols may increase atmospheric Hg deposition rates through Hg oxidation pathways (20). Volcanic emissions, as an important natural source of Hg to the environment (21), could also cause natural geographical gradients in atmospheric Hg deposition rates. Gradients of Hg deposition along geographical transects have also been used to imply long-range transport of Hg from industrial pollution sources (22, 23). Profiles from ombrotrophic peat cores are particularly good archives of atmospheric metal deposition as they receive input only from the atmosphere and are therefore free from the effects of groundwater or catchment input. Also, peat bogs are present in a wide latitudinal range (24). Peat bogs are becoming increasingly attractive archives because of their accessibility, lack of extreme analytical challenges, and reliability. Compared to snow and ice cores from polar regions, peat bogs are widely distributed around the globe, and trace element concentrations in peats are typically 103106 times greater than those found in polar ice. Thus, chemical analyses of peat samples are not plagued by the severe contamination problems that constrain the use of ice and snow archives. Whereas sediments appear to be affected by chemical diagenesis, which make them unreliable as archives (26-28), studies have shown that peat bogs provide a reliable record of changes in atmospheric Hg deposition rates (13, 17, 18, 29-33). However, it is thought (based on studies of German bogs) that ca. 15% of the Hg deposited to the surface of the bog from the atmosphere is lost againseither as particulate and colloidally bound Hg in lateral runoff (ca. 10%) or to the atmosphere because of revolatilization (ca. 5%) (34). Thus, peat accumulation rates of atmospherically deposited Hg should not be interpreted as being representative of atmospheric deposition rates but rather as net Hg accumulation rates (equal to total atmospheric deposition minus postdepositional losses): the changes in net accumulation rates should be proportional to the changes in rates of atmospheric Hg deposition (34). The main objective of this paper is to show that although peat bogs can provide chronologically and quantitatively reliable records of changes in net atmospheric Hg deposition, small differences between individual bogs such as their situation, surface topography, vegetation, and canopy cover can significantly affect the efficiency with which they accumulate Hg from the air. While still allowing comparisons of the ratio of natural to modern net Hg accumulation rates, VOL. 37, NO. 2, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Location of the coring sites at Etang de la Grue` re and La Tourbie` re de Genevez in the Swiss Jura Mountains. The approximate extent of the two bogs is shown. this indicates that comparisons between the accumulation rates of Hg in different bogs along geographical transects should be approached with great care. The implications for such studies, illustrated here for Hg, are likely also to be applicable to similar studies of other metals as well as organic contaminants.

Experimental Section Study Sites. The two ombrotrophic Sphagnum peat bogs studied lie just 3.5 km apart (Figure 1) in the Franches Montagnes region of the Swiss Jura Mountains (35, 36). The Franches Montagnes consist of a generally flat, calcareous plateau with abundant karst phenomena. The present climate in the area is moist continental. The average annual temperature at both sites is 5.5 °C, and annual precipitation is modeled to be 1500 mm. Both sites are underlain by clays and marls and have similar present-day vegetation (37). Also, the pollen zones at the two sites are synchronous (38). Both cores were taken in August 1991. (a) Etang de la Grue` re. The first study site was the peat bog at Etang de la Grue`re (EGR), which is situated ca. 1005 m above sea level (asl). Peat formation began ca. 14 500 years ago in an Oxfordian clay- and marl-based hollow (18, 39). The bog has now grown to cover an area of 22.5 ha and has reached a thickness of 6.5 m on the dome of the peninsula. The dome surface is raised about 4 m above the edge of the bog (40). Although some parts of the bog have been affected in the past by drainage, grazing, and flooding [the lake is probably not natural (40)], the coring location was on the lawn at the top of the dome on the peninsula. This area is 236

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thought to have remained unaffected by human activities (41). The core studied here, EGR2G, is 67 cm in length. The entire core consists of ombrotrophic Sphagnum peat. Ash contents in the preindustrial section of the core are below 3% but increase to over 6% in peat that formed during the industrial period. (b) La Tourbie` re de Genevez. The second study site was the peat bog at La Tourbie`re de Genevez (TGE), which is situated ca. 1020 m asl. Peat formation has taken place on an outcrop of Tertiary clays and marls (39) that is elevated above the surrounding karstic terrain and surrounded by sinkholes (35). This situation combined with the generally flat topography of the area prevents penetration of water from the surrounding area into the bog. The bog covers an area of 7.2 ha, and maximum peat accumulation on the dome is 140 cm. From the edge of the peatland, the increase in elevation N-S to the apex of the dome is 2.5 m (36). At the coring site, Sphagnum peat is present from the surface to ca. 40 cm depth, followed by Sphagnum-Eriophorum peat down to ca. 70 cm. From ca. 70-140 cm, the peat is Carexdominated (36). The 87-cm-long core studied here (TGE270891) is geochemically ombrotrophic only above ca. 20 cm depth because of upward diffusion of mobile trace elements such as strontium (Sr) and calcium (Ca) (36). Ash contents range from 0.7 to 7.2% in preindustrial sections of the peat and reach a maximum of 12.5% in peat that formed during the industrial period. Sample Collection. Cores EGR2G and TGE were taken using a stainless steel Wardenaar corer (42) on August 27, 1991, at coordinates 7°2′57′′ E, 47°14′22.6′′ N and 7°5′42.8′′

FIGURE 2. Mercury concentration profiles in nanograms per gram of the two peat cores (a) EGR2G and (b) TGE. E, 47°14′30.4′′ N, respectively. Both cores were taken on the lawns present at the centers of the domes of the two bogs, although the lawn at TGE has a smaller and much less welldefined lawn than EGR (Figure 1). The EGR core measured 10 × 10 × 67 cm, and the TGE core measured 10 × 10 × 87 cm. The cores were frozen after collection. In December 2000, the cores were sliced frozen into 1-cm slices with a stainless steel bandsaw. The slices were kept at -18 °C until analysis. Trace Element Analysis. The edges of the cores were removed and dried at 105 °C overnight before being pulverized in an Ultracentrifugal Mill ZM 1-T (F. K. Retsch GmbH and Co., Germany). The powders were stored in airtight, acid-washed plastic beakers. This powder was analyzed for 19 trace elements including bromine (Br) by X-ray fluorescence spectroscopy (XRF; 43). Ca.1 g of the peat powder was burned at 550 °C to obtain the ash content of the peat. The centers of the peat slices were subsampled using a sharpened stainless steel tube of 16 mm diameter. Four plugs were taken from each slice. Three of these plugs were dried at room temperature in a class 100 clean-air cabinet overnight, ground together, and measured for Hg concentration by atomic absorption spectroscopy (AAS) using a Leco AMA254 according to the procedure described in Roos-Barraclough et al. (44). As some material was lost during cutting, rather than assuming a slice thickness of 1 cm, the bulk density was calculated using the height of each individual bulk density plug (measured to an accuracy of 0.1 mm). The accuracy and precision of the AAS results were checked using the plant standard reference materials NIST 1547 Peach Leaves (certified value: 0.031 ( 0.007 µg/g Hg, measured value: 0.035 ( 0.003 µg/g Hg) and NIST 1575 Pine Needles (certified value: 0.15 ( 0.05 µg/g Hg, measured value: 0.132 ( 0.008 µg/g Hg). The remaining subsample was used for bulk density determination. Dating. The surface layers of the cores have been dated using two independent methods. Powdered peat samples were dated radiometrically by γ-assay at Liverpool University, England, using a constant rate of supply (CRS) model of 210Pb (45). As an independent check on this dating method, macrofossils of Sphagnum moss were extracted, cleaned of roots and detritus, and dated by acceleration mass spectroscopy (AMS) using the 14C bomb pulse method (46) at Århus University, Denmark. Macrofossils from deeper samples (9 samples from EGR and 7 samples from TGE) were

dated using the conventional AMS 14C dating technique (47) at the ETH, Zurich, Switzerland. Details of the dated samples and results for EGR2G and TGE are found in Tables1 and 2 in the Supporting Information, respectively. Humification. Percentage light absorption at 550 nm wavelength was measured using colorimetry on alkaline peat extracts and used as a proxy of peat humification, as described in Roos-Barraclough et al. (48).

Results and Discussion Mercury and Bromine Concentration Profiles. The Hg concentration profiles obtained from the two cores are shown in Figure 2. Both profiles are similar in shape, showing a stable background followed by an increase in concentration from ca. 40 cm depth upward and a peak in concentrations between 20 and 30 cm depth. In EGR, another distinct peak is visible at 8.5 cm depth. Notably though, the concentrations at TGE are much higher than those at EGR; average Hg concentrations in peat which formed below 39.7 cm at TGE (corresponding to peat which accumulated prior to ca. 1340 AD) are 68.4 ( 15.0 ng g-1, whereas below 31.5 cm at EGR (also corresponding to peat which accumulated prior to ca. 1340 AD) they are only 24.7 ( 4.3 ng g-1. Also, there is a general increase in Hg concentrations toward the bottom of the profile from ca. 45 cm depth at TGE, which could be interpreted as being due to the influence of Hg in groundwater or upward diffusion of Hg from the sediment in this minerotrophic section of peat. In the next section, we show that this is in fact not the case for peat above 80 cm. Bromine concentrations on the other hand are very similar in both profiles, being 16.3 ( 2.8 and 15.6 ( 2.9 µg g-1 in the pre1340 AD sections of EGR and TGE, respectively. However, the changes in element concentration profiles do not necessarily represent changes in atmospheric deposition rates as changes in peat accumulation rates are not taken into account. Variation in peat accumulation rates within a profile and differences between profiles can be caused both by differences in surface growth rates and in decomposition rates because peat compaction increases with decomposition. To calculate the time-resolved net atmospheric deposition rate profiles, an age-depth model must be constructed. Age-Depth Model EGR. For EGR, the radiometric 210Pb dates were in excellent agreement with the 14C AMS bomb pulse dates (see Table 1 in Supporting Information). ThereVOL. 37, NO. 2, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Age-depth models for the two profiles (a) EGR2G and (b) TGE. Dated points and their error bars are shown. fore, an age-depth model was constructed that took all sets of dates into account (Figure 3a). The model consisted of two consecutive 5th degree polynomial regressions. The first (upper regression) was based on all 14C AMS bomb pulsedated points and all radiometrically dated points, and the second (lower regression) was based on all conventional 14C AMS-dated points and the deepest radiometric date (22 cm, 1881 ( 18 AD). Because the dates for depths 31.5 and 38.5 cm lay very close together, their error bars were used in the construction of the regression. The resulting age-depth relationship indicates a period of slow peat accumulation rate from ca. 40-25 cm depth. This could be due to increased compaction because of the high levels of decomposition, which are observed in the humification profile (Figure 4a,b). This high level of decomposition is thought to most likely have been caused by hydrological disturbance (48). See for comparison the humification profile of TGE (Figure 4c), which has remained hydrologically undisturbed. Age-Depth Model TGE. For TGE, the radiometric dates did not agree well with the 14C AMS bomb pulse dates. Radiometric dates indicated a lower peat accumulation rate than the bomb pulse dates. At ca. 13 cm depth, the difference between the two sets of dates was ca. 10 yr; radiometric dating indicated an age of 1950 ( 7 AD at 13 cm, whereas 14C AMS bomb pulse dating indicated an age of 1959 ( 1 or 1961 ( 1 AD. (Two dates can be possible for one point using the 14C AMS bomb pulse dating technique if it is unclear on which side of the pulse the point lies; see ref 46). Because of this discrepancy, only the 14C AMS bomb pulse dates were used to construct the age-depth model for the surface samples. A second degree polynomial regression was used, based on the 14C AMS bomb pulse dates only. The gap to the beginning of the conventional 14C AMS dates was bridged by using a linear regression from the last 14C AMS bomb pulse date to the last available radiometric date, at 22 cm (1901 ( 29 AD). This was followed by another second degree polynomial regression, based on the last radiometric date and the nine conventional 14C AMS dates for the deeper samples (Figure 3b). Calculation of Net Accumulation Rates. The age-depth models described above were used to predict the average age of each peat slice. These ages were then used to calculate the growth rate for each slice. Net accumulation rates for trace elements could then be calculated using

AR ) 10[E] × BD × GR 238

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where AR (µg m-2 yr-1) is the net accumulation rate of the element in the peat, [E] is the concentration of the element (ng g-1), BD is bulk peat density (g cm-3), and GR is growth rate (cm yr-1). This calculation takes into account changes both in peat accumulation rates due to differences in surface growth rates and in compaction due to differing rates of peat decomposition. In both cores, the range of Hg AR in peat that formed before ca. 1340 AD is very limited until 80 cm. This suggests that the increase in Hg concentrations toward this point in the TGE profile was in fact due to changes in peat accumulation rate rather than the influence of groundwater or upward diffusion from the sediment. However, below 80 cm in the TGE profile, Hg concentrations and AR increase sharply (Figure 5). As this could be due to the minerotrophic nature of this section of the peat, samples below 80 cm depth at TGE were not included in further calculations. However, the peat at TGE is known to be chemically minerotrophic to some extent up to 20 cm depth because of the upward diffusion of mobile trace elements (e.g., Ca and Sr from the underlying sediment). This can be concluded from the AR profiles of these elements, which increase from ca. 20 cm depth downward. Unlike these mobile minerotrophic elements, the Hg AR in the peat section above 80 cm up to 39.7 cm (ca. 1340 AD) does not show an increasing trend with depth (Figure 5). Therefore, it can be concluded that Hg is not diffusing up from the minerotrophic section into the ombrotrophic section of the core. Average Hg AR at EGR before 1340 AD is 1.0 ( 0.3 µg m-2 yr-1, and at TGE it is 1.6 ( 0.4 µg m-2 yr-1. A slow increase in Hg AR is observed in both profiles during the Middle Ages followed by a sharp increase at the top of the profile in peat stemming from the anthropogenic period (AP). Calculation of Hgex. In a previous paper (18), we showed and discussed a correlation that exists between Hg and Br concentrations in the peat at EGR, which has lasted throughout the entire Holocene (i.e., the past 10 000 yr). Plotting [Br] against [Hg] for samples from a 6-m-long EGR peat core containing peat from ca. 500-14 500 calendar years before present (cal yr BP) gave a wide but well-defined band of points, with fewer than 5% outliers. The outliers all occurred during periods corresponding to regional volcanic eruptions or during the modern period. Thus, the band was assumed to represent the normal natural range of Hg concentrations as compared to bromine concentrations. This relationship was used to calculate Hgex, an indicator of either anthro-

FIGURE 5. Plot of Sr (gray) and Hg AR (black) profiles for the TGE core. Sr AR increases with depth (toward the minerotrophic peat zone) but Hg AR does not. This indicates that mercury has not diffused up into the ombrotrophic peat section from the minerotrophic zone.

FIGURE 4. Humification profiles obtained from each of the peat cores. The EGR2G profile (a) showed a strong linear trend (dashed line). The profile was delineated to reveal a highly humified section in the middle of the record (b) that may be due to the construction of a dam prior to 1650. The humification profile of the TGE core however (c) indicates that the site has remained hydrologically undisturbed. pogenic Hg contamination or extreme natural events (such as regional volcanic eruptions). Hgex points are those outlying points that do not lie on the correlation band. The amount of Hgex is defined as the difference between the total Hg concentration and the edge of the correlation band. Here, Hgex was calculated for both the EGR and the TGE Wardenaar cores. In the cores studied here, this Hg-Br relationship makes it possible to distinguish samples that contain a significant amount of mercury from anthropogenic sources. When Hg AR is plotted against Br AR for the Wardenaar cores from EGR and TGE, the points from the pre-1340 AD section are seen to be clustered together, whereas points from the section

that accumulated after 1340 AD splay away from the clusters (Figure 6a,b). To allow Hgex to be calculated, [Br] vs ln [Hg] was plotted for both cores. In both cases, the pre-1340 AD points were grouped together (Figure 6c,d). Equations defining the lower and upper limits of the bands (eqs 1 and 2, respectively) were calculated by fitting a linear regression to the points that lie along the edges of the pre- 1340 AD group. Hgex was calculated for outlying points using

ln[Hgex] ) ln[Hgtotal] - ln[Hgeq 2] [Hgex] can then be converted into Hgex AR as described above and plotted against the age of the peat (Figure 7c). The appearance of Hgex in the profiles can be interpreted as marking the beginning of significant anthropogenic contamination (18). Hence, here we define the pre-anthropogenic period by the occurrence of samples where Hgex AR e 0.0 µg m-2 yr-1. In other words, these samples contain no (or very little, as in the case of samples from the Roman period in the TGE record) Hg in excess of the natural variability in Hg/Br. In contrast, we define the anthropogenic period (AP) as the time when Hgex AR > 0.0 µg m-2 yr-1. In these samples, the accumulation rates of Hg are in excess of the natural variation VOL. 37, NO. 2, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Plots of EGR (a) and TGE (b) Hg AR vs Br AR show clearly the contrast between the grouped pre-AP samples (pre-1340 AD, gray triangles) and the spread-out AP samples (black dots). In panels c (EGR) and d (TGE), the relationships of Hg to Br concentration in the peat at EGR are shown, including the regressions that were used to calculate Hgex (equation 2 in both cases).

in Hg/Br and are attributed to diverse anthropogenic sources. The AP, as defined here, begins at ca. 1340 AD in each core. Interpretation of Net Atmospheric Deposition Rate Profiles. (a) The Period Prior to Anthropogenic Impact. Mercury is to thought to have been purified for use by man since at least the 15th century BC, from which time the Egyptian tomb at Kurna, in which a crucible (or crucibles) filled with Hg was reportedly discovered (49). Mercury was used by the Greeks for religious ceremonies and for medicinal purposes, and the Romans inherited most of the Greek’s technical knowledge and extended applications of Hg into commercial uses. For example, vermilion, a red, Hgcontaining pigment, was used to decorate Roman villas and as a constituent in beauty products (50). Mercury is also known to have been used as an amalgamator for use in gold purification and guilding of silver and brass since Roman times (51). By 77 AD, the Romans consumed over 4500 kg of Hg per year (50). Although Hgex does not appear during the Roman period in the EGR profile, there are two small flashes of Hgex during this period in the TGE core. Thus, Hg pollution due to Roman mining and use of Hg was just barely sufficient to increase Hg deposition beyond its natural boundaries in this region. With the fall of Rome, Hg production declined considerably (50). In both profiles, Hg AR begins to increase at ca. 1150 AD (Figure 7a). This is coincident with the first European medieval writings about uses of mercury; Theophilus the 240

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Monk, who lived during the late 11th or early 12th century devoted several chapters of his work De Diversis Arbitus to processes in which mercury is used for gold refining, guilding, and also for putting a finish on tin jugs. Hgex appears a little later on during the Middle Ages: at EGR at ca. 1348 AD (30.5 cm), and at TGE at ca. 1344 AD (38.6 cm). Note that the remarkable agreement with respect to time but not to depth indicates how faithfully the bogs have preserved the change in Hg deposition. During this time (around 1340 AD), Hg was being used industrially in Europe for tasks such as gold extraction from the sands of the Rhine (from at least 1140 AD) and also to recover any metallic silver from finely ground slags and other process debris. During the 16th century, books (Pirotechnia, Biringuccio, 1540, and the Probierbu ¨ chlein, Germany, ca. 1520) were published that contained references to a process which used mercuric chloride, salt, vinegar, and copper acetate to extract silver from slags, workshop dust, and spent cement (52). The De re Metallica (53) provides illustrations of apparatus used to mine gold by density separation and mercury amalgamation and describes the gold recovery process. Large quantities of Hg were also used in the patio silver-refining process in Spanish America (an average of 612 ton per anum between 1580 and 1900; 54). The long atmospheric residence time of elemental Hg (2) means that it is possible that some of the increase in Hg AR pre-dating the Industrial Revolution in the profiles was caused by long-

FIGURE 7. Net accumulation rate profiles calculated for EGR (- -) and TGE (s) of Hg (a), Br (b) and Hgex (c). range pollution from this process. Thus, the appearance of Hgex in the 14th century, indicating the beginning of significant Hg pollution in Central Europe, and the subsequent gradual increase in Hg AR toward the Industrial Revolution fits well with historical records of Hg use. (b) Mercury AR in the Anthropogenic Period As Compared to the Pre-anthropogenic Period. Moving on to the AP, it can be seen in Figure 7b that bromine deposition rates have increased dramatically since the Industrial Revolution. In addition to natural sources such as algal halocarbon emissions (55), litter decomposition (56), biomass burning (57), and abiotic oxidation of organic matter (58), there are also industrial sources of atmospheric bromine. For instance, bromine-containing halons are used as fire suppressants (59, 60), and car exhausts are a source of fine-particle Br (61, 62) because of Br additives used to scavenge lead. It seems likely that these industrial sources have caused the sharp increase in Br AR at the top of the profiles. Therefore, Hgex cannot be used for the post-Industrial Revolution period as an indicator of the proportion of anthropogenic pollution. Instead, Hg AR can be compared to its natural range (values prior to 1340 AD). Comparisons between the pre-AP (natural) and AP mercury ARs at EGR and TGE are shown in Table 3. The pre-AP mercury AR at TGE (1.6 ( 0.4 µg m-2 yr-1) is higher than at EGR (1.0 ( 0.3 µg m-2 yr-1). A comparison of background Hg concentrations in two cores from EGR gave very similar values (EGR2K: 25.5 ( 2.8 ng/g, 65-104 cm; EGR2G: 25.4 ( 5.2 ng/g, 34-67 cm). Thus, the inter-bog variation shown here between EGR and TGE exceeds the observed intra-bog variation. As the two bogs have very similar climate regimes and are very close together (ca. 3.5 km) and at similar altitudes, an actual difference in natural atmospheric Hg deposition rates at the two sites is extremely unlikely. Thus, there must be a difference in the efficiency with which Hg is captured and/or retained by the two bogs.

TABLE 3. Comparison of the Deposition Rates and Increase Factors Calculated from the Two Peat Core Records site range of pre-AP (natural) Hg AR (µg m-2 yr-1) avg pre-AP (natural) Hg AR (µg m-2 yr-1) avg AP Hg AR (µg m-2 yr-1) max Hg AR (µg m-2 yr-1) max increase factor (max AP/ avg pre-AP Hg AR) avg increase factor (avg AP/ avg pre-AP Hg AR)

EGR

TGE

0.5-1.9

1.0-3.0

1.0 ( 0.3

1.6 ( 0.4

12.6 ( 7.7 28.9 30×

17.0 ( 13.1 42.8 28×

12×

11×

Hummocks and hollows are not well developed at EGR or TGE, so microtopography is not likely to be a very influential factor. TGE has slightly greater tree cover on the lawn surface and also a smaller opening in the tree canopy on the lawn, which could be responsible for an increased “canopy effect” (63, 64). The canopy effect involves elemental and particulate Hg being removed from the atmosphere by impaction on the trees and then deposited on the bog surface by throughfall water or by falling needles. [Pinus mugo twigs and needles were found to be high in Hg concentration as compared to other bog plants collected from the surface of the bog at EGR, (44)]. Increased tree cover also shades the bog surface from solar radiation, which is known to increase Hg volatilization from soils (65). However, a dendroecological study of TGE (66) has indicated that trees only colonized the bog after 1835 AD, so this point may only be relevant for peat that accumulated during the last ca. 150 yr. Another possibility is that more Hg is lost as lateral runoff from the bog at EGR than at TGE because of its more highly domed structure [the dome center is 4 m higher than the bog edge at EGR but only 2.5 m higher at TGE (36)]. Also, although the surface vegetation is similar at both sites (38), species proportions VOL. 37, NO. 2, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. (a) Detail of the increase factors calculated for the EGR and TGE profiles for the late 19th and 20th centuries. (b) Detail of the net mercury accumulation profiles for EGR (striped) and TGE (solid) profiles. The profiles match well with both global (s) and regional (- -) mercury emission chronologies. are not identical. At TGE there is a higher proportion of Sphagnum recurvum, which was found to be particularly high in Hg as compared to other Sphagnum mosses (44). In summary, differences in tree cover, surface vegetation, and relief could all contribute to the higher Hg AR at TGE. Except for the point regarding volatilization, these arguments may also be true for other elements. The AP Hg ARs are also higher at TGE than at EGR with the exception of the peak centered ca. 1910, for which Hg ARs are very similar (Figure 8b). The pollution sources for the past ca. 50 yr are not thought to be local. Therefore, the mechanism that causes the bog at TGE to be a more effective capturer/retainer of atmospherically deposited Hg than the bog at EGR appears to still be operating. Comparison of the ratios of the average pre-AP and average AP Hg ARs shows that the increases at the two sites are in general agreement (Figure 8a). An increase from average pre-AP to average AP values by factors of 13 and 11 is observed at EGR and TGE, respectively. Also, the ratios of maximum AP values to preAP values are similar for the two sites. Maximum increase factors of 30× at EGR and of 28× at TGE were observed (Table 3). Thus, despite the fact that the two bogs differ in their ability to accumulate atmospheric Hg, have different pH gradients (67), and also have different trophic states in their deeper layers (36, 68), they both indicate similar chronologies of Hg contamination (with anthropogenic emissions forcing Hg AR out of its natural range during the 14th century) and 242

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similar factors of increase from the pre-AP to the AP. This supports the use of peat bogs as archives of changing atmospheric Hg deposition. However, these findings also show that the absolute value of the Hg accumulation rates in the peat depend to a large extent on minor, bog-specific details that are as yet poorly understood. Therefore, peat bog archives should not be used as indicators of actual atmospheric Hg deposition rates but rather as (i) indicators of natural variations in atmospheric Hg deposition rates and (ii) indicators of the extent to which atmospheric Hg deposition rates have changed since the advent of anthropogenic emissions. This also means that comparisons of Hg ARs in different bogs along geographical transects may yield misleading results, with the differences between sites being due to minor differences between the bogs themselves rather than to differences in the rates of atmospheric Hg deposition at the different sites. Particular care should be taken when comparing sites with a significant tree canopy, as in the case of continental raised bogs (69). (c) The Late 19th and Early 20th Centuries. Figure 8 shows the Hg AR for EGR and TGE during the 20th century in detail. Both profiles match well with the chronology of past world Hg production (50) (Figure 8b). The chronologies of the two profiles at EGR and TGE do not match one another perfectly though. For the peak at the beginning of the century, this could be due to the lack of trustworthy dates for this period (only the basal 210Pb date was used due to the disagreement between 14C AMS bomb pulse dates and 210Pb

FIGURE 9. Map showing the location and active periods of the steam railways that surrounded the bogs in the past. radiometric dates for this core) increasing error in the agedepth relationship. The peak centered around ca. 1910 AD is probably due to pollution from coal burning (70). This can be concluded from the suite of elements that peak at this point in the peat (Pb, As, Se, Br, Ti, Ni, Cu, Zn, and Ga) and the lead isotope data. However, it is unlikely that the pollution is the result of long-range pollution from industrialized nations surrounding Switzerland: The peak in Hg and other trace elements continues to decline following World War I (19141918), whereas coal production in all nations surrounding Switzerland (with the exception of Austria) increased between the wars to reach levels even greater than before World War I (71). Therefore, it is more likely that this peak represents a local pollution source. The most likely local source of pollution from coal burning for this period is steam trains. The timing of the introduction of steam trains to the area (1874-1904 AD) matches the chronology shown in the peat core records. Also, as steam trains in Switzerland were electrified following World War I (72), the decline in the trace element ARs from ca. 1910 onward also fits the historical record. Figure 9 shows the steam railways that exist around the sites and their dates of construction and electrification. A local source such as coal burning in steam trains could also explain the fact that this is the only period for which the EGR and TGE profiles have similar Hg ARs. As can be seen in Figure 9, EGR is closer to the steam railways than TGE. As the prevailing wind and precipitation direction is southwesterly (40), airborne particles are transported first toward EGR and then TGE. EGR would therefore have experienced higher atmospheric Hg concentrations. This could have evened out the Hg ARs of the two bogs by counteracting the higher efficiency of TGE in scavenging/trapping atmospherically deposited Hg. (d) The Late 20th Century. Swiss Hg emission estimates are available for the entire 20th century (73). The estimated increase in Swiss Hg emissions (Figure 8) during the first half of the 20th century has been attributed to emissions from the chlor-alkali industry and open garbage incineration. The high Hg emissions during the 1970s and 1980s on the other hand are mainly attributed emissions from municipal garbage

incinerators. These emissions were reduced from 1980 onward, when measures were taken to reduce Hg levels in flue gas emissions. In France, Hg emissions have also gone into decline recently because of the introduction of measures to reduce Hg in flue gas emissions (74). However, the EGR and TGE profiles fit better to trends in world Hg production (50, 74) than the Swiss emissions estimates (Figure 8). This suggests that the high Hg AR shown by the two profiles for the latter 20th century is mainly due to transboundary pollution. A shoulder peak (Figure 8) appears on the latter side of the main late 20th century peak in both profiles in the 1980s. This peak corresponds in time to the period during which Swiss emissions were at their highest (72) and world Hg production was generally decreasing (74). Therefore, this shoulder peak is likely to be due to regional pollution. Comparisons of modern Hg ARs from the two profiles with background ARs and historical records have therefore provided (i) validation of peat bogs as reliable archives of changes in rates of atmospheric Hg deposition, (ii) an indication of the relative importance of natural and anthropogenic sources, and (iii) an indication of the relative importance of local and transboundary pollution sources to the region.

Acknowledgments Financial support for this work, including graduate student assistantship to F.R.-B., was provided by the Swiss National Science Foundation (Grants 21-55669.98 and 21-061688.00 to W.S.) The authors gratefully acknowledge N. Givelet, Dr. W. O. van der Knaap, Dr. P. Grosvernier, Dr. P. Steinmann, Dr. B. Eilrich, H. P. Ba¨rtschi, A. Leichti, Dr. A. Cheburkin, H. Kurzel, Dr. P. Appleby, Prof. J. Heinemeier, and Dr. G. Bonani for help with research, field, and laboratory work; B. Grose for administrative assistance; and three anonymous reviewers for their helpful critique.

Supporting Information Available Tables 1 and 2 show details of the dated samples and dating results from cores EGR2G and TGE. This material is available free of charge via the Internet at http://pubs.acs.org. VOL. 37, NO. 2, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Received for review July 22, 2002. Revised manuscript received October 21, 2002. Accepted October 25, 2002. ES0201496