Published in J. Environ. Qual. 34:192-197 (2005).
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TECHNICAL REPORTS
Atmospheric Pollutants and Trace Gases
Atmospheric Supply of Trace Elements Studied by Peat Samples from Ombrotrophic Bogs
E. Steinnesa,*,
O. Ø. Hvatumb,
B. Bølvikenc and
P. Varskogc
a Department of Chemistry, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
b Department of Soil and Water Sciences, Agricultural University of Norway, NO-1432 Ås, Norway
c Institute for Energy Technology, NO-2027 Kjeller, Norway
* Corresponding author (eiliv.steinnes{at}chem.ntnu.no)
Received for publication February 4, 2004.
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ABSTRACT
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Concentrations of Fe and 12 trace elements in peat from ombrotrophic bogs were used to estimate the atmospheric deposition of these elements on a temporal and spatial scale. Peat samples were collected at 21 different sites in Norway encompassing large geographical differences in marine influence and air pollution. The study demonstrates that surface peat is an excellent medium to study geographical differences in heavy metal deposition, provided that effects of the surface plant cover are properly considered. Long-range atmospheric transport of pollutants is the main source for As, Cd, Pb, Sb, and Zn, and to a lesser extent for Cu and Se. Biogenic emissions from the ocean appear to be the main source of Se to the peat. The metals Co, Cr, Fe, and Ni are mainly associated with windblown local soil dust. Surface enrichment of Mn, and in part Zn, is mainly caused by nutrient circulation between the surface peat and vascular plants growing on it. Deposition of marine salts appears to be the main reason for lower Mn concentrations in the peat near the coast.
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INTRODUCTION
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OMBROTROPHIC BOGS are peatlands where the surface layer receives chemical substances only by dry and wet deposition from the atmosphere. The ombrotrophic surface layer of these bogs, most commonly occurring in the temperate zone, may be as much as several meters thick, in some cases allowing retrospective studies of atmospheric deposition over periods of 5 to 10 thousand years (Shotyk, 1996b). Many have used peat cores from ombrotrophic bogs to study the temporal development in heavy metal deposition (e.g., Lee and Tallis, 1973; Damman, 1978; Aaby et al., 1979; Glooschenko et al., 1986; Shotyk, 1996a; Shotyk et al., 2001). Most studies were on single cores. The use of peat for spatial comparisons of metal deposition has not been common.
Hvatum (1971) first showed the enrichment of Pb in peat surface layers due to atmospheric pollution. Subsequently, an extensive sampling of peat was performed in 1979 at 21 different sites across Norway (Fig. 1)
. Preliminary results from the chemical analyses were presented by Hvatum et al. (1983) and Steinnes (1997). In this paper the distribution of 13 elements (As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, Se, Zn) with depth is presented. In addition to discussing temporal trends in atmospheric deposition, the paper focuses on the feasibility of using surface peat for the study of spatial trends. Moreover, the results reveal information about natural sources in addition to the anthropogenic ones.
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MATERIALS AND METHODS
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Peat samples were collected during the summer of 1979 at 21 sites in Norway (Fig. 1). Samples were collected at regular depth intervals by digging a ditch and sampling peat from a vertical wall of the ditch with a stainless steel spoon, taking care to avoid contamination from overlying peat layers. At eleven sites (1, 3, 4, 6, 10, 11, 13, 15, 16, 18, 19, 20) samples were obtained from four depths: surface layer (referred to hereafter as 3 cm), 10 cm, 20 cm, and 50 cm. At Site 2 the peat was sampled at 10-cm intervals to a 100-cm depth. At the remaining sites (5, 7, 8, 9, 12, 14, 17, 21) only surface or near-surface samples were taken (Table 1). At each site four subsites at a 100-m distance along a line were sampled. The samples were processed and analyzed separately. Sites 1, 2, 3, 6, 10, 11, 13, 16, and 18 were clearly ombrotrophic based on the raised character of the bog. At the remaining sites the ombrotrophic character of at least the surface peat was evident from the character of the plants, in particular the moss Sphagnum fuscum (replaced by Sphagnum neoreum at some coastal sites). The peat surface was defined as the interface between living Sphagnum moss and peat.
The samples were transported to the laboratory in sacking bags, dried in the bags at 35°C, crushed, and sieved (2 mm). Ash content was determined on separate aliquots after dry ashing at 430°C for 19 h. The concentrations of Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn were determined by flame atomic absorption spectrometry after wet-ashing with 7 M HNO3. For the determination of As, Se, Sb, and Hg a radiochemical neutron activation method was used (Steinnes and Njåstad, 1995). At the time of analysis no appropriate reference samples for peat analysis were available, but the procedures used were regularly tested by analysis of international biological and geological reference samples.
The underlying relations between the chemical parameters for the peat samples were investigated using factor analysis. The analyses were performed separately for the 3-, 10-, 20-, and 50-cm depth layers using the "FACTOR" command in the statistical package SPSS (1999). The command was run using Principal Component extraction, 1 as the eigenvalue cut-off value, and VARIMAX rotation of the extracted factors.
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RESULTS AND DISCUSSION
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The ash contents and the concentrations of the 13 elements in the surface peat (3 cm) of the 21 bogs are shown in Table 1. Vertical distributions of the 13 elements in a continental bog in the south (2) and a coastal bog in the north (18) are shown in Fig. 2, 3, and 4
. The metal concentrations, in addition to varying with depth, generally show characteristic differences between south and north in the country and between near-marine and more continental sites.
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Fig. 2. Vertical distribution of Cr, Co, Mn, Ni, Fe, and Cu in a southern continental bog (Site 2, open bars) and a northern oceanic bog (Site 18, filled bars)
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Fig. 3. Vertical distribution of Zn, As, Se, and Cd in a southern continental bog (Site 2, open bars) and a northern oceanic bog (Site 18, filled bars).
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Fig. 4. Vertical distribution of Sb, Pb, and Hg in a southern continental bog (Site 2, open bars) and a northern oceanic bog (Site 18, filled bars).
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The ash content is generally less than 3% of the dry matter, consistent with the ombrogenous character of the samples. The surface samples in most cases have higher ash content than the deeper layers, most markedly in bogs with neighboring agricultural land (2, 6, 7, 13). The higher surface content of ash may be due to either of the following reasons: windblown soil dust from open agricultural fields or other areas with sparse vegetation, higher deposition of air pollutants during recent times than in preindustrial time, or greater concentrations of nutrients in the surface layer due to the action of vascular plants circulating them between the peat and living plant tissue (referred to hereafter as "vascular pump"). Below the 10-cm depth the ash level is low and relatively constant in most cases, a fact that indicates an ombrotrophic character of the peat at least down to the lowest sampled layer. Bogs near the ocean (e.g., 6, 10, 11, 17, 18, 19) generally have higher ash contents in the peat than those located farther away, possibly because of the higher input of marine anions and cations 
through wet and dry deposition and subsequent retention in the peat.
The Fe concentrations generally follow the ash content in the upper part of the peat, supporting the assumption that windblown soil dust is an essential factor in the cases of elevated surface ash. At several sites (8, 20, 21), increased Fe concentrations in deeper layers may indicate that the peat is not ombrogenous at this depth, or could mark a redox boundary between anoxic and oxic peat. Chromium, Co, and Ni have distribution patterns similar to that of Fe.
Among the trace elements, Pb shows a pattern that strongly supports the hypothesis (Hvatum, 1971) that air pollution is the strongly predominant source, because the surface concentration is substantially higher than at depth, and the likelihood of upward transport of Pb by plants through root uptake is very small. Moreover, isotopic studies have shown that Pb is nearly immobile in peat profiles (Shotyk et al., 1997). The 10- to 20-times-higher surface concentration in the south points to a main contribution from long-range atmospheric transport from other parts of Europe (Steinnes, 1997, 2001). Antimony, As, Cd, Zn, and to a lesser extent Cu and Se, show trends similar to that of Pb, supporting conclusions from work on aerosols (Amundsen et al., 1992) and atmospheric deposition (Schaug et al., 1990) that long-range atmospheric transport is a major source of these elements in Norway. For Zn the surface concentrations do not decrease as much with higher latitude as for the other elements, indicating that the vascular pump may be a second contributing factor for this element in the surface peat. A similar less steep south–north gradient for Cu may be explained by a smaller contribution from long-range atmospheric transport for this element than for the others (Steinnes, 2001). For Se, there is a coast-to-inland gradient, which had also been previously observed in samples of natural surface soils (Låg and Steinnes, 1974). More recent research (Cooke and Bruland, 1987; Amouroux and Donard, 1997) has shown evidence of biogenic emissions of volatile organoselenium compounds from the ocean, which may be the source of excess Se in the terrestrial environment near the coastline (Steinnes, 1997, 2003).
The distribution of Mn is unique, with a stronger surface enrichment than any of the other studied elements at most of the sites. No clear geographical trend is evident for this element except for very low surface concentrations at the most strongly oceanic sites (7, 9, 10, 11, 18) and 10-fold higher values at sites (20, 21) that have a more continental climate with low annual precipitation. Apparently Mn is very mobile in the peat, and the consistent strong enrichment in the 3-cm layer points to the vascular pump as the driving force. The surface depletion at coastal sites is most probably due to replacement by marine cations such as Mg2+ on the peat cation exchange complex.
The main factors resulting from the factor analysis and their interpretations are shown for each depth in the following (explained variance in parentheses):
- 3-cm depth:
- Factor 1 (54.3%): As, Cd, Pb, Sb, Zn > Cr, Cu, Hg, Ni, Se. Air pollution factor, dominated by contribution from long-range atmospheric transport.
- Factor 2 (27.6%): ash, Co, Fe. Factor related to windblown soil particles, with a particularly high score for Bog 6, situated next to agricultural land. The site is frequently exposed to strong wind, and snow cover in winter is not permanent.
- Factor 3 (9.4%): -Mn. Strong concentration in the surface layer due to the vascular pump.
- 10-cm depth:
- Factor 1 (44.0%): Cd, Sb, Pb, As, Zn, Cu > Ni, Hg. Air pollution factor, dominated by contribution from long-range atmospheric transport.
- Factor 2 (27.4%): ash, Co, Fe, > Cr. Windblown soil particles.
- Factor 3 (14.4%): -Mn, Se. Marine factor. Replacement of nutrient Mn by sea-salt cations, particularly at sites close to the sea, and enrichment of Se at coastal sites due to biogenic emission from ocean water.
- 20-cm depth:
- Factor 1 (34.3%): Co, Cr, Cu, Ni. High scores for Bogs 20 and 21, probably reflecting the bedrock in this area and thus indicating non-ombrotrophic behavior of these bogs at this depth.
- Factor 2 (25.4%): As, Cd, Pb, Sb, Zn. Air pollution factor, dominated by contribution from long-range atmospheric transport.
- Factor 3 (13.3%): Se > Hg. Atmospheric deposition from natural processes.
- Factor 4 (11.8%): ash, Mn > Fe. Windblown soil particles; Mn associated with a particulate phase in this case.
- 50-cm depth:
- Factor 1 (39.9%): Co, Cr, Cu, Fe, Hg, Ni. Same as Factor 1, 20 cm.
- Factor 2 (21.8%): -As, -Zn > ash, -Pb. This factor has high negative scores at Sites 1 and 2 where the influence of long-range transport of pollutants is significant, and positive scores at some of the more oceanic sites (6, 10, 15, 17, 18, 19).
- Factor 3 (11.8%): As. Difficult to explain.
- Factor 4 (11.1%): -Cd, Mn. Difficult to explain.
The samples from the present investigation were not dated, and thus cannot give accurate information about temporal trends. However, more recent age determinations of peats from ombrotrophic bogs in Norway by means of 210Pb (Dunlap et al., 1999) and 14C (work in progress) indicate that the ages of the layers sampled in this work were of the following order (years): 3 cm: 25–50, 10 cm: 250–480, 20 cm: 550–900, 50 cm: 1600–2400. The results from the factor analysis thus clearly indicate that long-range atmospheric transport of pollutants to southern Norway was significant already several hundred years before present. Traces of the elements associated with this factor occur even at a 50-cm depth, but the fact that the presumably more mobile As and Cd have higher loadings than the less mobile Pb indicates that the observed traces may at least partly be due to downward leaching. A similar investigation of ombrogenous peats in Sweden, however, employing Pb isotope ratios for the characterization of sources, found evidence of anthropogenic Pb deposition back to 2000 yr ago (Brännvall et al., 1997).
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CONCLUSIONS
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The results from the present work, in addition to providing information about the sources and geographical distribution of individual elements in the surface peat, give rise to the following general conclusions:- Temporal trends in the deposition of strongly bound elements can be determined from their depth distribution in vertical peat cores from ombrotrophic bogs.
- Peat from ombrotrophic bogs is also a suitable medium to elucidate the spatial distribution of elements from air pollution if sampled just below the surface. What is obtained is a value representative of the last few decades, depending on the thickness of the sampled peat layer.
- Similar information can be obtained for elements derived from natural processes such as biogenic Se from the ocean.
- Surface enrichment of some elements may be caused by vascular plants growing on the ombrotrophic bogs transporting nutrient elements to the growing plant from the subsurface peat through root uptake. Such elements (e.g., Mn) tend to be enriched in all surface samples, relative to samples from greater depth. For some elements (e.g., Zn) surface enrichment may be due to a combination of atmospheric supply and upward transport by vascular plants.
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REFERENCES
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- Aaby, B., J. Jacobsen, and O.S. Jacobsen. 1979. Pb-210 dating and lead deposition in the ombrotrophic peat bog. Aarbog Dan. Geol. Unders. 1978:45–68.
- Amouroux, D., and O.F.X. Donard. 1997. Evasion of selenium to the atmosphere via biomethylation processes in the Gironde estuary, France. Mar. Chem. 58:173–188.
- Amundsen, C.E., J.E. Hanssen, A. Semb, and E. Steinnes. 1992. Long-range transport of trace elements to southern Norway. Atmos. Environ. 26A:1309–1324.
- Brännvall, M.L., R. Bindler, O. Emteryd, M. Nilsson, and I. Renberg. 1997. Stable isotope and concentration records of atmospheric lead pollution in peat and lake sediments in Sweden. Water Air Soil Pollut. 100:243–252.
- Cooke, T.D., and K.W. Bruland. 1987. Aquatic chemistry of selenium: Evidence of biomethylation. Environ. Sci. Technol. 21:1214–1219.
- Damman, A.H.W. 1978. Distribution and movement of elements in ombrotrophic peat bogs. Oikos 30:480–495.
- Dunlap, C.E., E. Steinnes, and A.R. Flegal. 1999. A synthesis of lead isotopes in two millennia of European air. Earth Planet. Sci. Lett. 167:81–88.
- Glooschenko, W.A., L. Holloway, and N. Arafat. 1986. The use of mires in monitoring the atmospheric deposition of heavy metals. Aquat. Bot. 25:179–190.
- Hvatum, O.Ø. 1971. Strong lead accumulation in the surface layer of peat soils. (In Norwegian.) Tek. Ukebl. 118(27):40.
- Hvatum, O.Ø., B. Bølviken, and E. Steinnes. 1983. Heavy metals in Norwegian ombrotrophic bogs. Ecol. Bull. 35:351–356.
- Lee, J., and J. Tallis. 1973. Regional and historical aspects of lead pollution in Britain. Nature (London) 245:216–220.[Medline]
- Låg, J., and E. Steinnes. 1974. Soil selenium in relation to precipitation. Ambio 3:237–238.
- Schaug, J., J.P. Rambæk, E. Steinnes, and R.C. Henry. 1990. Multivariate analysis of trace element data from moss samples used to monitor atmospheric deposition. Atmos. Environ. 24A:2625–2631.
- Shotyk, W. 1996a. Natural and anthropogenic enrichments of As, Cu, Pb, Sb, and Zn in rainwater-dominated versus groundwater-dominated peat bog profiles, Jura Mountains, Switzerland. Water Air Soil Pollut. 84:1–31.
- Shotyk, W. 1996b. Peat bog archives of atmospheric metal deposition: Geochemical evaluation of peat profiles, natural variations in metal concentrations, and metal enrichment factors. Environ. Rev. 4:149–183.
- Shotyk, W., A.K. Cheburkin, P.G. Appleby, A. Frankhauser, and J.D. Kramers. 1997. Lead in three peat bog profiles, Jura Mountains, Switzerland: Enrichment factors, isotopic composition and chronology of atmospheric deposition. Water Air Soil Pollut. 100:297–310.
- Shotyk, W., D. Weiss, J.D. Kramers, R. Frei, A.K. Cheburkin, M. Gloor, and S. Reese. 2001. Geochemistry of the peat bog at Etang de la Gruère, Jura Mountains, Switzerland, and its record of atmospheric Pb and lithogenic trace metals (Sc, Ti, Y, Zr, and REE) since 12,370 14C yr BP. Geochim. Cosmochim. Acta 65:2337–2360.[ISI]
- SPSS. 1999. SPSS Base 9.0. SPSS, Chicago.
- Steinnes, E. 1997. Trace elements in ombrogenous peat profiles from Norway: Evidence of long-range atmospheric transport. Water Air Soil Pollut. 100:405–413.
- Steinnes, E. 2001. Metal contamination of the natural environment in Norway from long range atmospheric transport. Water Air Soil Pollut. Focus 1:449–460.
- Steinnes, E. 2003. Biogeochemical cycling of iodine and selenium and potential geomedical relevance. p. 57–60. In H.C.V. Skinner and A.R. Berger (ed.) Geology and health: Closing the gap. Oxford Univ. Press, New York.
- Steinnes, E., and O. Njåstad. 1995. Ombrotrophic peat bogs as monitors of trends in atmospheric deposition of pollutants: Role of neutron activation analysis in studies of peat samples. J. Radioanal. Nucl. Chem. 192:205–213.
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ABSTRACT
INTRODUCTION
