chapter 20 mercury in mires
TRANSCRIPT
Peatlands: Evolution and Records of Environmental and Climatic Changes
I.P. Martini, A. Martınez Cortizas, W. Chesworth, Editors
r 2006 Elsevier B.V. All rights reserved.
ISSN: 0928-2025 DOI: 10.1016/S0928-2025(06)09020-1473
Chapter 20
Mercury in mires
H. Biester�, R. Bindler and A. Martınez Cortizas
Institute of Environmental Geochemistry, University of Heidelberg, Im Neuenheimer Feld 236,
69120 Heidelberg, Germany
Introduction
Long-range transport of mercury (Hg) emitted from anthropogenic sources is welldocumented. Coal burning, waste incineration, and chlor-alkali plants located atmid-latitudes are common sources, and the atmospheric transport of mercury toremote areas and subsequent deposition are well studied (Nriagu and Pacyna, 1988;Steinnes and Andersson, 1991; Slemr and Langer, 1992; Mason et al., 1994; Pacynaand Keeler, 1995; AMAP, 1998; Hermanson, 1998). Although there are uncertaintiesover the importance of natural geologic sources for the mercury found in peat andlake sediments (Rasmussen, 1994; Fitzgerald et al., 1998), it is now well accepted thatthe evidence for an increase in anthropogenic mercury emissions relative to naturalsources since the Industrial Revolution, is unambiguous. In the early 1990s, Mason etal. (1994) estimated that 70–80% of the modern atmospheric mercury flux can beattributed to anthropogenic sources.
A better understanding of the behavior of mercury in the environment is neededfor a number of reasons. For example, increased biomagnification of mercury inaquatic food chains, especially in fish, and enhanced accumulation in remote areassuch as the Arctic have been observed in the last few decades. Mercury toxicity inaquatic ecosystems is of particular concern, with the role of methylmercury (MeHg)being critical. This compound can be concentrated by more than a million times inthe aquatic food chain (Grigal, 2002).
Biogeochemical studies and monitoring programs, which include direct measure-ments of wet deposition or indirect measurements based on biomonitoring of forestmosses, have established that anthropogenic activities have affected the global cy-cling of mercury. Although a precise link has yet to be made between the increasedcontent of mercury in biota and the increased accumulation rates observed in naturalenvironmental archives, such as peat, lake sediments, and glacial ice, there is broadconsensus that these archives provide a means to reconstruct atmospheric depositiontrends at local, regional, and global scales (Jackson, 1997; Fitzgerald et al., 1998).Most importantly, the natural archives allow us to look at aspects of the mercurycycle on timescales not available in monitoring programs.
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�Corresponding author. E-mail address: [email protected] (H. Biester).
This chapter gives an overview of the present knowledge of mercury enrichment inmires and bogs, which may be influenced both by external and internal processes,and progress in determining past deposition rates of mercury. Bogs in particular havebeen widely used to evaluate long-term records of atmospheric mercury depositionwith the goals of estimating the natural, pre-anthropogenic flux of mercury andquantifying the increase in atmospheric mercury fluxes during the industrial age. Theuse of data from bogs has definite advantages over other archives. For example, lakesediment cores, unlike ombrotrophic peat cores, are influenced by additional inputsto the lake from the surrounding catchment, and separating atmospheric fromcatchment contributions of mercury in lake sediment studies is a difficult and com-plicated task (Swain et al., 1992; Fitzgerald et al., 2005). Ice cores, on the other hand,have the disadvantage of a limited geographical distribution. Bogs are found atnearly all latitudes where precipitation is sufficient, and consequently, they havereceived increasing attention as archives of historical deposition of metals.
Atmospheric deposition and retention of mercury
Three major mercury species occur in the atmosphere: gaseous elemental Hg(0),gaseous inorganic Hg(II) compounds (reactive gaseous mercury), and particulate-phase mercury (Schroeder and Munthe, 1998). Elemental mercury is dispersed glo-bally, and its residence time in the atmosphere is estimated to be 1–2 years (Fit-zgerald and Mason, 1996), which is sufficiently long to allow some mixing betweenthe northern and southern hemispheres. Transport of Hg(II) and particulate mercuryranges between tens to hundreds of kilometers depending on particulate size andmass (Schroeder and Munthe, 1998). More than 95% of atmospheric mercury isHg(0), but the relative amounts of mercury species are source dependent and can bealtered by oxidation–reduction reactions (Iverfeldt and Lindqvist, 1986; Munthe,1992).
Although atmospheric mercury is dominated by Hg(0), gaseous Hg(II) is muchmore soluble and is the dominant form in precipitation (Porcella, 1994; Fitzgeraldand Mason, 1996). Wet deposition of mercury may vary to a large extent dependingon local emission sources and weather conditions, but most values for recent dep-osition in the literature range between 5 and 15 mgm�2 yr�1 in the northern hem-isphere, with a mean value of 10 mgm�2 yr�1 (Grigal, 2002). Data on dry mercurydeposition are comparatively rare, but Lamborg et al. (1995) estimated that about25% of the total mercury deposition directly to lakes in north central Wisconsin(USA) was dry deposition of particulate mercury, and Rea et al. (2001) estimatedabout 20% of the mercury deposited as throughfall in a mixed forest setting was drydeposition.
The retention of mercury by peat in general and more specifically under changingsurface wetness has not been investigated. Because of the high affinity of mercury tobind to humic substances it is most likely that mercury in peat is mainly retainedthrough binding to humified organic matter (Benoit et al., 1994), preferentially toreduced sulfur groups (Skyllberg et al., 2003), and that plant uptake is of minorimportance. Moreover, it is generally assumed that organically bound mercury is
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H. Biester, R. Bindler and A. Martınez Cortizas474
stable and immobile in peat as soon as it enters the catotelm, the permanently anoxicpart of a bog. Little is known about the occurrence of defined mercury species inpeat. Formation of Hg(0) as a result of Hg(II) reduction by humic acids (Alberts etal., 1974; Allard and Arsenie, 1991) and subsequent degassing most likely occurs as isthe case for soils. Martınez Cortizas et al. (1999) assumed that the existence of Hg(0)and labile Hg(II) species in peat is dependent on climatic conditions. They concludedfrom thermal release experiments that Hg(0) is the dominant form of Hg in peatformed during cold and dry climates, whereas Hg(II) species are dominant in warmerand wetter periods. Although production of methylmercury (CH3Hg+) in mires andits importance for surface waters is an important area of research (Branfireun et al.1999), there are few data on mono- or di-methylmercury in ombrotrophic peat. Inone Swedish bog (Bindler, unpublished data) concentrations of methylmercury are inthe range from 0.1 to 12 ng g�1, which represents from 1% to as much as 35% of thetotal Hg in the peat samples. This range is similar to values reported for uplandpeatlands and mires (r7 ngmeHg g�1; Branfireun and Roulet, 2002).
Mercury concentrations in peat
Concentrations of mercury in peat do not necessarily reflect atmospheric depositionbecause concentrations do not depend solely on atmospheric fluxes, but also on netpeat accumulation rates (Biester et al., 2003). Moreover, atmospheric mercurymainly exists in a gaseous form (Hg(0)) and only a small portion is bound to par-ticles. Because mercury and other crustal metals are completely decoupled, mercuryconcentrations cannot be normalized to conservative elements such as titanium oraluminum to compensate for changes in peat accumulation and to distinguish be-tween natural and anthropogenic fractions, as has been done for other metals such aslead (Shotyk et al., 2002).
Mercury concentrations in ombrotrophic peat dated to pre-industrial (prior toca. 150 cal yr BP) or pre-historical times (ca. 2000 cal yr BP or earlier) show generallylow values in the range of ca. 5–50 ngHg g�1. Important natural sources of atmos-pheric mercury are volcanic eruptions, degassing of the Earth crust, emission fromsoils, and biologically induced evasion of mercury from the oceans (Vandal et al.,1993). In some studies, increased mercury concentrations in pre-industrial peat sec-tions have been assigned to volcanic eruptions ( QA :1Roos-Barraclough et al., 2002), arelationship that can be seen in the recent glacial record from the Upper FremontGlacier in Wyoming, USA (Schuster et al., 2002), but other peat studies see no directlink between volcanic emissions and mercury enrichment in peat ( QA :2Pheiffer-Madsen,1981; Biester et al., 2003).
Higher mercury concentrations in the uppermost (youngest) peat are attributed toincreased atmospheric deposition caused by anthropogenic emission. In contrast,higher values in the deepest and often more minerogenic parts of peat cores areconsidered to be influenced by external factors such as influxes from groundwater orweathered bedrock. Mercury concentrations in peat formed in the past 200 yearsvary from o100 to 4700 ng g�1 (Table 20.1). Mercury concentrations are generallyhigher in the mid-latitudes of the northern hemisphere, where most of the emission
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Mercury in mires 475
sources are, and the enrichment of mercury in peat in northern Scandinavia (Jensenand Jensen, 1991; Bindler et al., 2004) and the Arctic indicates mercury transportfrom mid- to high latitudes.
Single peat cores are commonly used to reconstruct regional mercury depositionrates, because peat cores are assumed to reflect absolute deposition rates. However,in a comparison of mercury records from nine hummock cores collected from a2000m2 area in a Swedish bog, Bindler et al. (2004) showed that the maximummercury concentration and cumulative inventories (the total amount of mercuryaccumulated per square meter during the past 110 years) both vary by a factor of 4among the cores (maximum concentrations from 130 to 460 ngHg g�1 and 110-yearinventories of 0.85–3.4mgHgm�2). Lead concentrations and inventories varied also,but the difference among cores was within a factor of 2. This variation for mercurywithin the investigated bog was greater than the difference between any one of thesecores and single cores measured from other bogs located within a distance of 60 km.
QA :3Malmer and Wallen (1999) observed similar within-bog spatial variations for at-mospherically supplied 210Pb, which were related to micro-topography. Bindler et al.(2004) attributed the large variations in mercury concentrations and inventories tosmall spatial-scale differences in vegetation and micro-topography on the surface ofthe bog suggesting an influence on the interception and retention of mercury. Theyconcluded that reconstructions of past mercury accumulation that are based only onsingle peat cores do not necessarily provide a representative flux for the entire bog.To overcome the potential constraints of single records, they recommended that datafrom multiple sites or at least multiple cores be used to scale up to regionally validmodels of past mercury deposition, which is an approach commonly used in pale-olimnology (Lamborg et al., 2002; Rippey and Douglas, 2004).
In addition to these differences in solid-phase concentrations, Branfireun (2004)found that porewater chemistry (methylmercury, sulfate, and DOC) varied spatiallyin relation to micro-topography. Such a variation in chemistry suggests that internal
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Table 20.1. Geographical variability of mercury concentrations in bogs and fens.
Location Hgng g�1 Peat type Age(cal yr BP)
Reference
Denmark 67–741 Ombrogenic 200 Pheiffer-Madsen (1981)Denmark 5–280 Ombrogenic 3000 Shotyk et al. (2003)Greenland 15–180 Minerogenic 3000 Shotyk et al. (2003)Scandinavia 12–194 Ombrogenic 120 Jensen and Jensen (1991)Switzerland 10–280 Ombrogenic 12,000 Roos-Barraclough et al. (2002)Switzerland 40–280 Minerogenic 2000 Roos-Barraclough et al. (2002)Canada 20–100 Minerogenic 5900–800 Givelet et al. (2004)Sweden 10–260 Ombrogenic 5500 Bindler et al. (2003)S Chile 10–157 Ombrogenic 15,000 Biester et al. (2003)S Chile 60–570 Minerogenic 11,000 Franzen et al. (2004)Spain 20–405 Ombrogenic 5000 Martınez Cortizas et al. (1999)Minnesota 40–280 Ombrogenic 1750 AD (210Pb date) Benoit et al. (1994)Faroe Islands 70–500 Minerogenic 5240 Shotyk et al. (2005)Slovenia 74–380 Ombrogenic n.d. Biester (unpub.)
H. Biester, R. Bindler and A. Martınez Cortizas476
processes, such as the bacterially mediated processes that produce methylmercury,may be important for peat chemistry.
Mercury concentrations and peat decomposition
Mercury concentrations in peat can be strongly influenced by peat decomposition(Biester et al., 2003). Mercury is enriched in highly decomposed peat sections,whereas concentrations are lower in relatively less decomposed peat sections becausethe loss of peat mass is lower. Based on this, most changes in mercury concentrationsin bogs have been explained by differences in peat decomposition (mass loss). Dia-genetic effects within the peat are superimposed on any changes in mercury con-centration that may be related to past changes in atmospheric fluxes (wet and drydeposition). This relationship between decomposition and mercury concentrationscan be shown for bogs from different locations. Figure 20.1 shows depth profiles ofmercury concentrations and carbon/nitrogen (C/N) ratios in peat cores from Chile(Magellanic Moorlands). The C/N ratio is commonly used as an indicator for thedegree of decomposition of peat and mass loss (Kuhry and Vitt, 1996; Malmer andWallen, 2004), where lower C/N ratios indicate a higher degree of peat decompo-sition (greater carbon loss). In the three peat cores, lower C/N ratios are clearlyassociated with higher mercury concentrations.
Peat decomposition and mass loss in bogs is mainly controlled by changes in thewater table (surface wetness). Peat decomposition is highest during dry periods, whenformerly anaerobic peat sections become aerated and metabolized (Malmer andWallen, 2004). Thus, mercury is enriched in highly decomposed peat sections as aresult of mass loss (carbon) during mineralization of organic matter. Mercury en-richment results from aerobic decay related to a lowered water table, which affects
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Figure 20.1. Historical mercury concentrations in peat compared to peat decomposition (C/N ratios) in
historical peat profiles taken from bogs in southern Chile (Skyring, Patagonia), Spain (PDC), and Sweden
(Dumme Mosse).
Mercury in mires 477
previously deposited mercury and peat. This finding is opposite to what is expectedfrom changes in atmospheric deposition, where higher fluxes and higher concentra-tions of metals in peat are expected during wet periods and vice versa. Whereas thewater table in a bog may normally fluctuate by about 10–15 cm (Damman, 1978),which might affect peat that is tens to a hundred years in age; lower water tables indry periods and the aeration of formerly anaerobic peat can affect peat that is up to athousand years old; and the subsequent enrichment of mercury is always youngerthan the deposition of mercury.
The role of peat decomposition in the enrichment of mercury in peat also becomesclear when mercury concentrations in minerogenic mires (fens) are compared tothose in bogs located in the same area. An important assumption in such a com-parison is that the fens and bogs in question are sufficiently close that relativelysimilar levels of atmospheric deposition occur. A second assumption is that ground-water is not an important additional source of mercury, which is reasonable given thelow hydraulic conductivity and low porewater mercury concentrations typicallymeasured in peat (Branfireun et al., 1999). As compared to peat in bogs, peat in fensusually shows a higher degree of decomposition because the higher availability ofnutrients and the higher pH in fens support more intense decomposition by micro-organisms. The higher degree of peat decomposition and also the higher turnover ofbiomass in fens cause a stronger enrichment of mercury than in bogs. Figure 20.2shows historical mercury records derived from a fen (GC2) and a bog (GC1) inPatagonia located within ca. 1 km of each other. Mercury concentrations in the fenare on average four times higher than in the bog. Although some of the mercury inthe fen may be from groundwater, the most important explanation for a fourfoldhigher concentration is that the lower net mass accumulation in the fen peat ascompared to the bog indicates a higher turnover of organic mass, and that mercury isenriched in the fen peat during the mineralization of the organic matter.
Support for an enrichment of mercury in fen peat is provided by other studieswhere peat from mires that are geographically close have been examined. Table 20.1shows the generally higher mercury concentrations in minerogenic peat relative toombrotrophic peat from comparable locations. Even for raised bogs that were in-itially fens, higher mercury concentrations typically are found in the underlyingminerotrophic peat. For example, in Dumme Mosse (Fig. 20.1; Bindler, 2003) mer-cury concentrations in the underlying fen peat (4350 cm depth) are in the range of25–230 ng g�1 as compared to values of about 10 ng g�1 in the overlying ombrotro-phic peat (from 60 to 350 cm depth). Furthermore, Biester et al. (2003) observed asixfold increase in mercury concentrations in the peat ca. 25 cm above a tephra layerin a peat record from Patagonia. The tephra layer provided a ready source of nu-trients that contributed to increased microbial peat decomposition, which correlateswith a higher concentration of mercury. The peat section with enriched mercuryconcentrations, which dates to ca. 2700 cal yr BP, does not coincide with any regionalvolcanic activity, thereby excluding volcanic emissions as an explanation for thisenrichment (Biester et al., 2003).
The relationship between peat decomposition and mercury concentrations clearlyindicates that the concentration of mercury in peat is significantly influenced bydiagenetic processes (in Dumme Mosse, for example, C/N ratios and mercury
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H. Biester, R. Bindler and A. Martınez Cortizas478
concentrations are highly correlated (R2 of 0.67)). The effects of these processes haveto be considered if mercury concentrations in different bogs are to be compared andused to study differences in atmospheric fluxes.
Although the mass balance of mercury in lakes and lake sediments is well re-searched (Fitzgerald et al., 2005), in bogs it is not understood. For example, based onbackground concentrations in peat in the range of 10–30 ng g�1 and the fact thatdeeper peat only constitutes 10–20% of the original peat mass, the original mercuryconcentrations must have been in the range of only 0.5–3 ng g�1. This range is quitelow in comparison to the concentrations measured in the uppermost peat and livingplant material, typically ca. 40–200 ngHg g�1.
Mercury accumulation rates
Mercury accumulation rates in peat cores are considered to reflect atmospheric fluxesmore accurately than concentrations, because it is assumed that accumulation rates
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Figure 20.2. Historical records of mercury in a bog (GC1) and a fen (GC2) in the same area of the
Magellanic Moorlands, Chile (after Franzen et al., 2004).
Mercury in mires 479
are independent of net mass accumulation. Most of the data on mercury accumu-lation in bogs are from sites located in the northern hemisphere (Grigal, 2003; ref-erences in Table 20.2). Data from the southern hemisphere are still very limited.Median pre-industrial mercury accumulation rates in peat are about 1 mgm�2 yr�1
and vary within a comparatively small range of 0.6–1.7 mgm�2 yr�1 at most sites(Table 20.2). This accumulation in peat is about one-third of the background at-mospheric mercury deposition rate of 3–3.5 mgm�2 yr�1 estimated from lake sedim-ents, after the accumulation rate of mercury in lake sediments has been corrected forthe influence of catchment size and sediment focusing (Engstrom et al., 1994; Loreyand Driscoll, 1999; Lamborg et al., 2002; Fitzgerald et al., 2005). A clear relationshipbetween background mercury accumulation rates and climate conditions or geo-graphical location remains elusive.
Modern mercury accumulation rates in bogs at different locations vary across awide range from 16 to 184 mgm�2 yr�1, which corresponds to an average increase inthe industrial age by a factor of 69 (9–410, median 38) compared to the backgroundvalues in peat (Table 20.2). The highest mercury accumulation rates in the industrialperiod occur at higher latitudes, where they exceed those found in bogs of mid-latitudes by factors of ca. 3–4. The highest reported modern mercury accumulationrates are from a bog in Denmark (184 mgm�2 yr�1) and a fen in southern Greenland(164 mgm�2 yr�1) (Shotyk et al., 2003).
The high modern mercury accumulation rates reported for some Arctic sites, suchas southern Greenland, suggest a long-range transport of mercury from middle tohigher latitudes, as is also reported for other contaminants such as persistent organicpollutants (Mackay et al., 1995). However, the average mercury accumulation ratederived from bogs is a factor of 6 (up to 18) higher than the average mercury
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Table 20.2. Mercury accumulation rates derived from bogs and fens at different locations.
Location Background(mgm�2 yr�1)
Maximum(mgm�2 yr�1)
Factor ofincrease
Reference
SW Greenland 0.4 164 410 Shotyk et al. (2003)Patagonia, Chile 1 63 63 Biester et al. (2003)Ontario, Canada 1.4 54 39 Givelet et al. (2003)Ontario, Canada 1.4 89 63.4 Givelet et al. (2003)Ontario, Canada 1.4 141 101 Givelet et al. (2003)Minnesota, USA 4.3 38 9 Benoit et al. (1994)NW Spain 0.8 87 109 Martınez Cortizas et al. (1999)NW Spain 1.8 144 80 Martınez Cortizas et al. (1999)NW Spain 0.8 44 55 Martınez Cortizas et al. (1999)S. Sweden (average) 0.73 37 50 Bindler et al. (2004)DM 0.6 24 41 Bindler (2003)SM1 0.8 16 20 Bindler et al. (2004)SM2 0.8 44 55 Bindler et al. (2004)SM3 0.8 63 79 Bindler et al. (2004)TM 0.9 29 32 Bindler et al. (2004)LM 0.6 Bindler (2003)Denmark – 184 Shotyk et al. (2003)EGr, Switzerland 1 29 29 Roos-Barraclough and Shotyk (2003)TGe, Switzerland 1.6 43 27 Roos-Barraclough and Shotyk (2003)Mean 1.2 64 68Median 0.5 43 37
H. Biester, R. Bindler and A. Martınez Cortizas480
deposition rates in the 1990s of ca. 10 mgm�2 yr�1, determined from direct meas-urements of wet deposition (Grigal, 2002). There is also poor consistency in thetiming of the maximum Hg accumulation rate in the late 20th century derived frombogs at different locations. Whereas peat, lake sediment, and ice data show a con-sistent record for the past deposition of lead and its maximum influence about 1970(Weiss et al., 1999; Renberg et al., 2001), the peak accumulation of mercury variesfrom 1950 to the present both between and within regions. This temporal variationcould be the result of the proximity of specific sites to point sources and differences inemission histories, but could also result from differences in dating methodology andmodeling. For example, 14C bomb-pulse-dated cores generally suggest an earlierpeak in mercury deposition in the 1950s (Givelet et al., 2003; Shotyk et al., 2003),whereas 210Pb-dated cores suggest a peak two decades later (Norton et al., 1997;Roos-Barraclough and Shotyk, 2003). Dating of peat cores and modeling chronol-ogies are clearly important for detailed comparison of trends and accumulation rates.
Mercury accumulation rates and influence of peat diagenesis
The term ‘mercury accumulation rate’ (or more accurately, the net accumulationrate) describes the amount of mercury that has been retained in a peat section perunit area within a given time period. The calculation of mercury accumulation ratesis based on mercury concentration, sample thickness and density, and the time rep-resented by a peat layer. This definition neglects all processes that cause losses ofmercury through different pathways, such as the release of mercury by soluble or-ganic complexes or re-emission as volatile Hg(0). It does not consider any changes inmercury retention caused by different plants, changes in the bog topography (par-ticularly in regard to the specific coring site), or changing climatic conditions. The useof mercury accumulation rates to reconstruct past atmospheric mercury depositionassumes that all variables except deposition are not altered through time; that is, thenet accumulation of mercury is the same as the original deposition rate. As statedearlier, the common assumption is also that atmospheric mercury deposition and thesubsequent accumulation of mercury in peat are uniform across the surface area of abog. This requires that the factors used to calculate mercury accumulation rates (peatmass accumulation and mercury concentrations) show a linear relationship throughhundreds or thousands of years of peat formation and decomposition.
Organic matter in bogs undergoes dramatic changes during its diagenesis. One ofthe major problems affecting metal records in peat is the intense loss of mass duringhumification and mineralization of organic matter and the related enrichment ofmetals. The mass loss is most intense during the initial phase of organic matter decayin the acrotelm, where as much as 90% of the original plant mass is lost, whereasanother 50% of the remaining organic matter is mineralized within a period of ca.2000 years (Kuhry and Vitt, 1996; Malmer and Wallen, 1999, 2004).
In most bogs a strong increase in mercury accumulation rates occurs in the upperpeat sections. In Spain, where the mining of mercury began at least 2000 years ago,an increase in mercury accumulation rates in a bog in the northwest starts as early as2000 years ago, when it increased by 1.5–1.8 times the pre-historical background
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Mercury in mires 481
values (Martınez Cortizas et al., 1999). This early increase may be anthropogenic orit may be natural, but the lack of supporting evidence in other archives such as lakesediment, ice cores, or peat cores from other areas, makes this interpretation un-certain.
A possible explanation for major temporal changes in peat mercury records issuggested by Biester et al. (2003, 2004). They believe that accumulation rates (as wellas concentrations) of mercury and other organically bound elements are influencedby peat decomposition processes. They observed a pre-industrial twofold increase inaccumulation rates of mercury and other elements in bogs located in the MagellanicMoorlands (531S latitude), which could not be explained by pollution or climaticchanges. Since carbon (mass) accumulation rates show a parallel increase to mercuryand other organically bound elements, they concluded that the increase in mercuryaccumulation is probably influenced by the increase in mass accumulation. The re-lationship between mass and mercury accumulation especially in the uppermost peatsections is not fully understood. The parameters used to calculate mercury accumu-lation rates in bogs (density, thickness, and mercury concentration) can changethrough time in a non-linear way. Mass loss in bogs not only increases mercuryconcentrations, but it also prevents density from increasing in deeper peat layers,where compaction is assumed to increase. Another important question in this context
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Figure 20.3. Net accumulation rates of mercury, bromine, selenium, and carbon in a Spanish bog.
H. Biester, R. Bindler and A. Martınez Cortizas482
is, whether the peat mass/mercury relation stays constant through time and space orif there is, for example, lateral mass movement especially at the acrotelm/catotelmboundary, which might cause thinning of peat sections and consequently reductionof calculated accumulation rates. As a first attempt Biester et al. (2003) normalizedmercury accumulation to carbon accumulation rates (correcting for mass loss) andfound that the corrected mercury net accumulation rate in recent peat is about 3–5times greater than in the pre-industrial period. This increase agrees with values de-rived from lake sediments, which also indicate a 3–5 times increase in the late 20thcentury compared to pre-industrial values.
Similarly, increases in mercury accumulation rates of about 30 times in bogs inSpain (Martınez Cortizas et al., 1999) are reduced by 80% when corrected for carbonaccumulation. The method of correcting mercury accumulation rates based on car-bon accumulation could be also applied to other organically bound elements such asbromine or selenium (Fig. 20.3).
Hg accumulation and climate variation
Accumulation rates of mercury and bromine were found to show similar patterns inthe peat cores from Spain (Fig. 20.3), Patagonia (Biester et al., 2003, 2004), andDumme Mosse in Sweden (R2
¼ .55; unpublished data) as well as in peat cores fromSwitzerland and Canada (Roos-Barraclough and Shotyk, 2003; Givelet et al., 2004).For the Swiss and Canadian studies the authors interpreted the variations in bromineaccumulation as reflecting changes in past atmospheric fluxes. Assuming that there isa relationship between natural mercury deposition and bromine deposition, they thencorrected mercury accumulation rates for changes in natural atmospheric fluxes us-ing bromine in order to estimate the increase in mercury caused by anthropogenicemissions. If wet deposition is seen as the most important pathway of atmosphericmercury deposition to bogs, this approach would imply that mercury accumulationrates in peat should increase during wet periods. However, data from Spanish bogssuggest a contrary relationship with climate. The accumulation of mercury, bromine,and selenium correlate with climate proxies (pollen and non-pollen palynomorphs;Mighall et al., in press), indicating that elevated mercury accumulation rates oc-curred during relatively dry periods as a result of increased peat decomposition. Thehigher frequency of low water table events in bogs during dry periods causes in-creased peat mineralization and mass loss and a related increase in mercury con-centrations as a result of aeration of the upper peat layers. The observation that thisprocess affects all organically bound elements to a similar extent explains the goodcorrelation between mercury and bromine in many bogs.
The assumption that bromine can be used to estimate the natural fraction ofmercury deposition is also problematic because the long-term trends in bromineaccumulation in peat do not follow patterns observed in glacial records. This sug-gests that mechanisms other than atmospheric deposition influence the net accumu-lation of bromine in peat (Biester et al., 2004).
In other words, mercury accumulation rates in bogs are not predominately con-trolled by atmospheric deposition. Climatic conditions have an important influence
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on the concentration of mercury in bogs, mainly through changes in peat decom-position and to a lesser extent through variations in derived atmospheric fluxes.Resolving the effects of peat diagenesis on mercury accumulation rates in the peatwould likely improve the comparability of the mercury records in peat and lakesediments.
Outlook
In this chapter, we have focused on the dependency of mercury enrichment in bogson climate. Changes in climate may significantly alter the biogeochemical cycling ofmercury in wetlands and soils and influence its release to aquatic environments. It isclear that we need to understand better the relationship between mercury accumu-lation rates, peat diagenesis, and climate. There are obvious differences in the es-timates of atmospheric mercury deposition between lake sediments and bogs and alsoamong bogs. We suggest that careful incorporation of results from studies of carbondynamics and peat mass accumulation can be an important step for improving ourunderstanding of past mercury cycling in bogs.
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