Modeling the Past Atmospheric Deposition of Mercury
Using Natural Archives
H A R A L D B I E S T E R , * ,† R I C H A R D B I N D L E R , ‡
ANTONIO MARTINEZ-CORTIZAS,§ AND
DANIEL R. ENGSTROM|
Institute of Environmental Geochemistry, University of Heidelberg, INF 236,
69120 Heidelberg, Germany, Department of Ecology and Environmental Sciences,
Umeå University, 907 42 Umeå, Sweden, Department of Soil Science and
Agricultural Chemistry, Faculty of Biology, University of Santiago,
E-15782 Santiago de Compostela, Spain, and St. Croix Watershed Research
Station, Science Museum of Minnesota, St. Croix, Minnesota 55047
Historical records of mercury (Hg) accumulation in lake
sediments and peat bogs are often used to estimate human
impacts on the biogeochemical cycling of mercury. On
the basis of studies of lake sediments, modern atmospheric
mercury deposition rates are estimated to have increased
by a factor of 3-5 compared to background values:
i.e., from about 3-3.5 µg Hg m-2 yr-1 to 10-20 µg Hg
m-2 yr-1. However, recent studies of the historical mercury
record in peat bogs suggest significantly higher increases
(9-400 fold, median 40×), i.e., from about 0.6-1.7 µg
Hg m-2 yr -1 to 8-184 µg Hg m-2 yr -1. We compared
published data of background and modern mercury
accumulation rates derived from globally distributed lake
sediments and peat bogs and discuss reasons for the
differences observed in absolute values and in the relative
increase in the industrial age. Direct measurements of
modern wet mercury deposition rates in remote areas are
presently about 1-4 µg m-2 yr -1, but were possibly as
high as 20 µg Hg m-2 yr -1 during the 1980s. These values
are closer to the estimates of past deposition determined
from lake sediments, which suggests that modern mercury
accumulation rates derived from peat bogs tend to overestimate deposition. We suggest that smearing of 210Pb in the
uppermost peat sections contributes to an underestimation
of peat ages, which is the most important reason for
the overestimation of mercury accumulation rates in many
bogs. The lower background mercury accumulation
rates in peat as compared to lake sediments we believe
is the result of nonquantitative retention and loss of mercury
during peat diagenesis. As many processes controlling timeresolved mercury accumulation in mires are still poorly
understood, lake sediments appear to be the more reliable
archive for estimating historical mercury accumulation
rates.
Introduction
Many advances have been made in recent years in the
understanding of mercury biogeochemistry in the contem* Corresponding author e-mail: [email protected].
† University of Heidelberg.
‡ Umeå University.
§ University of Santiago.
| St. Croix Watershed Research Station.
10.1021/es0704232 CCC: $37.00
Published on Web 06/16/2007
 xxxx American Chemical Society
porary environment. For mercury such studies include
environmental manipulations (e.g., METAALICUS project,
(1)), the effects of the polar sunrise (2, 3), and the influence
of dissolved organic carbon and sulfate on methylation
processes (4-6). While most studies emphasize present-day
contamination and biogeochemistry, another area of research
has focused on estimating the natural contribution of mercury
prior to human impacts, which is based on the analysis of
natural environmental archives such as lake sediments
(7-24), peat (25-41), and also glacial ice (42, 43). We exclude
ice from further discussion because of its limited geographic
distribution and the small number of published records. Data
on metal deposition from monitoring programs are temporally very limited with reliable records spanning only the
past decade or two at best. Thus, natural archives provide
the only link between current and past mercury loading to
terrestrial and aquatic environments, and they give us insights
into timescales and spatial patterns not available in contemporary monitoring programs. There is broad consensus
that natural archives provide a means to reconstruct atmospheric deposition trends at local, regional, and even global
scales (23, 44), and that mercury deposition rates are related
to, for example, levels of fish contamination (45-47).
In addition, estimates of the natural background deposition rate of mercury and certain other elementssas well as
subsequent human impactssin lake sediment and peat also
provide us with insights on how natural environmental factors
have influenced deposition and accumulation. Such environmental factors include climate changes (warmer-colder
or wetter-drier) or volcanic activity (29, 43).
Both lake-sediment and peat records show an increase in
mercury accumulation which parallels increasing industrialization during the past two centuries. Whereas studies of
lake sediments suggest an increase in mercury atmospheric
deposition rates that are, in recent decades, about 3-5 times
above natural background (pre-industrial) rates in the
northern hemisphere, studies of peat suggest an increase in
deposition of 30 to as much as 500-fold in recent versus
pre-industrial times.
The disparity in past atmospheric mercury deposition
rates between the archives is problematic for understanding
anthropogenic perturbation of the global mercury cycle. It
affects not only our perception of the magnitude of human
impact, but also our understanding of emission sources,
atmospheric processes, and rates of exchange between
terrestrial, ocean, and atmospheric pools (44, 48, 49). Two
studies of mercury accumulation rates in lakes and a peat
deposit in Greenland serve as an initial example. Greenland
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is a remote area with no significant local anthropogenic
mercury sources, thus the increase in modern atmospheric
deposition of mercury in Greenland archives should reflect
long-range transport from anthropogenic sources in the
northern hemisphere as well as inter-hemispheric mixing.
Based on three lakes in western Greenland, Bindler et al. (19)
reported background mercury accumulation rates in the
range of 1-2 µg m-2 yr-1 and a maximum of modern mercury
accumulation of 5-10 µg m-2 yr-1, which corresponds
generally to a 3-5 fold increase during the industrial period.
Shotyk et al. (37) analyzed a single core from a peat deposit
in southern Greenland and reported lower background values
of 0.3-0.5 µg m-2 yr-1, but a maximum modern mercury
accumulation rate of 164 µg m-2 yr-1, which corresponds to
an increase of ∼300-500 times over background values. This
striking difference in the two archives could hardly be
explained by local differences in mercury deposition, and it
raises questions about the reliability of these archives to reflect
true atmospheric mercury fluxes and the magnitude by which
they have changed.
In this paper we review mercury data from a range of peat
and lake sediment studies that have reconstructed accumulation rates and subsequently modeled past atmospheric deposition rates of mercury. We discuss some
potential reasons for the differences observed between the
two archives. In addition to the mercury studies, we include
anthropogenic lead for comparative purposes.
Comparison of Lake Sediment and Peat
Properties of the Archives. Lake sediments and peat
constitute two substantially different media in terms of
composition, biogeochemistry, and hydrology. Lake sediments represent relatively closed systems once the sediment
has been buried below the active surface layer, which in
most lakes constitutes only the unconsolidated, uppermost
few centimeters. This active layer, where bioturbation and
redox processes can redistribute some sediment or elements,
typically comprises sediments accumulated over the past
one or a few decades. Hg in lake sediments is likely bound
to reduced sulfur groups of the organic matter or precipitated
as metacinnabar (HgS). The solubility of organo-Hg-sulfides
or of metacinnabar is very low, so that diffusion of Hg within
sediments is low. There might be some reflux of formed
Hg(0) during dissolution of Fe-oxides or decomposition of
organic matter at the sediment-water interface but once
recorded in the anaerobic zone diffusion appears to be
negligible. There is strong empirical and theoretical evidence
for the stability of inorganic mercury in lake sediments
including (i) the temporal coherence of mercury increases
among multiple cores and lakes (11), (ii) the preservation of
distinct peaks in mercury profiles from cores collected many
years apart in the same lake (50), (iii) the absence of mercury
redistribution in experimental core incubations (51), and
(iv) the strong solid-phase partitioning of mercury from
porewaters of intact cores (log Kd > 5) (51).
Because lake sediments represent an integrated record
reflecting both changes in direct atmospheric deposition of
mercury and other metals to the lake surface and changes
in transport from surrounding catchment soils, it is difficult
to model actual rates of depositionsas opposed to relative
changes in depositionsof mercury (or other metals) over
time. The rate of atmospheric metal deposition can be
modeled using lake sediments, but this requires timeconsuming mass-balance studies of multiple lakes (11, 23,
52) or a focusing correction of mercury flux data from multiple
lakes based on 210Pb inventories (24, 31), which thus far have
only been applied to the time-scale covered by radiometric
lead (210Pb) dating. From the perspective of making environmental reconstructions, peat cores from ombrotrophic
bogs benefit from the fact that they are supplied with nutrients
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and metals only via the atmosphere. Consequently, peat
potentially offers a less-complicated medium to model longterm changes in atmospheric inputs of mercury (53) and
lead to the environment, and many presume that the peat
archive corresponds to a record of absolute atmospheric
deposition rates. However, peat initially represents a more
open system than lake sediment because it is composed of
a build-up of organic matter that is supported by water, where
the uppermost sectionsthe acrotelmsis exposed to aeration
for decades or centuries before permanent incorporation
below the water table. In addition, organic matter is
continually lost from the peatsfirst rapidly in the acrotelm,
where 50% to as much as 80% of the mass can be lost before
burial in the catotelmsand thereafter much more slowly in
the catotelm, where a further 10% of the original mass can
be lost (54).
Historic Mercury Accumulation Rates. Most available
data of historic and modern mercury accumulation rates
(HgAR) are derived from archives located in the northern
hemisphere, whereas only a small number of studies are
available from the southern hemisphere (30, 31, 55). Figure
1 provides an overview of mercury accumulation rates derived
from lake sediments and peat bogs from different locations
worldwide. One might argue that data from different sites
are not readily comparable because of differences in distances
to local or regional pollution sources. However, most sites,
and all of those discussed in this review, are located in remote
areas far from local mercury sources. Because of mercury’s
long atmospheric residence time, these remote sites are
thought to represent mercury emissions on a hemispheric,
if not global, scale (44). Although we can rule out the effects
of local emission sources, there remains the issue of
geographic variability in mercury deposition rates (largely a
function of differences in precipitation) as well as the
amplification (or diminution) of the atmospheric signal
introduced by mercury transport and cycling within the
individual lake or peatland. However, assuming site-specific
factors remain constant over time, the relative change in
mercury accumulation from pre-industrial to modern times
(mercury flux ratio) should provide a robust and comparable
measure of the magnitude of change in atmospheric mercury
deposition (56).
As the increase of mercury deposition rates in modern
times (from ca. 1860 to present) is in many cases of particular
interest, we depict pre-industrial and modern mercury
accumulation rates separately. The data show that modern
mercury accumulation rates derived from bogs are systematically higher than those from lake sediments. Moreover,
the maximum mercury accumulation rates in bogs show
much greater variation as compared to those in lake
sediments. In the past 150 years the median peak HgAR in
peat bogs is about 40 µg m-2 yr-1 (8-184 µg m-2 yr-1) whereas
those in lake sediments are only ∼15 µg m-2 yr-1 (5-68 µg
m-2 yr-1), which is a factor of 2.5 lower (most lake sediment
data in Figure 1 are not corrected or normalized for catchment
inputs or sediment focusing). Far higher rates of modern
HgAR have been reported for lakes in urban and agricultural
areas (100-200 µg m-2 yr-1); however, such sites are clearly
impacted by local emission sources as well as large mercury
inputs from soil erosion (21) and are not comparable to the
remote sites discussed here. The highest peat mercury
accumulation rates in the industrial period are from higher
latitude sites, where they exceed those found in mid-latitude
bogs by a factor of ∼3-4. The highest reported modern
mercury accumulation rates are from a bog in Denmark (184
µg m-2 yr-1) and a fen in southern Greenland (164 µg m-2
yr -1), which Shotyk et al. (37) explain are the result of longrange mercury transport from mid-latitude sources.
The finding that peat bogs show significantly higher
maximum HgAR than lake sediments is surprising, because
FIGURE 1. Comparison of background and the modern maximum mercury accumulation rates as estimated from lake sediments and peat
bogs (only lake sediment values from Alaska, Nova Scotia, and Minnesota have been corrected for the influence of sediment focusing
or catchment size). The mean background and modern maximum accumulation rate for lake sediments (6.9 and 24 µg m-2 yr-1, respectively)
and peat (1.4 and 59 µg m-2 yr-1, respectively) are marked by a dashed line. For lake sediments accumulation rates corrected for the
influence of, e.g., sediment focusing are also indicated by a gray dashed line (3.5 and 12 µg m-2 yr-1, respectively). Data are from refs
11, 14, 15, 17, 19, 22, 23, 27-29, 31-33, 35-37, 39, 40, 80, 83, and 84.
lake sediments would be expected to show higher accumulation rates due to catchment inputs and sediment
focusing. Such amplification of the atmospheric mercury
flux by lake sediments would explain the differences in
background accumulation rates. The median background
HgAR in peat bogs is ∼1 µg m-2 yr-1 (0.4- 4 µg m-2 yr-1) as
compared to ∼5 (1.4-18 µg m-2 yr-1) in lake sediments. Based
on these data the median mercury flux ratio (increase from
background to modern HgAR) is 32 (9- ∼500) in peat but
only 3.6 (2-6.3) in lake sediments, which represents an order
of magnitude difference between the two archives. This large
disparity in flux ratios raises the question as to which archive
gives the more realistic estimates of past and modern
atmospheric mercury fluxes.
First of all, it is inconsistent that lake sediments should
have higher background mercury accumulations than peat,
but that the situation should be reversed in more recent
times (following industrialization). Possible reasons for this
inconsistency could be changes in peat decomposition
patterns, as discussed by Biester et al. (30, 32), or problems
with accurate dating of the uppermost peat sections (discussed below). An important first indication of the reliability
of the two geochemical archives is a comparison of accumulation rates with direct measurements of atmospheric
mercury deposition. Recent wet deposition rates for mercury
in remote areas geographically and climatically comparable
to those hosting mires, such as the northeastern United States,
show values (∼4-8 µg m-2 yr -1; 57) that are lower by a factor
of 3-6 (and as much as 18) than recent mercury accumulation
rates derived from mires. These wet deposition fluxes are
more similar to those determined in lakes. However, Lamborg
et al. (31) did find good agreement in Nova Scotia between
wet deposition of mercury and recent rates of mercury
accumulation in both lake sediment and peat, but only when
the peat was dated by Polytrichum increment-counting, and
not 210Pb. Although these results imply that 210Pb-dating of
peat is problematic (see below), it is possible that the modern
mercury fluxes reported for some peat cores may include
additional mercury inputs from dry deposition to the mire
surface. However, the significance of this flux is difficult to
evaluate, not only because we know little about dry deposition
in nonforested landscapes, but because a large portion of
terrestrial mercury inputs may be revolatilized back to the
atmosphere (58).
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Another important factor for the evaluation of the two
archives is the chronology of mercury accumulation, especially the timing of the maxima associated with peak
emissions in the late 20th century. Such a comparison might
be questionable for archives located comparatively close to
anthropogenic sources where atmospheric mercury deposition might vary considerably over short distances, but it
should work for remote sites that reflect mercury emission
trends at regional or continental scales. Dating of the
maximum mercury accumulation in bogs is different from
that derived from lake sediments. Lake-sediment records
generally indicate a peak in mercury deposition during the
1970s to 1990s (e.g., 14, 17, 20, 23, 24, 31), consistent with
mercury emission inventories for North America and Europe
(14, 59, 60). In contrast several peat studies suggest a peak
in deposition 10-20 years earlier (e.g., 37).
Dating of Peat and Estimation of Mercury Accumulation
Rates. The basis for calculating metal accumulation rates in
geochemical archives is accurate dating. For the upper recent
sections of peat and sediment cores there are two preferred
methods for continuous dating: these are 210Pb and bombpulse carbon-14, which span approximately the past 150 and
50 years, respectively. Dating using bomb-pulse C-14 is based
on matching the recent stratigraphic record of C-14 in peat
with the known changes in atmospheric C-14 caused by
weapons testing (61). Problems with this approach are
differential uptake of carbon (62) and redistribution and
recycling of carbon within the plants and substrate (63), which
can result in adjacent peat sections containing the same
amount of modern carbon and thus the same age.
Peat layers can also be dated by biological chronometers,
i.e., increment dating based on Polytrichum, which Lamborg
et al. (31) applied to recent peat (<10 years), but which has
also been used to date the past 150 years (e.g., 64, 65). For
older layers in both archives the primary dating tool is
radiocarbon (14C).
An important point regarding 210Pb chronologies is that
age uncertainties are only an estimated error based on
propagation of the analytical counting error, and there is no
calibration error calculated. This is quite unlike most models
used to reconstruct environmental changes (e.g., diatombased inference models for pH), which are typically based
on a transfer function that contains prediction errors. Lead210 dating contains no such prediction-error estimate, and
many studies take ages given by these age-depth models as
absolute ages, which in reality they are not (e.g., 66).
In undisturbed lake sediments 210Pb activity (concentration) typically follows the ideal monotonic log-linear decline
with increasing depth in accordance with its 22.3-year halflife (Figure 2; see also 31). In contrast, the shape of the 210Pb
profile in undisturbed peat cores oftentimes does not show
a monotonic decrease with depth. Instead the peak or peaks
in 210Pb activity occur below the surface. A non-monotonic
decrease of 210Pb activity is not restricted to peat archives
and may also occur in lake sediments, usually as a consequence of changing rates of sediment accumulation. One
modeling approach that can accommodate this problem is
the constant rate of supply (CRS) model developed by Appleby
and Oldfield (67). This model is based on the assumptions
that the flux of 210Pb to the sediment is constant, that once
deposited the 210Pb is immobile, and that a non-monotonic
decrease in 210Pb activity in sediments is caused solely by
changes in sedimentation rates. Although this model was
developed for lake sediments, it has been frequently applied
to date peat deposits. However, some studies have suggested
that the assumption that 210Pb is immobile in peat is incorrect
and that the often observed non-monotonic decrease of
210
Pb activities is for the most part not related to changes in
peat accumulation (31, 68, 69).
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The problems of dating peat using 210Pb seem to be mainly
related to the hydrological and biogeochemical characteristic
of bogs and the related mobility of 210Pb in the uppermost
peat layers. In contrast to the accumulation of lake sediments,
peat formation does not occur under permanently saturated
conditions; rather, the uppermost peat (acrotelm) is characterized by water table fluctuations, which cause frequent
changes in redox conditions. Water table changes not only
control the decomposition of the organic substrate, but in
combination with the usually high macroporosity in the
acrotelm they allow vertical (advection) and lateral movement
of trace elements and even particles such as pollen (70, 71).
Thus, it appears unlikely that atmospherically deposited
210
Pb is fixed on the bog’s surface and that a non-monotonic
decrease in 210Pb activity is solely attributable to changes in
peat accumulation. In fact it should be expected that 210Pb
and also trace elements are to some extent mobile in the
acrotelm.
Several studies have shown that some of the 210Pb deposited to the bog surface is washed downward and accumulates at the zone of the water table and partly smears through
the uppermost peat sections (31, 68, 69, 72). The consequence
of this advection is that the resulting 210Pb chronology reflects
ages of the 210Pb distribution (and migration) and not necessarily the true ages of individual peat increments. Mobility
in the acrotelm has not only been shown for 210Pb but also
for total Pb and other trace elements in bogs (73). Figure 3A
shows the distribution of 210Pb together with americium
(241Am) and 14C-ages in a peat profile from a blanket bog in
northern Spain. The 210Pb activity profile shows a nearly exponential decrease down to a depth of 30 cm suggesting that
accurate dating should be possible. However, 210Pb dating
(CRS model) indicates an age of 120-150 yrs (corresponding
to ∼5 half-lives of 210Pb) at 30 cm, whereas 14C-dating gives
an age of ∼500 years at this same depth; the 240 cm peat
profile has a basal age of about 4000 years. A similar discrepancy between 210Pb ages and 14C ages is observed in Caribou
Bog, Maine (40) (Figure 3B). The reason for the observed
discrepancy between 210Pb and 14C ages is readily explained
by a downward migration of 210Pb into older peat layers. The
effect of smearing of elements down to the water table is also
illustrated by the distribution of 241Am. The fall-out of 241Am
is attributed to nuclear weapons testing between ∼1952 and
1963, and in most sediment profiles this period can be identified as a sharp peak. In the Spanish peat profile, however,
the 241Am exhibits a broad maximum and could be measured
down to a depth of 20 cm, which has a 14C age of 110-335
years. In such cases 210Pb ages are likely to underestimate
the true ages of the peat and therefore highly overestimate
the accumulation rates of any trace element in the acrotelm.
Moreover, if 210Pb and mercury both migrate downward in
the peat but are retained at different depths, the chronology
of mercury accumulation is undefined and cannot be dated.
The living vegetation at the surface of a bog frequently
introduces another problem in 210Pb dating. Earlier studies
have shown that moss-increment and 210Pb chronologies
often fail to agree. The increment analyses indicate more
years than the 210Pb-derived chronology and thus the vegetation layer is assigned too few years. For example, in a surface
core from Store Mosse, Sweden, 210Pb dating of the uppermost
10-cm-thick moss layer indicates an age of only 2.2 years
(Figure 4). In contrast, typical Sphagnum growth rates in
southern Scandinavia are 0.8-1 cm yr-1 (64, 74, 75). Based
on such growth rates the 10 cm of vegetation corresponds
to about 10-12 years. According to the 210Pb chronology the
average mass accumulation rate would be 850 g m-2 yr-1,
which is 2-4 times higher than reported values for this type
of bog. In contrast, if we assume a typical growth rate of 1
cm yr-1, the estimated 10 years for this section returns a
FIGURE 2. (A) Typical exponential decline in 210Pb in undisturbed lake sediment cores (redrawn from ref 22). (B) Typical
in undisturbed peat cores (redrawn from ref 27).
more reasonable average accumulation rate of 190 g m-2
yr-1 (75).
A comparison of mercury and lead accumulation rates in
Store Mosse shows that both metals follow closely the changes
in mass accumulation, especially in the uppermost peat
sections (Figure 5A-C). Surprisingly, the highest accumulation rates were found in those peat sections where mercury
and lead concentrations were lowest. This pattern clearly
indicates that the calculated metal accumulation rates are
mainly determined by the mass accumulation rates, which,
as argued above, are unrealistically high.
Poor estimation of peat mass accumulation rates have
a serious impact on calculated mercury accumulation
rates. For example, the highest peat mercury accumulation rates are reported for sites in Denmark and Greenland, 165-185 µg Hg m-2 yr-1, which were dated based on
bomb-pulse 14-C as well as with 210Pb (37). Given the reported mercury concentrations and accumulation rates in
the peat records, the net peat mass accumulation rate
210
Pb profiles
required would be 1260 g m-2 yr-1 and 540 g m-2 yr-1, for
Greenland and Denmark, respectively (76). These values
represent net accumulation rates following a few decades
of decomposition, and thus the original organic matter
production must have been greater. Malmer and Wallén
(77) determined that the transfer rate of organic matter
from the living moss layer to the litter layer was e200
g m-2 yr-1 in southern Scandinavia, 50-200 g m-2 yr-1 in
northern Scandinavia (the sub-arctic zone and southern
Greenland), and 100-300 g m-2 yr-1 in North America.
Decomposition studies by Johnson et al. (78) indicate that
about 10-20% of the plant mass is lost in the first year,
whereas 50-80% of the original organic mass is lost during
subsequent decomposition in the acrotelm (77). Therefore,
the high peat mass accumulation rates required to produce
the high mercury accumulation rates in the peat from
Greenland and Denmark (165-185 µg Hg m-2 yr-1) are
unrealistic when compared to typical net peat mass accumulation rates.
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FIGURE 3. Distribution of 210Pb and Americium and 14C-ages in a
Spanish blanket bog located in northern Galicia and (B) 210Pb and
14
C chronologies from a bog in Maine (data are from ref 40).
Metal concentrations and accumulation do not necessarily
have to peak in the same peat section, but large differences
in their occurrence may indicate unrealistically large changes
in peat-mass accumulation or atmospheric metal deposition.
Increasing mass accumulation by a factor of 10-15 as
observed in the Store Mosse example requires that mercury
(or lead) concentrations decrease by the same factor to obtain
constant accumulation rates. Moreover, the mercury and
lead profiles in the Store Mosse peat core are nearly identical
for both accumulation rates and concentration, implying
that the timing of atmospheric pollution of both metals has
been the same. Such results make little sense given the
distinctly different emission histories of lead and mercury
over the last several decades. It is also surprising that the
concentration profiles of both metals are to a large extent
consistent with that of 210Pb activity (Figures 4 and 5). Such
close covariance strongly suggests that the vertical distribution of all three (mercury, lead, and 210Pb) is a consequence
of similar downward advection, rather than simultaneous
changes in atmospheric flux. Additional evidence for mercury
mobility in surface peat comes from the mercury-isotope
experiments of the METAALICUS project. Here, a dilute
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Hg spike applied to the peat surface was redistributed
several meters outside the experimental plot and uniformly
to a depth exceeding 15 cm below the water table within 3
months of application (79).
It is important to note that in some studies peat ages
obtained from 210Pb activity and CRS modeling have been
validated by other dating markers such as pollen stratigraphy
(67). Such results indicate that the mobility of trace elements
might be negligibly low in some bogs, presumably those with
a shallow acrotelm.
Correction/Normalization of Mercury Accumulation
Rates. Several recent papers (38, 40, 80) have proposed that
natural, background mercury deposition could be distinguished from anthropogenic mercury deposition using
bromine. This idea was based on the observation that the
accumulation rates of mercury and bromine, as well as
selenium, were very similar over a period of several thousand
years prior to industrialization. From this covariate pattern
these authors argued that natural changes in atmospheric
fluxes must have been the same for all three elements.
However, it is doubtful that the atmospheric chemistry of
these elements is similar enough to produce nearly identical
accumulation patterns in peat. Moreover, it is well-known
that all three elements have a high binding affinity for organic
matter and might therefore undergo similar changes during
peat decomposition. Recent studies have shown that mercury
and bromine concentrations in peat show a strong dependency on the degree of peat humification; that is, their
concentrations increase with increasing peat decomposition
(32, 81). In Caribou Bog, Maine, Roos-Barraclough et al. (40)
found that all three elements in the uppermost peat (the
past ∼100 years) show a strong increase in accumulation
rates by factors of ∼10 (bromine, Se) and ∼100 (Hg) compared
to background values. In the same section an increase in
mass accumulation by a factor of 4-10 could be observed.
A 10-fold increase in bromine and selenium accumulation
rates can hardly be explained by anthropogenic emissions
alone, and neither does the Greenland ice record for chloride
support such dramatic changes in aerosol deposition during
the Holocene (GISP2 and GRIP data).
The relationship between peat accumulation and trace
element accumulation, as discussed above, suggests that the
accumulation rates of mercury, bromine, and selenium in
the Maine bog are not determined only by changes in
atmospheric fluxes, but also more likely by changes in peat
mass accumulation and decomposition. This concern applies
not only to the uppermost peat, but also to older, deeper
sections that may have experienced post-burial changes in
peat decomposition during dry phases (32, 81). Thus, the
∼9-fold increase in Hg, Br, and Se accumulation rates (with
constant concentrations) between 6000 and 7000 BC in
Caribou bog (40) can only be achieved by decreasing density
or increasing peat accumulation by a factor of 9, which is not
reasonable. High mass losses, typical for fen peat, and release
or non-quantitative retention of mercury, bromine, and
selenium might be a possible explanation for the high trace
element accumulation rates at more or less constant
concentrations. We suggest that the correction or normalization of mercury accumulation records in peat by means
of bromine or selenium does not correct for changes in
atmospheric fluxes but instead for changes in peat massaccumulation rates. The records of mercury and selenium
from a Spanish peat bog, Peña da Cadela, (82) serve as an
example to illustrate this relationship (Figure 6A). In this
core mercury accumulation rates are 20-25-times greater in
the uppermost peat (past 500 years) as compared to
background values (∼1 µg m-2 yr-1). Mercury accumulation
rates were then corrected for changes in both selenium and
carbon accumulation rates. This correction leads to a 6-fold
reduction of the ratio between modern and pre-industrial
FIGURE 4. Non-exponential distribution of unsupported 210Pb in a Swedish peat bog (Store Mosse, Sweden; 34) typical for many peat
deposits. Low amounts of 210Pb could be detected in the plant layers (0-10 cm), which increase to three maxima between -10 and -15
cm. As a result of the low 210Pb activities in the upper peat calculation of 210Pb ages by means of the CRS-model leads to very low ages
of the uppermost peat increments (5-10 cm) compared to those of deeper layers which show up to 30 times higher ages.
FIGURE 5. Accumulation rates of peat mass, mercury, and lead calculated based on non-exponential decrease distribution of 210Pb and
CRS and Hg and Pb concentrations in the upper 20 cm of an ombrotrophic bog (Store Mosse, Sweden).
mercury accumulation rates to a value of 4-6. This corrected
flux ratio is in line with the average factor of increase (3-5)
found in lake sediments (44). The results are the same whether
the correction is based on selenium (correction for changes
in atmospheric fluxes) or carbon (correction for changes in
mass accumulation) (Figure 6B). It is therefore evident that
the observed variations in mercury accumulation in peat,
and especially the large increase in modern rates, are
influenced not only by anthropogenic inputs, but also by
changes in mass accumulation (32).
Although this correction appears to produce reasonable
results, little is known about the processes that would properly
explain it. Retention mechanisms, characteristics of binding,
and the fate of mercury, bromine, and selenium during peat
diagenesis are still poorly understood. Thus, normalization
of mercury accumulation rates to those of other atmospherically derived elements is a questionable procedure until the
fate of trace elements in peat is better understood. From
what is currently known we believe that correction or
normalization of mercury accumulation rates should be
based on carbon or mass accumulation rather than on other
atmospherically derived trace elements.
Relative Changes in Mercury Accumulation. Preindustrial mercury accumulation rates calculated from peat
cores are lower than those in sediments by a median factor
of 5 (Figure 1). It can be argued that mercury accumulation
rates in lake sediments should generally be higher due to
catchment inputs. For example, Swain et al. (11) calculated
that 20-60% of the mercury load in sediments of seven
headwater lakes originated from their respective catchments.
On the other hand, mercury losses through surface evasion
and outflow, by reducing the portion of the atmospheric
load retained in the sediments, should have the opposite
effect. Evasive losses can represent a large portion of direct
mercury deposition to the lake surfacesabout 45% of an
added isotopic spike in the METAALICUS experiment (1)s
but probably a much smaller fraction of total mercury loading
when watershed inputs are factored in. Outflow losses can
be large in lakes with short residence time (83), but are
generally not significant in the small headwaters lakes
commonly used to assess atmospheric deposition.
Mercury losses from volatilization or outflow are likely in
peatlands as well. Short-term tracer experiments (METAALICUS; 1) indicate only about 8% of an isotopic spike was lost
VOL. xx, NO. xx, xxxx / ENVIRON. SCI. & TECHNOL.
9
G
FIGURE 6. Accumulation rates of mercury, selenium, and carbon in an ombrotrophic Spanish mire (Peña da Cadela) and enrichment factors
of background to modern mercury accumulation corrected for changes in selenium and carbon accumulation rates.
following application to a small upland catchment, but
roughly a third of native mercury deposition was volatilized
over the same period. In the case of peatlands, additional
mercury losses are likely to occur following burial when
50-80% of the original carbon is lost through decomposition.
However, mercury losses in peat are not proportional to those
of carbon, as we do see mercury enrichment in more highly
decomposed peat (32). Although it is not known how much
of the wet or dry deposited mercury is initially retained in
the surface layer of a bog, other trace elements, such as
bromine or iodine, which also bind to organic carbon, show
retention of only 35-50% (81). Due to the high porosity of
acrotelm peat it is reasonable that considerable amounts of
mercury are lost through lateral transport through the surface
layer, especially during times of high precipitation or
snowmelt. Lateral transport may also explain the large
difference in mercury retention observed between and within
hummocks and hollows (28, 34).
Deriving actual rates of mercury deposition from lakes
and bogs clearly requires a mass-balance assessment of these
other fluxes, which is seldom done and has its own large
uncertainties (e.g., 23). But in most cases what is important
is the magnitude of change in mercury deposition, and for
this assessment all that is required is that those factors that
contribute to mercury retention in peat and lake sediments
remain relatively constant (55). Evidence that this is so for
lake sediments comes indirectly from the uniformity of
mercury flux ratios (the increase from pre-industrial to
modern mercury accumulation) from a very large number
of lakes and cores throughout the Northern Hemisphere (44).
Such convergence argues strongly that secular changes in
global mercury emissions and deposition are the primary
signal in these records, and that other fluxes (runoff, evasion,
outflow) that might alter the accumulation record for
individual lakes are either unimportant or have remained
H
9
ENVIRON. SCI. & TECHNOL. / VOL. xx, NO. xx, xxxx
relatively constant. The same cannot be said for peat records,
which show flux ratios that span more than an order of
magnitude.
Although there are complications with both types of
archives, it seems clear that lake sediments, as closed systems,
are internally more consistent and less problematic than peat
records. However, recent advances in our understanding of
peat diagenesis and spatial variability hold promise for
improved modeling of mercury deposition from ombrotrophic bogs. The 3-fold increase in mercury deposition since
pre-industrial times recorded in lake sediments is significant
because it indicates anthropogenic emissions have altered
mercury fluxes to about the same degree throughout North
America and Europe (and the world at large), even though
contemporary mercury deposition exhibits large temporal
and spatial variability. These results also confirm core
assumptions of several global-scale mercury models (31, 49),
including a long atmospheric residence time for Hg0 and the
relative size of anthropogenic and natural mercury fluxes.
Acknowledgments
Data for the Spanish bogs included in this paper was obtained
under project REN2003-09228-C02-01 funded by the Spanish
Ministry of Education and Science.
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