Atmospheric Environment 43 (2009) 2012–2017
Contents lists available at ScienceDirect
Atmospheric Environment
journal homepage: www.elsevier.com/locate/atmosenv
Impact of mercury emissions from historic gold and silver mining:
Global modeling
Sarah Strode a, Lyatt Jaeglé a, *, Noelle E. Selin b
a
b
Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195, USA
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 July 2008
Received in revised form
7 January 2009
Accepted 8 January 2009
We compare a global model of mercury to sediment core records to constrain mercury emissions from
the 19th century North American gold and silver mining. We use information on gold and silver
production, the ratio of mercury lost to precious metal produced, and the fraction of mercury lost to the
atmosphere to calculate an a priory mining inventory for the 1870s, when the historical gold rush was at
its highest. The resulting global mining emissions are 1630 Mg yr1, consistent with previously published
studies. Using this a priori estimate, we find that our 1880 simulation over-predicts the mercury
deposition enhancements archived in lake sediment records. Reducing the mining emissions to
820 Mg yr1 improves agreement with observations, and leads to a 30% enhancement in global deposition in 1880 compared to the pre-industrial period. For North America, where 83% of the mining
emissions are located, deposition increases by 60%. While our lower emissions of atmospheric mercury
leads to a smaller impact of the North American gold rush on global mercury deposition than previously
estimated, it also implies that a larger fraction of the mercury used in extracting precious metals could
have been directly lost to local soils and watersheds.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Mercury
Mining
Gold rush
North America
Deposition
Sediment cores
1. Introduction
Gold and silver mining in North, Central, and South America,
and in Australia, which used mercury amalgamation to extract the
precious metals, is estimated to have released approximately
156,000 Mg of mercury into the atmosphere and over 250,000 Mg
of mercury into the environment between 1580 and 1900 (Nriagu,
1994). The North American gold and silver rushes that began when
gold was discovered in California in 1847 represent a particularly
intense period of mining. Silver production rose from 1.2 Mg yr1 in
1850 to 940 Mg yr1 in 1880 (Bureau of the Census, 1989). From
1850 to 1900 atmospheric mercury emissions from gold and silver
mining in the United States averaged 780 Mg yr1 (Nriagu, 1994),
compared to present day U.S. anthropogenic emissions of approximately 100 Mg yr1 (Pacyna et al., 2006). Because mercury is both
toxic and persistent in the environment, this mercury release is still
a concern today. Sites contaminated with mercury during the
historic gold rush continue to cause mercury contamination in
California (Alpers et al., 2005) and Nevada (Wayne et al., 1996)
watersheds.
* Corresponding author. Tel.: þ1 206 685 2679; fax: þ1 206 543 0308.
E-mail address: [email protected] (L. Jaeglé).
1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2009.01.006
Sediment cores provide a record of changes in mercury deposition through history. Lake sediments suggest that modern
mercury deposition in the northern hemisphere is 2–4 times larger
than the pre-industrial background value (Lindberg et al., 2007;
Biester et al., 2007). For the historic North American gold rush
period, however, the record is less clear. Schuster et al. (2002) found
a factor of 5 enhancement in total mercury concentration and
deposition in a Wyoming glacier ice core layer corresponding to the
late 19th century, which they attribute to gold-mining emissions. In
contrast, Lamborg et al. (2002) did not find a clear mining signal in
lake sediments from Nova Scotia and New Zealand, and suggest
that mercury from historic mining remained close to its source
rather than being deposited globally.
Modeling studies provide insight into the relationship between
mining emissions of mercury and the deposition changes recorded
in sediment core record. Hudson et al. (1995) included mercury
emissions of 2200 Mg yr1 at the peak of the North American gold
rush in a pre-technological to modern box-model simulation of the
mercury cycle. Compared to lake sediments from the upper Midwest United States (Swain et al., 1992), these mining emissions
result in too large a peak in mercury deposition around 1880
(Hudson et al., 1995). Pirrone et al. (1998) estimate that North
American mercury emissions peaked at 1708 Mg yr1 in 1879
S. Strode et al. / Atmospheric Environment 43 (2009) 2012–2017
mostly due to mining emissions, but do not find a corresponding
enhancement in this period in sediment cores from the Great Lakes.
In this study, we estimate mercury emissions from the North
American gold rush era based on records of gold and silver production. We then use the GEOS-Chem global atmosphere–ocean–land
mercury model to calculate the global impact of these emissions on
deposition. Finally, we examine the consistency between our estimates and historic core records from around the world.
2. Methods
2.1. Historic core records
Numerous studies have used lake sediment, peat bog, or ice
cores to interpret the history of mercury deposition and infer the
anthropogenic enhancement ratio (ER) given by the ratio of
modern to pre-industrial accumulation rates (ERmodern/p-i). Several
uncertainties are important in relating these records to atmospheric deposition, including variability in sedimentation rates
(Engstrom and Wright, 1984) and sediment focusing (Perry et al.,
2005), post-deposition mobility of mercury within the core (Gobeil
and Cossa, 1993), and the contribution of mercury from the
catchment area rather than from atmospheric deposition (Swain
et al., 1992). For this study, we select cores from the literature that
report mercury as accumulation rate rather than concentration to
reduce the influence of variable sediment flux, and we exclude
studies that report significant post-depositional redistribution of
mercury. Since we are comparing with a global model, we use cores
from areas where atmospheric deposition is expected to dominate
over runoff from local sources.
The long lifetime of total gaseous mercury (>6 months) implies
that large emissions from the North American gold rush mining era
should have affected deposition globally, and not just in North
America. We thus assemble a global dataset including sediment
core records from North America (Engstrom and Swain, 1997;
Fitzgerald et al., 2005; Kamman and Engstrom, 2002; Lamborg
et al., 2002; Landers et al., 1998, 2008; Lorey and Driscoll, 1999;
Swain et al., 1992), South America (Lacerda et al., 1999), Greenland
(Asmund and Nielsen, 2000; Bindler et al., 2001), Europe (Verta
et al., 1989), Siberia (Landers et al., 1998), and New Zealand (Lamborg et al., 2002). For greater geographic coverage, we also include
observations from an ice core in North America (Schuster et al.,
2002), and two peat bog records in South America (Biester et al.,
2002) and Europe (Roos-Barraclough and Shotyk, 2003).
As some of the cores do not extend back to pre-industrial
times and some lack the temporal resolution to provide an 1880
value, we define 3 enhancement ratios: ERmodern/p-i, ER1880/p-i,
and ER1880/modern. ER1880/p-i is the product of the other two ratios.
Modern is defined as the sediment data point closest to the year
2000; p-i represents the pre-industrial period, which includes
data from before 1840; and 1880 is the date chosen to represent
the North American gold and silver rush. If multiple cores lie in
the same model grid box or within 2 degrees latitude and
longitude of each other, we average them together. Table S1 in
the supplemental materials shows the dataset used for this study.
2.2. A priori mining emissions
We derive our global a priori mining emission inventory for the
1870s using the following equation:
Fmining ¼
Pgold þ Psilver RHg=metal fatmos
(1)
where Fmining is the total mass of elemental mercury (Hg0) released
to the atmosphere. Pgold is the mass of gold produced, Psilver is the
2013
mass of silver produced, RHg/metal is the mass ratio of mercury lost
to gold or silver produced and fatmos is the fraction of mercury
released to the atmosphere. Mitchell (2003a,b) reports the mass of
gold and silver produced by country for each year. Silver dominates the total production, with the United States producing
760 Mg silver compared to 50 Mg gold in 1875 (Mitchell, 2003b).
To determine Pgold þ Psilver, we sum Mitchell’s (2003a,b) gold and
silver production numbers and average from 1870 to 1879 to
account for the long atmospheric lifetime of Hg0 and the temporal
resolution of the sediment cores. Mining emissions are distributed
evenly across each country, except in the United States, where we
assume that the emissions occurred in the western part of the
country.
The ratio of mercury lost to gold or silver produced is uncertain.
In 18th century South American silver mining, RHg/metal was estimated to be approximately 2:1, although in some regions the ratio
was closer to 1.5:1 or 1:1 and the ratio varied greatly between
regions and years depending on the ore and the availability of
mercury (Brading and Cross, 1972; Fisher, 1977; Nriagu, 1994).
Nriagu (1994) estimate that the ratio was approximately 1:1 in 19th
century South and Central America. Pfeiffer and Lacerda (1988)
estimated a ratio of 1.3:1 for modern gold miners using mercury
amalgamation in Brazil. Other modern estimates usually lie
between 1:1 and 1.5:1 (Lacerda, 2003). Given that the published
estimates for the ratio of mercury lost to gold and silver overlap, we
choose a common value of RHg/metal ¼ 1.5:1 for our a priori 1870s
emissions estimate.
Estimates also vary for the fraction of mercury released to the
atmosphere during the amalgamation process, fatmos. Pfeiffer and
Lacerda (1988) estimated that 55% of mercury from modern
Amazon gold mining enters the atmosphere, and Lacerda and
Salomons (1991) report that 65%–83% is lost to the atmosphere in
this region. In contrast, Swain et al. (2007) estimate that only 30% of
the mercury used in small-scale gold mining is released directly to
the atmosphere. For historic mining, Nriagu (1994) estimates a 60%
loss to the atmosphere, and Pirrone et al. (1998) used this 60% value
to estimate emissions from North American gold and silver mining.
Following these studies, we set fatmos to 0.6 for our a priori
emissions.
2.3. Global model
The GEOS-Chem chemical transport model (Bey et al., 2001)
simulates mercury in the atmosphere–ocean–land system (Selin
et al., 2007, 2008; Strode et al., 2007). The simulation includes
tracers for elemental mercury (Hg0), divalent mercury (HgII), and
particulate mercury (Hgp), with both oxidation of Hg0 to HgII and
in-cloud reduction of HgII occurring in the atmosphere. We use here
model version 7.04 (http://www.harvard.as.edu/chemistry/trop/
geos) with updates described in Selin and Jacob (2008).
We conduct model simulations for 3 different sets of mercury
emissions: pre-industrial, 1880, and modern day. For each emission scenario, we run the model until it reaches steady state. The
model has a horizontal resolution of 4 latitude by 5 longitude,
and 30 vertical levels. It is driven by assimilated meteorological
fields from the NASA Goddard Earth Observing System (GEOS-4)
for 2004 for all simulations so that the ER is not affected by interannual variability in precipitation. We compare the ER values
from the cores with modeled deposition enhancement ratios for
the same time periods. Comparing ER values rather than absolute
deposition rates normalizes out some site-specific factors such as
average local precipitation (Biester et al., 2007). Note that modern
oxidant concentrations (OH and O3) were not modified for the
pre-industrial and 1880 simulations. Thus, we do not address the
effects of changing oxidant concentrations on deposition.
2014
S. Strode et al. / Atmospheric Environment 43 (2009) 2012–2017
Selin et al. (2008) describes the pre-industrial and modern
simulations. Briefly, the pre-industrial simulation includes emissions of 1260 Mg yr1 from the ocean and 1540 Mg yr1 from land
(Table 1). The ocean model simulates the coupled interactions of
the mixed layer with the atmosphere and deep ocean, as well as
conversion among three aqueous mercury species: elemental,
divalent and non-reactive. The mechanistically parameterized land
source includes geogenic, evapotranspiration, and soil volatilization sources, and prompt recycling of deposition. The modern
simulation includes an additional 3400 Mg yr1 anthropogenic
source and 660 Mg yr1 biomass burning emissions. This increase
in emissions leads to increasing deposition to land and ocean
surfaces, and thus increases the cycling of mercury through these
reservoirs. With our interactive coupling of the atmosphere with
the land and ocean, we simulate increasing ocean and land emissions of 2960 Mg yr1 and 2180 Mg yr1, respectively (Table 1). The
modern mercury simulation has been validated against observations of atmospheric surface concentrations, wet deposition over
land, and oceanic aqueous concentrations, yielding a generally
unbiased simulation (Selin et al., 2007, 2008; Selin and Jacob, 2008;
Strode et al., 2007, 2008).
For the 1880 simulation, we add our mining emission estimate
for the 1870s (Section 2.2) to the pre-industrial simulation of Selin
et al. (2008). We will compare the results of this simulation to core
records for 1880, because of the averaging time between input to
the lake and deposition in the sediments.
3. Results and discussion
3.1. A priori mining emissions
Based on equation (1), we obtain global mercury emissions from
mining of 1630 Mg yr1 for the 1870s. We assume that these
emissions occur as Hg0. For the United States, we find mercury
emissions from gold and silver mining in the 1870s of 960 Mg yr1.
Our a priori mining emissions for the United States lie in the middle
of the range estimated by Nriagu (1994) and Pirrone et al. (1998)
(Table 2). Considering all of North America, we estimate mining
emissions of 1350 Mg yr1, 83% of global mining emissions.
Pirrone et al. (1998) estimated North American mercury emissions ranged from under 750 Mg yr1 in the early 1870s to over
1500 Mg yr1 during the late 1870s. For South America we estimate
mercury emissions of 220 Mg yr1, smaller than the 1821–1900
average of 525 Mg yr1 estimated by Nriagu (1994). In our inventory, mining emissions outside of America, occur in Japan, Australia,
and New Zealand, and total 60 Mg yr1.
Table 2
Atmospheric mercury emissions from gold and silver mining for the United States,
North America and the globe.
Years
1800–1920
1870–1880
1850–1900
1880
1870–1880
1870-1880
Emissions, Mg yr1
United States
North America
(25–1664)
396 (53–2331)
1200 (700–1708)
Reference
Global
780 (208–1660)
960
490
(200–810)
1350
660
(270–1100)
2200
1630
820
(330–1360)
Pirrone et al. (1998)
Pirrone et al. (1998)
Nriagu (1994)
Hudson et al. (1995)
This study, a priori
This study, revised
Reported emissions for the various studies are given as mean values for the time
period, with the range of emissions indicated in parentheses.
3.2. Modeled and observed ERs
Before constraining the 1870s mining emissions, we examine
the ability of the model to reproduce the observed ERmodern/p-i.
Selin et al. (2008) found good agreement between the modeled ER
and core records from the upper Midwest U.S. and New Zealand.
Fig. 1 compares the modeled ERmodern/p-i with a more extensive
dataset of core records, described in Section 2.1 and Table S1.
The model shows good agreement with core records from areas
such as the upper Midwest U.S. (observations: 3.2, model: 3.2) and
Chile (observations: 2.6, model: 2.9). It also captures the greater
anthropogenic impact in industrialized regions such as the northeast U.S. and Europe. The model greatly underestimates the
Wyoming ice core value of 11, but this value is much higher than
the values from lake sediments (1–4) and may relate to the difficulty in interpreting the ice core record. In addition, the model does
not capture much of the site-to-site variability seen in the cores.
This may be due to local sources not captured by the model or
uncertainty in the individual core records. The model is also unable
to reproduce the low enhancement ratios at high latitudes found by
Landers et al. (1998) in Siberia (1.1–1.3) and Alaska (1.0–1.3). The
model does reproduces the Alaskan observations reported by
Fitzgerald et al. (2005) (observations ¼ 3.2, model ¼ 2.8). A possible
explanation for the low ER values at some high latitude sites is
a large mercury source from erosion of naturally enriched soils. This
larger background input would reduce the relative impact of
atmospheric deposition and thus reduce the observed modern to
pre-industrial enhancement ratio (Fitzgerald et al., 2005). Another
Table 1
Mercury budget for North America and the globe in Mg yr1.
North Americaa
Global
Pre-industrial 1880 Modern Pre-industrial 1880 Modern
Emissions (Mg yr1)
Anthropogenicb
Biomass burning
Land
Ocean
0
0
330
–
660
0
340
–
180
30
390
–
0
0
1540
1260
820
0
1580
1400
Total Emissions
330
1000 600
2800
3800 9200
Deposition
(wet þ dry)
180
300
2800
3800 9200
a
570
3400
660
2180
2960
Emissions and deposition for North America are only over land.
Includes direct emissions from mining and other anthropogenic activities. The
1880 mining emissions are for our revised inventory (Section 3.3).
b
Fig. 1. Modeled ratio of modern to pre-industrial deposition (ERmodern/p-i). Core
records are shown by the circles, which are color coded according to the observed
ERmodern/p-i.
S. Strode et al. / Atmospheric Environment 43 (2009) 2012–2017
Table 3
Deposition enhancement ratios from historic cores and model for modern/preindustrial, 1880/pre-industrial and 1880/modern.
A priori model
Revised model
Observations
(mean s)
Sites
Global Model Sites
Global
bias
2.7 1.4
ERmodern/p-i
ER1880/p-i
1.4 0.3
ER1880/modern 0.5 0.2
3.1 0.3 3.1 0.7 þ51%
1.8 0.2 1.7 0.2 þ29%
0.6 0.2 0.5 0.1 þ39%
Model
Bias
–
–
–
1.4 0.1 1.3 0.1 0%
0.5 0.1 0.4 0.1 þ4%
Model values (mean s) are presented for the core site locations and the entire
world. The mean model bias is calculated for the core sites.
possible explanation is differences in the time period considered
‘‘pre-industrial’’ for the different studies.
Table 3 presents a comparison of observed and modeled ER’s.
The Wyoming ice core is excluded from the calculations, as it is an
outlier compared to other observations (Table S1). The mean (
standard deviation) observed ERmodern/p-i from 18 locations is
2.7 1.4 (Table 3), while the corresponding model ERmodern/p-i is
3.1 0.3. The model overestimate of the Landers et al. (1998) sites
in Siberia and Alaska drives the positive model bias of 51% shown in
Table 3 (model bias is defined as the mean normalized difference
between model and observations: [model-observations]/observations). Removing these sites increases the observed mean ratio and
reduces the bias in ERmodern/p-i to 3%.
Fig. 2 (left) shows the global distribution of modeled ER1880/p-i.
ER1880/p-i is largest over the western U.S. and Mexico, where it
exceeds 2 due to large mining emissions during the 1870s in
these regions. There is an inter-hemispheric gradient due to the
larger increase in source strength in the northern hemisphere.
The mean modeled ER1880/p-i is 1.8 in the northern hemisphere
and 1.6 in the southern hemisphere. In North America, the mean
observed ER1880/p-i is 1.4 0.3 while the mean model ER1880/p-i is
2.0 0.4. The model overestimates ER1880/p-i at remote sites in
Alaska and New Zealand. This model overestimate suggests that
mining emissions may be overestimated. An exception is the
Wyoming ice core, which predicts an ER1880/p-i of 5 for the gold
rush era compared to the modeled value of 2.7. The model also
under predicts the ER from the lake core record from Carajas
Mountain, Brazil (Lacerda et al., 1999). This may be due to a local
source not included in our model emissions.
Fig. 2 (right) shows the modeled and observed ER1880/modern.
Small ER1880/modern values in East Asia reflect the large modern
anthropogenic source in this region. The model generally overpredicts ER1880/modern, particularly in the western U.S. The observed
mean ER1880/modern in the western U.S. is 0.4 while the mean model
2015
ER1880/modern is 0.8. Table 3 shows a 29% positive mean model bias
in ER1880/p-i and a 39% positive bias in ER1880/modern, both suggesting
that mining emissions should be reduced.
3.3. Revised 1870s mining emissions
Given the disagreement between the observed and modeled
ER’s for the gold rush period, we derive a revised mining emissions
estimate to bring the model into better agreement with the global
core record. Reducing our global a priori mining emissions by
a factor of 2 from 1630 Mg yr1 to 820 Mg yr1 removes the model
bias in ER1880/p-i (Table 3) (minimizing the bias for ER1880/modern
requires reduction by a factor of 2.3, yielding mining emissions of
710 Mg yr1). Given the uncertainties in both the model and the
historic core records, we determine an uncertainty range for the
mining emissions by using the GEOS-Chem model to calculate
emissions consistent with the observed mean ER1880/p-i one
standard deviation (ER1880/p-i ¼ 1.1–1.7). This yields estimates of
mining emissions ranging between 330 and 1360 Mg yr1 (Table 2).
Fig. 3 maps ER1880/p-i and ER1880/modern using the revised mining
estimate of 820 Mg yr1, and displays improved agreement with
observations. In particular, the modeled ER1880/modern over North
America is reduced from 2.0 0.4 to 1.5 0.2, and compares well
with observations (1.4 0.3). Based on these revised global mining
emissions, we estimate that gold and silver mining in the United
States released 490 Mg yr1 of mercury to the atmosphere with an
uncertainty range of 200–810 Mg yr1 (Table 2).
Assuming that the production of gold and silver is relatively
well constrained, equation (1) implies that a reduction in mercury
released to the atmosphere can result from a reduction in RHg/metal
and/or fatmos. A reduction in RHg/metal implies lower total mercury
release to the environment, while a reduction in fatmos implies that
more mercury was deposited to local soil and watersheds while
less was exported to the global atmosphere. If our fatmos value of
0.6 is correct, then to obtain a 50% reduction in mining emissions,
RHg/metal must be reduced from 1.5 to 0.75, lower than most
published estimates. Alternatively, if the RHg/metal value is correct,
fatmos must be reduced from 0.6 to 0.3, which is within published
estimates, albeit at the low end. The total reduction could also be
obtained by smaller reductions in both RHg/metal and fatmos.
3.4. Mercury budget for 1880
Atmospheric emissions from gold and silver mining had
a significant impact on global deposition during the late 19th
century. Table 1 compares the mercury budget for the 1880
Fig. 2. Modeled ER1880/p-i (left) and ER1880/modern (right) with a priori mining emissions of 1630 Mg yr1. Observations are shown by the color-coded circles.
2016
S. Strode et al. / Atmospheric Environment 43 (2009) 2012–2017
Fig. 3. Same as Fig. 2, but with the revised mining emissions (820 Mg yr1).
simulation (with revised mining emissions) to the modern and preindustrial simulations. Both the global and North American budgets
are summarized. North American mercury emissions more than
tripled between the pre-industrial period and 1880, increasing
from 330 to 1000 Mg yr1. Globally, emissions increased by 35%
(Table 1). The resulting deposition increased by 60% (30%) over
North America (globally) in 1880.
Between 1880 and present time, North American emissions
(including biomass burning and land emissions) decreased by 40%
from 1000 to 600 Mg yr1. Over that same time period, however,
deposition to North America nearly doubled. This is due to the
factor of 2.4 increase in global emissions in the modern world
compared to 1880, and reflects the fact that a large fraction of
modern deposition over North America is due to the oxidation and
scavenging of Hg0 from the global pool (Selin and Jacob, 2008).
Globally, our 1880s emissions of 3800 Mg yr1 include a direct
mining source of 820 Mg yr1 and emissions from land and ocean
of 1580 Mg yr1 and 1400 Mg yr1, respectively. Relative to the preindustrial simulation, emissions from these two reservoirs are
enhanced by a total of 170 Mg yr1 because of the recycling of
deposited mining emissions.
Fig. 4 summarizes calculated ERmodern/p-i and ER1880/p-i for each
continent. While the majority of the mining emissions occurred in
North America, the impact on deposition is spread across all
continents because of the long lifetime of atmospheric Hg0.
ERmodern/p-i shows greater variability between continents than
ER1880/p-i in part because modern emissions occur not only as Hg0,
but also as HgII and Hgp, which can be deposited locally and
regionally. For North America and the global average, the modeled
ratios are close to the observed mean. Observations on other
continents are too sparse to calculate a continental average.
Several key uncertainties are important in the mining emission
estimates. We have assumed no anthropogenic emissions for the
1880 simulation, while in reality the industrial revolution had
started by this point. Consequently, our mining emission estimates
should be viewed as an upper limit on the true mining source. Preindustrial land and ocean emissions are also uncertain since there
are no direct measurements of these fluxes. While the model
reproduces well the mean ER values of the core record, cores taken
from nearby locations can show substantial variability not captured
by the model. The variability between cores may be due to either
local processes or uncertainties in the interpretation of the cores. To
better constrain historic mercury fluxes, a greater number of cores
from remote regions outside of North America would be very
useful.
4. Conclusions
Fig. 4. Deposition enhancement ratio spatially averaged over each continent and the
whole world. Light gray bars represent ERmodern/p-i and dark gray bars represent
ER1880/p-i. The 1880 simulation uses the revised mining estimate. The symbols show
the mean observed ratios from North American and global cores for ERmodern/p-i
(circles) and ERmodern/p-i (triangles) with error bars representing the standard
deviation. The filled black circles show the mean observed ERmodern/p-i for all cores
except the Wyoming ice core, while the open circles show ERmodern/p-i also exclude
Landers et al. (1998) observations in Alaska and Siberia as discussed in the text.
We estimate atmospheric mercury emissions from gold and
silver mining during the peak of the North American gold rush. Our
a priori mining emissions estimate is based on gold and silver
production averaged over the 1870s and multiplied by the ratio of
mercury lost to gold or silver produced and the fraction of mercury
emitted to the atmosphere. We include these emissions in a simulation of the global mercury cycle for the year 1880.
The modeled enhancement in mercury deposition between the
pre-industrial era and present compares well with the mean
enhancement seen in sediment core records, although it misses
much of the observed variability. However, the modeled enhancement due to mining emissions in the 1880 simulation overestimates the observed enhancement with a mean bias of 29%. To
improve the consistency with observations, we revise our estimate
of 1870s mining emissions of atmospheric mercury downward
from 1630 Mg yr1 to 820 Mg yr1. Globally, this leads to a deposition enhancement ratio of 1.3 for the 1880 versus pre-industrial
simulations.
Lower atmospheric emissions from 1870s mining imply
a smaller impact of the North American gold rush on global
mercury deposition. However, if a smaller fraction of the mercury
used in gold and silver mining was emitted to the global
S. Strode et al. / Atmospheric Environment 43 (2009) 2012–2017
atmosphere, then a larger fraction may have been deposited locally
to water and soil. More sediment cores from remote regions
throughout the world would be valuable for reducing the uncertainty in the global impact of historic gold and silver mining on
mercury deposition.
Acknowledgements
This work was supported by funding from the National Science
Foundation under grant ATM 0238530.
Appendix. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.atmosenv.2009.01.006.
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