Atmospheric Environment 43 (2009) 2012–2017

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Impact of mercury emissions from historic gold and silver mining: Global modeling Sarah Strode a, Lyatt Jaegle´ 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. Jaegle´). 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

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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.

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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

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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.

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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

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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. References Alpers, C.N., Hunerlach, M.P., May, J.T., Hothem, R.L., Taylor, H.E., Antweiler, R.C., De Wild, J.F., Lawler, D.A., 2005. Geochemical characterization of water, sediment, and biota affected by mercury contamination and acidic drainage from historical gold mining, Greenhorn Creek, Nevada County, California, 1999–2001. U.S. Geological survey scientific investigations report 2004-5251. Available from: http://pubs.usgs.gov/sir/2004/5251. Asmund, G., Nielsen, S.P., 2000. Mercury in dated Greenland marine sediments. Science of the Total Environment 245, 61–72. Bey, I., Jacob, D.J., Yantosca, R.M., Logan, J.A., Field, B.D., Fiore, A.M., Li, Q.B., Liu, H.G.Y., Mickley, L.J., Schultz, M.G., 2001. Global modeling of tropospheric chemistry with assimilated meteorology: model description and evaluation. Journal of Geophysical ResearchdAtmospheres 106 (D19), 23073–23095. Biester, H., Bindler, R., Martinez-Cortizas, A., Engstrom, D.R., 2007. Modeling the past atmospheric deposition of mercury using natural archives. Environmental Science and Technology 41 (14), 4851–4860. Biester, H., Kilian, R., Franzen, C., Woda, C., Mangini, A., Sholer, H.F., 2002. Elevated mercury accumulation in a peat bog of the Magellanic Moorlands, Chile (53 S) – an anthropogenic signal from the Southern Hemisphere. Earth and Planetary Science Letters 201, 609–620. Bindler, R., Renberg, I., Appleby, P.G., Anderson, N.J., Rose, N.L., 2001. Mercury accumulation rates and spatial patterns in lake sediments from West Greenland: a coast to ice margin transect. Environmental Science and Technology 35, 1736–1741. Brading, D.A., Cross, H.E., 1972. Colonial silver mining: Mexico and Peru. Hispanic America Historical Review 52 (4), 545–579. Bureau of the Census, 1989. Historical Statistics of the Unites States: Colonial Times to 1970. U.S. Department of Commerce, Kraus International Publications, White Plains, New York. Engstrom, D.R., Wright Jr., H.E., 1984. Chemical stratigraphy of lake sediments as a record of environmental change. In: Haworth, E.Y., Lund, J.G. (Eds.), Lake Sediments and Environmental History. University of Minnesota Press, Minneapolis, MN. Engstrom, D.R., Swain, E.B., 1997. Recent declines in atmospheric mercury deposition in the upper midwest. Environmental Science and Technology 31 (4), 960–967. Fisher, J.R., 1977. Silver Mines and Silver Miners in Colonial Peru, 1776–1824. Center for Latin American Studies, University of Liverpool, Liverpool. Fitzgerald, W.F., Engstrom, D.R., Lamborg, C.H., Tseng, C.-M., Balcom, P.H., Hammerschmidt, C.R., 2005. Modern and historic atmospheric mercury fluxes in northern Alaska: global sources and arctic depletion. Environmental Science and Technology 39, 557–568. Gobeil, C., Cossa, D., 1993. Mercury in sediments and sediment pore water in the Laurentian Trough. Canadian Journal of Fisheries and Aquatic Sciences 50, 1794–1800. Hudson, R.J.M., Gherini, S.A., Fitzgerald, W.F., Porcella, D.B., 1995. Anthropogenic influences on the global mercury cycle: a model-based analysis. Water, Air, and Soil Pollution 80, 265–272. Kamman, N.C., Engstrom, D.R., 2002. Historical and present fluxes of mercury to Vermont and New Hampshire lakes inferred from 210Pb dated sediment cores. Atmospheric Environment 36, 1599–1609. Lacerda, L.D., Salomons, W., 1991. Mercury in the Amazon: A Chemical Time Bomb? Dutch Ministry of Housing. Physical Planning and Environment Report Haren. Lacerda, L.D., Ribeiro, M.G., Cordeiro, R.C., Sifeddine, A., Turcq, B., 1999. Atmospheric mercury deposition over Brazil during the past 30,000 years. Environmental Biodiversity 51 (5/6), 363–371.

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