Legacy impacts of all-time anthropogenic emissions on the global mercury cycle 1  

Helen M. Amosa*, Daniel J. Jacoba,b, David G. Streetsc, Elsie M. Sunderlandb,d 2  

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aHarvard University, Department of Earth and Planetary Sciences, 20 Oxford St, Cambridge, 4  

MA, 02138, USA 5  

bHarvard University, School of Engineering and Applied Sciences, 29 Oxford St, Cambridge, 6  

MA, 02138, USA 7  

cArgonne National Laboratory, Decision and Information Sciences Division, 9700 S. Cass Ave, 8  

Argonne, IL, 60439 USA 9  

dHarvard School of Public Health, Department of Environmental Health, 401 Park Dr, Landmark 10  

Center West, Boston, MA, 02215, USA 11  

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Abstract 19  

Elevated mercury (Hg) in marine and terrestrial ecosystems is a global health concern 20  

because of the formation of toxic methylmercury. Humans have emitted Hg to the atmosphere 21  

for millennia and this Hg has deposited and accumulated into ecosystems globally. Here we 22  

present a global biogeochemical model with fully-coupled atmospheric, terrestrial and oceanic 23  

Hg reservoirs to better understand human influence on Hg cycling, and timescales for responses. 24  

We drive the model with a historical inventory of anthropogenic emissions from 2000 BC to 25  

present. Results show that anthropogenic perturbations introduced to surface reservoirs 26  

(atmosphere, ocean, or terrestrial) accumulate and persist in the subsurface ocean for decades to 27  

centuries. The present-day atmosphere in the model is enriched by a factor of three relative to 28  

1850 levels, consistent with sediment archives, and by a factor of seven relative to natural levels 29  

(2000 BC). Legacy anthropogenic Hg re-emitted from surface reservoirs accounts for 60% of 30  

present-day atmospheric deposition, compared to 27% from primary anthropogenic emissions, 31  

and 13% from natural sources. We find that only 17% of the present-day Hg in the surface ocean 32  

is natural, and that half of its anthropogenic enrichment originates from pre-1950 emissions. 33  

Although Asia is presently the dominant contributor to primary anthropogenic emissions, only 34  

18% of the surface ocean reservoir is of Asian anthropogenic origin, as compared to 31% of 35  

North American and European origin. The accumulated burden of legacy anthropogenic Hg 36  

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means that future deposition will increase even if primary anthropogenic emissions are held 37  

constant. Aggressive global Hg emissions reductions will be necessary just to maintain oceanic 38  

Hg concentrations at present levels. 39  

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1. Introduction 41  

Hg cycles naturally through geochemical reservoirs, but human activities such as mining 42  

and more recently fossil fuel combustion have been increasing the Hg flux from the deep mineral 43  

reservoir to the atmosphere for millennia [Nriagu, 1993; Nriagu, 1994; Lacerda et al., 1997; 44  

Camargo, 2002]. Here we present a global biogeochemical model of Hg that that dynamically 45  

couples the ocean, atmosphere, and terrestrial reservoirs. We force the model with a historical 46  

inventory of anthropogenic emissions [Streets et al., 2011], quantify the resulting present-day 47  

enrichment of the various global Hg reservoirs, and discuss implications for the future. 48  

Hg liberated from the deep mineral reservoir naturally (e.g. volcanoes, erosion) or by 49  

human activities cycles between the atmosphere and surface reservoirs (ocean and terrestrial 50  

ecosystems) on timescales of years to decades [Mason et al., 1994; Mason and Sheu, 2002; Selin 51  

et al., 2008]. It is eventually transferred to recalcitrant soil pools [Smith-Downey et al., 2010] 52  

and the deep ocean [Gill and Fitzgerald, 1988; Sunderland and Mason, 2007] over centuries, and 53  

back to the deep mineral reservoir over millennia [Andren and Nriagu, 1979]. Anthropogenic 54  

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enrichment of the ocean is of particular interest because Hg exposure for most human 55  

populations is from methylmercury in marine fish [Johansen et al., 2004; Sunderland, 2007; Kim 56  

et al., 2010]. 57  

Sedimentary and ice core records provide evidence of a two- to five-fold enrichment in 58  

present-day atmospheric Hg relative to preindustrial (ca. 1850) levels [Fitzgerald et al., 1998; 59  

Schuster et al., 2002; Biester et al., 2007; Lindberg et al., 2007]. This reflects growing emissions 60  

in Europe and North America over the 19th and 20th centuries, compounded by growth in Asian 61  

emissions over the past decades [Pacyna et al., 2003, 2006; Wu et al., 2006; Pacyna et al., 2010; 62  

Streets et al., 2011]. However, pre-1850 anthropogenic emissions were also large [Nriagu, 1994; 63  

Streets et al., 2011], as demonstrated by sediment records extending back thousands of years 64  

[Roos-Barraclough et al., 2002, 2006; Cooke et al., 2009; Elbaz-Poulichet et al., 2011]. This 65  

historical anthropogenic Hg continues to cycle through biogeochemical reservoirs, representing a 66  

legacy of Hg enrichment in the environment. 67  

  Understanding how the legacy of past historical anthropogenic emissions impacts the 68  

present-day Hg cycle lays the foundation for projecting future changes. Global anthropogenic 69  

emissions are expected to increase in the future in the absence of directed efforts to control Hg 70  

[Streets et al., 2009]. Developing a global treaty to reduce Hg emissions is presently a focus 71  

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activity of the United Nations Environment Program [UNEP, 2012]. Here we examine how the 72  

global ocean, atmosphere, and terrestrial ecosystems will respond to future changes in emissions.  73  

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2. Model description 75  

We develop a fully coupled, seven-reservoir box model of the global Hg cycle by 76  

building on previous models that focused on subcomponents of the cycle [Sunderland and 77  

Mason, 2007; Selin et al., 2008; Soerensen et al., 2010; Holmes et al.2010; Smith-Downey et al., 78  

2010]. The seven reservoirs include the atmosphere, ocean (surface, subsurface, deep), and 79  

terrestrial ecosystems (fast terrestrial, slow soil, armored soil). Hg transferred from the deep 80  

mineral reservoir to the atmosphere by geogenic or anthropogenic emissions is viewed as an 81  

external forcing since the residence time of Hg in the deep mineral reservoir is ~1 billion years 82  

[Andren and Nriagu; 1979]. Burial from the deep ocean returns Hg to the deep mineral reservoir. 83  

Figure 1 provides a schematic of the box model. Mass transfer between model reservoirs 84  

is represented by a system of coupled first-order differential equations:

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[Eq. 1] 86  

where m is the 7-component vector of reservoir masses (Mg of Hg), s is a vector describing the 87  

external forcing (Mg a-1) from the deep mineral reservoir (0 for all reservoirs except for the 88  

atmosphere), and K is a 7x7 matrix of first-order rate coefficients kij (a-1) describing the flow 89  

  6  

from reservoir i to reservoir j . Thus kij = Fij/mi, where Fij (Mg a-1) is the flow (sink) from 90  

reservoir i to reservoir j, and mi (Mg) is the mass of reservoir i. All mass transfers between 91  

reservoirs are governed by first-order processes with time invariant kij, which we take as the best 92  

assumption in the absence of contrary evidence. We derive values of kij from literature estimates 93  

of present-day masses and flows, as summarized in Table 1 and described below. These 94  

estimates account for Hg speciation in the different reservoirs but here we only track total Hg. 95  

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2.1 Atmosphere 97  

We adopt the present-day atmospheric reservoir and deposition estimates from the global 98  

model simulation of Holmes et al. [2010] (Table 1), which show no systematic biases against 99  

observations. The resulting atmospheric reservoir is 5000 Mg and the total deposition is 6900 100  

Mg a-1, corresponding to an atmospheric lifetime of 0.7 years. Other literature estimates give a 101  

range of 4600-5600 Mg for the atmospheric reservoir, 4200-9000 Mg a-1 for total deposition, and 102  

0.5-2 years for atmospheric lifetime [Slemr et al., 1985; Lindqvist and Rhode, 1985; Mason et al., 103  

1994; Shia et al., 1999; Mason and Sheu, 2002; Lamborg et al., 2002b; Seigneur et al., 2004; 104  

Selin et al., 2008; Amos et al., 2012]. 105  

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2.2 Ocean 107  

We partition the ocean into three reservoirs: (1) the surface ocean extending to the base 108  

of the mixed layer, (2) the subsurface ocean extending to the depth of the permanent thermocline, 109  

and (3) the deep ocean extending to the sea floor. Our surface ocean budget, including air-sea 110  

exchange, is based on Soerensen et al. [2010], who use a global mean depth of 54 m based on 111  

observations [Montegut et al., 2004]. Subsurface and deep ocean reservoirs are estimated from 112  

vertical Hg profile data following Sunderland and Mason [2007]. 113  

Hg flows/sinks from ocean reservoirs include evasion to the atmosphere, settling of 114  

particulate matter including eventual burial in deep-sea sediments, and vertical transport of 115  

seawater. Particle settling is based on carbon export fluxes (ocean rain) using the 116  

parameterization of Antia et al. [2001] and global ocean productivity from MODIS satellite data 117  

(see Soerensen et al. [2010] and Sunderland and Mason [2007] for details). 118  

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2.3 Terrestrial ecosystems 120  

The terrestrial Hg is partitioned into three reservoirs: fast terrestrial (vegetation, litter, 121  

labile organic carbon soil pools, ice and snow-covered terrestrial surfaces), slow soil, and 122  

armored soil following the Global Terrestrial Mercury Model (GTMM) by Smith-Downey et al. 123  

[2010]. . GTMM estimates Hg cycling between terrestrial pools in association with organic 124  

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carbon following the Carnegie Ames Stanford Approach (CASA) global carbon model [Potter et 125  

al., 2003; van der Werf et al., 2003]. 126  

Transfer of Hg from terrestrial reservoirs to the atmosphere takes place by soil respiration, 127  

photoreduction, and biomass burning; and to the surface ocean by river runoff. Respiration of Hg 128  

(740 Mg a-1) is from Smith-Downey et al. [2010]. Global estimates of photoreduction are in the 129  

range 260-1100 Mg a-1 [Selin et al., 2008; Smith-Downey et al., 2010; Holmes et al., 2010]. We 130  

adopt a value of 850 Mg a-1 to balance emissions and deposition in our atmospheric budget. 131  

Global biomass burning emission estimates range from 300 Mg a-1 [Holmes et al., 2010] to 675 132  

Mg a-1 [Friedli et al., 2009]. We use here 300 Mg a-1, but a higher biomass burning source could 133  

be accommodated in our model by correspondingly decreasing the photoreduction source. We 134  

assume that 95% of the biomass burning source is from the fast terrestrial reservoir (which 135  

includes vegetation) and the remaining 5% is partitioned among the fast, slow, and armored soils 136  

based on their respective soil organic carbon content, following Smith-Downey et al. [2010]. 137  

River runoff is 380 Mg a-1 from Sunderland and Mason [2007] and is partitioned among 138  

terrestrial pools in the same manner as biomass burning. 139  

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2.4 Geogenic emission 143  

We define geogenic emission as Hg liberated from the deep mineral reservoir by volcanic 144  

activity, crustal degassing, and weathering [Nriagu and Becker, 2003]. Andren and Nriagu 145  

[1979] estimated the size of the deep mineral reservoir to be 3x1011 Mg. Estimates of geogenic 146  

range from ~1 to 1000 Mg a-1 [Varrekamp and Buseck, 1986; Ferrara et al., 2000; Nriagu and 147  

Becker, 2003; Pyle and Mather, 2003; Bagnato et al., 2011], but the high end of these estimates 148  

is thought not to be representative of mean contemporary volcanic activity [Bagnato et al., 2011]. 149  

We use 90 Mg a-1 from Pirrone et al. [2010], which is consistent with the estimate of Bagnato et 150  

al. [2011] (95 Mg a-1) derived from atmospheric observations of volcanic plumes. 151  

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2.5 Anthropogenic emissions 153  

We force the model from 2000 BC to 2008 AD with primary anthropogenic emissions of 154  

total Hg from Streets et al. [2011], as shown in Figure 2a. “Primary” refers to direct transfer 155  

from the deep mineral reservoir to the atmosphere. Present-day (defined here as 2008) emissions 156  

are 2000 Mg a-1. Cumulative emissions from 1850 to 2008 are 215 Gg (Figure 2b). Streets et al. 157  

[2011] provide detail on emission sectors and source regions for 1850-2008. An estimated 137 158  

Gg was emitted prior to 1850 according to Streets et al. [2011]. Nriagu [1994] estimated that 70 159  

Gg was emitted between 1570 and 1820 from mining activities in South and Central America, 160  

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and we assume here a constant rate of 280 Mg a-1 between 1570 and 1850. Sediment cores 161  

suggest a history of over 3500 years of anthropogenic Hg contamination [Cooke et al., 2009; 162  

Elbaz-Poulichet et al., 2011]. We assume that anthropogenic emissions began in 2000 BC and 163  

linearly increased until 1570, emitting a cumulative pre-1570 total of 59 Gg and resulting in a 164  

pre-1850 anthropogenic total of 137 Gg. 165  

Streets et al. [2009] projected future Hg emissions out to 2050 using the IPCC SRES 166  

scenarios [Nakicenovic et al., 2000]. The A1B business-as-usual scenario features continued 167  

growth in emissions such that primary anthropogenic emissions in 2050 are a factor of 3.6 higher 168  

than present-day. The most optimistic SRES scenario (B1) has effectively constant emissions 169  

relative to present-day levels [Streets et al., 2009]. These scenarios assume no direct policy 170  

intervention for Hg control, but negotiations are currently underway through the United Nations 171  

Environment Program for a global treaty to reduce primary anthropogenic emissions [UNEP, 172  

2012]. To illustrate the potential benefit of emission controls, we consider a “mercury controls” 173  

scenario where primary anthropogenic emissions decline by 2050 to 50% of their present-day 174  

value. Achieving this level of reduction would require implementation of Hg-specific control 175  

technologies across multiple sectors (fuel combustion, mining, waste incineration, industry). 176  

Lastly, we consider a “zero emissions” scenario where primary anthropogenic emissions are 177  

completely eliminated as of 2015. This bounds the maximum achievable global benefit from 178  

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regulating emissions. Business-as-usual is assumed between 2008 and 2015, and all scenarios 179  

take effect in 2015. 180  

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3. Model evaluation and uncertainty 182  

3.1 Evaluation 183  

We first apply the model to simulate natural steady state conditions, defined by solving 184  

(1) for dm/dt = 0 with s = 90 Mg a-1 as the constant geogenic source. Table 2 shows the natural 185  

distribution of Hg between reservoirs as obtained by the steady-state solution to the model with 186  

no anthropogenic forcing. From this initial condition we force the model with historical 187  

anthropogenic emissions (2000 BC - present), shown in Figure 2a. Table 2 shows the resulting 188  

pre-industrial (1840) and present-day reservoir masses. The model is consistent with 189  

observational constraints, including pre-industrial-to-present anthropogenic enrichment of 190  

atmospheric deposition as discussed below. 191  

Figure 3 depicts our simulated present-day global budget. Reservoir masses and flows are 192  

comparable to the literature values of Table 1 used to construct the model, even though these 193  

literature values were used only to constrain the first-order rate coefficients in the model. This 194  

provides an independent check and demonstrates consistency between present-day anthropogenic 195  

enrichments and our historical inventory of anthropogenic emissions. The present-day 196  

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atmospheric Hg reservoir is simulated to be 5200 Mg, within the expected range of 4600-5600 197  

Mg (see Section 2.1). The mean ocean concentration above the thermocline (surface+subsurface) 198  

is 1.4 pM, consistent with the range of 0.5-2.5 pM given by Mason et al. [2012] from compiled 199  

observations. We find a modern (preindustrial-to-present) enrichment factor of 3.1 for 200  

atmospheric deposition, which is consistent with sediment core data indicating that atmospheric 201  

Hg deposition has increased by 2 to 5 times relative to pre-industrial levels [Lamborg et al. 202  

2002a; Biester et al., 2007; Drevnick et al., 2012]. We conclude that our box model provides a 203  

realistic representation of the present-day budget of Hg in the different geochemical reservoirs 204  

and a plausible reconstruction of the historical anthropogenic enrichment. 205  

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3.2 Uncertainty 207  

We apply perturbation analysis to examine how uncertainties in individual model terms 208  

affect simulated anthropogenic enrichments of the different Hg reservoirs. We find that 209  

enrichment is most sensitive to uncertainties in geogenic emissions and oceanic evasion. The 210  

magnitude of geogenic emissions dictates the natural Hg loading and the resulting relative 211  

enrichment from primary anthropogenic emissions. Based on the observational constraints and 212  

reasonable estimates for primary anthropogenic emissions, geogenic emissions are unlikely to be 213  

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more than four times larger than our adopted value of 90 Mg a-1 and could be much smaller. 214  

Smaller geogenic emissions will increase the relative anthropogenic enrichment and vice versa. 215  

Oceanic evasion is the largest single input of Hg to the atmosphere and is a major 216  

removal pathway of Hg from the surface ocean (Table 1 and Figure 3). Removal of Hg from the 217  

surface ocean is a competitive process between evasion and export to the subsurface ocean by 218  

particle scavenging. If evasion rates decrease, the net transfer to the subsurface ocean increases 219  

while atmospheric deposition decreases and there is an associated increase in the relative 220  

anthropogenic enrichment of atmospheric deposition. To be consistent with both the present-day 221  

atmospheric reservoir and the preindustrial-to-present enrichment in atmospheric deposition, the 222  

rate coefficient of ocean evasion in our model must be within ±30% of our best estimate from 223  

Figure 1. 224  

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4. Timescales for Hg cycling and response to perturbations 226  

We use an eigenanalysis of our seven-compartment box model (see Appendix) to 227  

determine the characteristic timescales involved in Hg global biogeochemical cycling. These 228  

timescales are shown in Figure 4 together with the principal reservoirs involved. They range 229  

from 0.2 years (exchange between the surface and subsurface ocean) to 6,000 years (transfer 230  

from the armored soil pool to the deep mineral reservoir via the atmosphere and the ocean). The 231  

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subsurface ocean is central in driving the biogeochemical Hg cycle, as highlighted in Figure 4. It 232  

is coupled to the atmosphere via deposition and air-sea exchange (0.8 years), to the fast and slow 233  

terrestrial reservoirs via river runoff and atmospheric cycling (6-70 years), and to the deep ocean 234  

(200 years) from which Hg is eventually buried in deep-sea sediments. 235  

Figure 5 shows the time-dependent fate of a unit pulse of Hg released in the atmosphere 236  

(top panel), surface ocean (center panel), and fast terrestrial reservoir (bottom panel). All 237  

reservoirs are initialized with zero mass. The pulse is released at t = 0 and there are no further 238  

releases for t > 0. We find that the pulse is transferred away from the reservoir of origin on a 239  

time scale of months (surface ocean, atmosphere) to a decade (fast terrestrial). In all three cases 240  

the transfer is principally to the subsurface ocean due to the fast timescales involved in 241  

communicating with that reservoir. 242  

Of the Hg that is initially released to the fast terrestrial reservoir, a greater fraction ends 243  

up in the subsurface ocean than in the slow soil reservoir. Terrestrial Hg can either be transferred 244  

to the surface ocean by re-emission to the atmosphere (photoreduction, respiration of organic 245  

matter, biomass burning) and subsequent deposition, or through river runoff. On a global scale, 246  

cycling through the atmosphere is the dominant pathway. Transfer of Hg from the fast terrestrial 247  

reservoir to more recalcitrant soil pools is slow and even then the armored soil reservoir is not a 248  

permanent sink of the Hg. In order for the initial Hg perturbation to be removed from the system, 249  

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Hg from the armored soil reservoir must be transferred to the deep ocean where it is 250  

subsequently buried in sediments and this process occurs on the timescale of millennia. 251  

Even though the armored soil is the dominant Hg reservoir under natural conditions 252  

(Table 2), most of the Hg perturbation resides in the ocean on timescales of decades to a 253  

millennium. Regardless of the location of the pulse of origin, after 1000 years we find that 254  

roughly 40% is in the ocean, 20% is in the soil, and 40% has been returned to the deep mineral 255  

reservoir. The preferential long-lived reservoir for the perturbed system is the deep ocean, 256  

imposing a continued legacy on the subsurface/surface ocean reservoirs through vertical 257  

exchange of seawater. 258  

Historically, large quantities of Hg have been released directly to terrestrial ecosystems 259  

[Streets et al., 2011]. The implication of our work is that a large fraction of anthropogenic Hg 260  

likely ends up in the subsurface ocean regardless of whether the Hg was released into the 261  

atmosphere, fast terrestrial reservoir, or surface ocean.  262  

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5. Anthropogenic enrichment and the importance of legacy Hg 264  

Table 2 shows that the present-day anthropogenic enrichment of the surface and 265  

subsurface ocean is similar to that in the atmosphere, reflecting the short timescales for coupling 266  

of these reservoirs. Even the deep ocean shows a factor of two enrichment. We find that the 267  

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present-day atmospheric Hg reservoir (and therefore deposition) is enriched 7 times relative to 268  

natural levels (Table 2, Figure 3). This enrichment is larger than the observed 3-fold 269  

anthropogenic enrichment from sediment cores that extend back to the mid-19th century (ca. 270  

1850) [Biester et al., 2007; Lindberg et al., 2007] and reflects anthropogenic Hg emissions prior 271  

to 1850 (described in Section 2.5). Several long-term archives corroborate the importance of 272  

anthropogenic Hg emissions prior to 1850 [Roos-Barraclough et al., 2002; Givelet et al., 2003; 273  

Roos-Baraclough et al., 2006; Cooke et al., 2009, 2011; Thevenon et al., 2011; Conaway et al., 274  

2012]. 275  

Figure 6 gives the source contributions to present-day atmospheric deposition and 276  

oceanic reservoirs. 27% of atmospheric deposition is from primary anthropogenic emissions, 277  

which is consistent with previous estimates [Mason and Sheu, 2002; Selin et al., 2008; Pironne 278  

et al., 2010; Corbitt et al., 2011]. We find that natural emissions contribute only 13% of present-279  

day deposition when accounting for pre-1850 anthropogenic emissions. Most (60%) of present-280  

day deposition is legacy anthropogenic Hg previously deposited and subsequently re-emitted. 281  

Past model studies had concluded that 1/3 of present-day deposition is natural and 1/3 is legacy 282  

anthropogenic, but this was built on the assumption of negligible anthropogenic emissions prior 283  

to 1850 and an imposed factor of 3 increase in deposition relative to 1850 based on soil core data 284  

[Mason et al., 1994; Mason and Sheu, 2002; Seigneur et al., 2004; Selin et al., 2008]. Our results 285  

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indicate that contribution from natural emissions to present-day deposition is much smaller than 286  

previously estimated, the contribution from legacy anthropogenic emissions much larger, and 287  

thus the anthropogenic perturbation to the natural Hg cycle is much greater than generally 288  

recognized. 289  

Figure 6 also shows the sensitivity of the present-day ocean reservoirs to anthropogenic 290  

emissions released in different historical periods. We find that 78% of the total anthropogenic Hg 291  

in the deep ocean originates from pre-1900 emissions. This legacy Hg exerts a sustained effect 292  

on the surface and subsurface ocean through vertical exchange of seawater. Pre-1950 293  

anthropogenic emissions account for more than half of the present-day enrichment in the surface 294  

and subsurface reservoirs. 295  

Lastly, Figure 6 shows the contributions of post-1850 anthropogenic emissions from 296  

different source continents (from Streets et al. [2011]) to the present-day anthropogenic 297  

enrichments of the different reservoirs. Although Asia presently accounts for 65% of global 298  

primary anthropogenic emissions [Streets et al., 2011], we find that Hg of Asian anthropogenic 299  

origin accounts for only 18% and 10% of the surface and subsurface ocean reservoirs, 300  

respectively. The legacy of past North American and European anthropogenic emissions together 301  

accounts for 25% of present-day deposition and 31% of surface ocean Hg. 302  

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6. Implications for the future 304  

We project future changes in the Hg cycle under four illustrative emission trajectories 305  

(Section 2.5, Figure 2a): (1) the business-as-usual A1B scenario of Streets et al. [2009] with 306  

continued increases in global emissions, (2) constant future emissions, (3) 50% decrease of 307  

emissions by 2050 (“mercury controls”), and (4) zero future anthropogenic emissions as a 308  

bounding case. These scenarios are assumed to take effect in 2015. 309  

Figure 2c shows the atmospheric deposition for each of the four future emission scenarios. 310  

Primary anthropogenic emissions increase by 2000 Mg a-1 between 2015 and 2050 under A1B, 311  

but atmospheric deposition increases by 4600 Mg a-1. Even if primary anthropogenic emissions 312  

stay constant between 2015 and 2050, atmospheric deposition increases by 30%. Under these 313  

scenarios new anthropogenic Hg is being added to the pool of legacy Hg faster than it can be 314  

sequestered into longer-lived reservoirs and the legacy contribution to deposition grows as a 315  

result. Corbitt et al. [2011] projected that atmospheric deposition would remain at present-day 316  

levels under the B1 scenario of Streets et al. [2009] (effectively constant emissions), but they 317  

assumed the legacy Hg pool to remain constant. Our work highlights the importance of 318  

accounting for changing legacy Hg in atmospheric Hg modeling simulations. 319  

We find that a decrease in Hg emission is needed just to maintain Hg deposition at its 320  

present-day level. Under our hypothetical ‘mercury controls’ scenario with 50% decrease in 321  

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primary anthropogenic emissions by 2050, deposition begins to decrease by 2020 as the legacy 322  

contribution stabilizes (reflecting a balance between new anthropogenic Hg inputs and transfer to 323  

the longer-lived reservoirs). In this scenario, primary emissions decrease by 1400 Mg a-1 from 324  

2015 to 2050 while deposition decreases by 1200 Mg a-1. In the bounding case of zero future 325  

anthropogenic emissions, there is an immediate 30% decrease in deposition (associated with 326  

primary anthropogenic emissions) and slower subsequent decrease limited by the time scale for 327  

transfer from the subsurface to the deep ocean (Figure 5). 328  

To illustrate the timescales required for dissipation of anthropogenic influence, Figure 7 329  

shows the time-dependent responses of the atmosphere and ocean Hg reservoirs to complete 330  

elimination of anthropogenic Hg emissions in 2016. We previously pointed out the immediate 331  

30% decrease in the atmosphere. There is also an immediate 10% decline in the surface ocean 332  

that it is dampened relative to the atmosphere by the persistent legacy influence from the 333  

subsurface ocean. Subsequent decline in the surface ocean is more gradual, with 65% of the 334  

initial loading remaining in 2100. The subsurface ocean begins to slowly decrease after a decade, 335  

mostly by transfer to the deep ocean, but the anthropogenic signal persists for centuries. The 336  

deep ocean reservoir continues to increase throughout the 21st century even in the absence of 337  

primary anthropogenic emissions. 338  

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Future Hg concentrations in the surface+subsurface ocean are of particular concern 339  

because they are critical for determining the reservoir available for oceanic methylmercury 340  

production [Mason et al., 2012] and associated concentrations in pelagic marine fish. Under the 341  

business-as-usual emissions trajectory, Hg concentrations in the combined surface+subsurface 342  

ocean will increase by 40% (relative to present day) by 2050. Under the ‘mercury controls’ 343  

scenario, the Hg concentration in the surface/subsurface will still increase by 20%. Our results 344  

suggest that stabilization of ocean concentrations at present-day levels will require aggressive 345  

reductions in primary anthropogenic emissions. 346  

347  

7. Conclusions 348  

We develop a seven-compartment biogeochemical box model to describe the global 349  

cycling of mercury (Hg) between the atmosphere, ocean, and terrestrial reservoirs. The model is 350  

based on our best current understanding from observations and process models, and is described 351  

by first-order rate constants (coupled ODEs) for the transfer of Hg between compartments. We 352  

force the model with time-dependent historical emissions (2000 BC – present), initializing from 353  

natural steady state, and show consistency with observational constraints for the present-day 354  

budget and anthropogenic enrichments. We use the model to analyze the timescales involved in 355  

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Hg cycling and quantify the impact of legacy anthropogenic Hg in the present-day global budget 356  

and under various future emission scenarios. 357  

We find that characteristic timescales for Hg global biogeochemical cycling range from 358  

less than a year for coupling between the subsurface/surface ocean and the atmosphere to 359  

thousands of years for transfer from the armored soil to the deep mineral pool via the deep ocean. 360  

The subsurface ocean, extending from the base of the ocean mixed layer down to the permanent 361  

thermocline, plays a central role in the global biogeochemical cycling of Hg. A perturbation to 362  

the surface reservoirs (atmosphere, ocean, fast terrestrial) propagates to the subsurface ocean on 363  

a time scale of years to decades, and persists there for decades to centuries. Under natural 364  

conditions, terrestrial reservoirs contain a larger Hg inventory than the ocean. However, 365  

perturbations to surface reservoirs first propagate to the deep ocean because transfer of Hg to the 366  

armored soil reservoir is extremely slow, and persist in the deep ocean for centuries to millennia 367  

with upwelling causing a sustained effect on the surface+subsurface ocean. The persistence of 368  

Hg in the ocean is of particular environmental consequence because of the importance of marine 369  

fish as source of methylmercury exposure for humans and wildlife. 370  

Atmospheric deposition in the model increases by a factor of three from 1840 to present, 371  

consistent with sediment archives. This factor of three has been regarded in previous studies as 372  

the anthropogenic enrichment relative to natural conditions, but in fact the anthropogenic 373  

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perturbation to the Hg cycle extends back for millennia. We find that all-time anthropogenic Hg 374  

emissions have enriched the present-day atmosphere, surface ocean, and deep ocean by factors of 375  

seven, six, and two, respectively, relative to natural conditions. This implies a much larger 376  

relative anthropogenic perturbation than is previously recognized. 377  

Model results show that most (60%) of present-day Hg atmospheric deposition is legacy 378  

anthropogenic Hg re-emitted from surface reservoirs. Natural emissions and primary 379  

anthropogenic emissions contribute 13% and 27% to present-day deposition, respectively. Over 380  

half of the anthropogenic enrichment of the surface ocean is from pre-1950 emissions, and most 381  

of the anthropogenic enrichment of the deep ocean is from pre-1900 emissions. Although Asia 382  

accounts for 65% of present-day primary anthropogenic emissions [Streets et al., 2011], Hg of 383  

Asian anthropogenic origin only accounts for 30% of present-day atmospheric deposition and 384  

18% of the surface ocean reservoir. By contrast, North America and Europe together account 385  

for 31% of the surface ocean reservoir. 386  

Understanding how the legacy of past anthropogenic emissions contributes to present-day 387  

Hg enrichment is essential for anticipating the effectiveness of future Hg emission reduction 388  

strategies. If anthropogenic emissions continue to rise, the resulting growth in atmospheric 389  

deposition will be greater than the increase in primary emissions due to sustained growth of the 390  

legacy component. Even if emissions stay constant, atmospheric deposition will continue to 391  

  23  

increase because new anthropogenic Hg will add to the surface+subsurface ocean reservoir faster 392  

than Hg can be transferred from that reservoir to the deep ocean. Our work shows that projecting 393  

future atmospheric deposition under changing emissions requires a full accounting of the 394  

coupling between the biogeochemical reservoirs. Aggressive reduction in primary anthropogenic 395  

emissions will be needed just to maintain oceanic Hg concentrations at present-day levels. 396  

We investigated the response of the global Hg cycle to zeroing primary anthropogenic 397  

emissions after 2015, as a bound on the maximum global benefits to be expected from regulating 398  

emissions. We find a nearly instantaneous 30% decrease in atmospheric deposition and 10% 399  

decrease in the surface ocean reservoir. Subsequent declines are slower, limited by the time 400  

scale for transfer to the deep ocean. By 2100 there is a 55% decrease in atmospheric deposition 401  

and 35% decrease in surface ocean loading. The subsurface ocean responds more slowly to the 402  

elimination of primary anthropogenic emissions and by 2100 has decreased by 25%. 403  

Our analysis has focused on the global background. However, Hg concentrations in the 404  

surface ocean and terrestrial reservoirs vary depending on local environmental conditions, and 405  

many historic Hg problems have occurred as site specific contamination issues from point 406  

sources. Ecosystem scale studies indicate large variability in biotic Hg response to total Hg 407  

loading, depending on conditions facilitating methylmercury production [Harris et al., 2007; 408  

  24  

Knightes et al., 2009]. The global information provided here needs to be combined with site-409  

specific information to understand the factors driving biological exposures. 410  

411  

Appendix; Eigenanalysis of timescales for Hg biogeochemical cycling 412  

Understanding the timescales involved in Hg biogeochemical cycling is critical to 413  

interpreting the effects of perturbations. Turnover times (τi = 1/ ) for individual reservoirs 414  

in our 7-box model provide a first estimate of these timescales but do not account for the 415  

coupling between reservoirs. A better approach is to perform an eigenanalysis following the 416  

method of Prather [1994; 1996]. For this purpose we compute the eigenvectors (x) and 417  

corresponding eigenvalues (λ) of the 7x7 mass transfer matrix K = (kij) in equation (1). Each 418  

eigenvector xp (p ∈ [1,7]) represents a separate normal mode of the model and any perturbation 419  

to that mode decays exponentially with a characteristic timescale 1/λp. 420  

Figure A1 shows the normal modes of our model and the corresponding characteristic 421  

timescales. Any initial-time perturbation Δm(0) to the model can be projected on the basis of 422  

these normal modes as 423  

[A1] 424  

where αp are coefficients. Thus the perturbation decays as a sum of exponentials following 425  

  25  

[A2] 426  

Different perturbations will excite different combinations of modes (and hence reservoir 427  

couplings). The eigenlifetimes 1/λp are the true timescales describing the dynamics of the model. 428  

Each mode in Figure A1 is defined by its eigenvector elements, describing the nature of 429  

the coupling between reservoirs for the corresponding timescale. Although all reservoirs are 430  

coupled on any of the timescales, there is always a dominant pair as is highlighted in the 431  

simplified schematic of Figure 4, which summarizes the information contained in Figure A1. We 432  

thus see from Figure A1 that the shortest Mode 1 (1/λ = 0.24 years) principally describes the fast 433  

exchange between the surface and subsurface ocean. Mode 2 (0.84 years) describes the coupling 434  

between the atmosphere and subsurface ocean via the surface ocean. . Modes 3 (6.0 years) and 4 435  

(66 years) couple respectively the fast and slow terrestrial pools to the subsurface ocean via the 436  

atmosphere and surface ocean. Mode 5 (190 years) couples the subsurface and deep ocean by 437  

ocean mixing. Mode 6 (1000 years) describes the return of Hg to the deep mineral reservoir by 438  

burial from the deep ocean. Mode 7 (5,900 years) describes the ultimate decay of the 439  

perturbation by transfer from the armored soil reservoir to the deep mineral reservoir via ocean 440  

burial. 441  

442  

  26  

Acknowledgements 443  

We acknowledge financial support from NSF Atmospheric Chemistry, NSF Chemical 444  

Oceanography, and the Electric Power Research Institute (EPRI). H.M.A. acknowledges support 445  

from NSF GRFP. We thank Anne Soerensen and Mauricio Santillana for thoughtful discussions. 446  

447  

448  

449  

450  

451  

452  

453  

454  

455  

456  

457  

458  

459  

460  

  27  

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  32  

Figure Captions 626  

Figure 1. Rate coefficients kij (a-1) driving our 7-compartment global biogeochemical box model 627  for Hg. kij defines the first-order transfer from reservoir i to reservoir j as Fij=kijmi, where Fij (Mg 628  a-1) is the Hg flow from reservoir i to reservoir j and mi (Mg) is the mass of Hg in reservoir 629  i.Values of kij are derived from best estimates of present-day flows and masses from the literature 630  (Table 1) and are assumed to be constant in time. The red arrow represents the external forcing 631  by primary emissions (geogenic or anthropogenic) from the deep mineral reservoir. Geogenic 632  emissions are constant (90 Mg a-1) and anthropogenic emissions are time-dependent. 633  

634  

Figure 2. History of global anthropogenic influence on environmental Hg. (a) Primary 635  anthropogenic emissions to the atmosphere, (b) corresponding cumulative emissions, (c) 636  atmospheric deposition, and (d), combined surface+subsurface ocean mercury concentrations. 637  1850-2008 emissions are from [2]. Pre-1850 emissions are described in the text. Post-2008 638  emissions assume the business-as-usual A1B scenario of Streets et al. [2011] from 2008 to 2015 639  and different hypothetical emission trajectories afterward as described in the text. 640  

641  

Figure 3. Simulated present-day global Hg budget and all-time anthropogenic enrichments (%). 642  Terrestrial-atmosphere exchange is given in the top panel for the sum of terrestrial reservoirs. 643  The bottom panel shows the breakdown and cycling between the different terrestrial reservoirs. 644  All terrestrial reservoirs receive inputs from Hg(II) deposition (fast = 780, slow = 500, armored = 645  270 Mg a-1). The fast reservoir also receives inputs from Hg(0) deposition (1500 Mg a-1). All 646  terrestrial reservoirs lose Hg through soil respiration (fast = 500, slow = 340, armored =30 Mg a-647  1) and biomass burning (fast = 320, slow = 8, armored = 5 Mg a-1). Photoreduction from the fast 648  terrestrial reservoir is 920 Mg a-1. 649  

650  

Figure 4. Characteristic timescales (years) for Hg biogeochemical cycling derived from 651  eigenanalysis of our seven-compartment box model (see Appendix). The arrows indicate the 652  principal reservoirs coupled over each timescale. These couplings often involve a combination or 653  succession of transfers through intermediate reservoirs. More detail is presented in Figure A1. 654  

655  

656  

  33  

Figure 5. Time-dependent fate of a pulse of Hg released to the atmosphere (top panel), fast 657  terrestrial pool (center), and surface ocean (bottom). A unit pulse of Hg is injected into the 658  corresponding reservoir at time t = 0 years and the resulting mass fraction from that pulse in 659  individual reservoirs is plotted as a function of time. Note the log scale for time. 660  

661  

Figure 6. Natural and anthropogenic contributions to present-day atmospheric deposition and 662  ocean reservoirs. The contribution from natural Hg is defined by steady state in our 663  biogeochemical model without anthropogenic emissions. The primary anthropogenic 664  contribution to deposition is from direct emissions (Figure 2a), while the legacy contribution is 665  from anthropogenic Hg previously deposited and then re-emitted by surface reservoirs. The 666  contribution from legacy Hg is calculated as total deposition minus primary anthropogenic 667  emissions and natural emissions. For the ocean reservoirs, we partition the anthropogenic 668  contribution by time period (left column) and region (right column). “Rest of world” includes 669  Africa, the Middle East, and Oceania. 670  

671  

Figure 7. Change in reservoir masses relative to 2015 under a scenario of zero primary 672  anthropogenic emissions after 2015. 673  

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  34  

Table 1. Present-day Hg reservoirs and flows used for box model constructiona 682  

Atmosphere: 5000 Mg Flows (Mg a-1) Hg(II) deposition to ocean 4000 Hg(0) deposition to ocean 40 Hg(II) deposition to landb 1500 Hg(0) deposition to landc 1500

Surface oceand: 2900 Mg Hg(0) evasion 3000 Particle settling to subsurface ocean 3300 Water transfer to subsurface ocean 5100

Subsurface oceane: 130,000 Mg Particle settling to deep ocean 480 Water transfer to surface ocean 7100 Water transfer to deep ocean 340

Deep ocean: 220,000 Mg Burial to deep sediments 210 Water transfer to subsurface ocean 180

Fast terrestrial pool: 9600 Mg Evasion due to respiration of organic carbon 460 Photochemical re-emission of deposited Hg 850 Biomass burningf 290 Transfer to slow pool 330 Transfer to armored pool 10 River runoff to surface oceang 365

Slow soil pool: 35,000 Mg Evasion due to respiration of organic carbon 330 Biomass burning 8 Transfer to fast pool 210 River runoff to surface ocean 10

Armored soil pool: 190,000 Mg Evasion due to respiration of organic carbon 10 Biomass burning 4 Transfer to fast pool 15 River runoff to surface ocean 5

Deep mineral reservoir: 3x1011 Mg Geogenic emission 90 Anthropogenic emissions 2000h

683  aReferences are Holmes et al. [2010] for the atmosphere and biomass burning, Soerensen et al. 684  [2010] for the surface ocean, Sunderland and Mason [2007] for the subsurface/deep ocean and 685  river runoff, Smith-Downey et al. [2010] for the terrestrial/soil pools, and Andren and Nriagu 686  [1979] and Pirrone et al. [2010] for the deep mineral reservoir. 687  

  35  

bPartitioned 50% to the fast terrestrial pool, 32% to the slow soil pool, and 18% to the armored 688  soil pool following Smith-Downey et al. [2010]. 689  cAll Hg(0) deposition is delivered to the fast terrestrial pool. 690  dExtending down to the base of the ocean mixed layer. 691  eExtending from the base of the ocean mixed layer down to the depth of the permanent 692  thermocline. 693  fTotal biomass burning is 300 Mg a-1 [Holmes et al., 2010] and is apportioned as 95% from 694  vegetation (fast terrestrial pool) and 5% from the three soil pools (fast, slow, armored) based on 695  carbon content [Smith-Downey et al., 2010]. 696  gTotal river runoff is 380 Mg a-1 [Sunderland and Mason, 2007] and is partitioned among the soil 697  pools in the same manner as biomass burning. 698  hAnthropogenic emissions are time-dependent (see Figure 2a). 2000 Mg a-1 corresponds to the 699  year 2008 from Streets et al. [2011]. 700   701  

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  36  

Table 2. Hg reservoir masses and historical anthropogenic enrichments 713   Reservoir mass (Mg) Enrichment factor Reservoir Natural Preindustrial

(1840) Present-day All-timea Modernb

Atmosphere 700 1,700 5,200 7.3 3.1 Terrestrial Fast 1,900 3,600 10,000 5.6 2.8 Slow 10,000 20,000 48,000 4.9 2.4

Armored 170,000 190,000 210,000 1.2 1.1 Ocean Surface 530 1,200 3,000 5.7 2.6 Subsurface 27,000 58,000 140,000 5.1 2.4

Deep 94,000 130,000 190,000 2.0 1.5 714  aRatio of present-day to natural reservoir mass 715   bRatio of present-day to preindustrial (1840) reservoir mass 716  

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728  Figure 1. 729  

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740  

Figure 2. 741  

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Figure 3. 751  

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Figure 4. 757  

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Figure 5. 767  

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Figure 6. 774  

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777  

Figure 7. 778  

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Figure A1. 788