Legacy impacts of all‐time anthropogenic emissions on the global mercury cycle
TL;DR: In this article, a global biogeochemical model with fully coupled atmospheric, terrestrial, and oceanic Hg reservoirs is presented to better understand human influence on Hg cycling and timescales for responses.
Abstract: [1] Elevated mercury (Hg) in marine and terrestrial ecosystems is a global health concern because of the formation of toxic methylmercury. Humans have emitted Hg to the atmosphere for millennia, and this Hg has deposited and accumulated into ecosystems globally. Here we present a global biogeochemical model with fully coupled atmospheric, terrestrial, and oceanic Hg reservoirs to better understand human influence on Hg cycling and timescales for responses. We drive the model with a historical inventory of anthropogenic emissions from 2000 BC to present. Results show that anthropogenic perturbations introduced to surface reservoirs (atmosphere, ocean, or terrestrial) accumulate and persist in the subsurface ocean for decades to centuries. The simulated present-day atmosphere is enriched by a factor of 2.6 relative to 1840 levels, consistent with sediment archives, and by a factor of 7.5 relative to natural levels (2000 BC). Legacy anthropogenic Hg re-emitted from surface reservoirs accounts for 60% of present-day atmospheric deposition, compared to 27% from primary anthropogenic emissions, and 13% from natural sources. We find that only 17% of the present-day Hg in the surface ocean is natural and that half of its anthropogenic enrichment originates from pre-1950 emissions. Although Asia is presently the dominant contributor to primary anthropogenic emissions, only 17% of the surface ocean reservoir is of Asian anthropogenic origin, as compared to 30% of North American and European origin. The accumulated burden of legacy anthropogenic Hg means that future deposition will increase even if primary anthropogenic emissions are held constant. Aggressive global Hg emission reductions will be necessary just to maintain oceanic Hg concentrations at present levels.
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Table 2. Hg Reservoir Masses and Historical Anthropogenic Enrichments 
Figure 3. Simulated present-day global Hg budget and all-time anthropogenic enrichments factors. Terrestrial-atmosphere exchange is given in the top panel for the sum of terrestrial reservoirs. The bottom panel shows the breakdown and cycling between the different terrestrial reservoirs. All terrestrial reservoirs receive inputs from Hg(II) deposition (fast = 800, slow= 510, and armored = 280Mg a 1). The fast reservoir also receives inputs from Hg(0) deposition (1600Mg a 1). All terrestrial reservoirs lose Hg through respiration (fast = 520, slow= 360, and armored =30Mg a 1) and biomass burning (fast = 320, slow = 9, and armored = 5Mg a 1). Photoreduction from the fast terrestrial reservoir is 950Mg a 1. 
Table 1. Present-Day Hg Reservoirs and Flowsa 
Figure 8. Change in reservoir masses relative to 2015 under a scenario of zero primary anthropogenic emissions after 2015. 
Figure 7. Natural and anthropogenic contributions to present-day atmospheric deposition and ocean reservoirs. The contribution from natural Hg is defined by steady state in our biogeochemical model without anthropogenic emissions. The primary anthropogenic contribution to deposition is from direct emissions (Figure 2a), while the legacy contribution is from anthropogenic Hg previously deposited and then re-emitted by surface reservoirs. The contribution from legacy Hg is calculated as total deposition minus primary anthropogenic emissions and natural emissions. For the ocean reservoirs, we partition the anthropogenic contribution by time period (left column) and region (right column). “Rest of world” includes Africa, the Middle East, and Oceania. 
Figure 1. Rate coefficients kij (a 1) driving our sevenreservoir global biogeochemical box model for Hg. kij defines the first-order transfer from reservoir i to reservoir j as Fij = kijmi, where Fij (Mg a 1) is the Hg flow from reservoir i to reservoir j and mi (Mg) is the mass of Hg in reservoir i. Values of kij are derived from best estimates of present-day flows andmasses from the literature (Table 1) and are assumed to be constant in time. The red arrow represents the external forcing by primary emissions (geogenic or anthropogenic) from the deep mineral reservoir. Geogenic emissions are constant (90Mg a 1), and anthropogenic emissions are time dependent (Figure 2). 
Figure 4. Normal modes of the Hg biogeochemical model. Each column describes an eigenvector of the mass transfer matrixK and represents a normal mode of the system. Perturbation to a normal mode decays exponentially with a timescale equal to the inverse of the corresponding eigenvalue. These timescales are displayed above the modes. The elements of the eigenvectors indicate the coupling between reservoirs associated with each timescale. For example, Mode 1 (0.2 years) describes a strong coupling between the surface and subsurface ocean reservoirs, and a weak coupling with the atmosphere. Mode 2 (0.8 years) describes a strong coupling between the atmosphere and subsurface ocean. Figure 5 gives a schematic summary of the information in this figure, highlighting the dominant couplings associated with each mode. 
Figure 5. Characteristic timescales (years) for Hg biogeochemical cycling derived from eigenanalysis of our sevencompartment box model (see section 4.1). The arrows indicate the principal reservoirs coupled over each timescale. These couplings often involve a combination or succession of transfers through intermediate reservoirs. More details are presented in Figure 4. 
Figure 6. Time-dependent rate of a pulse of Hg released to the atmosphere (top), fast terrestrial pool (middle), or surface ocean (bottom). A unit pulse of Hg is injected into the corresponding reservoir at time t = 0 years, and the resulting mass fraction from that pulse in individual reservoirs is plotted as a function of time. Note the log scale for time. ![Figure 2. History of global anthropogenic influence on environmental Hg: (a) primary anthropogenic emissions to the atmosphere, (b) corresponding cumulative emissions, (c) atmospheric deposition, and (d) combined surface + subsurface ocean mercury concentrations. The 1850–2008 emissions are from Streets et al. [2011]. Pre-1850 emissions are as described in the text. Post-2008 emissions assume the business-as-usual A1B scenario of Streets et al. [2009] from 2008 to 2015 and different hypothetical emission trajectories afterward as described in the text.](/figures/figure-2-history-of-global-anthropogenic-influence-on-16xvmtkg.png)
Figure 2. History of global anthropogenic influence on environmental Hg: (a) primary anthropogenic emissions to the atmosphere, (b) corresponding cumulative emissions, (c) atmospheric deposition, and (d) combined surface + subsurface ocean mercury concentrations. The 1850–2008 emissions are from Streets et al. [2011]. Pre-1850 emissions are as described in the text. Post-2008 emissions assume the business-as-usual A1B scenario of Streets et al. [2009] from 2008 to 2015 and different hypothetical emission trajectories afterward as described in the text.
![Figure 2. History of global anthropogenic influence on environmental Hg: (a) primary anthropogenic emissions to the atmosphere, (b) corresponding cumulative emissions, (c) atmospheric deposition, and (d) combined surface + subsurface ocean mercury concentrations. The 1850–2008 emissions are from Streets et al. [2011]. Pre-1850 emissions are as described in the text. Post-2008 emissions assume the business-as-usual A1B scenario of Streets et al. [2009] from 2008 to 2015 and different hypothetical emission trajectories afterward as described in the text.](/figures/figure-2-history-of-global-anthropogenic-influence-on-16xvmtkg.webp)
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TL;DR: Understanding of sources, atmosphere-land-ocean Hg dynamics and health effects are synthesized, and integration of Hg science with national and international policy efforts is needed to target efforts and evaluate efficacy.
Abstract: Mercury (Hg) is a global pollutant that affects human and ecosystem health. We synthesize understanding of sources, atmosphere-land-ocean Hg dynamics and health effects, and consider the implications of Hg-control policies. Primary anthropogenic Hg emissions greatly exceed natural geogenic sources, resulting in increases in Hg reservoirs and subsequent secondary Hg emissions that facilitate its global distribution. The ultimate fate of emitted Hg is primarily recalcitrant soil pools and deep ocean waters and sediments. Transfers of Hg emissions to largely unavailable reservoirs occur over the time scale of centuries, and are primarily mediated through atmospheric exchanges of wet/dry deposition and evasion from vegetation, soil organic matter and ocean surfaces. A key link between inorganic Hg inputs and exposure of humans and wildlife is the net production of methylmercury, which occurs mainly in reducing zones in freshwater, terrestrial, and coastal environments, and the subsurface ocean. Elevated human exposure to methylmercury primarily results from consumption of estuarine and marine fish. Developing fetuses are most at risk from this neurotoxin but health effects of highly exposed populations and wildlife are also a concern. Integration of Hg science with national and international policy efforts is needed to target efforts and evaluate efficacy.
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Abstract: The slope of the simple linear regression between log10 transformed mercury (Hg) concentration and stable nitrogen isotope values (δ15N), hereafter called trophic magnification slope (TMS), from several trophic levels in a food web can represent the overall degree of Hg biomagnification. We compiled data from 69 studies that determined total Hg (THg) or methyl Hg (MeHg) TMS values in 205 aquatic food webs worldwide. Hg TMS values were compared against physicochemical and biological factors hypothesized to affect Hg biomagnification in aquatic systems. Food webs ranged across 1.7 ± 0.7 (mean ± SD) and 1.8 ± 0.8 trophic levels (calculated using δ15N from baseline to top predator) for THg and MeHg, respectively. The average trophic level (based on δ15N) of the upper-trophic-level organisms in the food web was 3.7 ± 0.8 and 3.8 ± 0.8 for THg and MeHg food webs, respectively. For MeHg, the mean TMS value was 0.24 ± 0.08 but varied from 0.08 to 0.53 and was, on average, 1.5 times higher than that for THg with a...
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Cites background from "Legacy impacts of all‐time anthropo..."
...…using additional observational constraints (Streets et al. 2011; Horowitz et al. 2014) estimate higher present-day organic soil Hg pools (250–1000 Gg with a best estimate of 500 Gg) and propose that anthropogenic activities have doubled the Hg stored in organic soils (Amos et al. 2013, 2015)....
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...…of modeling tools that collapse the necessary detail from global simulations into more computationally feasible geochemical box models, enabling fully coupled simula- tions of the interactions among the land, atmosphere, and oceans over millennial time scales (Amos et al. 2013, 2014, 2015)....
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...…atmospheric Hg reservoir is estimated to be between 4400 and 5300 Mg, and is enriched by more than an order of magnitude relative to natural levels and by approximately three- to fivefold relative to 1850 levels (Amos et al. 2013; Engstrom et al. 2014; Horowitz et al. 2017; Streets et al. 2017)....
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...Amos et al. (2013) predicted that legacy Hg will be present in global ecosystems for periods ranging from decades to millennia, suggesting a large time lag in response to changes in anthropogenic emissions and climate....
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TL;DR: In this paper, the authors present an estimate of the total amount and spatial distribution of anthropogenic mercury in the global ocean based on oceanographic measurements of mercury and related parameters from several expeditions including data from recent GEOTRACES cruises.
Abstract: GEOTRACES sampling of deep water from the Atlantic, Pacific and Southern oceans allows an estimate of the amount (tripled in surface waters) and distribution (two-thirds increase in water less than a thousand metres deep) of anthropogenic mercury accumulating in the global ocean. Large amounts of the toxic trace metal mercury have been released into the environment as a result of human activities such as mining and burning of fossil fuels. Estimates of the amount of mercury that have reached the ocean as a result of such anthropogenic activities remain uncertain and are largely based on model studies. This paper presents an estimate of the total amount and spatial distribution of anthropogenic mercury in the global ocean based on oceanographic measurements of mercury and related parameters from several expeditions including data from recent GEOTRACES cruises. The findings suggest that there has been a tripling of the surface water mercury content and a ∼150% increase in the amount of mercury in the underlying thermocline water layer. Mercury is a toxic, bioaccumulating trace metal whose emissions to the environment have increased significantly as a result of anthropogenic activities such as mining and fossil fuel combustion1,2. Several recent models have estimated that these emissions have increased the oceanic mercury inventory by 36–1,313 million moles since the 1500s2,3,4,5,6,7,8,9. Such predictions have remained largely untested owing to a lack of appropriate historical data and natural archives. Here we report oceanographic measurements of total dissolved mercury and related parameters from several recent expeditions to the Atlantic, Pacific, Southern and Arctic oceans. We find that deep North Atlantic waters and most intermediate waters are anomalously enriched in mercury relative to the deep waters of the South Atlantic, Southern and Pacific oceans, probably as a result of the incorporation of anthropogenic mercury. We estimate the total amount of anthropogenic mercury present in the global ocean to be 290 ± 80 million moles, with almost two-thirds residing in water shallower than a thousand metres. Our findings suggest that anthropogenic perturbations to the global mercury cycle have led to an approximately 150 per cent increase in the amount of mercury in thermocline waters and have tripled the mercury content of surface waters compared to pre-anthropogenic conditions. This information may aid our understanding of the processes and the depths at which inorganic mercury species are converted into toxic methyl mercury and subsequently bioaccumulated in marine food webs.
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TL;DR: Atmosphere and at Atmospheric Interfaces: A Review and Future Directions Parisa A. Ariya, Marc Amyot, Ashu Dastoor, Daniel Deeds, Aryeh Feinberg, Gregor Kos, Andrei Ryjkov, Kirill Semeniuk, M. Subir, and Kenjiro Toyota are authors.
Abstract: Atmosphere and at Atmospheric Interfaces: A Review and Future Directions Parisa A. Ariya,*,†,‡ Marc Amyot, Ashu Dastoor, Daniel Deeds,‡ Aryeh Feinberg,† Gregor Kos,‡ Alexandre Poulain, Andrei Ryjkov, Kirill Semeniuk, M. Subir, and Kenjiro Toyota †Department of Chemistry and ‡Department of Atmospheric and Oceanic Sciences, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada, H3A 2K6 Department of Biological Sciences, Universite ́ de Montreál, 90 avenue Vincent-d’Indy, Montreal, Quebec, Canada, H3C 3J7 Air Quality Research Division, Environment Canada, 2121 TransCanada Highway, Dorval, Quebec, Canada, H9P 1J3 Department of Biology, University of Ottawa, 30 Marie Curie, Ottawa, Ontario, Canada, K1N 6N5 Department of Chemistry, Ball State University, 2000 West University Avenue, Muncie, Indiana 47306, United States Air Quality Research Division, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, Canada, M3H 5T4
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References
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Abstract: This paper presents a modeling approach aimed at seasonal resolution of global climatic and edaphic controls on patterns of terrestrial ecosystem production and soil microbial respiration. We use satellite imagery (Advanced Very High Resolution Radiometer and International Satellite Cloud Climatology Project solar radiation), along with historical climate (monthly temperature and precipitation) and soil attributes (texture, C and N contents) from global (1°) data sets as model inputs. The Carnegie-Ames-Stanford approach (CASA) Biosphere model runs on a monthly time interval to simulate seasonal patterns in net plant carbon fixation, biomass and nutrient allocation, litterfall, soil nitrogen mineralization, and microbial CO2 production. The model estimate of global terrestrial net primary production is 48 Pg C yr−1 with a maximum light use efficiency of 0.39 g C MJ−1PAR. Over 70% of terrestrial net production takes place between 30°N and 30°S latitude. Steady state pools of standing litter represent global storage of around 174 Pg C (94 and 80 Pg C in nonwoody and woody pools, respectively), whereas the pool of soil C in the top 0.3 m that is turning over on decadal time scales comprises 300 Pg C. Seasonal variations in atmospheric CO2 concentrations from three stations in the Geophysical Monitoring for Climate Change Flask Sampling Network correlate significantly with estimated net ecosystem production values averaged over 50°–80° N, 10°–30° N, and 0°–10° N.
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...[4] Sedimentary and ice core records provide evidence of a two- to five-fold enrichment in present-day atmospheric Hg deposition relative to preindustrial (ca. 1840) levels [Fitzgerald et al., 1998; Schuster et al., 2002; Biester et al., 2007; Lindberg et al., 2007]....
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...Although Asia is presently the dominant contributor to primary anthropogenic emissions, only 17% of the surface ocean reservoir is of Asian anthropogenic origin, as compared to 30% of North American and European origin....
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