Abstract
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.
Highlights
[1] Elevated mercury (Hg) in marine and terrestrial ecosystems is a global health concern because of the formation of toxic methylmercury
[3] Hg liberated from the deep mineral reservoir naturally or by human activities cycles between the atmosphere and surface reservoirs on timescales of years to decades [Mason et al, 1994; Mason and Sheu, 2002; Selin et al, 2008]
To be consistent with both the present-day atmospheric reservoir and the preindustrial-to-present enrichment in atmospheric deposition, we find that the rate coefficient of ocean evasion in our model must be within Æ30% of our best estimate from Figure 1
Summary
[2] Hg cycles naturally through geochemical reservoirs, but human activities such as mining and more recently fossil fuel combustion have been increasing the Hg flux from the deep mineral reservoir to the atmosphere for millennia [Nriagu, 1993, 1994; Lacerda, 1997; Camargo, 2002]. Transfer to fast pool River runoff to surface ocean Deep mineral reservoir: 3 Â 1011 Mg Geogenic emission Anthropogenic emissionsh. Mason [2007] for the subsurface/deep ocean and river runoff, Smith-Downey et al [2010] for the terrestrial/soil pools, and Andren and Nriagu [1979] and Pirrone et al [2010] for the deep mineral reservoir. [13] Hg from terrestrial reservoirs is transferred to the atmosphere by respiration of organic carbon, photoreduction, and biomass burning; and to the surface ocean by river runoff. We assume that 95% of the biomass burning source is from the fast terrestrial reservoir (which includes vegetation), and the remaining 5% is partitioned among the fast, slow, and armored soils based on their respective soil organic carbon content, following Smith-Downey et al [2010]. Business-as-usual is assumed between 2008 and 2015, and all scenarios take effect in 2015
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