Modeling Perspectives on the Environmental Legacy of Human and Natural Mercury Releases

Abstract

Mercury (Hg) is a naturally occurring heavy metal that has been mined and used by humans since antiquity. Exposure to Hg, particularly its organic form (methylmercury) poses health risks for humans and wildlife globally. This thesis quantitatively explores the natural biogeochemical Hg cycle and the extent of the perturbation from human activity. Chapter 1 focuses on the natural Hg cycle. Volcanism is the largest natural source of Hg to the biosphere. However, past Hg emission estimates have varied by three orders of magnitude. Here, we present an updated central estimate and interquartile range (232 Mg a−1; IQR: 170 - 336 Mg a−1) for modern volcanic Hg emissions based on advances in satellite remote sensing of sulfur dioxide (SO2) and an improved method for considering uncertainty in Hg:SO2 emissions ratios. Atmospheric modeling shows the influence of volcanic Hg on surface atmospheric concentrations in the extratropical Northern Hemisphere is 1.8 times higher than in the Southern Hemisphere. Spatiotemporal variability in volcanic Hg emissions may obscure atmospheric trends forced by anthropogenic emissions at some locations. This should be considered when selecting monitoring sites to inform global regulatory actions. Volcanic emission estimates from this work suggest the pre-anthropogenic global atmospheric Hg reservoir was 580 Mg, 7-fold lower than in 2015 (4000 Mg). Chapter 2 focuses on the future anthropogenic perturbation to the global mercury cycle. Mercury (Hg) is a naturally occurring element that has been greatly enriched in the environment by human activities like mining and fossil fuel combustion. Despite commonalities in some carbon dioxide (CO2) and Hg emission sources, the implications of long-range climate scenarios for anthropogenic Hg emissions have yet to be explored. Here, we present comprehensive projections of anthropogenic Hg emissions up to 2300 and evaluate impacts on global atmospheric Hg deposition. Projections are based on four Shared Socioeconomic Pathways (SSPs) ranging from sustainable reductions in resource and energy intensity to rapid economic growth driven by abundant fossil fuel exploitation. There is a greater than two-fold difference in cumulative anthropogenic Hg emissions between the lower-bound (110 Gg) and upper-bound (235 Gg) scenarios. Hg releases to land and water are approximately six times those of direct emissions to air (600 - 1470 Gg). At their peak, anthropogenic Hg emissions reach 2200 - 2600 Mg a−1 sometime between 2010 (baseline) and 2030, depending on the SSP scenario. Coal combustion is the largest determinant of differences in Hg emissions among scenarios. Decoupling of Hg and CO2 emission sources occurs under low- to mid-range scenarios, though contributions from artisanal and small-scale gold mining remain uncertain. Future Hg emissions may have lower gaseous elemental Hg (Hg0) and higher divalent Hg (HgII), resulting in a higher fraction of locally sourced Hg deposition. Projected reemissions of previously deposited anthropogenic Hg follow a similar temporal trajectory to primary emissions, amplifying the benefits of primary Hg emission reductions under the most stringent mitigation scenarios. Chapter 3 explores the cumulative impact of historical and future Hg releases on the global cycle. Humans have intentionally mined and released Hg from the Earth’s lithosphere over millennia. Here, we synthesize past, present, and future anthropogenic releases of Hg and explore its fate using a global geochemical box model. Future growth trajectories are based on the Shared Socioeconomic Pathways (SSPs). Results suggest that the upper bound for future anthropogenic Hg releases (SSP5-8.5) between 2010 and 2300 (1.7 Tg) could surpass historical anthropogenic releases over the past half millennium (1.5 Tg). In contrast, lower bound releases (SSP1-2.6; 0.7 Tg) highlight substantial effects of mitigation. We estimate that cumulative future (2010 - 2300) Hg releases from coal combustion will be ∼12 times higher under SSP5-8.5 than under SSP1-2.6. Observational constraints on global modeling suggest that most Hg released to land and water prior to 2010 remains sequestered at contaminated sites. Substantial oceanic enrichment by anthropogenic Hg (270%) has been driven mainly by atmospheric emissions, which totaled 0.36 Tg between antiquity and 2010. In the future, about 6-times more Hg is expected to be released to land and water than to the atmosphere. This pattern of Hg releases may result in localized Hg contamination issues but is unlikely to substantially impact Hg pollution in the ocean unless legacy Hg waste pools are mobilized by climate change. Modeling results suggest that by 2100 atmospheric Hg concentrations will be similar to present levels if society follows SSP5-8.5. Declines in the surface ocean (-19%) and atmosphere (-45%) are expected under SSP1-2.6, emphasizing the benefits of stringent regulatory controls on future Hg releases. The chapters presented in this work: (1) leverage satellite observations to reduce uncertainty in natural Hg emissions, (2) quantify the drivers of future anthropogenic Hg emissions, and (3) provide a framework for combining simple and complex models to gain new insight into the environmental fate of Hg following release. Together, these studies advance understanding of the key sources of Hg, the processes mediating its redistribution, and the timescales of its removal.