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Tundra uptake of atmospheric elemental mercury drives Arctic mercury pollution

Nature volume 547, pages 201–204 (2017)Cite this article

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Abstract

Anthropogenic activities have led to large-scale mercury (Hg) pollution in the Arctic1,2,3,4,5,6. It has been suggested that sea-salt-induced chemical cycling of Hg (through ‘atmospheric mercury depletion events’, or AMDEs) and wet deposition via precipitation are sources of Hg to the Arctic in its oxidized form (Hg(ii)). However, there is little evidence for the occurrence of AMDEs outside of coastal regions, and their importance to net Hg deposition has been questioned2,7. Furthermore, wet-deposition measurements in the Arctic showed some of the lowest levels of Hg deposition via precipitation worldwide8, raising questions as to the sources of high Arctic Hg loading. Here we present a comprehensive Hg-deposition mass-balance study, and show that most of the Hg (about 70%) in the interior Arctic tundra is derived from gaseous elemental Hg (Hg(0)) deposition, with only minor contributions from the deposition of Hg(ii) via precipitation or AMDEs. We find that deposition of Hg(0)—the form ubiquitously present in the global atmosphere—occurs throughout the year, and that it is enhanced in summer through the uptake of Hg(0) by vegetation. Tundra uptake of gaseous Hg(0) leads to high soil Hg concentrations, with Hg masses greatly exceeding the levels found in temperate soils. Our concurrent Hg stable isotope measurements in the atmosphere, snowpack, vegetation and soils support our finding that Hg(0) dominates as a source to the tundra. Hg concentration and stable isotope data from an inland-to-coastal transect show high soil Hg concentrations consistently derived from Hg(0), suggesting that the Arctic tundra might be a globally important Hg sink. We suggest that the high tundra soil Hg concentrations might also explain why Arctic rivers annually transport large amounts of Hg to the Arctic Ocean9,10,11.

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Figure 1: Cumulative atmospheric deposition of major Hg forms in the Arctic tundra.
Figure 2: Hg stable isotope composition of atmospheric Hg(0) and Hg in snowfall and snowpack, vegetation, organic and mineral soil horizons, and bedrock samples in the Arctic tundra.
Figure 3: Gaseous Hg(0) concentrations in the atmosphere, interstitial snow air, and tundra soil pores.

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Acknowledgements

We thank Toolik Field Station and Polar Field Services staff for their support in setting up the field site and maintaining its operation for two years, with special thanks to J. Timm. We thank O. Dillon and C. Pearson for support with laboratory analyses; A. Steffen and S. Brooks for providing additional instrumentation; J. Chmeleff for support with inductively coupled plasma mass spectrometry; and R. Kreidberg and J. Arnone for editorial and technical assistance in manuscript preparation. The project was funded primarily by a US National Science Foundation (NSF) award (PLR 1304305), with additional support provided by further NSF (CHN 1313755) and US Department of Energy (DE-SC0014275) awards. The Hg isotope work was funded by H2020 Marie Sklodowska-Curie grant agreement no. 657195 to M.J., and European Research Council grant ERC-2010-StG_20091028 and CNRS-INSU-CAF funding (PARCS project) to J.E.S.

Author information

Authors and Affiliations

  1. Department of Environmental, Earth and Atmospheric Sciences, University of Massachusetts, Lowell, Massachusetts, 01854, USA

    Daniel Obrist

  2. Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, 89512, Nevada, USA

    Daniel Obrist, Yannick Agnan, Christine L. Olson & Christopher W. Moore

  3. Milieux Environnementaux, Transferts et Interactions dans les Hydrosystèmes et les Sols (METIS), UMR 7619, Sorbonne Universités UPMC-CNRS-EPHE, 4 place Jussieu, Paris, F-75252, France

    Yannick Agnan

  4. Géosciences Environnement Toulouse, CNRS/OMP/Université de Toulouse, 14 Avenue Edouard Belin, Toulouse, 31400, France

    Martin Jiskra & Jeroen E. Sonke

  5. Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, 4001 Discovery Drive, Boulder, 80309, Colorado, USA

    Dominique P. Colegrove, Jacques Hueber & Detlev Helmig

  6. Gas Technology Institute (GTI), 1700 South Mount Prospect Road, Des Plaines, 60018, Illinois, USA

    Christopher W. Moore

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Contributions

D.O. and D.H. initiated and designed this project, and M.J., J.E.S. and D.O. designed and developed the isotope component. All authors were involved in all field sampling and/or laboratory analyses. Y.A. led data analysis of flux data, and M.J. led stable isotope sampling and analysis with support from J.E.S. D.O. led manuscript writing with major support from M.J., Y.A., J.E.S. and C.W.M.

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Correspondence to Daniel Obrist or Detlev Helmig.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks J. Blum and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Research site and measurement systems used in this study.

a, The research site was characterized as Tussock tundra, and is located at Toolik Field Station in northern Alaska, USA (68° 38′ N, 149° 36′ W). b, A temperature-controlled research laboratory was set up to house analytical sensors, control systems, and data acquisition. Heated conduits were installed to run electrical lines and trace-gas lines to the tundra and to protect lines from bite damage and freezing. c, Continuous gaseous Hg(0) flux measurements were conducted using an aerodynamic method based on gradient measurements at heights of 61 cm and 363 cm above ground. d, A snow tower, consisting of five trace-gas inlets, was established over the tundra before the onset of snowfall. Wintertime snowfall buried this tower and allowed sampling of snow air from the respective air inlets without disturbing the snowpack. e, A soil trace-gas system, consisting of six trace-gas wells at various depths and locations, was deployed in tundra soils to allow three daily extractions of soil pore gas and analysis for Hg(0) gas and auxiliary trace gases. Panel e shows an example of the wells used in year one; these were replaced with a different system in year two but detected similar gas magnitudes and seasonal patterns.

Extended Data Figure 2 Measured atmospheric Hg(ii) concentrations from February to September 2016, and estimated Hg(ii) dry deposition.

Hg(ii) concentrations were measured by using differential measurements of gaseous Hg(0) and total atmospheric Hg, which passed through a glass-tube inlet directly into a pyrolyzer oven set at 650 °C. Cumulative Hg(ii) deposition was calculated on the basis of reported deposition velocities for Hg(ii) forms of 1.5 cm s−1.

Extended Data Figure 3 Example of a springtime AMDE at Toolik Field Station, from 1–3 April 2016.

This atmospheric Hg measurement sequence shows corresponding concentration depletions of Hg(0) (in blue) and O3 (in red), along with the formation of atmospheric Hg(ii) (in green). Hg(0) concentrations reached values below the detection limit when Hg(ii) concentrations increased to 0.5 ng m−3. At the same time, O3 mixing ratios dropped down from 45 ppb to less than 10 ppb.

Extended Data Figure 4 Mass-independent anomalies of even-mass-number Hg isotopes (Δ200Hg) and mass-dependent Hg isotopic signatures (δ202Hg) for different Hg sources and ecosystem sinks.

Symbols for sources include: circles for atmospheric Hg(0); filled blue triangles for Hg(ii) in snow deposited before January/February 2016; open blue triangles for Hg measured in surface snow during periods of AMDEs (March/April 2016); and grey inverted triangles for geogenic Hg in bedrock samples. Symbols for tundra components include: filled diamonds for bulk vegetation; filled squares for organic (O horizon) soils; and open squares with a cross for mineral soil A horizons (which have more than 10% organic matter) or without cross for B horizons (which have less than 10% organic matter). Measurement uncertainties, calculated as 2 s.d. of replicate standards, are shown on the lower right.

Extended Data Figure 5 Mass-independent Hg isotopic anomalies (Δ199Hg and Δ201Hg) for different Hg sources and ecosystem sinks.

Symbols for sources include: circles for atmospheric Hg(0); filled blue triangles for Hg(ii) in snow deposited before January/February 2016; open blue triangles for Hg deposited during AMDEs (March/April 2016); and grey inverted triangles for geogenic Hg in bedrock samples. Symbols for tundra components include: filled diamonds for bulk vegetation; filled squares for organic (O horizon) soils; and open squares with a cross for mineral soil A horizons (more than 10% organic matter) or without a cross for B horizons (less than 10% organic matter). Measurement uncertainties, calculated as 2 s.d. of replicate standards, are shown on the lower right. All data are within analytical uncertainty, with the straight line representing the 1:1 Δ199Hg to Δ201Hg slope, thought to be representative of photochemically induced mass-independent fractionation by magnetic isotope effects44.

Extended Data Table 1 Hg stable isotope compositions of atmospheric gaseous Hg(0) collected on iodated carbon traps
Full size table
Extended Data Table 2 Hg isotope compositions of vegetation
Full size table
Extended Data Table 3 Hg stable isotope compositions of soil and rock (geogenic) samples
Full size table
Extended Data Table 4 Hg stable isotope compositions of soil samples along a transect to the Arctic Ocean along the Dalton Highway
Full size table
Extended Data Table 5 Stable isotope ratios of precipitation (snow) samples
Full size table

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Obrist, D., Agnan, Y., Jiskra, M. et al. Tundra uptake of atmospheric elemental mercury drives Arctic mercury pollution. Nature 547, 201–204 (2017). https://doi.org/10.1038/nature22997

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