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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-225
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-225
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Jun 2019

Research article | 21 Jun 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Insights from mercury stable isotopes on terrestrial – atmosphere exchange of Hg(0) in the Arctic tundra

Martin Jiskra1,2, Jeroen E. Sonke1, Yannick Agnan3,4, Detlev Helmig5, and Daniel Obrist3,6 Martin Jiskra et al.
  • 1Laboratoire Géosciences Environnement Toulouse, CNRS/IRD/Université de Toulouse, Toulouse, 31400, France
  • 2Environmental Geosciences, University of Basel, Basel, 4056, Switzerland
  • 3Division of Atmospheric Sciences, Desert Research Institute, Reno, 89512, USA
  • 4Sorbonne Université, CNRS/EPHE/UMR/METIS, Paris, F-75252, France
  • 5Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, 80309, USA
  • 6Department of Environmental, Earth, and Atmospheric Sciences, University of Massachusetts, Lowell, 01854, USA

Abstract. The tundra plays a pivotal role in the Arctic mercury (Hg) cycling by storing atmospheric Hg deposition and shuttling it to the Arctic Ocean. A recent study revealed that 70 % of the atmospheric Hg deposition to the tundra occurs by gaseous elemental mercury (GEM or Hg(0)) uptake by vegetation and soils. Processes controlling land – atmosphere exchange of Hg(0) in the Arctic tundra are therefore central, but remain understudied. Here, we combine Hg stable isotope analysis of Hg(0) in the atmosphere, interstitial snow and soil pore air, with Hg(0) flux measurements in a tundra ecosystem at Toolik field station in northern Alaska (USA). In dark winter months, planetary boundary layer (PBL) conditions and Hg(0) concentrations were generally stable throughout the day and small Hg(0) net deposition occurred. In spring, halogen-induced atmospheric mercury depletion events (AMDE's) occurred, with fast re-emission of Hg(0) after AMDE's resulting in net emission fluxes of Hg(0). During the short snow-free growing season in summer, vegetation uptake of atmospheric Hg(0) enhanced atmospheric Hg(0) net deposition to the Arctic tundra. At night, when PBL conditions were stable, ecosystem uptake of atmospheric Hg(0) led to a depletion of atmospheric Hg(0). The night time decline of atmospheric Hg(0) was concomitant with a depletion of lighter Hg(0) isotopes in the atmospheric Hg pool. The enrichment factor, ε202Hg = −4.2 ‰ ± 1.0 ‰ was consistent with the preferential uptake of light Hg(0) isotopes by vegetation. Hg(0) flux measurements indicated a partial re-emission of Hg(0) during daytime, when solar radiation was strongest. Hg(0) concentrations in soil pore air were depleted relative to atmospheric Hg(0) concentrations, concomitant with an enrichment of lighter Hg(0) isotopes in the soil pore air (ε202Hgsoilair-atmosphere = −1.00 ‰ (±0.25 ‰) and E199Hgsoilair-atmosphere = 0.07 ‰ (±0.04 ‰)). These first Hg stable isotope measurements of Hg(0) in soil pore air are consistent with the fractionation previously observed during Hg(0) oxidation by natural humic acids suggesting abiotic oxidation as a cause for observed soil Hg(0) uptake.

Martin Jiskra et al.
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Short summary
The tundra plays a pivotal role in Arctic mercury cycling by storing atmospheric mercury deposition and shuttling it to the Arctic Ocean. We used the isotopic fingerprint of mercury to investigate the processes controlling atmospheric mercury deposition. We find that the uptake of atmospheric mercury by vegetation was the major deposition source. Direct deposition to snow or to soils played only a minor role. These results improve our understanding of Arctic mercury cycling.
The tundra plays a pivotal role in Arctic mercury cycling by storing atmospheric mercury...
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