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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 20
Biogeosciences, 15, 6297-6313, 2018
https://doi.org/10.5194/bg-15-6297-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue: Progress in quantifying ocean biogeochemistry – in honour...

Biogeosciences, 15, 6297-6313, 2018
https://doi.org/10.5194/bg-15-6297-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 26 Oct 2018

Research article | 26 Oct 2018

A model of mercury cycling and isotopic fractionation in the ocean

David E. Archer1 and Joel D. Blum2 David E. Archer and Joel D. Blum
  • 1Department of the Geophysical Sciences, University of Chicago, Chicago, 60637, USA
  • 2Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan, 48109, USA

Abstract. Mercury speciation and isotopic fractionation processes have been incorporated into the HAMOCC offline ocean tracer advection code. The model is fast enough to allow a wide exploration of the sensitivity of the Hg cycle in the oceans, and of factors controlling human exposure to monomethyl-Hg through the consumption of fish. Vertical particle transport of Hg appears to play a discernable role in setting present-day Hg distributions, which we surmise by the fact that in simulations without particle transport, the high present-day Hg deposition rate leads to an Hg maximum at the sea surface, rather than a subsurface maximum as observed. Hg particle transport has a relatively small impact on anthropogenic Hg uptake, but it sequesters Hg deeper in the water column, so that excess Hg is retained in the model ocean for a longer period of time after anthropogenic Hg deposition is stopped. Among 10 rate constants in the model, steady-state Hg concentrations are most sensitive to reactions that are sources or sinks of Hg(0), the evasion of which to the atmosphere is the dominant sink term in the surface ocean. Isotopic fractionations in the interconversion reactions are most strongly expressed, in the isotopic signatures of dissolved Hg, in reactions that involve the dominant dissolved species, Hg(II), including mass independent fractionation during Hg photoreduction. The Δ199Hg of MMHg in the model, subject to photoreduction fractionation, reproduces the Δ199Hg of fish in the upper 1000m of the ocean, while the impact of anthropogenic Hg deposition on Hg isotope ratios is essentially negligible.

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Humans have had a huge impact on the mercury cycle in the biosphere, but it is difficult to follow the mercury cycle because mercury has so many mobile forms, as gases in the atmosphere and solutes in water. Mercury isotopes constrain mercury fluxes and sources, because mercury has many stable isotopes, and different fractionation mechanisms have different fingerprints in those isotopic compositions. We present the first model of mercury isotopic composition in the ocean.
Humans have had a huge impact on the mercury cycle in the biosphere, but it is difficult to...
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