Mercury stable isotopes constrain atmospheric pathways to the ocean

18 November 2021

Mercury is one of the least concentrated trace elements in the ocean. Yet, the small seawater Hg levels biomagnify million times along the marine food chain, to reach harmful levels in predatory fish and their human consumers. Natural and anthropogenic Hg sources to the ocean remain ill-constrained. It took 10 years of development to come up with a method to measure Hg stable isotopes of seawater at subpicomolar levels. The Hg isotopic signatures in the ocean suggest the gas uptake (Hg⁰) and ionic deposition (Hg^II) are equally important (1:1), which is contrary than our current understanding (1:3)[1]. Hg^II deposition is likely overestimated by 2 to 3-fold, and the ocean receives less Hg overall. The study’s results hold promise that the implementation of anti-pollution measures under the Minamata Convention[2] will likely result in a faster decrease of oceanic Hg levels than previously thought.

**Figure:** Summary of marine Hg^II deposition and Hg⁰ air-sea exchange fluxes. Gross fluxes (solid arrows, Mg y^-1) are based on published model estimates. Hg⁰exchange is bidirectional. Marine Δ²⁰⁰Hg signatures of 0.04‰ indicate a relatively more important contribution of the atmospheric Hg⁰ end-member to marine Hg than current 3D models suggest. This indicates that either 3D model Hg^II deposition is likely overestimated (black dotted arrows, indicating 2–3 times lower, required to fit Δ²⁰⁰Hg data).

Reference:

Jiskra, M., Heimbürger-Boavida, L., Desgranges, M., Petrova, M., Dufour, A., Ferreira-Araujo, B., Masbou, J., Chmeleff, J., Thyssen, M., Point, D., Sonke, J. E. Mercury stable isotopes constrain atmospheric sources to the ocean. Nature, https://doi.org/10.1038/s41586-021-03859-8

[1] https://www.unep.org/resources/publication/global-mercury-assessment-2018

[2] https://www.mercuryconvention.org/en

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