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Methylmercury photodegradation influenced by sea-ice cover in Arctic marine ecosystems

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Abstract

Atmospheric deposition of mercury to remote areas has increased threefold since pre-industrial times. Mercury deposition is particularly pronounced in the Arctic. Following deposition to surface oceans and sea ice, mercury can be converted into methylmercury, a biologically accessible form of the toxin, which biomagnifies along the marine food chain. Mass-independent fractionation of mercury isotopes accompanies the photochemical breakdown of methylmercury to less bioavailable forms in surface waters. Here we examine the isotopic composition of mercury in seabird eggs collected from colonies in the North Pacific Ocean, the Bering Sea and the western Arctic Ocean, to determine geographical variations in methylmercury breakdown at northern latitudes. We find evidence for mass-independent fractionation of mercury isotopes. The degree of mass-independent fractionation declines with latitude. Foraging behaviour and geographic variations in mercury sources and solar radiation fluxes were unable to explain the latitudinal gradient. However, mass-independent fractionation was negatively correlated with sea-ice cover. We conclude that sea-ice cover impedes the photochemical breakdown of methylmercury in surface waters, and suggest that further loss of Arctic sea ice this century will accelerate sunlight-induced breakdown of methylmercury in northern surface waters.

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Figure 1: Common and thick-billed murre colony locations and sea-ice conditions at the beginning of the 2002 breeding season.
Figure 2: Mercury three-isotope diagrams illustrating variations in mercury MDF and MIF in murre eggs.
Figure 3: Ecological effects on mercury MDF and MIF in murre eggs.
Figure 4: Effect of latitude on mercury MIF.
Figure 5: Influence of sea ice on egg mercury MIF.
Figure 6: Odd-isotope anomalies in Δ201Hg versus Δ199Hg space.

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Change history

  • 21 January 2011

    In the version of this Article originally published online, 'Arctic sea' should have read 'Arctic sea ice' in the last sentence of the abstract. This error has now been corrected in all versions of the text.

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Acknowledgements

The samples used in this study were obtained from the Marine Environmental Specimen Bank (MESB) through the Seabird Tissue Archival and Monitoring Project (STAMP), a long-term collaborative effort by the Alaska Maritime National Wildlife Refuge (AMNWR), the National Institute of Standards and Technology (NIST), the US Geological Survey Biological Resources Division (USGS-BRD) and the Bureau of Indian Affairs Alaska Region Subsistence Branch (BIA-ARSB). We thank AMNWR and University of Alaska—Fairbanks (UAF) biologists, members of the St George Traditional Council, the Native Village of Point Hope IRA Council and residents of St George, Savoonga and Point Hope for collecting the eggs.

We also thank K. S. Simac (USGS-BRD) for processing the eggs, M. B. Ellisor (NIST) for cryohomogenizing and banking the eggs and F. Poitrasson and Y. Godderis for providing helpful comments on the manuscript. This work is part of International Polar Year (IPY) Research Activity No. 439 ‘MERSAM’ (MERcury Seabird Arctic Monitoring). Financial support for this research was provided by NIST, the French Centre National de la Recherche Scientifique, and Research Grant ANR-09-JCJC-0035-01 from the French Agence Nationale de Recherche.

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P.R.B., O.F.X.D., D.P. and R.D.D. designed the study; D.G.R. obtained the scientific collecting permits, made arrangements to collect the eggs and coordinated field logistics; and S.S.V. managed sample processing and banking. A.J.M. and R.S.P. were responsible for specimen processing, cryogenic banking and cryogenic homogenizations. Mercury isotopes were measured by D.P., R.D.D. and J.E.S. R.D.D. measured total mercury and K.H.H. measured nitrogen stable isotopes. D.P. and J.E.S. prepared the manuscript and all of the authors reviewed it.

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Correspondence to D. Point.

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Point, D., Sonke, J., Day, R. et al. Methylmercury photodegradation influenced by sea-ice cover in Arctic marine ecosystems. Nature Geosci 4, 188–194 (2011). https://doi.org/10.1038/ngeo1049

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