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Mass-independent fractionation of mercury isotopes in Arctic snow driven by sunlight


After polar sunrise in the Arctic, sunlight-induced reactions convert gaseous elemental mercury into compounds that are rapidly deposited to the snowpack. These atmospheric mercury depletion events occur repeatedly until snowmelt1,2. Following deposition, the mercury can be reduced by sunlight-induced reactions and emitted as a gas3,4,5,6, or can be retained in the snowpack7,8, where it may affect Arctic ecosystems following snowmelt. However, the proportion of mercury that remains in the snowpack is uncertain. Here, we measured the mercury isotopic composition of snow samples collected during an atmospheric mercury depletion event in Barrow, Alaska. We report large negative mass-independent fractionation of mercury isotopes in the Arctic snow. Results from a flux chamber experiment suggest that mass-independent fractionation is coupled to the re-emission of elemental mercury to the atmosphere, and is triggered by sunlight-induced reactions. On the basis of the above, we estimate that photochemical reactions triggered the release of a significant portion of the mercury deposited during this atmospheric mercury depletion event.

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Figure 1: Mass-dependent and mass-independent mercury isotope compositions of snow samples, chamber experiment samples and gaseous samples.
Figure 2: Mass-independent mercury isotope compositions of snow samples and chamber experiment samples.
Figure 3: Rayleigh fractionation models showing mercury isotope changes in snow and emitted gas.


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Financial support for this research was provided by the National Science Foundation Office of Polar Programs (Grants ARC-0435989 and ARC-0435893). L.S.S. is financially supported by a National Defense Science and Engineering Graduate Fellowship through the Department of Defense, Office of Naval Research. We are grateful to M. W. Johnson, L. Alvarez-Aviles, M. Sturm, R. Prevost, P. W. Sherman and the members of BEIGL for assistance and helpful discussions.

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J.D.B. initiated this study along with T.A.D. and G.J.K. K.P.J., J.D.B., T.A.D., J.A.B. and G.J.K. collected the snow samples and conducted the chamber experiment. L.S.S. and K.P.J. conducted the laboratory analyses and L.S.S. wrote the manuscript. All authors contributed to discussions of results and conclusions.

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Correspondence to Laura S. Sherman.

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The authors declare no competing financial interests.

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Sherman, L., Blum, J., Johnson, K. et al. Mass-independent fractionation of mercury isotopes in Arctic snow driven by sunlight. Nature Geosci 3, 173–177 (2010).

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