Mercury isotopes in the Arctic snow

Since industrialization, the amount of mercury circulating in the air, water, and living organisms has roughly tripled. But the physical and chemical processes involved in that Hg cycle are poorly understood. In 2007 Bridget Bergquist and Joel Blum of the University of Michigan in Ann Arbor discovered that Hg’s two stable magnetic isotopes, ¹⁹⁹Hg and ²⁰¹Hg, sometimes exhibit slightly different behavior from that of the five stable nonmagnetic isotopes. The difference is attributed mostly to coupling between nuclear and electronic spins, which facilitates electron spin flips in certain photoinitiated reactions. Consequently, the varying isotopic compositions of natural samplescan provide information about the Hg cycle and sources of Hg pollution. (See PHYSICS TODAY, December 2008, page 25 .) Now Blum, Laura Sherman, and colleagues have studied Hg isotope fractionation in Arctic snow. The Hg is deposited seasonally: Shortly after the annual polar sunrise, sunlight induces precipitation of almost all the Hg in the polar atmosphere. How and to what extent Hg leaves the snow before the snow melts is uncertain, but the researchers found that of the snow samples they collected, those that had been exposed to more sunlight contained less ¹⁹⁹Hg and ²⁰¹Hg. They conclude that Hg is emitted from the snow via a photochemical reaction that favors the odd-numbered isotopes. And they suggest that any Hg that enters Arctic ecosystems from the melted snow could be identified by its isotopic composition. (L. S. Sherman et al., Nat. Geosci. 3, 173,2010 .) --Johanna Miller