Mercury-laced food

intense cold, sea spray and long dark winters are stripping toxic mercury from the atmosphere and pushing it into the fragile Arctic food chain where it is accumulating, new research suggests.

The sudden burst of sunlight after long polar winters drives chemicals in sea salt to react with normally inert mercury vapour in the air, depositing it onto snow, report Julia Lu and colleagues at the Meteorological Service of Canada and Toronto, Canada. On melting, this snow injects a pulse of mercury into the Arctic ecosystem at a time when it is most vulnerable to contamination -- the end of the winter -- when most animals and plants are growing fastest.

"Just as the Arctic prepares for its two-month growing season, the atmosphere throws down all this mercury," says Alexandra Steffen, member of the research team. Burining of fossil-fuel releases about 4,000 tonnes of mercury into the atmosphere every year, mostly as vapour. Although toxic at high concentrations, mercury vapour doesn't pass easily from the atmosphere into the environment. Only oxidised mercury enters the ecosystem, which then gets converted in to methyl mercury. This becomes concentrated as it passes through the food chain. Levels of methyl mercury are particularly high in Arctic fish and mammals and in the Inuit people of North America who eat the fish.

Lu's team combined measurements of peak mercury deposition with the degree of surface ozone depletion at a research station high in the Canadian Arctic zone. Ozone depletion occurs each spring when amassed bromine and chlorine oxides from seawater break down ozone. The poles are shunting about 150 tonnes of mercury into the environment each year. The timing of mercury deposition and ozone depletion matched perfectly, so Lu's team is confident that bromine and chlorine oxides are also oxidizing mercury vapour. "As soon as the sun comes up it initiates the photochemical reaction," says Lu ( www.nature.com , 3 September).