Cracks in arctic sea ice that expose seawater to cold polar air pump toxic atmospheric mercury down to the surface and into the food chain, U.S. scientists say. Vigorous mixing of the air above large cracks in the sea ice is what pulls the mercury down to the surface, they said. The scientists detected increased concentrations of mercury near ground level after sea ice off the coast of Barrow, Alaska, split open, creating seawater channels called leads, NASA's Jet Propulsion Laboratory in Pasadena, Calif., said Wednesday. "None of us had suspected that we would find this kind of process associated with leads," JPL scientists Son Nghiem said. The mercury-pumping reaction takes place because open water in a lead is much warmer than the air above it, and that temperature difference caused the air above the lead to churn like the air above a boiling pot, researchers said. "The mixing is so strong, it actually pulls down mercury from a higher layer of the atmosphere to near the surface," study lead author Chris Moore of the Desert Research Institute in Reno, Nev., said. The mixing is marked by dense clouds spewing out of the leads to a height of about a quarter-mile, he said. Arctic atmospheric mercury pollution originates almost entirely in nations as far south as the tropics from sources such as wildfires, coal burning and gold mining, the researchers said.