South Pole's 'hot' snow: Chernobyl source?

South Pole's 'hot' snow: Chernobyl source?

Scientists studying Antarctic snow have discovered radioactive isotopes they think may have come from the April 1986 accident at the Chernobyl nuclear power plant in the Soviet Union. But others question whether significant amounts of Chernobyl's fallout could cross the equator and reach the South Pole.

Geochemist Jack E. Dibb and his colleagues from the University of New Hampshire in Durham analyzed samples collected from a snow pit about 38 kilometers from the South Pole. As expected, the deeper portion of the pit held radioactive layers corresponding to the years 1955 through 1974 -- the peak period of above-ground nuclear bomb testing. Yet the researchers also measured a radiation "spike"--about 20 to 30 times above the background level -- in snow near the top of the pit, deposited during late 1987 and early 1988. They traced the radioactivity to cesium-137, an isotope that does not form in nature but does form during nuclear explosions and in reactors.

The radioactive snow fell on Antarctica about 20 months after the Chernobyl accident -- a lag consistent with the time it took isotopes to reach the South Pole from bomb tests in the Northern Hemisphere. In a letter in the May 3 NATURE, Dibb's group proposes that some of Chernobyl's radioactive isotopes penetrated the stratosphere, crossed the equator and then fell in snowflakes on central Antarctica. They speculate that special wind patterns above the Antarctic might explain why the South Pole is the only spot in the Southern Hemisphere where scientists have detected excess cesium-137 following the Chernobyl event.

Some atmospheric scientists remain skeptical about this route of transport. Unlike a nuclear or volcanic explosion, the Chernobyl accident did not spew debris directly up through the atmospheric barrier at the base of the stratosphere, 10 to 15 km above the ground, they say. Thus, if isotopes did travel through the stratosphere, only weather patterns could have lofted them high enough. But in that case, water condensation in the rising air should have washed the cesium out, contends Jerry D. Mahlman of the Geophysical Fluid Dynamics Laboratory in Princeton, N.J.

Mahlman raises another possibility: Small amounts of isotopes might have crossed the equator via the troposphere, the atmosphere's lowest layer. However, he doubts significant levels of this radiation would have reached the pole, because tropospheric weather patterns tend to block movement across the equator.