60 years after the end of the tests, radioactive isotopes of the chlorine element were found in the ice.
By Ingrid Luisa
Oct 17, 2019, 8:26 pm – Posted on Oct 17, 2019, 8:20 pm
During the Cold War, the Pacific Ocean was the scene of several nuclear tests. Places like the Marshall Islands still bear deep marks from that time – the atolls of Bikini and Enewetak are more radioactive today than Chernobyl and Fukushima, though it has been more than 60 years since the nuclear tests ended.
These bombs left no trace only where they exploded: the various chemicals released reacted with other elements in the air. This generated high concentrations of radioactive isotopes, such as chlorine-36.
Now scientists from the Russian base Vostok in Antarctica have discovered that this isotope has gone further than previously thought: it has traveled the globe through the stratosphere and into Antarctica, where it has been deposited on ice and is still today.
In fact, the discovery happened by chance. Researchers were examining concentrations in different parts of Antarctica to better understand how chlorine behaves over time in areas where annual snowfall is high versus areas where snowfall is low. This is useful because scientists use isotopes such as chlorine-36 to determine the ages of deep-frozen ice samples.
Although radioactive and unstable, a low concentration of this isotope is naturally produced when argon gas reacts with cosmic rays in the Earth's atmosphere. Just remembering: radioactive isotopes are versions of atoms that have a very large amount of neutrons, which destabilizes them. They release radiation to achieve more stable states.
To study, the researchers collected ice samples from a snow pit at Vostok, a Russian research station in East Antarctica that receives little snow, and ice from Talos Dome, a large ice dome approximately 1400 kilometers away that receives a lot of snow accumulation every year.
The researchers tested the chlorine-36 concentration in ice at both sites based on samples previously collected at the sites. The results were clear: while in Talos Dome the concentration gradually decreased over time, Vostok ice showed very high levels of chlorine-36, with the top of the snow pit reaching up to 10 times the expected natural concentration.
This suggests that the Vostok snowblock is still releasing radioactive chlorine stored during nuclear tests in the 1950s and 1960s. Today, this amount of radioactivity is too small to affect the environment, but the results are surprising because scientists had hoped that the concentrations of this gas had already normalized. The results of this analysis were published in the Journal of Geophysical Research: AG Atmos.