Antarctic ice sheets are still releasing a radioactive element as a result of nuclear weapons tests conducted by the US 70 years ago, according to new research published in the Journal of Geophysical Research: Atmospheres.
Chlorine-36 is a naturally occurring radioactive isotope that can form when argon gas reacts with cosmic rays in the atmosphere. It can also form during nuclear explosions over the ocean when neutrons react with chlorine found in seawater, which can evaporate to the stratosphere, travel around the globe, and ultimately become deposited on Antarctic ice and snow where it can become permanently stored.
“There is no more nuclear chlorine-36 in the global atmosphere. That is… why we should observe natural chlorine-36 levels everywhere,” said study author and geoscientist Mélanie Baroni in a statement.
The US conducted multiple nuclear weapons tests in the Pacific Ocean in the 1950s and 1960s. In less than 20 years, a total of 19 operational test series were conducted, resulting in more than 230 detonations. Similar findings of chlorine-36 have been found in ice core samples from a US glacier located in the Wind River Mountain Range of Wyoming.
To determine how chlorine-36 behaves over time, particularly in areas with varying levels of snowfall, researchers took ice core samples dated between 1949 and 2007 from a snow pit at Vostok, a Russian research station in East Antarctica that gets very little snow. They then compared it against samples from 1910 to 1980 taken from Talos Dome, a region located 1,400 kilometers (870 miles) away that receives a lot of snow every year.
Ice from Talos Dome was shown to decrease over time and only held about four times the level of natural chlorine-36 in 1980. Vostok, on the other hand, showed very high levels – the top snow was 10 times the expected level of naturally occurring chlorine-36 suggesting that snow here is still releasing radioactive isotopes from marine nuclear tests and that chlorine-36 is more mobile than previously anticipated, moving upwards from the depths.
The authors caution that “the atmosphere of Vostok remains polluted by anthropogenic chlorine today. This pollution results from gaseous mobility at low accumulation sites and implies re‐emission of chlorine-36 from the snowpack that is not observed at Talos Dome.”
Researchers hope to next drill for a 1.5-million-year-old ice core in order to better understand how Vostok releases chlorine-36 and to help inform how it builds up in snowpack over time, informing how we date ancient ice and understand Earth’s climate.