Scientists studying Antarctic ice have made a surprising discovery: manmade chlorine remains at unusually high levels within parts of the ice.
The chlorine stems from nuclear testing in the 1950s and 60s.
Studying chlorine levels lets scientists paint a picture of the Earth’s climate history. The team wants to drill deeper and discovery if there are any other surprises in the past.
We still have a lot to learn about how ice stores radioactivity. Scientists from the European Centre for Research and Teaching in Geosciences and the Environment (CEREGE) in Aix-en-Provence, France have shown that Antarctica’s ice sheets are still releasing radioactive chlorine from Cold War-era marine nuclear weapons tests.
During the 1950s and 60s, various governments tested nuclear weapons across the planet. The U.S. tested its weapons in a variety of locations, including off islands in the Pacific Ocean. During 1962’s Operation Dominic, for example, over 100 aircraft, 40 warships, and 28,000 uniformed service members tested nuclear weapons with explosive yields 700 times the size of the weapons that dropped on Hiroshima just 17 years prior.
These tests generated high levels of concentrations of isotopes like chlorine-36, which can also occur naturally. The radioactive isotopes from these explosions floated up to the Earth’s stratosphere, at which point they began to circumnavigate the globe. Some of that gas ended up in Antarctica, where it found a home of sorts.
Over the decades, other isotopes generated through the testing fell back to pre-bomb levels. Scientists expected the same of chlorine-36, which in its natural state helps scientists interpret the age of ice within ice cores. Researching the Vostok region of Antarctica, the scientists discovered the manmade chlorine still exists within the ice and is still being released into the atmosphere.
“There is no more nuclear chlorine-36 in the global atmosphere. That is … why we should observe natural chlorine-36 levels everywhere,” said Mélanie Baroni, a geoscientist at CEREGE and coauthor of the new study, in a press statement.
Studying chlorine levels within ice gives scientists a better sense of the history of Earth’s climate. Focusing on the extreme climates of Antarctica allowed the team to observe how “chlorine behaves over time in areas where annual snowfall is high versus areas where snowfall is low,” according to the press release.
Scientists picked two locations for their study: the Russian Vostok research station, which receives little snow, and the Talos Dome, a large ice dome approximately 870 miles away that gets a heavy snowfall each year.
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Over time, the artificially raised chlorine levels at Talos Dome have fallen. By 1980, the scientists were able to detect only four times the natural levels of chlorine in at the dome. But Vostok, which has been in operation since 1957, showed 10 times the natural chlorine levels as recently as 2008.
While this radioactivity is too small to have any impact on Earth’s environment, it shows that in the right conditions, chlorine can be surprisingly sturdy.
The next step for scientists is to get deeper into the ice. They plan to drill for a 5 million-year-old ice core in the Antarctic to learn how the area surrounding Vostok stores and releases chlorine. Understanding that chlorine release could be crucial to getting a complete picture of the Earth’s past climate.