When humans mess around with radiation, the residue has a nasty habit of hanging around and now researchers think we may have left an enduring biomarker in marine sediments that could be used to mark the beginning of the Anthropocene, a geological epoch where we as a species started having a significant impact on the planet’s ecosystems. How did we do it? By letting off a load of atomic weapons.
Plutonium isotopes were detected in a study published in Scientific Reports that looked at corals and marine sediments in the northwest Pacific Ocean. The site is close to the coast of a region of Japan where in the 1950s atomic tests were being carried out.
There was a clear plutonium increase in the sedimentary record, samples revealed, beginning in the 1950s, spiking in the 1960s, and dropping off dramatically into the 1970s. Based on their measurements, the paper’s authors estimate that the Anthropocene epoch could be said to have begun in 1954, and they are confident as to the reliability of the sediment record found in Beppu Bay.
“The nearshore sediments from anoxic seafloor environments that preserve annual layers are one of the best archives for recording changes in local and global climate relevant to anthropogenic activities,” wrote the authors.
The researchers paired this approach with studies looking at coral radiocarbon using samples taken from Ishigaki island. The coral skeletons they collected were also found to contain fallout from the nuclear testing, and these fixed marine remains aligned with the (easily dispersed) sediment’s timeline of contamination meant the researchers could build a clearer picture of when the Nuclear Era really kicked off here.
"Our task was to find clear indications of fallout from the 1950s up to 1963 when testing largely stopped," said geoscientist Yusuke Yokoyama from the University of Tokyo in Japan to Science Alert.
"It was challenging to analyze plutonium within our samples, as during the period in question, three tons of plutonium were released into the sea and atmosphere, but those three tons dispersed far and wide. So, we're actually looking for incredibly small signatures.”
As for what this means for Earth’s timeline, it’s still widely regarded that we’re living in the Holocene epoch, which kicked off over 11,000 years ago.
Whether the official record will move into the Anthropocene and adopt the researchers’ given date remains to be seen, but for now, they hope their work can contribute to climate science and marine models which explore threats to human life including the climate crisis and tsunamis.