The human race is leaving an indelible mark on our planet in many damaging ways: global deforestation, an island of plastic the size of Texas in the middle of the Pacific Ocean and industrial farming that is washing fertilisers into rivers. How can we assess the long-term effects we’re having on the Earth?
Some scientists believe a new geological epoch is under way – a time in which humankind is permanently altering the ecosystems of our planet, and fundamentally changing the Earth system. They call this period the Anthropocene epoch. The problem is that, until we find evidence to prove that we are making geologically long-term or permanent changes to Earth, we cannot be certain that a new era has actually begun.
But researchers at the University of Southampton have demonstrated one such indicator: the presence of plutonium in the Earth’s sedimentary deposits.
Since the industrial revolution, pollution from fossil fuels, and from agricultural and technological development, has impacted on the world’s sediment infrastructure, and resulting changes can be seen in the beds of lakes, seas and rivers through different concentrations of elements, compounds and isotopes. These changes are particularly marked from the 1950s, due to the rapid and widespread increase in human activity from the mid-20th century.
Plutonium is one of these purely man-made elements that wasn’t detectable before mankind developed nuclear weapons and nuclear energy production.
Pawel Gaca, Senior Radiochemist at the University of Southampton
Plutonium, unlike carbon or other naturally occurring elements, is artificially produced and so provides a strong marker of the changes that have taken place. Wherever it is found on the Earth (apart from in very rare instances), it is there as a result of human behaviour – as fallout from nuclear weapons testing and as a by-product of the nuclear energy industry. Where it settles in sediment layers, it will stick around for tens of thousands of years, according to Dr Pawel Gaca, Senior Radiochemist at the University of Southampton.
“Plutonium falls out onto the ground, is washed into rivers, lakes and seas, and it has fairly high chemical stability in these systems,” he explains. “If you take a sample from anywhere in the world, you can find its signature.”
The half-life of plutonium-239, the main component of plutonium from nuclear weapons, is 24,000 years and the element can already be detected in coral samples, polar ice, marine and lake sediments, and other geological materials. “It will be measurable for longer than our civilization will probably last,” Pawel says. That makes it the perfect candidate to mark the beginning of the Anthropocene epoch.
In Canada, scientists (led by the team at Brock University) have found a location, Crawford Lake in Ontario, where the presence of plutonium can be accurately measured year by year within its very distinct sediment layers. Now Pawel, colleagues at Southampton and researchers from the University of Vienna are using accelerator mass spectrometry to measure distinct annual changes in the concentration of plutonium in the sediment at the bottom of the lake.
Those shifts are so clear that the researchers can map the progress of human history from the Second World War onwards.
Pawel says: “We can see signatures for when nuclear weapons testing began, and how levels increased and then decreased following the test ban treaty of 1963.” Concentrations are lower now but remain present in every yearly sediment deposit.
Of course, the presence of plutonium doesn't mark the beginning of human history on planet Earth, nor does it mark the beginning of the industrial revolution when hun1ankind began to shape the world. However, it does chart when humans began to make what Pawel describes as a "really significant impact – often referred to as a “Great Acceleration” by researchers associated in the Anthropocene Working Group, on the functioning of the planet's ecosystems - and to leave their lasting mark there too.
That is a reason why the presence of plutonium is a top candidate for making the case that the so-called Anthropocene epoch has already begun – an argument that is not yet won in the global scientific community but one that University for Southampton researchers are actively making.
Even more importantly for Pawel, the information they are gathering can be used to prove to governments and nuclear energy companies that their actions are not without permanent consequences for the planet.
