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The Fallout From Old Nuclear Tests Might Help Scientists Mark a New Geologic Age

Scientists say we've entered a new geologic epoch called the Anthropocene, but they disagree on when it began.

by Matt Smith
Jul 2 2015, 8:37pm

Photo via Wikimedia

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It's been 70 years since the first atomic bomb lit up the New Mexico desert. Now the radioactive residue from the nuclear tests that followed may help scientists define the start of a new geologic age.

Researchers are debating when we entered what they've dubbed the Anthropocene—the epoch in which mankind began to reshape the planet around us. Was it in the 1600s, when Europeans brought cattle and smallpox to the Americas and spread crops like potatoes and corn around the globe? Perhaps the late 1700s, when the steam engine ushered in the widespread burning of fossil fuels? How about post-World War II trade boom, when lightweight metals like aluminum and new substances like plastics became common worldwide?

It's more than just an academic debate. Previous epochs are usually defined by their extinctions—think of how dinosaurs gave way to the rise of mammals and birds. And recent studies show animals around us are disappearing at a rapid clip.

"It's almost saying that we're in the early stages of that change in the species on the planet which we would expect to see on a major event such as the Anthropocene," geologist Colin Waters told VICE News. "At what point do you say it's become so significant that we want to call it part of a new epoch? It's very difficult to do that."

'Domesticated animals and pets — those species that mankind has favored — in effect represent 97 percent of the weight of vertebrate animals on the planet.'

The most recent epoch, known as the Holocene, dates back to the end of the last ice age nearly 12,000 years ago. It's the geologic age in which humans learned to domesticate animals, grow food, build cities, and write down their thoughts, creating what's often called "civilization as we know it."

But that civilization has brought about such an extensive reordering of animal life that pound-for-pound, barely 3 percent of living vertebrates can still considered wildlife, said Waters, a member of the British Geological Survey and part of a committee of scientists tasked with defining the Anthropocene.

"Domesticated animals and pets — those species that mankind has favored — in effect represent 97 percent of the weight of vertebrate animals on the planet," he told VICE News. Chickens and cows, dogs and cats, and the crops raised to feed them "spread across all the continents and become the dominant biota," he said.

Meanwhile, Waters said, about one species a year has gone extinct over most of the past few hundred years — but that rate has gone up sharply in the past century. In a study released in late June, US and Mexican scientists warned that a new mass extinction could be at hand, with more than vertebrate species disappearing since 1900. And Waters told VICE News that many others are "effectively extinct."

"There's so many species that are in such small numbers that if you're a geologist looking at the geological record sometime way into the future, looking back at this present day, what are the chances of finding, say, a tiger bone?" he said. "Maybe a century ago they would be quite common, but nowadays, they're extremely rare."

Related: Humans are causing the sixth mass extinction in the Earth's history, says study

Being able to spot something tangible in the ground beneath our feet — whether it's a tiger bone or something else — is what geologists look for when they're trying to mark the start of a new age. Which brings us back to the Bomb, and the fallout that led the world's leading powers to ban nearly all above-ground nuclear testing in 1963.

In March, researchers at two British universities proposed two years for starting the clock on the Anthropocene: 1964, when levels of radioactive carbon from the bomb tests peaked; or 1610, when atmospheric carbon dioxide was at its lowest point in recent geologic history and a cold snap known as the "Little Ice Age" followed. Simon Lewis, a plant ecologist at University College London, said both dates could be considered what scientists have dubbed a "golden spike" in the historical record — a change that can be measured in a geological deposit that correlates with other changes around the world. 

"The date of the first nuclear detonation does not fit that, as there is no geological evidence," Lewis told VICE News. "But by the 1950s following more nuclear testing there is some evidence, with that evidence seen most clearly at 1964, when the partial test-ban treaty kicks in. Similarly, at 1610 the same criteria are also met."

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In a response published this week, the committee on which Waters sits poured cold water on the earlier date. Human activity had nothing to do with the dip in carbon dioxied, it concluded, and its effects don't show up as a clear point in the geologic record. It was more sympathetic to the idea of using radioactive materials, though it suggested a different year and isotope.

The first nuclear explosion was set off on July 16, 1945. Two atomic bombs were dropped on Japan less than a month later, ending World War II. In the 18 years that followed, the United States, the Soviet Union, Britain, and France conducted more than 400 above-ground nuclear bomb tests.

One of the most common elements in the resulting fallout was Plutonium-239, which has a radioactive half-life of more than 24,000 years. Only tiny amounts of plutonium appear in nature; the rest is man made, and it's likely to be detectable in sediments for 100,000 years, Waters told VICE News.

"It's a novel material more emblematic of the Anthropocene," he said. He suggested the date should be set in 1952, when tests of the more powerful hydrogen bomb began to spew fallout into the upper atmosphere, where it was quickly transported around the globe.

"That signature really starts to rise around 1952, peaks about 1963, and has declined ever since," he said. "But it represents a clear marker in sediment that we can distinctively say is post-Second World War."

Lewis said that the arbiters of the Anthropocene should publish some guidelines for determining which event would be most definitive and let scientists sort it out. But he told VICE News, "There is much more we agree on than disagree. Twelve years or even 350 years is nothing in geological terms." 

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