(CN) — Scientists have long speculated why the globe experienced rapid cooling almost 13,000 years ago. According to one hypothesis, a comet or meteor exploded in the Earth’s atmosphere, setting off the sudden temperature drop. But scientists published evidence Friday that a series of major volcanic eruptions better explain the ancient global cooling.
Scientists learned about this cooling event, known as the Younger Dryas, when ice cores drilled from Europe revealed that temperatures had dropped as much as 14 degrees Fahrenheit in the northern hemisphere 12,900 years ago.
“The Younger Dryas was a cold period that occurred as our planet was warming up from the last glaciation — which was about 25,000 years ago,” said Steven Forman, a Baylor University geoscientist and co-author of the study published Friday in the journal Science Advances. “And we warmed up pretty close to our present conditions by about 9,000 years ago.”
The Younger Dryas lasted about 1,200 years, ending 11,700 years ago. During this period, tundra vegetation overtook forests, and life across the planet was disrupted.
“Associated with this Younger Dryas period are a number of extinctions of large mammals, such as mammoths and mastodons, and there are some cultural changes in human beings that appear, such as in the Clovis, an early human culture,” Forman said in an interview.
Scientists know that the Younger Dryas was kicked off by a number of processes: glacial dams retained freshwater in massive North American lakes, and as the planet gradually warmed before the Younger Dryas, the pressure mounted until these dams burst.
All that freshwater poured into the North Atlantic Ocean, impeding ocean circulation and cooling the planet. But other, simultaneous processes also brought along the millennium of cooling.
“One of the reigning theories has been, basically, [that] an airburst comet … caused massive cooling,” Forman said. “It came in, put a lot of dust in the air, was highly explosive and cooled off the planet during the Younger Dryas. They said it would have been in the area of Michigan, which was ice-covered at the time, so it’s hard to get evidence for it.”
Forman was one of five scientists from Baylor, the University of Houston and Texas A&M University who examined sediments collected from Hall’s Cave, located in the rural Texas Hill Country north of San Antonio, to examine the rare elements within the cave samples and test the comet hypothesis.
They found that the sediment’s composition matched not what a meteor or asteroid would bring to Earth, but rather the remains of volcanic eruptions.
“Our results show that basically there are about five different peaks in terms of the osmium isotopes and also these iridium and platinum peaks, dating about 16,000 to 8,000 years ago. There should be just one peak if it were [caused by] a comet, but we found five peaks, actually,” Forman said. “People had been misidentifying a geochemical signature in the sediment and saying ‘Oh, that’s comet.’ But it’s not unique, it occurs four other times.”
And because these trace amounts of rare elements were aerosolized, the scientists were counting them in terms of parts per trillion. Co-lead author Alan Brandon, a University of Houston geoscientist, used an isotope mass spectrometer to measure the density of the elements in the group’s samples.
The volcanic explanation wasn’t the researchers’ first choice, but the evidence before them was undeniable.
“I was skeptical. We took every avenue we could to come up with an alternative explanation or even avoid this conclusion,” Brandon said in a statement. “A volcanic eruption had been considered one possible explanation but was generally dismissed because there was no geochemical fingerprint.”
The group can’t know for sure which volcanoes may have erupted and spewed the rare elements across the Earth, but candidates include Mount St. Helens, the Trans-Mexican Volcanic Belt and volcanoes located in Germany.
Is it possible that volcanic eruptions could have coincided with an impact event?
“It would be an incredible coincidence that these two phenomena occurred at the same time,” Forman said. “The lesson that we’ve learned is that you can have very quick and extreme climate change with Earth’s internal drivers of climate. And what caused the Younger Dryas was that the ice sheets were melting; they were recooling or refrigerating the North Atlantic Ocean, and that led to an increase in snow cover, and there may have been some volcanic eruptions that caused cooling at the very start of it.”
He added, “Those three things happening tipped the threshold, and the planet became cooler for a thousand years.”
Clues to the causes of global climate change, in other words, may be found beneath our feet rather than in the skies above.
“I think this study is the start of something about how we look at the evidence for extraterrestrial impacts on the climate system. It gives a much more rigorous way of assessing the sources of these geochemical anomalies,” Forman said. “And we’re just starting down that pathway.”