Researcher looks at comet as potential trigger of 'Big Freeze'

LAWRENCE — Scientists have long speculated about the cause of the Younger Dryas, a sudden cold period that befell Earth as it emerged from the last Ice Age between about 12,800 and 11,500 years ago. Sometimes, this millennium-long temperature glitch is called the “Big Freeze.”

“The world suddenly began to cool again, quite rapidly,” said Adrian Melott, professor of physics and astronomy at the University of Kansas. “Around this time, many large animals disappeared, especially from North America, and the Clovis culture of Paleo-Indians disappeared as well.”

Researchers have pointed to a shutdown of the North Atlantic conveyor or other possible reasons for the sudden cooling and loss of biodiversity. Now, in a just-published article in Earth and Planetary Science Letters, Melott examines the possibility that a strike by a long-period comet touched off the Younger Dryas.

“The conveyor shutdown is the ‘standard model’ for this event,” Melott said, “but I’ve also heard discussion of changes in the jet stream. The Younger Dryas hypothesis is that the comet impact destabilized the ice sheet and that, in turn, shut down the conveyor. We wanted to test this idea.”

Along with first author Drew Overholt (now an assistant professor at MidAmerica Nazarene University), who received his doctorate from KU last spring, Melott looked for signals that a comet might have hit Earth at the onset of the Younger Dryas by measuring isotopes associated with long-period comets. These isotopes could have been spread out over the planet at the time of such an impact, held within ice and sediments from that time.

“People have already observed an increase in the abundance of the isotopes carbon-14 and beryllium-10 around the time of the Younger Dryas,” Melott said. “Various explanations have been proposed for the two isotopes, but we have shown that a long-period comet, being exposed to a lot of radiation in the outer solar system, would contain enough of both isotopes to show up in the terrestrial record due to what is distributed into the atmosphere. Much of this ends up being snowed out and preserved in ice in places like Greenland.”

Overholt added: “A third isotope (aluminum-26) may offer the key to determining whether this impact took place. A detectable amount should be present as a layer in Greenland ice cores, whether the object was an asteroid or comet. This isotope is incapable of being produced in large quantities in any other way and has yet to be tested for during this time period. This could be the smoking gun researchers have been seeking for the Younger Dryas impact hypothesis.”

Melott said that such a comet would have measured a few kilometers in diameter, and that before hitting Earth the comet may have broken up in space into hundreds of large pieces.

“If so, the comet could have been far larger,” he said, adding that simultaneous impacts in North America and the Middle East could have been possible.

“The main idea is that there was an airburst or airbursts, possibly over the ice sheet that covered most of down into what is now the U.S.,” said Melott. “Because of the ice sheet, an impact into it would likely have left no crater.”

The KU researcher added that one good reason to determine if a comet brought on the Younger Dryas is to help weigh the danger to humanity from such comets today.

“A long period comet comes from way out, far beyond the orbit of Pluto,” Melott said. “The rate of impacts from them is unknown. The danger is that, unlike asteroids, we can’t predict an impact a long time ahead. In fact, we don’t usually even see them until they’re a couple of years out from the Earth’s orbit. The British astronomer William Napier believes that most of the really big impacts have been comets, and a recent paper from a group at Dartmouth argues that there is evidence that the [dinosaur-killer] KPg object was a comet. That object did leave a big crater at the Yucatan, but it still could have been a comet. A big snowball coming straight down will still leave a crater.”

Indeed, the cause of the Younger Dryas could serve as a warning to future generations.

“Due to being wildly unpredictable and having such a high velocity, long-period comets pose the largest threat of all extraterrestrial impactors,” Overholt said. “This work may be the first step in finding a rate for their impacts and airbursts.”