What happened to the great land mammals of North America nearly 13,000 years ago? A University of Maine professor is among those working to find out.
The research by Paul Mayewski, director of the Climate Change Institute, and other scientists into this question is the subject of “Last Extinction,” presented on “Nova” at 8 tonight on PBS.
The program explores the question of what wiped out such animals as woolly mammoths, saber-toothed cats, giant ground sloths and the armadillolike glyptodonts.
One theory is that a comet broke apart in the atmosphere and devastated North America 12,900 years ago.
Mayewski was a natural to be contacted, as his past findings dovetail well with what scientists studying such a theory have found.
After all, the UMaine climatologist was field leader for the ground-breaking Greenland Ice Sheet Project 2, which used instrumentally calibrated ice cores to create a high-resolution record for climate change over the past 300,000 years. That research also verified the existence of abrupt climate changes throughout history.
One such change that Mayewski pointed to was a return to ice-age conditions 12,900 years ago, at the start of the Younger Dryas period. The change in climate took place in less than two years, and continued for the next 1,200 years.
“That’s the equivalent of going from three months of winter to 11 months of winter in two years,” he explained.
This is also the time after which any geologic record of large land mammals ceased to exist.
In Science magazine in 1993, Mayewski wrote a description of what happened in the atmosphere at that time, which included the largest amount of biomass burning during the entire record of 300,000 years.
About a year and a half ago, he was asked to take part in a paper which would summarize evidence that might exist pointing to an extraterrestrial event, such as a meteorite impact.
He began comparing notes with another contributor to that paper, geologist James Kennett of University of California, Santa Barbara, who wondered if there might be evidence of such an impact in the Greenland Ice Sheet.
The ice sheet serves as a frozen geologic record, with each layer of ice encapsulating atmospheric conditions in a given time period, much like rings of a tree.
So, as Mayewski explained, finding the correct layer was like looking for a needle in a haystack. Still, aided by Danish climatologist J.P. Steffensen and his own UMaine colleague Andrei Kurbatov, Mayewski found the right spot at the edge of the ice sheet.
“We found the ice that we want to look for, and analyzed it to be sure it was from the right time period,” Mayewski said.
Examing the samples taken by Mayewski and Kurbatov, Kennett and Arizona geologist Allen West struck gold, er, diamonds, in this case nano-diamonds. A cup of water melted from these samples would contain billions and billions of nano-diamonds.
Nano-diamonds fall from the sky every day. But the quantities contained in Mayewski’s samples were abnormally large. Also, there were many hexagonal-shaped ones, which can only be formed under the heat and pressure of an extraterrestrial impact.
“This showed that this was a unique event, with such a heavily concentrated layer found so close to the beginning of Younger Dryas,” Mayewski said.
Other theories have been offered for the mammals’ disappearance. One is a sudden climate change, but Mayewski doesn’t buy that: “That doesn’t hold a lot of water, because the animals involved had been adapting for tens of thousands of years.”
Another is that the animals were overhunted by the spear-wielding Clovis people of the Stone Age. But Mayewski wondered: “Why would hunting pressures all of the sudden lead to extinction in a matter of decades?”
Next, to gather more evidence of what went on early in the Younger Dryas period, Mayewski plans to return with a crew to Greenland this summer.
“Now that we’ve found the site, we’ll go back and sample in much more detail,” he said.