Ice Age time machine

The Snowmass excavation team uncovers a heap of Mastodon bones, including five pelvises, two tusks, and two skulls, on May 27, 2011. The remains were wrapped in a protective cast of plaster for transport to the Denver Museum of Nature and Science. Once in Denver, three-dimensional photographs were taken of the remains, so scientists in the future can see everything just as they found it./Photo Denver Museum of Nature and Science

Snowmass discovery provides crystal clear glimpse into the pleistocene

by Allen Best

Do earthquakes explain all those mastodon bones discovered at Snowmass in 2010 and 2011? Not likely, say scientists, although they haven’t completely shelved the idea.

And did humans kill a mammoth 50,000 years ago and cache the meat for later use? Circumstantial evidence suggests that was the case. If so, it would rank as one of the major scientific discoveries of the decade, putting people on North America some 36,000 years earlier than is now generally agreed by archaeologists.

That intriguing idea also remains on the shelf, just beyond touch for lack of corroborating evidence.

Discouraged? Hardly. Scientists still think the 6,000 bones plus other organic matter retrieved in two bursts of intense digging in 2010 and 2011 ranks as among the preserved from 50,000 to 130,000 years ago, the last glacial interlude.

“It’s one of the premier finds of the last decade, and arguably – because of the high elevation and the quality of preservation ... one of the five top Pleistocene sites in North America,” says Ian Miller, curator of paleontology at the Denver Museum of Nature and Science. In fact, he said the Snowmass site ranks up there in importance with the famed mammoth site at Hot Springs, S.D., and L.A.'s La Brea Tar Pits.

A layer of clay created the exceptionally well-preserved archive of life during the last interglacial period. Spades upturned leaves still green and insects iridescent after 50,000 years. Logs that had gathered along the ancient shoreline were so well preserved that chain saws were used to slice them.

“It’s really a crystal clear picture into the ecosystem between 50,000 and 130,000 years ago,” says Miller.

Occasionally there were shouts of glee as scientists triumphantly hoisted bones. They found the remains of 40 different animals, from mice to a camel, a mighty bison now extinct and a ground sloth many times the size of what now exists.

Mostly they found elephants: the grass-eating mammoths, 9 feet tall at the shoulder but with long, curving tusks and trunks shorter than those found in elephants today. Deeper in the sediments and more plentiful were mastodons, which are slightly shorter and have teeth better adapted to eating leaves and twigs.

As the bones of at least 34 different animals were pulled out, it’s among the biggest mastodon discoveries in North America.

Just how so many mastodon bones came to exist in one place perplexed the scientists. One early hypothesis was that an earthquake, or perhaps several, had occurred while the mastodons were in the lake. Earthquakes can have the effect of liquefying lake sediment, anchoring whatever animals are stuck in them like quicksand.

If they have not discarded that idea, scientists now favor a simpler explanation of individual deaths from natural causes, the bodies then scavenged by various opportunists. “Think of a watering hole in Africa,” says Miller.

But a mammoth found near the surface poses a mystery. Rocks were intermixed among the bones in ways that didn't seem natural. Scientists wondered if the rocks had been put there by people to cache the meat for later use.

If that were the case, this would be one of the top scientific discoveries of the decade. Archaeologists have only been able to confirm human existence in the Western Hemisphere to about 14,000 to 15,000 years ago, near the end of the last ice age. This would, if proven, extend human habitation another 36,000 years.

With the need to get out of the lake in 2011 while it was converted into a reservoir, museum personnel wrapped the bones, peat and rocks into a protective cast of plaster for transport to the museum in Denver. Once in Denver, they took three-dimensional photographs, so that scientists in the future can see everything just as they found it.

Archaeologists from around the country were invited to examine the remains. “Everybody was super intrigued,” says Miller. “Nobody was like, ‘You guys are out to lunch.’ But at the same time everybody said, ‘You need to find extraordinary evidence to indeed prove that it was a human cache.”

So far, that extraordinary evidence is wanting – and may remain so. “It will likely remain a mystery but sometimes that’s science!” says Miller.

An unprecedented window

The greater significance of the Snowmass find is likely to be its ability to document the changing climate of that last interglacial period, when humans had little or no influence on the planetary environment. That record, in turn, may help us better evaluate our own era, when humans more clearly are driving changes.

“That’s the million dollar question,” says Miller, “and after spending $1 million (on the dig), we would like to have a very good answer to that.”

The site of the excavation is on a ridge, surrounded by aspen trees. The ridge was created about 130,000 years ago when glaciers of the Bull Lake advance receded, leaving the moraine. The lake where the mastodons and mammoths died was created as the result of an oddity of this geography.

From 130,000 years ago to about 50,000 years ago was a time of relative warmth, some times much warmer than our own. This is the period in which the mammoth and mastodon bones were left, as well as some creatures that can still be found in the Rocky Mountains.

By about 50,000 years ago, glaciers had begun to wax again. This time, they didn't advance quite as far down the valleys, but they substantially cooled the climate and lowered timberline.

That last glacial period peaked about 16,000 years ago – then the ice rapidly receded. Temperature suddenly warmed 12-13 degrees in just a decade about 10,000 to 11,000 years ago. Presto: the Earth was out of the ice age and in what we now like to think of as normal.

Even that period of stability, however, has been a roller coaster. Temperatures have swung widely. One pronounced cooling for several centuries froze the Thames River during winter months, stalled the advance of civilization, and aborted Norse settlement of Greenland. Now, the planet seems to be heating again – rapidly so, say some scientists, taking note of the rapid breakup of Arctic sea ice.

Important to the value of the Snowmass site is its elevation, nearly 9,000 feet. High elevations are more sensitive to climate changes than lower elevations.

Jeff Pigati of the U.S. Geological Survey, says the quality of the preservations allows for sharp dating, which will enhance clarity for the emerging story of the last interglacial period, called the Eemian.

“How did the plant communities change? How did the animal communities begin to change?” says Pigati. That, he explains, is where the Snowmass site will provide new understanding.

“This site opens up a window that we have not been able to see into,” he says.

The new insights will soon become apparent. Some 47 to 50 scientists of various disciplines continue their tests and are scheduled to begin submitting their reports, some of which will be presented in October at the Geological Society of America conference in Denver. The papers are expected to be published in early 2014.

Pigati describes himself like a child at Christmas, eager to unwrap the boxes. “It’s kind of like coming down and seeing all the gifts under the tree,” he says.