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© Heather Rousseau, courtesy Denver Museum of Nature & Science
Dr. Kirk Johnson, chief curator at the Denver Museum of Nature & Science, inspects the Ice Age bison skull. Upon inspecting the skull, Johnson said, “I’m trying to think of a cooler fossil that I’ve ever seen in my life.”
Changing climate observed in the Rocky Mountains

Scientists who study prehistoric remains know they have a huge find at Snowmass, probably the biggest of their careers.

Digging furiously for 18 days last fall as winter closed in, they uncovered the remains of eight to 10 American mastodons, four Columbian mammoths and a Jefferson's ground sloth, the first ever found in Colorado and the highest elevation sample anywhere in North America.

Although a herbivore, the ground sloth was the size of a grizzly bear and "capable of ripping your face off if you got too close to it," said Kirk Johnson, chief curator at the Denver Museum of Nature and Science, at a recent press conference.

That's conjectural, as ground sloths and most other species found at the Snowmass site disappeared 10,000 years ago, soon after the last great ice sheets retreated. Other species remain, such as the camel, whose tooth was found at Snowmass. But scientists aren't sure whether the genus of the extinct Camelops, had a hump, as camels today do, or lacked one, like their modern relative the llama.

At a recent press conference in Snowmass, Johnson showed a picture of the horns of an Ice Age bison retrieved from the site. Tip to tip, the horns are close to two metres, explained Johnson, who himself stands that tall. The extinct species also stood eight and 2.6 metres at the shoulder. The bison, said Johnson, was "really the most compelling animal from this whole site."

Picking up where they left off last fall, scientist and volunteers found another 100 bones, including more mastodon remains, in just the first week of digging in May.

"These giant fossils, the skulls and pelvises each the size of a car door, are in one large one bed among massive shoulders," explained Johnson. "We are just baffled as to why they are all in one place."

As captivating as these bones are, the more interesting story yielded by the muck at Snowmass may be of changing climates in the Rocky Mountains. Adding relevance is the location, elevation 2,700 metres. It is, said Johnson, "the best high-elevation site in North America."

Johnson and other scientists have a long way to go. Even after digging ends on July 1, they have probably two years of work ahead as they take the many discoveries to create a broad picture of life and change over what may have been a 100,000-year period.

At the start of digging this spring, 16 tusks had been uncovered, so well preserved that the annual growth rings could be studied, testifying not only to the age of the animals but even to the season of death. "These really start to tell the story," said Johnson.

From white spruce and subalpine fir, wood has also been abundant. In some, beetle-bored chambers can be seen, much like in diseased trees common across the west today, but in this case they're more than 50,000 years old. In some cases there is bark, a rarity at such sites.

"I was stunned when I found a lot (of wood) with bark on it," said Johnson.

Johnson called Snowmass the "best wood site in North America," and researchers hope to create a chronology based on the evidence left in the rings of wood, much as tree rings have allowed dendrochronologists to recreate sequences of wet and dry years in the American Southwest going back 2,000 years.

Circumstantial evidence suggests the depositions occurred between 50,000 and 150,000 years ago. Radiocarbon dating, the most common tool for identifying the age of organic matter, is of no use, at least so far. Two separate labs reported that samples submitted last winter were "radiocarbon dead," meaning that there is so little radioactive carbon 14 left in the samples that it is no longer measurable. That, said Johnson, means the remains are likely older than 45,000 or 50,000 years old.

Geologists believe the site can be no older than 150,000 years. Before that, during a phase called Bull Lake, after a site in Wyoming, giant glaciers that extended from Capitol Peak, a 4,300 metres peak, ground down the valley, depositing rocks, soil and other detritus in a ridge called a moraine. The lake was created on top of the moraine. Given the time Bull Lake glaciation ended elsewhere, they reason the lake and deposits are newer than 130,000 to 150,000 years.

Johnson said scientists would use several other techniques in an effort to get their thumbs more securely on dates within that band of 50,000 and 150,000 years ago. One technique, optically simulated luminescence, a technique developed at Simon Fraser University in B.C., measures how long a grain of sand has been exposed to sunlight.

Pollen records may also be useful, and they may be correlated to climate records revealed by ice cores retrieved from Greenland glaciers.

"There are at least four techniques, and the four of them together we hope will give us a real nice tight (range of) age," said Johnson.

"It would be really great if we had all of that range of 50,000 to 150,000 years ago," he added.

Already, however, the 36 scientists from 18 different universities and other institutions have concluded that the nine metres of earth exhumed has yielded material from five distinct layers. The remains may be from both ice ages and inter-glacial periods.

The ancient lake site was being scraped of the peat and mud last October when a bulldozer operator for Gould Construction noticed the bone of what turned out to be a juvenile Columbian mastodon. Work was halted, although museum officials worked out an agreement with the Snowmass Water and Sanitation District to delay continued work on the site until July. The enlarged reservoir is intended to provide assurances of water for Snowmass as the new real estate component, called Base Village, builds out.

The peat on the top layer has done an extraordinary job of preserving. Few of the bones were actually fossils, in which minerals have filled in to preserve the original structure. They are the bones themselves. And at least one of the mastodon tusks was so well preserved that it was still white when pulled from the mud.

"It was as though this was sealed in a container for more than 50,000 years," said Johnson.

Leaves are still green when pulled out of the bog, even though they lose their greenness within 30 seconds to a minute once exposed to the air, said Ian Miller, the Denver museum's curator of paleobotany at an appearance in Aspen in February.

The muck also presents a challenge. The boots of several volunteers had become so stuck that they had to get assistance. Johnson himself got stuck one day in May for 90 minutes, buried to his thighs in the viscous material that he compared to concrete.

"I can sympathize with the animals," he said.