Preliminary estimates say an ancient ridge-top lake in the US might have persisted for as long as 1,00,000 years before windblown dust filled it in to become a typical-looking alpine meadow. Researchers have found many fossils at this spot, writes Kirk Johnson
Two different time scales collided in this place. Over 1,30,000 years ago in the chilled depths of the Illinoian ice age, a glacier left a hole atop a 9,000-foot-high ridge near what would later be the town of Aspen in the US Colorado Rockies. The depression filled with snowmelt, and for tens of thousands of years, the lake attracted the giants of the Pleistocene – mammoths, mastodons, ground sloths, supersize bisons, camels and horses. The second time scale was more like a runner’s sprint.
On October 14 last year, workers on a reservoir dam turned over the first fossil bones (of a young female mammoth, named Snowy). Recently, work on the reservoir resumed.
A race to find the fossils
The result was a race to find and catalog everything possible before the site was entombed once more by water. The breakneck pace of the fossil dig was matched only by what scientists said was the extraordinary richness of the site, one of the best windows into the thundering megafauna of its time. “The speed of this thing is so unlike normal science – from discovery to completion of one of the biggest digs ever in less than nine months,” said Kirk R Johnson, the chief curator of the Denver Museum of Nature and Science, who oversaw the project.
The ancient Snowmass clock was measured in the untold lives of the creatures that roamed and roared in a place and period poorly recorded in the scientific record: The high reaches of Rocky Mountains during the Sangamonian interglacial, a time of very warm weather around the globe, 75,000 to 125,000 years ago. Other well-known ice age fossil sites, by contrast, like the La Brea Tar Pits in California and Hot Springs, S.D., have been dated to between 10,000 and 40,000 before present, and no well-preserved site has ever been found, scientists said, at this altitude in North America.
At Snowmastodon, as the site is called, the human clock ran partly on adrenaline, with 50 or more shovel-wielding scientists, volunteers and interns from the Denver Museum pawing the lake bed on a typical day. Their goal: sift 7,000 tons of sediment – 35 feet worth to the bottom of the glacial scrape – by the deadline. Something very big – a mammoth tusk, a partial mastodon skull – was turning up every few days. Over 4,500 fossil specimens from 20 different animals were hauled out.
Preliminary estimates say the ancient ridge-top lake – unusual in having no stream inlet to bring in sediment – might have persisted for as long as 1,00,000 years before windblown dust filled it in to become a typical-looking alpine meadow, a state it had reached 50,000 years or more before humans came to the Americas.
The fossil bed has a long climate record in its pollens, buried plants and windborne particles, as well as a long yardstick of the animals and what might be deduced about their lives. The sediment layers suggest periods when the lakeside landscape was tundra – too cold for trees – and others when great forests hugged the shore.
“I think at the end of the day that’s what’s going to be so valuable – you’ve got this crystal-clear glimpse into the Rockies before humans show up,” said Ian Miller, curator of paleobotany at the Denver museum.
There are still many unanswered questions about what happened here – most pointed is, when did the animals actually die? Because the site is too old for radiocarbon dating, which is only useful to about 50,000 years before present, other complicated methods, all of which take longer to work out, will have to be used. Ancient pollen, for example, was collected from the mud to compare against other climate indicators. Core samples will be examined for markers like volcanic dust, which might be dated using radiometric dating techniques based on argon 40-39 or uranium-lead geochronology.
Secondly, the animals did not march to their deaths in a steady procession over the centuries. There are sediment layers with few bones, followed by layers with many bones – indicating, Johnson said, that the lake may have multiple stories to tell. The remains of young animals found in the pit could suggest, for example, that at least through part of its history the lake was a trap, with slippery slopes or lethal leg-sucking goo, like the La Brea Tar Pits.
The climate connection
The third great question is how the great shifts of climate recorded by the mud affected the lives and habits of the creatures that roamed here. Was the climate warm enough in the interglacial period, which peaked in temperature around 1,10,000 years ago, that elephant-family relatives and other animals like camels and sloths could live year round at high altitude, or were there migratory patterns – highlands in the summers, lowlands in winter – that might emerge? For instance, will the growth rings of mastodon or mammoth tusks found here differ from those of cousins found at low altitude sites, hinting at permanent mountain residence? “That’s the kind of question we couldn’t even ask before this site was discovered,” Johnson said.
Some researchers are hoping the finds will yield DNA that might give a glimpse into the genes of ice age mammals. Genetic diversity, or uniformity, can suggest how big a population was at the time of an individual’s death. “The interglacial period wasn’t a great time for stuff to be preserved,” said Beth Shapiro, an associate professor of biology at Pennsylvania State University who studies ancient DNA. “So this is not just a window into a time, but a whole group of animals we’ve never been able to get before.”
Author: Kirk Johnson | Source: Deccan Herald [July 23, 2011]
 |
| A giant sloth humerus [Credit: Rick Wicker/Denver Museum of Nature and Science] |
On October 14 last year, workers on a reservoir dam turned over the first fossil bones (of a young female mammoth, named Snowy). Recently, work on the reservoir resumed.
A race to find the fossils
The result was a race to find and catalog everything possible before the site was entombed once more by water. The breakneck pace of the fossil dig was matched only by what scientists said was the extraordinary richness of the site, one of the best windows into the thundering megafauna of its time. “The speed of this thing is so unlike normal science – from discovery to completion of one of the biggest digs ever in less than nine months,” said Kirk R Johnson, the chief curator of the Denver Museum of Nature and Science, who oversaw the project.
The ancient Snowmass clock was measured in the untold lives of the creatures that roamed and roared in a place and period poorly recorded in the scientific record: The high reaches of Rocky Mountains during the Sangamonian interglacial, a time of very warm weather around the globe, 75,000 to 125,000 years ago. Other well-known ice age fossil sites, by contrast, like the La Brea Tar Pits in California and Hot Springs, S.D., have been dated to between 10,000 and 40,000 before present, and no well-preserved site has ever been found, scientists said, at this altitude in North America.
At Snowmastodon, as the site is called, the human clock ran partly on adrenaline, with 50 or more shovel-wielding scientists, volunteers and interns from the Denver Museum pawing the lake bed on a typical day. Their goal: sift 7,000 tons of sediment – 35 feet worth to the bottom of the glacial scrape – by the deadline. Something very big – a mammoth tusk, a partial mastodon skull – was turning up every few days. Over 4,500 fossil specimens from 20 different animals were hauled out.
Preliminary estimates say the ancient ridge-top lake – unusual in having no stream inlet to bring in sediment – might have persisted for as long as 1,00,000 years before windblown dust filled it in to become a typical-looking alpine meadow, a state it had reached 50,000 years or more before humans came to the Americas.
The fossil bed has a long climate record in its pollens, buried plants and windborne particles, as well as a long yardstick of the animals and what might be deduced about their lives. The sediment layers suggest periods when the lakeside landscape was tundra – too cold for trees – and others when great forests hugged the shore.
“I think at the end of the day that’s what’s going to be so valuable – you’ve got this crystal-clear glimpse into the Rockies before humans show up,” said Ian Miller, curator of paleobotany at the Denver museum.
There are still many unanswered questions about what happened here – most pointed is, when did the animals actually die? Because the site is too old for radiocarbon dating, which is only useful to about 50,000 years before present, other complicated methods, all of which take longer to work out, will have to be used. Ancient pollen, for example, was collected from the mud to compare against other climate indicators. Core samples will be examined for markers like volcanic dust, which might be dated using radiometric dating techniques based on argon 40-39 or uranium-lead geochronology.
Secondly, the animals did not march to their deaths in a steady procession over the centuries. There are sediment layers with few bones, followed by layers with many bones – indicating, Johnson said, that the lake may have multiple stories to tell. The remains of young animals found in the pit could suggest, for example, that at least through part of its history the lake was a trap, with slippery slopes or lethal leg-sucking goo, like the La Brea Tar Pits.
The climate connection
The third great question is how the great shifts of climate recorded by the mud affected the lives and habits of the creatures that roamed here. Was the climate warm enough in the interglacial period, which peaked in temperature around 1,10,000 years ago, that elephant-family relatives and other animals like camels and sloths could live year round at high altitude, or were there migratory patterns – highlands in the summers, lowlands in winter – that might emerge? For instance, will the growth rings of mastodon or mammoth tusks found here differ from those of cousins found at low altitude sites, hinting at permanent mountain residence? “That’s the kind of question we couldn’t even ask before this site was discovered,” Johnson said.
Some researchers are hoping the finds will yield DNA that might give a glimpse into the genes of ice age mammals. Genetic diversity, or uniformity, can suggest how big a population was at the time of an individual’s death. “The interglacial period wasn’t a great time for stuff to be preserved,” said Beth Shapiro, an associate professor of biology at Pennsylvania State University who studies ancient DNA. “So this is not just a window into a time, but a whole group of animals we’ve never been able to get before.”
Author: Kirk Johnson | Source: Deccan Herald [July 23, 2011]