Overhunting. Abrupt climate change. Disease.

Scientists have cited those and other theories in their decades-old debate about why mammoths, mastodons, sloths, saber-toothed cats, camels, horses and other large creatures disappeared from North America at the end of the last ice age.

The researchers say the impact also may have wiped out or fragmented the prehistoric Clovis people who flourished in North America at the time.

UO archaeologists Douglas Kennett and Jon Erlandson are among about two dozen scientists who say the impact also could have led to the abrupt cooling known as the Younger Dryas, which lasted 1,200 years.

"I initially thought this idea for a comet impact was preposterous," Erlandson said. "But as I started reviewing the data from multiple locations, the concept began to capture my imagination. There was dramatic change going on at that time. A comet contributing to that change is just a hypothesis at this point, and it's probably going to take at least 10 more years of research to figure out if that occurred."

The researchers laid out their proposed theory recently in the Proceedings of the National Academy of Sciences. They say the evidence is in a 2-inch-thick carbon-rich black mat found in about 50 sites across North America.

Their article says the layer contains grains with iridium, carbon spherules and fullerenes packed with concentrations of helium 3 -- "all of which are evidence for an extraterrestrial impact" about 12,900 years ago and the raging wildfires that followed.

The scientists studied samples of black mats from 10 Clovis archaeological sites in Canada, California, Arizona and South Carolina. Evidence of mammoths and other animals and early human hunters are found beneath the black mat but are missing within or above the strip.

They suggest that heat from the detonation or impact may have set off wildfires across the continent and destabilized the vast Laurentide Ice Sheet that covered much of Canada and the northern United States. The melting of ice may have led to a flood of fresh water, changing ocean currents in the North Atlantic and contributing to the cooling climate that reduced human populations and added to the demise of the animals.

No giant crater has been found from the impact, but the scientists say the comet may have broken into smaller pieces and detonated in various places.

"If a comet hit the Laurentide Ice Sheet, it would not have left a big crater," Erlandson said. "It's ice on ice, and it's going to throw up enormous amounts of shattered ice and debris."

Geological features known as Carolina Bays -- hundreds of thousands of small depressions from New Jersey to Florida -- may have been caused by the disintegrating object. Samples from 15 of the depressions contained the iridium-rich magnetic grains and carbon spherules.

The elliptical depressions are oriented in a northwest-southeast direction that point toward Canada and the Great Lakes, suggesting the object disintegrated in that area.

"The initial impact would have been very heavy in eastern North America, and western North America would have been more of a refuge," Erlandson said. "Some of the initial modeling of this impact indicated that it would have created an enormous rolling fireball and force wave that engulfed the entire continent and wiped out everything. Archaeology tells us that it didn't do that; there were animals and people that survived."

Erlandson said the reduced populations of large animals in the West probably would have been under tremendous pressure from changes in the environment and from hunting by remnants of Clovis populations.

The tell-tale black mats have not been found in the Northwest, Kennett said, but scientists "have some leads on sites to visit and sample." They include a site near Mill Creek in Woodburn's Legion Park, where a treasure trove of remains from extinct ice age mammals has been uncovered in recent years.

His father, James Kennett, a paleoceanographer at the University of California at Santa Barbara, introduced the impact idea with Richard Firestone of the Lawrence Berkeley National Laboratory and Allen West, an Arizona researcher.

Douglas Kennett is in Spain looking for potential effects of the impact in Europe and soon will look at sites in the Netherlands.

One of the archaeological sites being investigated is about 30 miles off the Southern California coast on Daisy Island, where Erlandson has uncovered evidence that humans used the islands about 13,000 years ago. Erlandson plans to return to the island with other researchers to determine whether there's additional evidence that would support the comet impact concept.

Idea has its critics

The new hypothesis already has drawn critics.

"I think we would all like to see a smoking gun," said Alison Stenger, a Portland archaeologist who has led excavations in Woodburn and McMinnville that have turned up ice age animals. "We have not seen any evidence of a black mat at these sites, and we've examined a lot of sediment."

Not all large animals died off at the end of the ice age, and many "were alive and well" at Woodburn and Rancho La Brea tar pits in Los Angeles nearly 2,000 years after the proposed impact.

"I'm convinced that a group of things led to the extinction of these animals," she said. "A comet certainly would be tidy explanation, but I'm not seeing the physical evidence that this occurred."

Donald Grayson, a University of Washington anthropology professor who has studied the animal extinctions, also is skeptical.

Grayson said there are other explanations for the black mats, which "are most assuredly not an indicator of an impact event." There's no evidence of massive burning at the end of the last ice age, he said, and there's no evidence suggesting that "post-Clovis human densities in North America were less than Clovis ones."


Comment: This is clearly untrue, given the evidence presented in the article:
In the 1990s, W. Topping (14) discovered magnetic microspherules and other possible ET evidence in sediment at the Gainey PaleoAmerican site in Michigan (see also ref. 15), and Lougheed (16) and Bi (17) reported that late Pleistocene glacial drift contained similar cosmic spherules. We now report substantial additional data from multiple, well dated stratigraphic sections across North America supporting a major ET airburst or collision near 12.9 ka. Directly beneath the black mat, where present, we found a thin, sedimentary layer (usually 5 cm) containing high concentrations of magnetic microspherules and grains, nanodiamonds, iridium (Ir) at above background levels, and fullerenes containing ET helium. These indicators are associated with charcoal, soot, carbon spherules, and glass-like carbon, all of which suggest intense wildfires. Most of these markers are associated with previously recorded impacts, but a few are atypical of impact events. We identify this layer as the YD boundary (YDB), and we refer to this incident as the YD event.

...

Ten Clovis and equivalent-age sites were selected because of their long-established archeological and paleontological significance, and, hence, most are well documented and dated by previous researchers (see SI Table 2). Two are type-sites where unique PaleoAmerican projectile point styles were first named: the Clovis-point style at Blackwater Draw, NM, and the Gainey-point style at Gainey, MI. Three of the sites are confirmed megafaunal kill sites, and six of 10 have a black mat overlying the YDB. At Blackwater Draw and Murray Springs, the YDB is found directly beneath the black mat and overlying Clovis artifacts with extinct megafaunal remains.

The other sample sites were in and around 15 Carolina Bays, a group of 500,000 elliptical lakes, wetlands, and depressions that are up to10 km long and located on the Atlantic Coastal Plain (SI Fig. 7). We sampled these sites because Melton, Schriever (20), and Prouty (21) proposed linking them to an ET impact in northern North America. However, some Bay dates are reported to be 38 ka (22), older than the proposed date for the YD event.

©PNAS
Fig. 7. Aerial photo (U.S. Geological Survey) of a cluster of elliptical and often overlapping Carolina Bays with raised rims in Bladen County, North Carolina. The Bays have been contrast-enhanced and selectively darkened for greater clarity. The largest Bays are several kilometers in length, and the overlapping cluster of them in the center is ≈8 km long. Previous researchers have proposed that the Bays are impact-related features.

It's interesting to note that the 38 Ka dates of some of the Carolina Bays correspond to two 19,000 year precessional periods.


The skepticism is natural "given the magnitude of this hypothesis and its potential significance in the history of our planet," and more research is needed, Douglas Kennett said. "The scale of the inquiry is also massive, and we have just started to scratch the surface "

Richard L. Hill: 503-221-8238; richardhill@news.oregonian.com