Geologic evidence supports theory that major cosmic impact event occurred approximately 12,800 years ago

Geologic evidence supports theory that major cosmic impact event occurred approximately 12,800 years ago
The researchers found evidence of cosmic impact at  the Pilauco dig site in a suburb of the Osorno province in Chile Credit: Courtesy image

When UC Santa Barbara geology professor emeritus James Kennett and colleagues set out years ago to examine signs of a major cosmic impact that occurred toward the end of the Pleistocene epoch, little did they know just how far-reaching the projected climatic effect would be.

"It's much more extreme than I ever thought when I started this work," Kennett noted. "The more work that has been done, the more extreme it seems."

He's talking about the Younger Dryas Impact Hypothesis, which postulates that a fragmented comet slammed into the Earth close to 12,800 years ago, causing rapid climatic changes, megafaunal extinctions, sudden human population decrease and cultural shifts and widespread wildfires (biomass burning). The hypothesis suggests a possible triggering mechanism for the abrupt changes in climate at that time, in particular a rapid cooling in the Northern Hemisphere, called the Younger Dryas, amid a general global trend of natural warming and ice sheet melting evidenced by changes in the fossil and sediment record.

Controversial from the time it was proposed, the hypothesis even now continues to be contested by those who prefer to attribute the end-Pleistocene reversal in warming entirely to terrestrial causes. But Kennett and fellow stalwarts of the Younger Dryas Boundary (YDB) Impact Hypothesis, as it is also known, have recently received a major boost: the discovery of a very young, 31-kilometer-wide impact crater beneath the Greenland ice sheet, which they believe may have been one of the many comet fragments that impacted Earth at the onset of the Younger Dryas.

Now, in a paper published in the journal Nature Scientific Reports, Kennett and colleagues, led by Chilean paleontologist Mario Pino, present further evidence of a cosmic impact, this time far south of the equator, that likely lead to biomass burning, climate change and megafaunal extinctions nearly 13,000 years ago.

"We have identified the YDB layer at high latitudes in the Southern Hemisphere at near 41 degrees south, close to the tip of South America," Kennett said. This is a major expansion of the extent of the YDB event." The vast majority of evidence to date, he added, has been found in the Northern Hemisphere.

This discovery began several years ago, according to Kennett, when a group of Chilean scientists studying sediment layers at a well-known Quaternary paleontological and , Pilauco Bajo, recognized changes known to be associated with YDB impact event. They included a "black mat" layer, 12,800 years in age, that coincided with the disappearance of South American Pleistocene megafauna fossils, an abrupt shift in regional vegetation and a disappearance of human artifacts.

"Because the sequencing of these events looked like what had already been described in the YDB papers for North America and Western Europe, the group decided to run analyses of impact-related proxies in search of the YDB layer," Kennett said. This yielded the presence of microscopic spherules interpreted to have been formed by melting due to the extremely high temperatures associated with impact. The layer containing these spherules also show peak concentrations of platinum and gold, and native iron particles rarely found in nature.

"Among the most important spherules are those that are chromium-rich," Kennett explained. The Pilauco site spherules contain an unusual level of chromium, an element not found in Northern Hemisphere YDB impact spherules, but in South America. "It turns out that volcanic rocks in the southern Andes can be rich in chromium, and these rocks provided a local source for this chromium," he added. "Thus, the cometary objects must have hit South America as well."

Other evidence, which, Kennett noted, is consistent with previous and ongoing documentation of the region by Chilean scientists, pointed to a "very large environmental disruption at about 40 degrees south." These included a large biomass burning event evidenced by, among other things, micro-charcoal and signs of burning in pollen samples collected at the impact layer. "It's by far the biggest burn event in this region we see in the record that spans thousands of years," Kennett said. Furthermore, he went on, the burning coincides with the timing of major YDB-related burning events in North America and western Europe.

The at Pilauco contain a valuable record of pollen and seeds that show change in character of regional vegetation—evidence of a shifting climate. However, in contrast to the Northern Hemisphere, where conditions became colder and wetter at the onset of the Younger Dryas, the opposite occurred in the Southern Hemisphere.

"The plant assemblages indicate that there was an abrupt and major shift in the vegetation from wet, cold conditions at Pilauco to warm, dry conditions," Kennett said. According to him, the atmospheric zonal climatic belts shifted "like a seesaw," with a synergistic mechanism, bringing warming to the Southern Hemisphere even as the Northern Hemisphere experienced cooling and expanding sea ice. The rapidity—within a few years—with which the climate shifted is best attributed to impact-related shifts in atmospheric systems, rather than to the slower oceanic processes, Kennett said.

Meanwhile, the impact with its associated major environmental effects, including burning, is thought to have contributed to the extinction of local South American Pleistocene megafauna—including giant ground sloths, sabretooth cats, mammoths and elephant-like gomphotheres—as well as the termination of the culture similar to the Clovis culture in the north, he added. The amount of bones, artifacts and megafauna-associated fungi that were relatively abundant in the soil at the Pilauco site declined precipitously at the impact layer, indicating a major local disruption.

The distance of this recently identified YDB site—about 6,000 kilometers from the closest well-studied site in South America—and its correlation with the many Northern Hemispheric sites "greatly expands the extent of the YDB impact event," Kennett said. The sedimentary and paleo-vegetative evidence gathered at the Pilauco site is in line with previous, separate studies conducted by Chilean scientists that indicate a widespread burn and sudden major climate shifts in the region at about YDB onset. This new study further bolsters the hypothesis that a cosmic impact triggered the atmospheric and oceanic conditions of the Younger Dryas, he said.

"This is further evidence that the Younger Dryas climatic onset is an extreme global event, with major consequences on the animal life and the human life at the time," Kennett said. "And this Pilauco section is consistent with that."

Explore further

A cataclysmic event of a certain age

More information: Mario Pino et al, Sedimentary record from Patagonia, southern Chile supports cosmic-impact triggering of biomass burning, climate change, and megafaunal extinctions at 12.8 ka, Scientific Reports (2019). DOI: 10.1038/s41598-018-38089-y
Journal information: Scientific Reports

Citation: Geologic evidence supports theory that major cosmic impact event occurred approximately 12,800 years ago (2019, March 13) retrieved 23 July 2019 from
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Mar 14, 2019
Very good work.

Mar 14, 2019
Oh boy, here we go again. @torbjorn will be along shortly with a comment no doubt.

Mar 14, 2019
So much left to discover under the seas, under the ice, along flooded coastlines which were dry during a very important phase of human expansion and settlement.

What we find could change everything.

Mar 14, 2019
"What we find could change everything." - Otto

Ya, it might prove that disease really is caused by bad odors, and angels might really be real.

Everything might change. White and black could become white and black again.

Who knows what magic might happen.

Mar 14, 2019
Oh boy, here we go again........

The immortal words of Da Schitts, the "meat" loving, knob gobbler, when goes down on all fours for his boyfriend.


Mar 14, 2019
I don't understand the resistance to YBD. Convincing evidence is mounting and yet, from reading some of the comments here, you'd think it was being proposed that the Younger Dryas was caused by little green men in flying saucers. Honestly, is YBD not following much the same path of Walter and Luis Alvarez that ultimately led us to Chicxulub?

Mar 20, 2019
@rhugh1066 - The evidence is not convincing as at least in the Northern Hemisphere, the timing of megafaunal extinctions and the disappearance of the Clovis culture are not simultaneous. In fact, archaeologists do not think the Clovis culture disappeared so much as moved on to new technologies for hunting and gave up the characteristic Clovis point spearheads. a paper published last year has convincing evidence that the draining of large glacial lakes in North America into the Arctic, which had the effect of shutting down the The Atlantic Meridional Overturning Circulation (AMOC) by pouring freshwater into the Atlantic caused the Younger Dryas in the Northern Hemisphere. The impact supporters major thesis is that the biomass burning caused cooling of the planet. This article provides no mechanism for warming in the opposite hemisphere. So is the evidence convincing - not by a long shot.

"The bigger they are, the bigger they are."

Gosh, extending the biomass burning and extinction into the Southern Hemisphere is beginning to revive my initial suspicion of the 450 km Nastapoka Arc as the primary impact structure, not to say impact crater, with cushioning by the relatively-compressible ice sheet ice, clamping the impact shock wave pressure below the melting point of silicates.

Water (ice) is 10 times as compressible as granite (silicates), such that an impact shock wave will do 10 times as much compressive work on target ice, compared to target-or-extraterrestrial silicates, dramatically reducing the impact shock wave pressure. The shock-absorber effect of a direct impact on the Laurentide ice sheet would have dramatically lowered the peak power of impact by extending its duration, exactly like compressive styrofoam in a crash helmet.

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