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

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Old March 10th 20, 04:08 PM posted to alt.astronomy
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Default theory that major cosmic impact event occurred approximately 12,800years ago


Geologic evidence supports theory that major cosmic impact event
occurred approximately 12,800 years ago
by Sonia Fernandez, University of California - Santa Barbara

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

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 archaeological site, 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 sedimentary layers 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

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

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