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by Julie Cohen
August 27, 2014
from
PHYS Website
A transmission
electron microscopy image
of carbon spherules
from the Younger Dryas Boundary
30 cm below the
surface in Gainey, Michigan.
Credit: UCSB
Most of North America's megafauna - mastodons, short-faced bears,
giant ground sloths, saber-toothed cats and American camels and
horses - disappeared close to 13,000 years ago at the end of the
Pleistocene period.
The cause of this massive extinction has
long been debated by scientists who, until recently, could only
speculate as to why.
A group of scientists, including UC Santa Barbara's James Kennett,
professor emeritus in the Department of Earth Science, posited that
a
comet collision with Earth played a
major role in the extinction.
Their hypothesis suggests that a
cosmic-impact event precipitated
the Younger Dryas period of global
cooling close to 12,800 years ago.
This cosmic impact caused abrupt
environmental stress and degradation that contributed to the
extinction of most large animal species then inhabiting the
Americas. According to Kennett, the catastrophic impact and the
subsequent climate change also led to the disappearance of the
prehistoric Clovis culture, known for its big game hunting, and to
human population decline.
In a new study published this week in the Journal of Geology (Nanodiamond-Rich
Layer across Three Continents Consistent with Major Cosmic Impact at
12,800 Cal BP), Kennett and an international group of
scientists have focused on the character and distribution of
nanodiamonds, one type of material produced during such an
extraterrestrial collision.
The researchers found an abundance of
these tiny diamonds distributed over 50 million square kilometers
across the Northern Hemisphere at the Younger Dryas boundary (YDB).
This thin, carbon-rich layer is often visible as a thin black line a
few meters below the surface.
Kennett and investigators from 21 universities in six countries
investigated nanodiamonds at 32 sites in 11 countries across North
America, Europe and the Middle East.
Two of the sites are just across the
Santa Barbara Channel from UCSB:
The solid line
defines the current known limits
of the Younger Dryas
Boundary field of cosmic-impact proxies,
spanning 50 million
square kilometers.
Credit: UCSB
"We conclusively have identified a
thin layer over three continents, particularly in North America
and Western Europe, that contain a rich assemblage of
nanodiamonds, the production of which can be explained only by
cosmic impact," Kennett said.
"We have also found YDB glassy and
metallic materials formed at temperatures in excess of 2200
degrees Celsius, which could not have resulted from wildfires,
volcanism or meteoritic flux, but only from cosmic impact."
The team found that the YDB layer also
contained larger than normal amounts of cosmic impact spherules,
high-temperature melt-glass, grapelike soot clusters, charcoal,
carbon spherules, osmium, platinum and other materials.
But in this paper the researchers
focused their multi-analytical approach exclusively on nanodiamonds,
which were found in several forms, including cubic (the form of
diamonds used in jewelry) and hexagonal crystals.
"Different types of diamonds are
found in the YDB assemblages because they are produced as a
result of large variations in temperature, pressure and oxygen
levels associated with the chaos of an impact," Kennett
explained.
"These are exotic conditions that
came together to produce the diamonds from terrestrial carbon;
the diamonds did not arrive with the incoming meteorite or
comet."
James P. Kennett,
professor emeritus in
UCSB's Department of Earth Science.
Credit: Sonia
Fernandez
Based on multiple analytical procedures, the researchers determined
that the majority of the materials in the YDB samples are
nanodiamonds and not some other kinds of minerals.
The analysis showed that the
nanodiamonds consistently occur in the YDB layer over broad areas.
"There is no known limit to the YDB
strewnfield which currently covers more than 10 percent of
the planet, indicating that the YDB event was a major cosmic
impact," Kennett said.
"The nanodiamond datum recognized in
this study gives scientists a snapshot of a moment in time
called an isochron."
To date, scientists know of only two
layers in which more than one identification of nanodiamonds has
been found:
the YDB 12,800 years ago and the
well-known Cretaceous-Tertiary boundary 65 million years ago,
which is marked by the mass extinction of the dinosaurs,
ammonites and many other groups.
"The evidence we present settles
the debate about the existence of abundant YDB nanodiamonds,"
Kennett said.
"Our hypothesis challenges some
existing paradigms within several disciplines, including
impact dynamics, archaeology, paleontology and
paleoceanography/paleoclimatology, all affected by this
relatively recent cosmic impact."
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