The Great Dying
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The Great Dying
250 million years ago something unknown wiped out
most life on our planet. Now scientists are finding buried clues
to the mystery inside tiny capsules of cosmic gas.
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January 28, 2002: It
was almost the perfect crime.
Some perpetrator -- or perpetrators -- committed murder on a
scale unequaled in the history of the world. They left few clues
to their identity, and they buried all the evidence under layers
and layers of earth.
The case has gone unsolved for years -- 250 million years,
that is.
But now the pieces are starting to come together, thanks to a team of NASA-funded sleuths who have found the "fingerprints" of the villain, or at least of one of the accomplices
Above: Life was flourishing on the Earth about 250 million years ago, then during a brief window of geologic time nearly all of it was wiped out. This image is an artist's impression of a Lower Permian swamp in Texas.
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Somehow, most of the life on Earth perished in a brief moment of geologic time roughly 250 million years ago. Scientists call it the Permian-Triassic extinction or "the Great Dying" -- not to be confused with the better-known Cretaceous-Tertiary extinction that signaled the end of the dinosaurs 65 million years ago. Whatever happened during the Permian-Triassic period was much worse: No class of life was spared from the devastation. Trees, plants, lizards, proto-mammals, insects, fish, mollusks, and microbes -- all were nearly wiped out. Roughly 9 in 10 marine species and 7 in 10 land species vanished. Life on our planet almost came to an end.
Scientists have suggested many possible causes for the Great Dying: severe volcanism, a nearby supernova, environmental changes wrought by the formation of a super-continent, the devastating impact of a large asteroid -- or some combination of these. Proving which theory is correct has been difficult. The trail has grown cold over the last quarter billion years; much of the evidence has been destroyed.
Left:
The Earth was engulfed in widespread volcanism at the time of
the extinction. Photo courtesy Dick Rasp/National Park Service.
"These rocks have been through a lot, geologically speaking,
and a lot of times they don't preserve the (extinction) boundary
very well," says Luann Becker, a geologist at the University
of California, Santa Barbara. Indeed, there are few 250 million-year-old
rocks left on Earth. Most have been recycled by our planet's
tectonic activity.
Undaunted, Becker led a NASA-funded science team to sites
in Hungary, Japan and China where such rocks still exist and
have been exposed. There they found telltale signs of a collision
between our planet and an asteroid 6 to 12 km across -- in other
words, as big or bigger than Mt. Everest.
Many paleontologists have been skeptical of the theory that an
asteroid caused the extinction. Early studies of the fossil record
suggested that the die-out happened gradually over millions of
years -- not suddenly like an impact event. But as their methods
for dating the disappearance of species has improved, estimates
of its duration have shrunk from millions of years to between
8,000 and 100,000 years. That's a blink of the eye in geological
terms.
"I think paleontologists are now coming full circle and
leading the way, saying that the extinction was extremely abrupt,"
Becker notes. "Life vanished quickly on the scale of geologic
time, and it takes something catastrophic to do that."
Such evidence is merely circumstantial -- it doesn't actually
prove anything. Becker's evidence, however, is more direct and
persuasive:
Deep inside Permian-Triassic rocks, Becker's
team found soccer ball-shaped molecules called "fullerenes"
(or "buckyballs") with traces of helium and argon gas
trapped inside. The fullerenes held an unusual number of 3He
and 36Ar
atoms -- isotopes that are more common in space than on Earth.
Something, like a comet or an asteroid, must have brought the
fullerenes to our planet.
Above: The carbon atoms in a fullerene molecule are arranged in a spherical pattern similar to a geodesic dome. (Geodesic domes were invented by Buckminster Fuller, hence the name of the molecules.) This shape allows the fullerenes to trap gases inside. Image courtesy Luann Becker.
Becker's team had previously found such gas-bearing buckyballs
in rock layers associated with two known impact events: the 65
million-year-old Cretaceous-Tertiary impact and the 1.8 billion-year-old
Sudbury impact crater in Ontario, Canada. They also found fullerenes
containing similar gases in some meteorites. Taken together,
these clues make a compelling case that a space rock struck the
Earth at the time of the Great Dying.
But was an asteroid the killer, or merely an accomplice?
Many scientists believe that life was already struggling when the putative space rock arrived. Our planet was in the throes of severe volcanism. In a region that is now called Siberia, 1.5 million cubic kilometers of lava flowed from an awesome fissure in the crust. (For comparison, Mt. St. Helens unleashed about one cubic kilometer of lava in 1980.) Such an eruption would have scorched vast expanses of land, clouded the atmosphere with dust, and released climate-altering greenhouse gases.
Above: What the world looked like 250 million years ago. Image credit: Chris Scotese. [more]
World geography was also changing then. Plate tectonics pushed the continents together to form the super-continent Pangea and the super-ocean Panthalassa. Weather patterns and ocean currents shifted, many coastlines and their shallow marine ecosystems vanished, sea levels dropped.
"If life suddenly has all these different things happen to it," Becker says, "and then you slam it with a rock the size of Mt. Everest -- boy! That's just really bad luck."
Was the "crime" then merely an accident? Perhaps so. Nevertheless, it's wise to identify the suspects -- an ongoing process -- before it happens again.
Editor's note: Becker's colleagues include Robert Poreda and Andrew Hunt from the University of Rochester, NY; Ted Bunch of the NASA Ames Research Center; and Michael Rampino of New York University and NASA's Goddard Institute of Space Sciences. Funding for the research was provided by NASA's Astrobiology and Cosmochemistry programs and the National Science Foundation.
Web LinksNASA Astrobiology Institute -- home page
What happens when a 6 to 12 km space
rock hits Earth? The
initial blast from the
impact would sweep over a large area of the planet, killing
everything in its path. It would throw up millions of tons of
debris to obscure the Sun, hindering the photosynthesis that
drives most ecosystems. The heat would release large volumes
of CO2 and SO2 from the vaporization of
rocks, and ignite the world's forests -- releasing still more
CO2 and inducing long-term global warming. As the
atmospheric debris slowly rains back down onto the surface, toxic
compounds might poison much of what life remains.
Right: Artist Don Davis's rendition of a large asteroid striking Earth.
Mounting Evidence: A research team headed by
Kunio Kaiho of the Tohoku University in Sendai, Japan, recently
announced their measurements of sulfur
isotopes in rocks from the Permian-Triassic
boundary. The P-T
boundary layer is enriched with "light sulfur" -- much
like material from our planet's mantle. The finding suggests
that some event 250,000,000 years ago (like an asteroid impact)
spread material from the mantle over the surface of the land.
"Their data suggest that an asteroid or comet hit the ocean
and caused a rapid, massive release of sulfur and other noxious
materials from the Earth's mantle to the ocean-atmosphere system,"
agrees Ted Bunch, a research scientist at NASA's Ames Research
Center and a member of Becker's science team.
Reference: "End-Permian catastrophe by a bolide impact:
Evidence of a gigantic release of sulfur from the mantle,"
K. Kaiho, et al., Geology, Sept. 2001, 29, 815.
Massive Volcanic Eruptions in Siberia Linked with Mass Extinction -- scientists present evidence linking the largest extinction of animals in Earth's history with rapid and extensive volcanic activity in Siberia
How did the formation of a supercontinent at the end of the Permian play a part in mass extinction?
Permian-Triassic extinction -- an article and video about the extinction, from the Public Broadcasting System
Theories of Causes of the PT extinction -- details about other theories on the cause of the extinction, including volcanism, a supernova, and climate changes.
Permian/Triassic life -- a listing of prominent types of life on Earth both before and after the great extinction. Includes pictures.
Buckyballs and Gases May Mark Massive Extinction -- an article about the fullerene evidence for an asteroid or comet impact
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