The biggest extinction in history was probably caused by a space rock that changed the climate
AS EVERY schoolchild knows, the
dinosaurs were wiped out in an instant,
when a rock from outer space hit what
is now southern Mexico. That
happened 66m years ago, at the end of
the Cretaceous period. Well-informed
schoolchildren also know that this
mass extinction was neither unique nor
the biggest. The geological record
speaks of four others since animal life
became complex at the beginning of
the Cambrian period 541m years ago.
What neither these clever
schoolchildren nor anyone else knows,
however, is whether these extinctions
had similar causes. But evidence is
accumulating that the biggest
extinction of all, 252.3m years back, at
the end of the Permian period, was
indeed also triggered by an impact.
Nevertheless, though the trigger was
the same, the details are significantly
different, according to Eric Tohver of
the University of Western Australia.When the dinosaurs vanished they were
accompanied by more than 70% of the
other animal species on Earth. At the
end of the Permian, the extinction
figure was more than 80%. And just as
the Cretaceous slate-clearing permitted
the rise of a hitherto obscure group
called the mammals (including,
eventually, one now referred to by
biologists as Homo sapiens ), so the
Permian clearance permitted the rise of
the reptiles, one branch of which
turned into Tyrannosaurus, Diplodocus
and all the other names familiar from
childhood.
The idea that an impact caused the
Permian extinction has been around
for a while. As at the end of the
Permian, as at the end of the
Cretaceous, huge volcanic eruptions
had been going on for hundreds of
thousands of years. These may have
weakened the world’s ecosystems,
making them vulnerable to an external
shock. But the abruptness of both
extinctions indicates that the coup de
grâce was administered by something
else, and in the case of the Permian
some fragments of meteorite of the
correct age, found in rock in
Antarctica, suggest that, as with the
Cretaceous, that something was an
asteroid or a comet. What was missing
from the story, though, was a suitable
crater.
Fracking hell!
Last year Dr Tohver and his colleagues
thought they might have found it. They
redated a hole that straddles the
border of the states of Mato Grosso
and Goiás in Brazil, called the
Araguainha crater, to 254.7m years,
with a margin of error of plus or minus
2.5m years. Previous estimates had
suggested Araguainha was 10m years
younger, but Dr Tohver has put it
within geological spitting distance of
the extinction date, which itself has a
margin of error of plus or minus
200,000 years.
Which would all be fine and dandy,
except most people think Araguainha is
too small to be the culprit. It is a mere
40km (25 miles) across. The Chicxulub
crater in Mexico, which did for the
dinosaurs, is 180km in diameter, and it
may have been paired with an even
bigger impact in the Indian Ocean.
(This could have happened if the
incoming object was a comet that
broke up in a close encounter with the
sun.)
Dr Tohver, however, has an answer to
this criticism. His latest paper, just
published in Palaeogeography,
Palaeoclimatology, Palaeoecology ,
describes the rock in the area in which
Araguainha resides.
After an extensive geological survey, he
and his team discovered that a sizeable
amount of this rock is oil shale. Any
hydrocarbons in the crater would
certainly have been vaporised. More
intriguingly, the researchers calculate
that the impact would have generated
thousands of earthquakes of up to
magnitude 9.9 (significantly more
powerful than the largest recorded by
modern seismologists) for hundreds of
kilometres around. In effect, it would
have been the biggest fracking
operation in history, releasing oil and
gas from the shattered rock in
prodigious quantities.
The upshot, Dr Tohver believes, would
have been a huge burp of methane into
the atmosphere. Since methane is a
powerful greenhouse gas, that burp
would have resulted in instant global
warming, making things too hot for
much of the planet’s animal life. Presto!
The Permian mass extinction is
explained.
Determining whether this was really
what happened will take a lot more
digging, of course. Even now, there are
those who think the formation of the
Chicxulub crater was a coincidence,
and that what did for the dinosaurs
was actually the volcanoes, so Dr
Tohver will have to work hard to
convince the sceptics. If he does,
though, he will have proved himself a
great geological detective, for he will
have been responsible for solving one
of the biggest puzzles in palaeontology.
