Image: Christine Daniloff |
Since
the first organisms appeared on Earth approximately 3.8 billion years ago, life
on the planet has had some close calls. In the last 500 million years, Earth
has undergone five mass extinctions, including the event 66 million years ago
that wiped out the dinosaurs. And while most scientists agree that a giant
asteroid was responsible for that extinction, there’s much less consensus on
what caused an even more devastating extinction more than 185 million years
earlier.
The
end-Permian extinction occurred 252.2 million years ago, decimating 90% of
marine and terrestrial species, from snails and small crustaceans to early
forms of lizards and amphibians. “The Great Dying,” as it’s now known, was the
most severe mass extinction in Earth’s history, and is probably the closest
life has come to being completely extinguished. Possible causes include immense
volcanic eruptions, rapid depletion of oxygen in the oceans, and—an unlikely
option—an asteroid collision.
While
the causes of this global catastrophe are unknown, a Massachusetts Institute of
Technology (MIT)-led team of researchers has now established that the
end-Permian extinction was extremely rapid, triggering massive die-outs both in
the oceans and on land in less than 20,000 years—the blink of an eye in
geologic time. The researchers also found that this time period coincides with
a massive buildup of atmospheric carbon dioxide, which likely triggered the
simultaneous collapse of species in the oceans and on land.
With
further calculations, the group found that the average rate at which carbon
dioxide entered the atmosphere during the end-Permian extinction was slightly
below today’s rate of carbon dioxide release into the atmosphere due to fossil
fuel emissions. Over tens of thousands of years, increases in atmospheric carbon
dioxide during the Permian period likely triggered severe global warming, accelerating
species extinctions.
The
researchers also discovered evidence of simultaneous and widespread wildfires
that may have added to end-Permian global warming, triggering what they deem “catastrophic” soil erosion and making environments extremely arid and
inhospitable.
The
researchers presented their findings in Science, and say the new
timescale may help scientists home in on the end-Permian extinction’s likely
causes.
“People
have never known how long extinctions lasted,” says Sam Bowring, the Robert R.
Schrock Professor of Earth, Atmospheric and Planetary Sciences (EAPS) at MIT. “Many people think maybe millions of years, but this is tens of thousands of
years. There’s a lot of controversy about what caused [the end-Permian
extinction], but whatever caused it, this is a fundamental constraint on it. It
had to have been something that happened very quickly.”
Rocks in a hard place
Bowring worked with a group of American and Chinese researchers to pinpoint the
extinction’s duration. The group analyzed volcanic ash beds from Meishan, a
region in southern China
where an old limestone quarry exposes rocks containing abundant fossils from
the Permian period, as well as the very first fossils that signified a recovery
from extinction, during the Triassic period. The rocks of the region have been
widely studied as the best global example of the Permian-Triassic Boundary
(PTB).
The
group collected clay samples from ash beds both above and below rock layers
from the PTB. In the laboratory, they separated out zircon, a robust mineral
that can survive intense geological processes. Zircon contains trace amounts of
uranium, which can be used to date the rocks in which it is found. Bowring and
his colleagues analyzed 300 of the “best-looking” grains of zircon, and found
the rocks above and below the mass-extinction period spanned only a 20,000-year
phase.
Bowring
says now that researchers are able to precisely date the end-Permian
extinction, scientists will have to re-examine old theories. For example, many
believe the extinction may have been triggered by large volcanic eruptions in
Siberia that covered 2 million square kilometers of Earth—an area roughly three
times the size of Texas.
“In
the old days you could say, ‘Oh, it’s about the same time, therefore it’s cause
and effect,'” Bowring says. “But now that we can date [the extinction] to plus
or minus 20,000 years, you can’t just say ‘about the same.’ You have to
demonstrate it’s exactly the same.”
‘Something unusual going on’
The group also analyzed carbon-isotope data from rocks in southern China and found
that within the same period, the oceans and atmosphere experienced a large
influx of carbon dioxide. Dan Rothman, a professor of geophysics in EAPS,
calculated the average rate at which carbon dioxide entered the oceans and
atmosphere at the time, finding it to be somewhat less than today’s influx due
to fossil fuel emissions.
“The
rate of injection of carbon dioxide into the late Permian system is probably
similar to the anthropogenic rate of injection of carbon dioxide now,” Rothman
says. “It’s just that it went on for … 10,000 years.”
Rothman
says the total amount of carbon dioxide pumped into Earth over this time period
was so immense that it’s not immediately clear where it all came from.
“It’s just not easy to imagine,” Rothman says. “Even if you put all the
world’s known coal deposits on top of a volcano, you still wouldn’t come close.
So something unusual was going on.”
