Many scientists accept that an impact on Mexico’s Yucatán Peninsula
is to blame for the dinosaurs’ demise 65 million years ago. But as more
research is conducted on a 3,000-meter-thick lava flow in India called the Deccan
Traps, some geologists grow more convinced that the environment was already
significantly perturbed when that projectile struck, and that the impact was
simply a final straw. New dates from the flood basalts further that cause, suggesting
that Earth would not have had time to recover between eruptions.
The Deccan Traps in west-central India is one of the largest flood basalt plains
in the world, covering an area of 500,000 square kilometers. The volume of basalt
was likely several million cubic kilometers when it originally erupted (the
volume of the 1980 eruption of Mount St. Helens in Oregon was about 1 cubic
kilometer).
The eruption, which consisted of several strong pulses, occurred over about
3.5 million years, ending at the mass extinction event, says Asish Basu of the
University of Rochester in New York. The questions have always been, he says,
when exactly the traps erupted and how much material was erupted in each pulse.
To answer those questions, Vincent Courtillot and Anne-Lise Chenet, of the Laboratoire
de Paleomagnetisme at the Institut de Physique du Globe de Paris, and colleagues
have looked at magnetic minerals left behind in the upper 600 meters of the
Deccan Traps basalt. Throughout time, Earth’s polarity reverses, so that
the positive and negative magnetic poles flip between north and south. When
that happens, magnetic minerals record a datable signature. Thus, looking at
the magnetic orientation of layers of lava, the researchers could determine
dates for volcanic pulses that occurred close in time to one another, as Chenet
reported at a joint meeting of the Geological Society of America and the Geological
Association of Canada in Calgary, Alberta, on Aug. 10.
The 600-meter segment contains seven distinct volcanic pulses over a 30,000-year
span, without evidence of any long periods of volcanic inactivity during which
Earth’s environment could have recovered, Chenet says. Those pulses could
have released up to 100,000 megatons of sulfur into the atmosphere, she says.
Thus, “atmospheric injection of sulfur dioxide may have drastically disturbed”
Earth’s climatic system, Chenet says, leading the researchers to believe
that the extinction was caused by more than just the impact event.
Chenet and colleagues seem to have built a good argument, based on the establishment
of the timing of the eruptions and extinction, as well as of the duration of
the eruptions, says József Pálfy of the Hungarian Natural History
Museum in Budapest, who co-chaired the meeting session. “I think it’s
pretty likely” that the Deccan Traps “prepped Earth’s environment
and paved the way for the impact to be as powerful” as it was.
“I don’t think there is any doubt that the volcanism was causing environmental
stress,” Basu says, but “there’s also no doubt that the impact
was the primary” killer. Sankar Chatterjee of Texas Tech University in
Lubbock agrees, citing the many dinosaur bones and eggs that have been found
between layers of lava, which, he says, indicate “that the dinosaurs thrived
in the midst of this chaos, right up until we see the iridium anomaly that indicates
the impact hit.” So the eruption “couldn’t have been a proximate
or the main cause of the extinctions,” he says.
Still, “this is one of the best timing studies ever done,” Chatterjee
says, “as for a long time, we didn’t know how long the Deccan Traps
erupted.” Even so, he says, the dates need to be pinned down further.
Megan Sever
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