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Geomagnetism
Volcanic prepping for dinosaur extinction

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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