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Currents--University of Rochester newspaper

Team unearths 'bookend' to dinosaur age


The same kind of cosmic collision that helped wipe out dinosaurs also may have given the giant reptiles their first foothold on Earth by triggering what scientists call the "Great Dying" of 251 million years ago, according to a new study co-authored by University researchers.

The study, published in the journal Science, indicates that an asteroid or comet smacked into Earth 251 million years ago, sparking the biggest extinction in Earth history--a time when more than 90 percent of marine animals and about 70 percent of land vertebrates were wiped out.

Estimated to be anywhere from six to 12 kilometers, the asteroid or comet, known as a "bolide," would have released unimaginable fury upon impact, says Robert Poreda, associate professor of earth and environmental sciences and one of the authors.

"The impact of a bolide of this size releases an amount of energy that is basically about 1 million times the largest earthquake recorded during the last century," Poreda says. "It was like a magnitude 12.0 earthquake on the Richter scale."

Poreda, joined by scientists from the University of Washington, NASA, and New York University, published their findings in the February 23 issue of Science.

Their evidence for the impact comes in the form of cosmic stowaways--helium and argon molecules formed elsewhere in the universe that survived a journey through space and crashed into Earth as part of the impact.

By making sensitive measurements of different forms of the gases, the scientists determined that the ratios of helium and argon molecules are characteristic of meteorites and comets and must have been formed in space.

The gas measurements were made by Poreda and Rochester postdoctoral associate Andrew Hunt, who were part of a team headed by Luann Becker of the University of Washington.

The impact and rapid extinction occurred simultaneously with some of the most extensive volcanic activity the world has ever seen. More than 1.6 million cubic kilometers of lava, enough to cover the entire planet with 10 feet of lava had it spread evenly around the globe, oozed out of the ground in Siberia in a relatively short time, less than 1 million years.

"It was the proverbial blast from the double-barreled shotgun," Poreda says. "We're not sure of all the environmental consequences, but with both the impact and with the volcanic activity, we do know that Earth was not a happy place. It may be that the combined effects of impact and volcanism are necessary to cause such a tremendous extinction."

Several scientists have suggested that extensive volcanic activity could contribute to a large extinction by releasing tons of sediment and ash, as well as massive amounts of carbon dioxide into the atmosphere.

The coupling of a massive impact and widespread volcanic activity also occurred in the more widely publicized extinction event that wiped out the dinosaurs 65 million years ago, a period known as the K/T (Cretaceous/Tertiary) boundary. Scientists generally agree that an impact on Mexico's Yucatan Peninsula played a role in that extinction.

At the same time, there was a dramatic outpouring of lava in present-day India.

"These two extinctions are like bookends for the age of the dinosaurs," Poreda says. "The P/T boundary helped to usher in the age of the dinosaurs, and the K/T boundary snuffed it out."

The project was funded by NASA and the National Science Foundation. Also contributing to the research were Theodore Bunch of NASA Ames Research Center in California and Michael Rampino of New York University.

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