A new study may have "pardoned" the giant asteroid that crashed into the Gulf of Mexico some 66 million years ago as the chief suspect in the extinction of dinosaurs, with researchers suggesting a giant flood of volcanic lava should take its place in the dock.

The study has put precise dates on the age of the Deccan Traps, mountain-high outpourings of lava in India that covered an area the size of France and which are now known to have occurred some 250,000 years before the Chicxulub space rock smashed into Mexico's eastern Yucatan Peninsula.

The dating supports the theory held by many scientists that the Cretaceous-Paleogene extinction that saw off the dinosaurs was the result of volcanic gases that changed the global climate, making the Earth inhospitable to 75 percent of the species inhabiting our planet at the time.

"If models of volatile release are correct, we're talking about something similar to what's happening today: lots of carbon dioxide being emitted into the atmosphere very rapidly," says MIT graduate student Michael Eddy, co-author of the study. "Ultimately what that can do is lead to ocean acidification, killing a significant portion of plankton -- the base of the food chain. If you wipe them out, then you'd have catastrophic effects."

While the asteroid impact has long been the leading theory for the extinctions, the fossil record shows many plants and animals vanished from the Earth before that impact, just at the time of the earlier Deccan Traps eruptions that piled up lava nearly 10,000 feet deep in some places.

A precise dating of the rocks of the Traps shows the eruption began 250,000 years before the asteroid strike and continued for 500,000 years after the giant impact, pouring out a total of almost a half million square miles of lava, the researchers report in the journal Science.

Such massive volcanic eruptions were involved in at least two other mass extinctions, the end-Permian and the end-Triassic, the researchers point out.

Still, there is considerable evidence in the geologic record to keep the asteroid impact scenario alive, they acknowledge.

"I don't think the debate will ever go away," says study co-author Sam Bowring, an MIT professor of Earth and Planetary Science. "The [asteroid] impact may have caused the extinction. But perhaps its effect was enhanced because things were softened up a bit by the eruption of these volcanoes."

Bringing the major eruptions in India into the era of the end-Cretaceous extinction by exact dating suggests that possibility, he says.

"The story that is emerging is that perhaps both might have been involved," Bowring says. "Perhaps the end of the dinosaurs was caused by a one-two punch."

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