An incredible outpouring of lava 66 million years ago could have set off environmental changes that killed off the dinosaurs, a new study finds.
The research reports precise dates for India's Deccan Traps, mountain-high piles of basalt lava flows that cover as much territory as France. The youngest lava flows emerged 66.29 million years ago, about 250,000 years before the Chicxulub space rock crashed into eastern Mexico's Yucatan Peninsula. The findings could revive the idea that the Deccan Traps caused the Cretaceous-Paleogene extinction — a hypothesis long pushed aside in favor of the asteroid impact.
"Now we can say when the Deccan Traps started," said lead study author Blair Schoene, a geologist at Princeton University in New Jersey. "For me, it's not whether the impact or the Deccan traps caused the extinction. The point is that without a precise timeline, you can't understand what happened." [Wipe Out: History's Most Mysterious Extinctions]
The study was published today (Dec. 11) in Science Express, the online edition of the journal Science.
The dinosaurs and 75 percent of other Cretaceous species suddenly vanished after the giant asteroid slammed into the Yucatan 66.04 million years ago. The big extinction and the Chicxulub impact are separated by about 30,000 years, an instant in geologic time, according to a 2013 study. The prevailing view credits the impact for the massive die-off.
But animals and plants had already started disappearing from the fossil record before the asteroid impact, at the same time the Deccan Traps eruptions began. The Deccan Traps derive their name from the Dutch word for stairs (trap). The steplike basalt lava flows stack up nearly 9,800 feet (about 3,000 meters) high. Proponents of the volcanic extinction model argue that climate-altering volcanic gases made Earth inhospitable for many species by changing temperatures and ocean acid levels.
However, until now, no one could closely tie the volcanic activity to the Cretaceous extinctions. "The Deccan Traps had never really been well dated," said Paul Renne, director of the Berkeley Geochronology Center in California, who was not involved in the study.
Renne has linked other large lava outpourings to mass extinctions — for instance, tying huge flood basalts in Siberia to the Permian period's Great Dying, when 95 percent of all species on Earth were obliterated.
"There's really only one established case where there is an extinction coincident with an impact," Renne said. "So one could make the case that here, maybe the extinction would have happened even without the impact."
Schoene and his co-authors nailed down the age of the Deccan Traps with a painstaking search for zircons, tiny minerals that are rarely found in basalt lavas. The researchers did find a few zircons in the lava flows, but many of their samples came from volcanic ash trapped in between the basalt, in layers called red boles. The red boles are zones of intensely weathered rock and sediment sandwiched between the lava.
Tests on the zircons show that volcanic activity began 66.288 million years ago, and 80 to 90 percent of the entire volume of lava flooded out within about 750,000 years, the researchers reported. The Deccan Traps generated about 123,000 cubic miles (512,000 cubic kilometers) of lava.
With new dates from only the top and bottom of the lava flows, the researchers have not yet worked out how quickly the molten rock oozed from the region's broad volcanoes and fissures. Earlier studies indicated that the lava tap turned on and off over time, with pulses of activity. Work in progress by Renne and others suggests there was a major surge soon after the Chicxulub impact. Schoene's group will return to India in January to search for more zircons, in hopes of further correlating volcanic activity with the mass extinction.
"We don't know which of the flows correlates with the extinction," Schoene said. "There's a lot more work to be done."