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Scientists conclude asteroid ended the age of the dinosaurs

Fairbanks, Alaska-University of Alaska Fairbanks scientist Michael Whalen is part of a team of distinguished scientists who recently compiled a wide swath of evidence striking a definitive blow in the ongoing battle over what killed the dinosaurs.

In a review published in the March 5 issue of the journal Science, the research group reaffirmed the recently challenged theory that an asteroid ended the age of the dinosaurs.

Scientists first proposed the asteroid impact theory of dinosaur mass extinction 30 years ago. The discovery of a massive crater at Chicxulub [CHICK-shuh-loob], in Mexico's Yucat√°n Peninsula in 1991, strengthened that hypothesis. The Chicxulub crater is more than 120 miles wide-about the distance from Fairbanks to the Arctic Circle-and scientists believe it was created when an asteroid more than six miles wide crashed into Earth 65 million years ago. The cataclysmic impact-a million times more powerful than the largest nuclear bomb ever tested-triggered massive earthquakes, atmospheric discharge and oceanic upheaval. The ensuing mass extinction ended both the reign of the dinosaursand the Cretaceous period, which gave way to the Paleogene period. This theory, having steadily accumulated evidence, was thought to be a near-consensus view.

Recently, however, in a series of articles, researchers posed an alternate hypothesis for the mass extinction. Some scientists claim that long-term volcanic activity at the Deccan Traps, in what is now India, caused acid rain and global cooling, gradually making life untenable for the dinosaurs and other large animals. They also suggest that the Chicxulub impact occurred some 300,000 years before the mass extinctions.

The alternate hypothesis spurred Whalen and other Chicxulub impact proponents to respond. The current Science article dispels the Deccan Traps hypothesis, arguing that the geological record favors the Chicxulub impact event theory.

"It's as tight a case for a synchronous chain of events as we can find in the fossil record," Whalen said.

Whalen is an associate professor at the UAF geology and geophysics department and the Geophysical Institute. He first began studying the Chicxulubsite in 2002. His expertise is in carbonate rock, or limestone-ahandy specialty, as limestone forms the layers above the Cretaceous-Paleogene geological boundary in the Chicxulub crater. He studied a 2001 corefrom the crater and compared it to seismic data gathered in 2006. His analysis offered insight on the geography of the area prior to impact, how it changed during the impact and the eventual infill of the crater by limestones deposited after the impact event.

ON THE WEB: The full text of the paper is available at http://www.sciencemag.org/cgi/reprint/327/5970/1214.pdf.

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