K-Pg Transition Dated within 11,000 Years. Chicxulub Still Guilty

The demise of the dinosaurs is the world's ultimate whodunit. Was it a comet or asteroid impact? Volcanic eruptions? Climate change?

In an attempt to resolve the issue, scientists at the Berkeley Geochronology Center (BGC), the University of California, Berkeley, and universities in the Netherlands and the United Kingdom have now determined the most precise dates yet for the dinosaur extinction 66 million years ago and for the well-known impact that occurred around the same time.

The dates are so close, the researchers say, that they now believe the comet or asteroid, if not wholly responsible for the global extinction, at least dealt the dinosaurs their death blow.

"The impact was clearly the final straw that pushed Earth past the tipping point," said Paul Renne, BGC director and UC Berkeley professor in residence of earth and planetary science. "We have shown that these events are synchronous to within a gnat's eyebrow, and therefore the impact clearly played a major role in extinctions, but it probably wasn't just the impact."

The revised dates clear up lingering confusion over whether the impact actually occurred before or after the extinction, which was characterized by the almost overnight disappearance from the fossil record of land-based dinosaurs and many ocean creatures. The new date for the impact – 66,038,000 years ago – is the same within error limits as the date of the extinction, said Renne, making the events simultaneous.

He and his colleagues will report their findings in the Feb. 8 issue of the journal Science.

The extinction of the dinosaurs was first linked to a comet or asteroid impact in 1980 by the late UC Berkeley Nobel Laureate Luis Alvarez and his son, Walter, who is a UC Berkeley professor emeritus of earth and planetary science. A 110-mile-wide crater in the Caribbean off the Yucatan coast of Mexico is presumed to be the result of that impact. Called Chicxulub (cheek'-she-loob), the crater is thought to have been excavated by an object six miles across that threw into the atmosphere debris still found around the globe as glassy spheres or tektites, shocked quartz and a layer of iridium-enriched dust.

Renne decided last year to re-date the dinosaur extinction, which occurred at the boundary between the Cretaceous and Tertiary periods – the KT boundary – after recalibrating the 20-year-old accepted date and discovering that it now occurred 180,000 years BEFORE the impact. That earlier date was obtained in 1993 by BGC researchers using the same argon-argon method, which relies on the decay rate of a radioactive isotope of potassium.

"Everybody had always looked at the age for the KT boundary and compared it with the ages that we had gotten for the tektites and the melt rock from the Chicxulub crater and said, 'Ooh yeah, this is pretty much the same age,'" Renne said. "But they are not. They differ by 180,000 years, actually. So, from simply this esoteric calibration issue, I started to realize, 'Wow, there is a real problem here.'"

"Accurately dating the major Cretaceous-Paleogene extinction, including that of the dinosaurs, has been controversial," says H. Richard Lane, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research.

Renne and his BGC colleagues dated tektites from Haiti, analyzing them using a recalibrated argon-argon technique to determine how long ago the impact occurred. The tektite results agreed with recalibrated previous data but were more precise. They did the same for altered volcanic ash collected from the Hell Creek Formation in Montana, the source of many dinosaur fossils and one of the best sites to study the change in fossils from before and after the extinction.

The new extinction date is precise to within 11,000 years.