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, an international team of scientists have 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 new extinction date is precise to within 11,000 years.

In a paper published today in the journal Science, researchers from the Berkeley Geochronology Center (BGC), the University of California, Berkeley, the Scottish Universities Environmental Research Centre (University of Glasgow) and Vrije University, conclude that the dates of the events are so close that it’s likely the impact of the comet or asteroid, if not wholly responsible for the global extinction, at least dealt the death blow to the dinosaurs.

“The impact was clearly the final straw that provided the tipping point,” said Paul Renne, BGC director and professor in residence of earth and planetary science at the University of California, Berkeley. “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.

At the Scottish Universities Environmental Research Centre in East Kilbride, a team led by Dr Darren Mark conducted independent argon-argon analyses which confirmed the results from the US. Argon-argon dating uses a mass spectrometer to measure the ratio of radioactive potassium in a sample of rock to its decay product, argon. The amount of argon in a sample allows researchers to use rocks as incredibly slow clocks.

Dr Mark said:  “This study shows the power of high precision geochronology. Many people think precision is just about adding another decimal place to a number, but it's far more exciting than that. It's more like getting a sharper lens on a camera. It allows us to dissect the geological record at greater resolution and piece together the sequence of Earth history.

“We’re proud to have contributed to this important study. It’s another success for SUERC which has recently re-dated the super-eruption deposits of Yellowstone showing the super-eruption deposit was a product of multiple eruptions.”

The extinction of the dinosaurs was first linked to a comet or asteroid impact in 1980 by the late UC Berkeley professors Luis and Walter Alvarez. 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, 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.

The international team 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 argon-argon method.

Renne and his BGC colleagues dated tektites from Haiti 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.

Despite the synchronous impact and extinction, Renne cautions that this doesn’t mean that the impact was the sole cause. Dramatic climate variation over the previous million years, including long cold snaps amidst a general Cretaceous hothouse environment, probably brought many creatures to the brink of extinction and the impact kicked them over the edge.

One cause of the climate variability could have been a sustained series of volcanic eruptions in India that produced the extensive Deccan Traps. Renne and Mark plan to re-date those volcanic rocks to get a more precise measure of their duration and onset relative to the dinosaur extinction.

Renne and his colleagues also dated rocks above the KT boundary where previous researchers had looked at carbon isotopes, and concluded that Earth’s atmospheric carbon cycle returned to normal within about 5,000 years of the impact. This is in stark contrast to the world’s oceans, which studies show took between 1 and 2 million years to return to normal. Renne attributes this to a sluggish recovery of pre-impact ocean circulation patterns, though he concedes that this remains poorly understood.

The study’s results also clarify some inconsistencies between different estimates for the age of the KT boundary based on Earth’s orbital rhythms recorded in sedimentary rocks. The new independent results agree within the margins of error with an age of 65,957,000 years determined using this approach by Dutch colleagues Frederik J Hilgen of Utrecht University and Klaudia F. Kuiper of Vrije University.

Renne’s colleagues, in addition to Mark, Hilgen and Kuiper, are UC Berkeley graduate student William S. Mitchell III; BGC scientists Alan L. Deino and Roland Mundil; Leah E. Morgan of the Scottish Universities Environmental Research Center; and Jan Smit of Vrije University in Amsterdam.

The work was supported by the Ann and Gordon Getty Foundation, UC Berkeley’s Esper S. Larsen Jr. Fund and National Science Foundation.

The Scottish Universities Research Centre’s Argon Isotope Facility is funded by the Natural Environment Research Council (NERC).

For more information contact Ross Barker in the University of Glasgow Media Relations Office on 0141 330 8593 or email ross.barker@glasgow.ac.uk or Robert Sanders, UC Berkeley media relations, on (510) 643 6998 / rsanders@berkeley.edu

First published: 7 February 2013

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