Dinosaur extinction link
to crater confirmed
Science reporter, BBC News, The Woodlands, Texas
The dinosaurs were one of many groups to go extinct
An international panel of experts has strongly endorsed evidence that a space impact was behind the mass extinction event that killed off the dinosaurs.
They reached the consensus after conducting the most wide-ranging analysis yet of the evidence.
Writing in Science journal, they rule out alternative theories such as large-scale volcanism.
The analysis has been discussed at the 41st Lunar and Planetary Science Conference (LPSC) in the US.
A panel of 41 international experts reviewed 20 years' worth of research to determine the cause of the Cretaceous-Tertiary (K-T) mass extinction, around 65 million years ago.
The extinction wiped out more than half of all species on the planet, including the dinosaurs, bird-like pterosaurs and large marine reptiles, clearing the way for mammals to become the dominant species on Earth.
Their review of the evidence shows that the extinction was caused by a massive asteroid or comet smashing into Earth at Chicxulub on Mexico's Yucatan Peninsula.
When the 10km-15km space rock struck the Yucatan, the explosive energy released was equivalent to 100 trillion tonnes of TNT - over a billion times more explosive than the bombs dropped on Hiroshima and Nagasaki.
The huge crater that remains from the event is some 180km in diameter and surrounded by a circular fault about 240km in diameter.
"You can actually trace debris right up to the rim of the crater from across the world," Co-author Dr David Kring, from the Lunar and Planetary Institute in Houston, told BBC News.
"You can start in Europe, cross the Atlantic and it just thickens as you approach the Chicxulub impact crater."
In the new study, scientists examined the work of palaeontologists, geochemists, climate modellers, geophysicists and sedimentologists who have been gathering evidence about the K-T extinction.
They conclude that the Chicxulub space impact is the only plausible explanation for the devastation evident in geological records.
The initial impact would have triggered large-scale fires, huge earthquakes, and continental landslides which generated tsunamis.
Dr Gareth Collins, one of the review's co-authors from Imperial College London, said the asteroid hit Earth "20 times faster than a speeding bullet".
He added: "The explosion of hot rock and gas would have looked like a huge ball of fire on the horizon, grilling any living creature in the immediate vicinity that couldn't find shelter."
Dr Joanna Morgan, another co-author from Imperial, commented: "The final nail in the coffin for the dinosaurs happened when blasted material was ejected at high velocity into the atmosphere. This shrouded the planet in darkness and caused a global winter, killing off many species that couldn't adapt to this hellish environment."
The ejected debris (white) can be seen in rocks from 65 million years ago
The review confirms that a unique layer of debris ejected from a crater is compositionally linked to the Mexican crater and is also coincident with rocks associated at the Cretaceous-Tertiary (K-T) boundary.
The team also says that an abundance of shocked quartz in rock layers across the world at the K-T boundary lends further weight to conclusions that a massive meteorite impact happened at the time of the mass extinction. This form of the mineral occurs when rocks have been hit very quickly by a massive force. It is only found at nuclear explosion sites and at asteroid impact sites.
"Combining all available data from different science disciplines led us to conclude that a large asteroid impact 65 million years ago in modern day Mexico was the major cause of the mass extinctions," said author Dr Peter Schulte, assistant professor at the University of Erlangen in Germany.
David Kring explained: "I have been invited to give colloquia at a number of universities across North America and I had always been surprised by the number of people who didn't think the connection was as firm as it was.
"I think it was very important for this distinguished panel of experts from around the world who have seen the evidence from their own geographic quarter to debate the issue and come to a final resolution. I think it is that international consensus that is so important in this case."
Peninsula, but weaknesses in the overlying rock have
produced a ring of slumping that is visible from space
Scientists have previously argued about whether the extinction was caused by a space impact or by volcanic activity in the Deccan Traps in India, where there were a series of super-volcanic eruptions that lasted approximately 1.5 million years.
These eruptions spewed more than 1,000,000 cu km of basaltic lava across the Deccan Traps - enough to fill the Black Sea twice. These were thought to have caused a cooling of the atmosphere and acid rain on a global scale.
Despite evidence for relatively active volcanism in the Deccan Traps at the time, marine and land ecosystems showed only minor changes within the 500,000 years before the time of the K-T mass extinction.
Furthermore, computer models and observational data suggest the release of gases such as sulphur into the atmosphere after each volcanic eruption in the Deccan Traps would have had a short-lived effect on the planet.
The panel also discounted previous studies that suggested the Chicxulub impact occurred 300,000 years prior to the mass extinction event.
Scientists estimate that this type of impact occurs on average about once every 100 million years; about five have occurred during the evolution of complex life on Earth.
The importance of Chicxulub was cemented by the announcement in 1991 of the discovery of shocked quartz in a 1.6km-deep drill hole from the crater.
David Kring, Alan Hildebrand and William Boynton presented their results at that year's LPSC, then held at Nasa's Johnson Space Center in Houston.
Dr Kring explained that he was "elated" with the consensus about the link between Chicxulub and the K-T mass extinction.
BBC SCIENCE BLOG:
It's official: It really was an asteroid, and not massive volcanic activity, that wiped out the dinosaurs (and more than half the other species on earth) at the Cretaceous-Tertiary boundary some 65 million years ago.
That's the conclusion of a painstaking review of all the available evidence by a panel of 41 international experts and published today in the journal Science.
But you knew that already, right? So what's all the fuss about? And, more importantly, why do we need a panel of experts to spend so much time and money exhaustively reviewing the evidence and producing a ruling on an issue that has long since been laid to rest?
Well (with apologies to Donald Rumsfeld), the fact is that we suspected we knew, but we didn't know that we knew for sure.
The problem is that the asteroid extinction theory is such a powerful explanation, such a catchy story, that it passed into the common narrative of the dinosaurs' demise long before the evidence to substantiate it had been consolidated.
But the asteroid theory is not the only explanation for the Cretaceous-Tertiary mass extinction.
Behind the scenes a dedicated band of geologists, geochemists and palaeontologists have argued that a series of massive volcanic eruptions across the Deccan Traps in India could well have been the real culprit.
These eruptions, spread over more than a million years, spewed billions of tonnes of lava - enough to fill the Black Sea twice over - across the earth's surface, blackening the sky with clouds of ash and triggering acid rain on a global scale.
It all comes down to a question of timing, and of interpretation of the geological record.
The supporters of the Deccan Traps theory argue that the asteroid impact at Chicxulub in Mexico, actually occurred some 300 000 years before the KT extinction boundary.
If that's true then the devastation it caused can only have been a contributory factor in the demise of the dinosaurs - just one of many smoking guns.
But the review team, lead by Peter Schulte at the University of Erlangen-Nurnberg in Germany, has discounted the idea that there was a significant gap between the Chicxulub impact and the rapid disappearance of fossils from the geological record, which they claim is based on a misinterpretation of the rock strata around the impact site.
The strongest evidence for the impact extinction theory has always been the layer of iridium deposits in geological samples dating from the KT boundary.
Iridium is very rare in the earth's crust, but a common component in asteroids. The latest research shows the decline in fossil abundance and species variety correlates very closely with the iridium layer, indicating that the extinction event followed immediately after the asteroid impact.
According to Dr Joanna Morgan from the Department of Earth Sciences at Imperial College London and a co-author on the review.
"We now have great confidence that an asteroid was the cause of the KT extinction," she says.
The impact would have triggered large-scale wildfires, massive earthquakes, and continental landslides which in turn created huge tsunamis, she argues, but the final nail in the dinosaurs coffin would have been the huge volume of material blasted into the atmosphere at high velocity.
"This shrouded the planet in darkness and caused a global winter, killing off many species that couldn't adapt to this hellish environment".
It does seem to be the end for one of science's great known unknowns.