Scientists probing a vast crater off Mexico’s Yucatan peninsula questioned a popular theory about dinosaurs on Monday, saying the collision that formed the crater happened too far back in time to have caused their extinction by itself.
Much evidence points to the idea that an asteroid or comet gouged the Earth around 65 million years ago, triggering volcanic and climate changes that eventually wiped out the dinosaurs.
When the huge, mostly underwater crater was found off Yucatan, it seemed the perfect candidate.
“Since the early 1990s the Chicxulub crater on Yucatan, Mexico, has been hailed as the smoking gun that proves the hypothesis that an asteroid killed the dinosaurs and caused the mass extinction of many other organisms at the Cretaceous-Tertiary (K-T) boundary 65 million years ago,” the researchers wrote in this week’s issue of the Proceedings of the National Academy of Sciences.
But they said a core drilled out of the middle of the crater suggests it dates back more than 300,000 years before the K-T boundary and “thus did not cause the end-Cretaceous mass extinction as commonly believed.”
Dates don't match up precisely
The researchers, led by Gerta Keller of Princeton University and including experts from Germany, Switzerland and Mexico, studied a sample that extends 5,000 feet (1,500 meters) below the current surface, in the middle of the more than 125-mile-wide (200-kilometer-wide) crater.
Other samples have included tiny pieces of glasslike rock that could have been melted during an asteroid impact, and which seem to date to the 65-million-year point, give or take a few hundred thousand years.
But their core sample showed fossils that suggest the crater was blasted out 300,000 years before the K-T boundary. Magnetic evidence also suggests it is older than previously believed.
More than one disaster?
This finding would support an alternative theory that the dinosaurs and other forms of life were wiped out in a series of disasters that changed Earth’s climate, Keller’s team said.
They noted there are other craters dating to around this time. None is big enough to have caused world-altering changes by itself.
But the meteors or asteroids hit at the same time of a busy period of volcanic activity known as Deccan volcanism, as well as when greenhouse-type atmospheric warming and major extinctions occurred.
“The Chicxulub impact occurred at a time of massive volcanism which led to greenhouse warming,” Keller said in an interview conducted by e-mail.
A surge of carbon dioxide
The name Deccan comes from an area of what is now India where a massive amount of molten material surged up from near the Earth’s core 65 million years ago.
It would have brought vast amounts of carbon gases to Earth’s surface, causing a warming effect that would have wiped out many species of plants and animals.
“This finding suggests that the K-T boundary impact (and volcanism) may have been the straw that broke the camel’s back, rather than the catastrophic kill of a healthy thriving community,” the researchers concluded.
Now they need to find the actual crater left by whatever made this final blow. Perhaps one known as the Shiva crater in India, dating to around the same period, is the one, they suggested.
“There is evidence for a third impact, which occurred about 150,000 years after the K-T mass extinction,” Keller said.
This impact may have made it harder for plants and animals to recover from the worldwide effects of the blasts from space and from within the planet.
