December 1, 2006
WASHINGTON (Reuters) -- A single, gigantic asteroid slammed into Earth 65 million years ago, dooming the dinosaurs and many other species, scientists said on Thursday in a new study rebutting theories that multiple impacts did the deed.
An examination of rock sediments drilled from five sites at the bottom of the Atlantic Ocean strongly supports the notion that one massive hunk of space rock caused the mass extinction, a research team led by University of Missouri-Columbia geology professor Ken MacLeod found.
"It's a completely straightforward, single-impact scenario," MacLeod, whose findings appear in the Geological Society of America Bulletin, said in an interview. "It was a haymaker that nobody saw coming. One shot, and that's all you need to explain it."
Scientists believe that an asteroid about 6 miles wide hurtled to Earth 65.5 million years ago at the end of the Cretaceous Period, plunging into what is now Mexico's Yucatan Peninsula to carve out the Chicxulub crater measuring about 110 miles across.
To put it mildly, it was a bad day to live on Earth.
The impact triggered a worldwide environmental catastrophe, many scientists believe, expelling vast quantities of rock and dust into the sky, unleashing giant tsunamis, sparking global wildfires and leaving Earth shrouded in darkness for years.
The dinosaurs, which had ruled for 160 million years, were wiped out. So were large marine reptiles like the mosasaurs and the plesiosaurs, the flying reptiles known as pterosaurs, the tentacled ammonites that populated the seas and many species of marine plankton. The birds suffered losses but survived.
The mammals made it through as well, allowing these warm-blooded, furry little creatures to eventually dominate the land and ultimately setting the stage for the rise of human beings.
Evidence of the single-asteroid calamity, in the form of debris from the impact scattered worldwide, is contained in rocks dating back to 65 million years ago.
Scientists 26 years ago found a band of iridium -- a metal rare on Earth but common in meteorites -- dating to the end of the Cretaceous Period that suggested a big space rock had smashed into the Earth and blasted its remains around the globe. The subsequent discovery of the Chicxulub crater, dating to the same time, was hailed by many as the smoking gun.
But a group of researchers led by Princeton University's Gerta Keller has advanced a competing theory that the impact that created the Chicxulub crater actually predated the end of the dinosaurs by 300,000 years and did not cause the mass extinction.
They propose that one or more additional big hunks of space rock later hit the Earth and finished the job, but the impact craters they would have left behind have not yet been found.
MacLeod's team examined sediment drilled far below the sea surface about 2,800 miles from the Yucatan impact site, a location they believed to be ideal.
Any rock samples taken too close to the crater may be altered by events that occurred immediately after the impact, like waves, earthquakes and landslides. Samples taken too far away may contain too little debris evidence from the impact.
The samples they examined boasted a telltale layer of impact-related material, but there was none on top or below -- indicating, they argued, there were no other impacts.
Keller said MacLeod's research does not settle the matter.
"Unfortunately, these claims are rather hyper-inflated and do not withstand close examination," Keller said by e-mail.