* Asteroid impact preceded extinction by 33,000 years
* Previous dating techniques flawed
By Irene Klotz
CAPE CANAVERAL, Fla., Feb. 7 (Reuters) - Dinosaurs died off about 33,000 years after an asteroid hit the Earth, much sooner than scientists had believed, and the asteroid may not have been the sole cause of extinction, according to a study released Thursday.
Earth's climate may have been at a tipping point when a massive asteroid smashed into what is now Mexico's Yucatan Peninsula and triggered cooling temperatures that wiped out the dinosaurs, researchers said.
The time between the asteroid's arrival, marked by a 110-mile-(180-km-)wide crater near Chicxulub, Mexico, and the dinosaurs' demise was believed to be as long as 300,000 years.
The study, based on high-precision radiometric dating techniques, said the events occurred within 33,000 years of each other.
Other scientists had questioned whether dinosaurs died before the asteroid impact.
"Our work basically puts a nail in that coffin," geologist Paul Renne of the University of California Berkeley said.
The theory that the dinosaurs' extinction about 66 million years ago was linked to an asteroid impact was first proposed in 1980. The biggest piece of evidence was the so-called Chicxulub (pronounced "cheek'-she-loob") crater off the Yucatan coast in Mexico.
It is believed to have been formed by a six-mile-(9.6-km-) wide object that melted rock as it slammed into the ground, filling the atmosphere with debris that eventually rained down on the planet. Glassy spheres known as tektites, shocked quartz and a layer of iridium-rich dust are still found around the world today.
Renne and colleagues reanalyzed both the dinosaur extinction date and the crater formation event and found they occurred within a much tighter window in time than previously known. The study looked at tektites from Haiti, tied to the asteroid impact site, and volcanic ash from the Hell Creek Formation in Montana, a source of many dinosaur fossils.
NEW DATING TECHNIQUE
"The previous data that we had ... actually said that they (the tektites and the ash) were different in age, that they differed by about 180,000 years and that the extinction happened before the impact, which would totally preclude there being a causal relationship," said Renne, who studies ties between mass extinctions and volcanism.
He and colleagues were comparing a new technique to date geologic events when they realized there was a discrepancy in the timing - the so-called 'K-T boundary' - the geological span of time between the Cretaceous and Paleocene periods when the dinosaurs and most other life on Earth died out.
"I realized there was a lot of room for improvement. Even though many people had locked in their opinions that the impact and the extinctions were synchronous or not, they were basically ignoring the existing data," Renne said.
The study, published in Science, resolves existing uncertainty about the relative timing of the events, notes Heiko Pälike of the Center for Marine Environmental Sciences at the University of Bremen, Germany.
Renne, for one, does not believe the asteroid impact was the sole reason for the dinosaurs' demise. He says ecosystems already were in a state of deterioration due to a major volcanic eruption in India when the asteroid struck.
The asteroid strike "provided the coup-de-grace for the final extinctions," Renne said, adding that the theory was speculative, but backed by previous ties between mass extinction events and volcanic eruptions.
About 1 million years before the impact, Earth experienced six abrupt shifts in temperature of more than 2 degrees in continental mean annual temperatures, according to research cited by Renne and his co-authors.
The temperature swings include one shift of 6 to 8 degrees that happened about 100,000 years before the extinction.
"The brief cold snaps in the latest Cretaceous, though not necessarily of extraordinary magnitude, were particularly stressful to a global ecosystem that was well adapted to the long-lived preceding Cretaceous hothouse climate. The Chicxulub impact then provided a decisive blow to ecosystems," Renne and his co-authors wrote in Science. (Editing by Tom Brown and Stacey Joyce)