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 new study.
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
180km-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
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 9.6km-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
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.