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Dinosaur extinction caused by asteroid: study
A new study strengthens the claim that an asteroid impact ended the reign of the dinosaurs 65 million years ago.

The finding by an international team of 41 researchers is published today in the journal Science.

Fossil records clearly show a mass extinction event across the planet 65 million years ago, in which 70% of known species suddenly vanished.

The change is so dramatic, geologists use it to define the boundary, between the end of the Cretaceous and the start of the Palaeogene periods, as the K-T boundary event.

The idea that an asteroid was responsible for the end of the age of the dinosaurs was first proposed 30 years.

The first clue was the discovery of large amounts of the element iridium - rare on Earth, but common in meteorites and asteroids - appearing in a layer across the globe at the time of the K-T boundary event.
Smoking gun

In 1991, the discovery of a 200-kilometre wide impact crater at Chicxulub on Mexico's Yucatan Peninsula provided what many scientists believe is the 'smoking gun' supporting the asteroid impact theory.

But some scientists challenge this hypothesis, suggesting deposits of tiny glass-like blobs of melted impact material around Chicxulub, predate the extinction event by 300,000 years.

They suggest the Deccan Traps, unusually active volcanoes in what is now India, led to global cooling and acid rain, which caused the mass extinction.
Geologic record

In this latest study, researchers examined the sedimentary material around the impact site and found it was far too violently churned up to provide a reliable dating record.

They found that farther away from the impact site, the sedimentary material becomes a single layer at the K-T boundary, matching the composition of rocks at Chicxulub.

They also determined that despite evidence for relatively active volcanism in India, ecosystems showed only minor changes within the 500,000 years before the K-T boundary event.

Only at the boundary do things suddenly change.

They say the Deccan hypothesis is further weakened by atmospheric chemistry models, which show that while significant volumes of sulphur were emitted during each volcanic eruption, forming aerosols in the stratosphere, they dropped rapidly with only short-lasting environmental effects.

The researchers say the Chicxulub impact saw much larger volumes of sulphur, dust and soot released in a much shorter time frame, leading to extreme environmental changes such as darkening and cooling.
More questions

Palaeontologist Dr Steven Salisbury of the University of Queensland says extinction events are complicated.

"My own work in Antarctica shows little change at the K-T boundary," he says. "It's as if the asteroid impact came and went and didnít really change much."

Salisburyís research shows species numbers declined thousands of years before and after the K-T boundary event.

"That indicates a wider combination of factors contributed to the extinction rather than a single event," he says. "To claim otherwise is too big a bow to draw."

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