By Paul Rincon
BBC News Online science staff
It is unlikely the dinosaurs perished in a global firestorm triggered by the asteroid strike on Earth 65 million years ago, scientists have claimed.
Did the dinosaurs go out in a blaze of glory?
A popular theory suggests the impact, which was centred on Chicxulub in Mexico, generated enough energy to set off a raging worldwide inferno.
But a new study shows rocks laid down at the time contain little charcoal - a possible tell-tale record of fires.
The researchers have published details of their work in the journal Geology.
The wildfires theory had grown up from previous research. One study had even found evidence of soot in rocks from around the Earth dating to the time of the impact.
It is thought that in addition to the devastation these fires caused, the soot thrown up into the atmosphere as a result of the cataclysmic event may have helped block sunlight, causing global cooling and a shut-down of photosynthesis.
Plants not consumed in the inferno would have just shrivelled away - so the theory goes.
But now Claire Belcher, of Royal Holloway, University of London in Egham, has come forward with research that challenges this particular view of dinosaur Armageddon.
CHICXULUB IMPACT CRATER
Approximately 180 km across
Now buried under one km of carbonate sediments
Asteroid responsible for Chicxulub was 10 km wide
She studied six sites in a transect through the western interior of North America.
Each site dates to the end of the Cretaceous Period when the impact occurred.
Each of these sites records a geological boundary dividing the end of the Cretaceous period from the beginning of the Tertiary.
This Cretaceous-Tertiary boundary, or K-T boundary, marks the extinction of the dinosaurs and is thought to be associated with the impact of a large space object because the sedimentary rocks of this layer contain large quantities of the element iridium, which is most commonly found in meteorites.
She and her colleagues looked for traces of charcoal in these rocks, which could only have been produced by burning biomass, such as vegetation. But they found very little charcoal in these layers.
"It's significant because most people model the K-T boundary in terms of the thermal energy released," Belcher told BBC News Online. "It's often said that temperatures on the ground reached 1,000 [Celsius].
Other environmental changes may have led to the dinosaurs' extinction
"But 40% of species survived the impact. How could a small mammal survive temperatures of 1,000 [Celsius]?"
The researchers conclude that North America, close to the site of the impact, could not have been engulfed by wildfires, as some have suggested.
Belcher and her co-authors acknowledge that rocks from the K-T boundary contain soot, but argue there could be other reasons for it than wildfires.
For instance, the impact could have vaporised hydrocarbons in the rocks at Chicxulub, creating soot. The fireball that rose over the impact site could have expanded through the Earth's atmosphere, spreading soot across the globe.
Professor Wendy Wolbach of DePaul University, Chicago, US, who linked the soot with wildfires in research published 13 years ago, said she had concerns about the conclusions of the Belcher study.
"I'm not convinced that they can tell the difference between coal and charcoal. The rocks they studied are loaded with coal.
"If there's any difficulty in making that recognition, their conclusions are not sound," said Professor Wolbach.
She added that it was possible Belcher and her colleagues had expertise she did not. But Professor Wolbach pointed out that the team's procedures for identifying charcoal were not made explicit enough in their published scientific paper.
The impact rock layer contains the rare element iridium
"The new study is very exciting. If correct, it may help narrow the field. But I wouldn't be so quick to exclude the wildfire theory; it still needs to be looked at," said Kevin Pope, chief scientist at Geo Eco Arc Research in Aquasco, US.
"The problem with the wildfire theory is that it is based on computer modelling and theoretical arguments."
Pope said he favoured the theory that the asteroid strike released sulphate aerosols from impacted rocks. In the atmosphere, they would have reacted with water to form sulphuric acid clouds.
These clouds could then have expanded over most of the Earth to block out the Sun, causing global cooling and a shut down of photosynthesis.