Dino heatwave recorded in leaves

Fresh evidence to show an impact from space lay behind the demise of the dinosaurs has been published by scientists.

The only thing that can explain such a large and sudden jump in CO2 would be this idea of a space impact

Prof David Beerling, Sheffield University

The researchers say analysis of fossil leaves from 65 million years ago shows there was a sudden and dramatic rise in carbon dioxide in the Earth's atmosphere.

Only the impact of a large asteroid, vaporising billions of tonnes of limestone rocks, could have released so much gas so quickly into the environment, they believe.

Their calculations suggest the change in CO2 levels would have led to catastrophic global warming, making it impossible for the ancient reptiles and countless other lifeforms to continue.

Competing theories

Scientists have long suspected that an extraterrestrial impact played a major role in the mass extinction of creatures recorded in rocks at the end of the Cretaceous Period.

An impactor from space may have led to dinosaur extinction

They have even identified what they think is the 100-kilometre-wide crater left by the space object, at Chicxulub on the Yucatan Peninsula.

But suspicions still remain that a vast "flood" of lava and gas in India at about the same time may have been the more decisive factor in "poisoning" the planet's biology.

Now, researchers from the University of Sheffield, UK, and Southwest Texas State University and Pennsylvania State University, US, have estimated atmospheric carbon dioxide levels for the period.

They studied leaf fossils of gingkoes and ferns that grew around the time of the dinosaurs' demise.

Shallow sea

The number of carbon dioxide-absorbing pores in the fossils reflects the amount of carbon dioxide in the air: the fewer the pores, the more carbon dioxide.

By using computer simulations and doing real experiments on plants, the scientists can show there was a sudden, five-fold increase in CO2 at the end of the Cretaceous.

This can only be explained, they believe, by the sudden vaporisation of between 6,400 and 13,000 billion tonnes of carbon - a substantial component of the limestone rocks that lined the shallow sea that existed at Chicxulub 65 million years ago.

Such an injection of CO2 into the atmosphere could have created blistering heatwave, raising global temperatures by as much as 7.5 Celsius.

Time dependent

"We estimate that the CO2 levels were four to five times higher for 10,000 years after the impact," Sheffield's Professor David Beerling told BBC News Online.

"The trouble with the [volcanism in India] is that it is spread over two million years. If you release that much CO2 into the atmosphere at that rate, the oceans will just suck it straight back out.

"So, the only thing that can explain such a large and sudden jump in CO2 would be this idea of a space impact."

Professor Beerling said other research teams had found evidence of rapid warming at the time.

"A Dutch team has shown that salt-water organisms, dinoflagellates, migrated to polar regions; there was this rapid spreading of warm-water organisms to the poles, which is indicative of a biological response to a strong warming.

"You see it also in the isotopic records from ocean cores."

The fossil leaf research is published in the journal Proceedings of the National Academy of Sciences (PNAS).