It seems that dinosaurs did not die out after a global forest fire triggered by an asteroid smacking into earth 65 million years ago – so what did happen? asks CLARE O'CONNELL.
AS SCIENTIFIC stories go, it’s worthy of an Oscar. It features giant beasts, an asteroid slamming into earth and a massive extinction event that changed the nature of life on the planet. But a mystery still remains: what exactly was it that wiped out the dinosaurs 65 million years ago?
New clues published this week are stirring the debate about how the creatures became extinct. In particular, the findings have poured cold water on theories that the dinosaurs died in the intense heat of global wildfires. Instead it appears that they could have suffered cold weather, gone hungry and inhaled noxious fumes as they died out.
The dinosaurs’ time was finally up when an asteroid hit the Yucatán peninsula in Mexico, leaving a crater about 180km across and heralding the end of the Cretaceous period and the start of the Tertiary, otherwise known as the “K-T” boundary. After the impact, as many as 80 per cent of the planet’s living species quickly became extinct.
To figure out what happened, scientists look at the rocks that were laid down around the time the asteroid hit, explains palaeobiologist Dr Claire Belcher, who currently holds a Marie Curie Postdoctoral fellowship at University College Dublin’s school of biology and environmental science.
“An asteroid impact could have various consequences on life on earth and undoubtedly released a significant amount of thermal energy by hitting into rock, which if it vapourises will release lots of dust into the atmosphere,” she says.
The discovery in the 1980s of a global layer of soot in 65-million-year-old rock layers sparked the theory that the asteroid’s heat had also ignited worldwide wildfires, she explains. But in recent years Belcher’s PhD work at Royal Holloway University of London suggested otherwise. She sampled K-T rocks at a series of locations in North America and searched for fossilised charcoal, an indication of wildfire activity.
“I found that there were lower amounts of charcoal in the K-T boundary rock than there were in the Cretaceous or the Tertiary on either side of the boundary event,” says Belcher, who published the findings in 2003. “So that suggested there was unlikely to have been extensive wildfires.”
But some observers still clung to the wildfire theory, suggesting that maybe tell-tale charcoal had been burned away at the time, or hadn’t been preserved in the fossil record, recalls Belcher.
However, the results of further analysis of the rock samples could silence those voices. This week in the online edition of the Proceedings of the National Academy of Sciences(US), Belcher and Royal Holloway colleagues Andrew Scott, Margaret Collinson, Paul Finch and Nathalie Grassineau reveal the K-T rocks contain chemical signatures that shed light on how the soot actually arose.
The researchers analysed 21 pyrosynthetic polycyclic aromatic hydrocarbons (pPAHS) within the non-marine rocks and found evidence that the K-T soot came from burning oil or coal rather than living vegetation.
“They showed no relationship to the burning of biomass at all,” says Belcher. “So we believe that the impact vapourised the rocks around it and combusted pockets of oil or coal deposits, and that is what is responsible for the soot that we see at the K-T boundary, not wildfires.”
However, the impact event still spelled bad news for living creatures. Dust and soot in the atmosphere could have blocked out sunlight, causing temperatures to drop and food sources to become scarcer, says Belcher. “So the dinosaurs weren’t burned to death but they could have ended up suffering from cold, or poisoning by some of the noxious gases.”
It appears to have been the last straw for the dinosaurs, who were past their prime by the time the asteroid wreaked its havoc, notes Belcher. “The dinosaurs were in significant decline before the K-T event,” she says. “They were much lower in numbers, they were on the way out anyway and the K-T finished them off.”