Burnt oil and gas, not vegetation, may have caused the soot layer at the end of the Cretaceous period.
The impact of a huge asteroid or comet at the end of the Cretaceous period 65 million years ago is generally held responsible for the sudden demise of 60–80% of all species on Earth. But new results challenge the common idea that the extinctions were partly caused by global wildfires triggered by the violent impact.
Claire Belcher and colleagues at Royal Holloway University of London in Surrey, UK, say in a paper in the Proceedings of the National Academy of Sciences USA1 that the widespread soot deposits in sedimentary rocks formed at the time of the putative impact are not, as previously asserted, evidence of runaway fires caused by the meteorite's impact.
They have analysed the mixtures of carbon-based molecules called polycyclic aromatic hydrocarbons (PAHs) in the sooty material from these rocks, and find that the compositions of the mixtures don't match those typically produced by burning vegetation. Instead, they resemble those formed when hydrocarbons such as gas and oil are burnt.
The researchers think the soot comes from combustion of hydrocarbons within the rocks of the impact site itself — thought to be the region around Chicxulub on the north coast of the Mexican Yucatán peninsula, where a now partly submerged crater about 180 km across has been dated to the time of the mass extinction that separates the Cretaceous from the Tertiary period.
A global layer of soot in rocks of this age was discovered in the late 1980s, and was interpreted as showing that the heat of the impact ignited wildfires all over the world. According to this hypothesis, vast swathes of land plants went up in flames, possibly roasting many animal species including the dinosaurs.
But for several years now, Belcher and her colleagues have been casting doubt on the idea that the Earth was engulfed in flames for years after the impact. In 2003 they reported that rock strata in North America dating to the Cretaceous–Tertiary boundary showed little evidence of charcoal, which would be expected to be produced from burning vegetation2. Instead, they speculated that the soot in these layers came from combustion of hydrocarbons.
Now the team claim to have clinching proof of that: chemical fingerprints of the source of the soot, in the form of 21 different PAHs separated and identified using the technique of gas chromatography.
Belcher says the new results also answer criticisms of their earlier work on the apparent lack of charcoal in the soot. Some other researchers have suggested that the wildfires might have been too intense to leave any charcoal. But Belcher says the PAHs she finds have molecular structures characteristic of relatively low-temperature formation.
Ashes to ashes
Nevertheless, she and her colleagues could still have some persuading to do. Bernt Simoneit, an organic geochemist at Oregon State University in Corvallis, questions whether the proportions of different PAHs in combustion products are a sufficiently discriminating signature of the fuel source.
He also says that sources of petroleum hydrocarbons at shallow depths "are very scarce now or in past times", and that the biomass of vegetation far surpasses the amount of near-surface oil. Belcher and colleagues, however, point out that Chicxulub is very close to Mexico's largest oil reservoir, the Cantarell Field.
Regardless of where the soot came from, it seems clear that huge amounts of it were thrown into the atmosphere by the impact, blocking out sunlight and perhaps triggering global cooling - causing an 'impact winter'.
"The soot itself undoubtedly had a significant impact on life at the time, but it is unlikely to represent the signature of global wildfires", says Belcher. She says there are clear signs that plant life was severely disrupted, but that this might have been due to the heat of the impact fireball and the global darkness, cold and poisoning (from toxic products of burning hydrocarbons) that might have followed. "I think that the global wildfire idea is beginning to be doused," she adds.
Belcher, C. M., Finch, P., Collinson, M. E., Scott, A. C. & Grassineau, N. V. Proc. Natl Acad. Sci. USA [doi:10.1073/pnas.0813117106] (2009).
Belcher, C. M., Collinson, M. E., Sweet, A. R., Hildebrand, A. R. & Scott, A. C. Geology 31, 1061–1064 (2003).
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Ball, P. 'Dinosaur-killing' impact did not start global wildfires. Nature (2009). https://doi.org/10.1038/news.2009.112