[meteorite-list] 'Dinosaur-killing' Impact Did Not Start Global Wildfires

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 26 Feb 2009 16:05:24 -0800 (PST)
Message-ID: <200902270005.QAA24909_at_zagami.jpl.nasa.gov>

http://www.nature.com/news/2009/090223/full/news.2009.112.html

'Dinosaur-killing' impact did not start global wildfires

Burnt oil and gas, not vegetation, may have caused the soot layer at the
end of the Cretaceous period.

Philip Ball
Nature Magazine
February 23, 2009

Meteor impactThe impact thought to have wiped out the dinosaurs may not
have led to global wildfires.NASA

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 USA 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.

Burning issue

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 Yucat??n 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
vegetation. 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.
    
Corrected:

In an earlier version of this story we incorrectly stated that a
meteorite impact caused the mass extinctions at the end of the
Cretaceous period.

          References

         1. 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).
         2. Belcher, C. M., Collinson, M. E., Sweet, A. R., Hildebrand,
            A. R. & Scott, A. C. Geology 31, 1061???1064 (2003).
Received on Thu 26 Feb 2009 07:05:24 PM PST