Published online 13 May 2009 | Nature | doi:10.1038/news.2009.477


Asteroid impact may have gassed Earth

Did dinosaur-killing space rock create enough carbon monoxide to trigger extreme global warming?

impactCarbonates in the crosshairs: how would a massive impact change the Earth's climate?

A burst of carbon monoxide triggered by an asteroid impact may have been a key factor in the mass extinction which saw off the dinosaurs 65 million years ago.

The claim comes from Japanese scientists who have simulated the impact that created the massive crater at Chicxulub on Mexico's Yucatán Peninsula.

The Chicxulub impact, which occurred around the end of the Cretaceous period and the beginning of the Tertiary (the K-T boundary), is thought to have blackened the skies with dust, blocking photosynthesis. The impact would also have triggered intense global firestorms, boosting the atmospheric concentration of the greenhouse gas carbon dioxide and warming the Earth.

But the shock of the impact may also have released significant amounts of greenhouse gasses by breaking down carbonate rocks such as calcite. Previous estimates have suggested that the Chicxulub impact could have released enough carbon dioxide from carbonate rocks at the site to cause a global warming of about 1-2 ºC.1,2

The Japanese team, led by Ko Kawaragi of the University of Tokyo, now suggest in Earth and Planetary Science Letters3 that the shocked carbonates would have released much more carbon monoxide than carbon dioxide, leading to a global warming of 2-5 °C for several years after the impact.

Blast off

In their simulation, the scientists directed a high-energy laser at a piece of metal foil to blast out a thin disc less than a millimetre wide. This speeding disc smashed into a calcite sample to create pressures up to 91 gigapascals, roughly the pressure estimated for a Chicxulub-like impact.

Using mass spectrometry to analyse the gasses given off by the collision revealed that twice as much carbon monoxide than carbon dioxide was released. On the basis of simulations of the Chicxulub impact, that's up to 2 trillion tonnes of carbon monoxide, the researchers say — a similar amount to the mass of carbon monoxide that could have been generated by firestorms.

Kawaragi and his colleagues argue that this burst of carbon monoxide would have changed the atmosphere's photochemistry in a way that led to an increase in atmospheric methane and ozone, much more potent greenhouse gases than carbon dioxide. That's because the carbon monoxide would react with hydroxyl radicals in the atmosphere — radicals that would otherwise be responsible for breaking down methane and ozone, they say.

The research could have implications in understanding other impact events in Earth's history, says planetary geologist David Kring of the Lunar and Planetary Institute in Houston, Texas, as about 25% of major impacts would be expected to have hit carbonate rocks. However, only those rare impacts that were on the same scale as Chicxulub would be likely to release enough gas to have a major effect on climate, he adds.

The Japanese simulation is also very different from the Earth's atmosphere 65 million years ago, making it difficult to assess how reliable their estimates are. "Impact ejecta raining through the atmosphere will severely disturb the normal structure of the atmosphere and will even destroy the tropopause in some regions of the globe, so the microphysics and atmospheric chemistry that occurs is complex," says Kring.

And Brian Toon, an atmospheric chemist at the University of Colorado at Boulder, also questions the photochemical reactions that the Japanese team used in their models. Immediately after the impact the skies would have been "pitch black because of dust, which would have shut off most photochemical reactions," he says.

"They might have figured out what was initially injected into the atmosphere, but trying to speculate what it means is a pretty big reach," he adds. "There could have been lots of surprises." 

  • References

    1. Pope, K. O., Baines, K. H., Ocampo, A. C. & Ivanov, B. A. J. Geophys. Res. 102, 21645-21664 (1997). | Article | PubMed | ChemPort |
    2. Pierazzo, E., Kring, D. A. & Melosh, H. J. J. Geophys. Res. 103, 28607-28625 (1998). | Article | ChemPort |
    3. Kawaragi, K. et al. Earth Planet. Sci. Lett. 282, 56-64 (2009). | Article | ChemPort |
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