Seismic images show dinosaur-killing meteor made bigger splash

January 23, 2008

The most detailed three-dimensional seismic images yet of the Chicxulub crater, a mostly submerged and buried impact crater on the Mexico coast, may modify a theory explaining the extinction of 70 percent of life on Earth 65 million years ago.

The Chicxulub crater was formed when an asteroid struck on the coast of the Yucatan Peninsula. Most scientists agree the impact played a major role in the “KT Extinction Event” that caused the extinction of most life on Earth, including the dinosaurs.

According to Sean Gulick, a research scientist at the Institute for Geophysics at The University of Texas at Austin’s Jackson School of Geosciences and principal investigator for the project, the new images reveal the asteroid landed in deeper water than previously assumed and therefore released about 6.5 times more water vapor into the atmosphere.

The impact site also contained sulfur-rich sediments called evaporites, which would have reacted with water vapor to produce sulfate aerosols. According to Gulick, an increase in the atmospheric concentration of the compounds could have made the impact deadlier in two ways: by altering climate (sulfate aerosols in the upper atmosphere can have a cooling effect) and by generating acid rain (water vapor can help to flush the lower atmosphere of sulfate aerosols, causing acid rain). Earlier studies had suggested both effects might result from the impact, but to a lesser degree.

“The greater amount of water vapor and consequent potential increase in sulfate aerosols needs to be taken into account for models of extinction mechanisms,” says Gulick.

The results appear in the February 2008 print edition of the journal Nature Geosciences.

An increase in acid rain might help explain why reef and surface dwelling ocean creatures were affected along with large vertebrates on land and in the sea. As it fell on the water, acid rain could have turned the oceans more acidic. There is some evidence that marine organisms more resistant to a range of pH survived while those more sensitive did not.

Gulick says the mass extinction event was probably not caused by just one mechanism, but rather a combination of environmental changes acting on different time scales, in different locations. For example, many large land animals might have been baked to death within hours or days of the impact as ejected material fell from the sky, heating the atmosphere and setting off firestorms. More gradual changes in climate and acidity might have had a larger impact in the oceans.

Gulick and collaborators originally set out to learn more about the trajectory of the asteroid. They had hoped the crater’s structure in the subsurface would hold a tell-tale signature. Instead, the structure seemed to be most strongly shaped by the pre-impact conditions of the target site.

“We discovered that the shallow structure of the crater was determined much more by what the impact site was like before impact than by the trajectory of the impactor,” says Gulick.

If scientists can determine the trajectory, it will tell them where to look for the biggest environmental consequences of impact, because most of the hazardous, shock-heated and fast-moving material would have been thrown out of the crater downrange from the impact.

Researchers at Imperial College in London are already using computer models to search for possible signatures in impact craters that could indicate trajectory regardless of the initial surface conditions at the impact site.

“As someone who simulates impact events using computers, this work provides valuable new constraints on both the pre-impact target structure and the final geometry of the cratered crust at Chicxulub,” says Gareth Collins, a research fellow at Imperial College.

Source: University of Texas at Austin

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2.3 / 5 (4) Jan 23, 2008
Dinosaurs would have had to have already become extinct some time before the Yucatan impact.
Had this impact exterminated the dinosaurs then many, especially the larger dinosaurs would have been found with their bones protruding through the KT boundary.
The residue that forms the KT boundary would have fallen onto the bodies and or bones of the dinosaurs and have been washed off by rains.
But this has not happened.
All dinosaur skeletons are found beneath the KT boundary so that other sediment had formed over their bones before the KT boundary material settled
This could mean a time gap of anything from a century or so, to millenniums.
The extinction of the dinosaurs may have been as undramatic as the change in plant life. If the herbivores died out it would only be a matter of time before the carnivores followed them into extinction. You do not last long when your food source is gone.
Or it may have been caused by some other equally undramatic event or series of events.
Other extinctions may well be directly due to the Yucatan impact, especially marine, but not the dinosaurs.
1 / 5 (1) Jan 24, 2008
dating the K/T boundary is accurate to about 100k years. Chicxulub happened about 300k years before the K/T extinctions. There was a 2nd impact causing iridium layer. Dont forget the Deccan traps also.
1 / 5 (1) Feb 11, 2008
The Deccan traps were caused by Chicxulub. Get a globe of the period and draw a line from Chicxulub to the earth's center. Extend the line and you exit the globe's surface at the Traps.
1 / 5 (1) Apr 28, 2009
Re: impact direction. Don't the very existence of Cuba / Bahamas / etc. as peaks of ridges thrown up by the impact indicate a southwest -- northeast trajectory?

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