Dinosaurs didn't die from asteroid strike

Posted by Emma Woollacott

Princeton (NJ) - So it wasn't an asteroid after all. New research has shown that the dinosaurs died out a full 300,000 years after a giant meteor struck what is now Mexico.

The crater, discovered in 1978 in northern Yucutan, measures about 180km in diameter. When spherules from the impact were found just below the Cretaceous-Tertiary (K-T) boundary, it was quickly identified as the "smoking gun" responsible for the mass extinction of the dinosaurs approximately 65 million years ago.

However, many scientists have disagreed with this interpretation. This latest research, led by Dr Gerta Keller at the University of Princeton and Thierry Adatte of the University of Lausanne in Switzerland, uses evidence from Mexico to suggest that the Chicxulub impact predates the K-T boundary by 300,000 years. "From El Penon and other localities in Mexico we know that between four and nine metres of sediments were deposited at about two to three centimetres per thousand years after the impact. The mass extinction level can be seen in the sediments above this interval" said Keller.

Advocates of the Chicxulub impact theory suggest that the impact crater and the mass extinction event only appear far apart in the sedimentary record because of earthquake or tsunami disturbance that resulted from the impact of the asteroid.

"The problem with the tsunami interpretation is that this sandstone complex was not deposited over hours or days by a tsunami; deposition occurred over a very long time period," said Keller.

The study found that the sediments separating the two events were characteristic of normal sedimentation, with burrows formed by creatures colonising the ocean floor, erosion and transportation of sediments, and no evidence of structural disturbance.

As well as this, they found evidence that the Chicxulub impact had nothing like the dramatic impact on species diversity that has been suggested. At one site at El Peon, the researchers found 52 species present in sediments below the impact spherule layer, and counted all 52 still present in layers above the spherules. In contrast, at a site at La Sierrita which records the K-T boundary, 31 out of 44 species disappeared from the fossil record.

"We found that not a single species went extinct as a result of the Chicxulub impact… these are astonishing results that have been confirmed by more studies in Texas," said Keller.

In place of Chicxulub, Keller suggests that the massive volcanic eruptions at the Deccan Traps in India may be responsible for the extinction, releasing massive amounts of dust and gases that could have blocked out sunlight and brought about a significant greenhouse effect.