BioEd Online

The widely held theory that a particular meteorite strike on Mexico wiped out the dinosaurs is under sharp attack, again.

The asteroid that created the Chicxulub crater in the Yucatán peninsula in Mexico arrived too early to have caused the Cretaceous-Tertiary mass extinction, according to evidence given on 23 May at an American Geophysical Union conference in New Orleans, Louisiana.

A team led by palaeontologist Gerta Keller of Princeton University, New Jersey, reported that a sediment core drilled in east Texas emphatically confirms a study that the group released two years ago. Sediments of glass sprayed out by the Chicxulub impact are separated from fossils killed during the mass extinction by a 300,000-year gap, they argue.

"I believe this is the mortal wound for the Chicxulub theory," says Keller. Scientists should mount a search for the crater left by the meteorite that was really responsible for the mass extinction, she adds.

Many geophysicists remain unswayed. Sean Gulick of the University of Texas at Austin doubts the report because it means another huge asteroid must have hit the Earth in the same era, about 65 million years ago. "The odds of that are highly unlikely," said Gulick, who chaired the Chicxulub symposium at the conference.

However, some sedimentologists are being persuaded by the core specimens. Paul Wignall of the University of Leeds in Britain calls Keller's evidence "quite convincing", although he didn't attend the meeting.

Original thought

Two years ago, Keller stunned a symposium at an American Geophysical Union meeting in Nice, France, with an analysis of a section of a 1,500-metre core drilled in the Yucatán, only 60 kilometres from the Chicxulub crater.

The Yucatán core, called Yaxcopoil-1, was the result of an international project designed to provide the most advanced record of events at the Cretaceous-Tertiary boundary. But it was beset by strife over access to the core and subsequent interpretations (see ' Hot Tempers, Hard Core').

Keller claimed the crater preceded mass extinction by 300,000 years¹. Her critics say the sediment layers she sees are actually rubble from collapsing crater walls. But her team argues that palaeomagnetic dating and minute fossil analysis rules this out.

Northern exposure

To settle the dispute, Keller drilled 2,000 kilometres north of the crater to get a sedimentary view unaffected by backwash.

The Brazos River Valley, Texas, is widely accepted as the best location to check Chicxulub impact debris from afar. In March, three 50-metre-deep holes were drilled near the small town of Rosebud to extract sediment from the time of the mass extinction.

From a 2-metre section of the best core, the Keller team charted what they say shows the 300,000-year gap. First, there is a 2 centimetre-thick layer of altered glass that is the ejected material from the Chicxulub impact. About 50 cm above that lie the sediment beds left by storms that followed the asteroid's impact. Finally, a full 1.2 metres above these beds, there is the detritus of the mass extinction, represented by fossils of tiny plants and animals that died.

The National Science Foundation has given Keller US$40,000 to drill another core in autumn 2006. This one will be on the opposite side of the Chicxulub crater, some 7,800 kilometres south near the city of Recife in Brazil. Keller hopes to find evidence that will finally quiet her critics.