'Quick' demise for the dinosaurs

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'Quick' demise for the dinosaurs
Graphic BBC
By BBC News Online's Jonathan Amos



The dinosaurs may have met a very quick end.

New research suggests the fearsome creatures could have been wiped from the face of the Earth in as little as 10,000 years.

The evidence comes from a study of rocks in Italy and Tunisia. The work lends support to the idea that a single, giant impact of an asteroid or comet was responsible for the mass extinction of life that occurred 65 million years ago.

In so doing, the research also undermines the popular, alternative theory for the demise of the dinosaurs: climate change brought on by huge volcanic eruptions.

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Geological timeline

Sujoy Mukhopadhyay and colleagues studied sedimentary rocks that mark the so-called K-T boundary, the line of separation between the zones of geological time referred to as the Cretaceous period and the Tertiary sub-era.



Crater map American Geophysical Union
Measurements of gravity anomalies trace the outline of the now buried crater
(Picture: American Geophysical Union)
It is at this K-T boundary, timed at about 65 million years ago, that 70% of all species, including the dinosaurs, suddenly disappear from the fossil record.

But scientists have struggled to determine how long the deposition of the clay minerals that make up the K-T boundary lasted - and by inference how long it took to kill off the dinosaurs. It would also indicate how long it took for life to turn the corner - the end of the boundary shows the beginnings of a recovery.

Mukhopadhyay's US-Italian team believe they can now measure the length of this dark period in Earth history.

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Interplanetary dust

They analysed the amount of helium-3 in the rocks of the K-T boundary.

Levels of this type, or isotope, of the element reflect the amount of interplanetary dust that settles on Earth and, properly calibrated, can be used as a tool to time the rate at which rocks are laid down.

"If you can show that the flux is not changing, you can use it as a marker to figure out how quickly sediments are depositing," Sujoy Mukhopadhyay, from the California Institute of Technology, told BBC News Online.

The research suggests the K-T boundary was deposited in about 10,000 years. He said the short period lent support to the theory that the dinosaurs were wiped out in a sudden, catastrophic event such as the impact of an extraterrestrial body.

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Single impact

The constant rate of accumulation of helium-3 also indicates that the impactor was not part of a comet shower or bombardment.

"Comets are dusty objects and if you have several comets coming into the inner Solar System, you will increase the dust flux to the Earth," Mukhopadhyay said.

"If you increase the dust flux, you increase the helium-3 flux. And so if you don't see an increase in the helium-3 in these sediments, it rules out a large number of comets coming in."

Scientists think they know where the suspected asteroid or comet came down. Several studies have now detailed traces of a crater beneath the Yucatan Peninsula in Mexico.

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'Smoking gun'

Even so, some scientists still hold to the theory that massive volcanic eruptions at about the same time were a more likely cause for the end of the dinosaurs.



Graphic BBC
The Deccan Traps, the flood basalts of western India, are often cited as the alternative "smoking gun" for the death of the dinos.

Certainly, this type of volcanism would have had a major effect on global climate, and probably did disrupt the food chains upon which much life depended, but Mukhopadhyay said the event occurred over too long a period.

"The Deccan Traps erupted over much longer timescales - over 500,000 years or more. If the recovery of life starts after only 10,000 years, it is hard for us to see how the traps are influencing the mass extinction."

The helium-3 research is published in the journal Science.

http://news.bbc.co.uk/2/hi/science/nature/1209870.stm