Domingo 19 de Mayo de 2002, Ip nš 15

Cosmic impact could have started dinosaur age
Scientists have long thought that a massive asteroid strike some 65 million years ago doomed the dinosaurs to extinction. Now new evidence suggests that another cosmic impact about 200 million years ago ushered in the age of the dinosaurs in the first place.

Paul Olsen of Columbia University and colleagues used new, highly sensitive instruments to detect small but significant levels of an element called iridium in earth's sediment layers dating back about 200 million years, to the boundary between the Triassic and Jurassic periods.

While iridium is rare on Earth, it is abundant in space objects such as asteroids. When researchers find iridium in the geological record, they consider it strong evidence that a space object hit the Earth in that time period.

Additionally, the research team looked closely at dinosaur footprints and bone fragments found at over 70 sites throughout eastern North America. Analysis of those artifacts suggests populations of large dinosaurs began increasing rapidly just 10,000 years into the Jurassic period.

"Finding the element iridium, which is common in space objects, creates a time marker for comet or asteroid impacts," said Dennis Kent, a geologist with Rutgers University and a co-author of the study.

"Correlating the finds with evidence of plant and animal life helps to tell us what happened."

The scientists also discovered a spike in fern spores in sediment layers dating to the same time frame, which they say is typical after a cosmic impact. Ferns are among the first plants to revive after an environmental disaster.

The notion that an asteroid or comet collision could have ushered in the age of the dinosaurs is not new; it was first proposed decades ago.

But up until now, scientists had been unable to detect any iridium at the Triassic-Jurassic boundary. Olsen and his team used a modified version of an instrument called a mass spectrometer, which allowed them to detect the presence of iridium down to the parts-per-trillion level.

As to why a major cosmic impact might have led to a dinosaur population boom, scientists think it would have killed off other animals in great numbers, giving fledging dinosaur species a competitive edge in adapting to the new environment.

"Our research adds to the speculation that there was a comet or asteroid impact about 200 million years ago, followed relatively quickly by the rising dominance of dinosaur populations of the Jurassic period," said Kent. "Dinosaurs went on to dominate for the next 135 million years."

The research is published in this week's edition of the journal Science.


  16/05/2002. CNN.