segunda-feira, 17 de setembro de 2012

Did the Reign of Dinosaurs Begin, as Well as End, with a Meteorite Strike?

Western New Jersey holds one of the most visible examples of the Triassic–Jurassic boundary, where evidence could settle the debate about what caused a mega-extinction event that paved the way for the age of dinosaurs

A huge meteorite strike may have helped the dinosaurs rise as well as fall. That's what a small crew of mud-spattered researchers who drilled down hundreds of feet in New Jersey this summer wanted to discover.

Roughly 200 million years ago, at least half the species on Earth died off over the course of about 100,000 years, both on land and in sea. This mass extinction, at the boundary between the Triassic and Jurassic periods and one of five known such events in Earth's geologic history, set the stage for dinosaurs to rise to prominence and dominate the planet's terrestrial life for the next 135 million years.

The quest for answers regarding dinosaurs and the end-Triassic mass extinction had previously led paleontologist Paul Olsen and his colleagues at Columbia University's Lamont –Doherty Earth Observatory (LDEO) to globe-trot to around cities in Morocco and along sea cliffs in the U.K . digging up clues from that past era. This summer, it brought them to the Kell family's yard in western New Jersey—the area holds one of the most visible examples of the Triassic– Jurassic boundary.

The sub surface dig for revealing sediment cores required a lightweight, portable drill called a Winkie. D iamond-tipped and gas-powered , it can be carried and operated by only two people—often paleomagnetist Dennis Kent and geochemist Morgan Schaller, both at Rutgers University. The goal was to unearth evidence of what might have triggered the mass extinction.

"The dinosaurs had actually first evolved about 25 million years before the mass extinction, but after their competition got wiped out, it looks like they came in like gangbusters," Olsen says. Such competition included extinct relatives of modern crocodilians, such as the large and carnivorous land-based rauisuchians and semiaquatic phytosaurs as well as  plant-eating aetosaurs and revueltosaurs.

Many scientists blame the end-Triassic mass extinction on exceptionally massive volcanic eruptions over the course of less than 20,000 years that occurred about when the one-time supercontinent of Pangea began rifting apart. These eruptions coated what was to become Africa and the Americas with a million cubic kilometers of lava and doubled the level of carbon dioxide in the air causing massive global warming, "about a 3- degree Celsius increase on average in temperature, if the climate system was as sensitive as models suggest," Olsen says, citing research published by Schaller, Kent and their colleague James Wright in 2011.

Those 3 degrees "could have translated to lethally high summer temperatures, especially for some kinds of broad-leafed vegetation, which in turn could have led to extinctions of animals dependent on the plants," Olsen explains. "In the oceans, a rapid rise in carbon dioxide would have resulted in acidifying the oceans and badly impacting animals that make calcium carbonate skeletons, such as corals, bivalves and ammonites, all of which suffered massive drops in diversity."

Volcanic eruptions also release large amounts of sulfur-laden compounds that reflect sunlight, however, causing cooling. "The cooling only lasts a short time because the sulfur is removed quickly from the atmosphere, but the effect can be very intense, and for a world in which there were no ice caps and there were forests at the poles, such dramatic drops in temperature cold have been devastating on land," Olsen says.

"One can envision the carbon dioxide raising temperatures multiple times for tens of thousands of years, fading away over 100,000 years or so, but these periods of great warmth would be punctuated by many intervals of abrupt and intense cooling caused by the sulfur," Olsen continues. "This one-two punch may have been too much for terrestrial ecosystems and the warmth and acidity in the oceans too much for marine life. Result: mass extinction."


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