Washington: Scientists believe that a set of gigantic volcanic might have eruptions resulted in the disappearance of half of earth`s species 200 million years ago.
The eruptions may have caused climate changes so sudden that many creatures were unable to adapt-possibly on a pace similar to that of human-influenced climate warming today, they said.
The extinction opened the way for dinosaurs to evolve and dominate the planet for the next 135 million years, before they, too, were wiped out in a later planetary cataclysm
Lead researcher Terrence Blackburn (then at MIT; now at the Carnegie Institution) used the decay of uranium isotopes to pull exact dates from basalt, a rock left by eruptions.
The basalts analyzed in the study all came from the Central Atlantic Magmatic Province (CAMP), a series of huge eruptions known to have started around 200 million years ago, when nearly all land was massed into one huge continent.
The eruptions spewed some 2.5 million cubic miles of lava in four sudden spurts over a 600,000-year span, and initiated a rift that evolved into the Atlantic Ocean; remnants of CAMP lavas are found now in North and South America, and North Africa.
Blackburn and his colleagues showed that the eruption in Morocco was the earliest, with ones in Nova Scotia and New Jersey coming about 3,000 and 13,000 years later, respectively.
Among the creatures that vanished were eel-like fish called conodonts, early crocodilians, tree lizards and many broad-leaved plants. The dating is further strengthened by a layer of sediment just preceding the extinction containing mineral grains providing evidence of one of earth`s many periodic reversals of magnetic polarity.
Co-author Paul Olsen, a geologist at Columbia University`s Lamont-Doherty Earth Observatory, and some others have long contended that the earth`s precession-a cyclic change in the orientation of the axis toward the sun and resulting temperature changes-consistently created layers reflecting the alternate filling and drying of large lake basins on a fairly steady 20,000-year schedule.
"By correlating the precisely dated basalts with surrounding sedimentary layers, the new study shows that precession operated pretty much the same way then, allowing dates with a give or take of 20,000 years to be assigned to most sediments holding fossils," Olsen said.