火山爆发造成恐龙时代前物种大灭绝

Scientists examining evidence across the world from New Jersey to North Africa say they have linked the abrupt disappearance of half of earth's species 200 million years ago to a precisely dated set of gigantic volcanic eruptions. 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. 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（灾难）. In recent years, many scientists have suggested that the so-called End-Triassic Extinction and at least four other known past die-offs were caused at least in part by mega-volcanism and resulting climate change. However, they were unable to tie deposits left by eruptions to biological crashes closely in time. This study provides the tightest link yet, with a newly precise date for the ETE--201,564,000 years ago, exactly the same time as a massive outpouring of lava. "This may not quench（熄灭） all the questions about the exact mechanism of the extinction itself. However, the coincidence in time with the volcanism is pretty much ironclad（装甲的，坚固的）," said coauthor Paul Olsen, a geologist at Columbia University's Lamont-Doherty Earth Observatory who has been investigating the boundary since the 1970s.

The new study unites several pre-existing lines of evidence by aligning（调整） them with new techniques for dating rocks. Lead author Terrence Blackburn (then at Massachusetts Institute of Technology; 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. The scientists analyzed samples from what are now Nova Scotia, Morocco and the New York City suburbs. (Olsen hammered one from a road cut in the Hudson River Palisades, about 1,900 feet from the New Jersey side of the George Washington Bridge.)

Previous studies have suggested a link between the CAMP eruptions and the extinction, but other researchers' dating of the basalts had a margin of error of 1 to 3 million years. The new margin of error is only a few thousand years -- in geology, an eye blink. 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. Sediments below that time contain pollen（花粉）, spores（孢子） and other fossils characteristic of the Triassic era; in those above, the fossils disappear. Among the creatures that vanished were eel-like fish called conodonts（牙形石）, 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. This particular reversal, labeled E23r, is consistently located just below the boundary, making it a convenient marker, said coauthor Dennis Kent, a paleomagnetism expert who is also at Lamont-Doherty. With the same layers found everywhere the researchers have looked so far, the eruptions "had to be a hell of an event," said Kent.