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回答题：Older Volcanic Eruptions Volcanoes were more destructive in ancient history, not because theywere bigger，but because the carbon dioxide（二氧化碳）theyreleased wiped out life with greater ease.
Paul Wignall from the University of Leeds was investigating the linkbetween volcanic eruptions and mass extinctions. Not all volcanic eruptionskilled off large numbers of animals, but all the mass extinctions over the past300 million years coincided with huge formations of volcanic rock. To hissurprise, the older the massive volcanic eruptions were, the more damage theyseemed to do. He calculated the "killing efficiency" for thesevolcanoes by comparing the proportion of life they killed off with the volumeof lava (熔岩) that they produced. He found that sizefor size, older eruptions were at least 10 times as effective at wiping outlife as their more recent rivals
The Permian (二叠纪)extinction, for example, which happened 250 million years ago, is marked byfloods of volcanic rock in Siberia that cover an area roughly the size ofwestern Europe. Those volcanoes are thought to have pumped out about 10gigatonnes (十亿吨) of carbon as carbon dioxide. The globalwarming that followed wiped out 80 percent of all marine genera (种类)at the time, and it took 5 million years for the planet to recover. Yet 60million years ago, there was another huge amount of volcanic activity andglobal warming but no mass extindtion. Some animals did disappear but thingsreturned to normal within tens of thousands of years. "The most recentones hardly have an effect at all," Wignall says. He ignored theextinction which wiped out the dinosaurs (恐龙) 65million years ago, because many scientists believe it was primarily caused bythe impact of an asteroid (小行星). Hethinks that older volcanoes had more killing power because more recent lifeforms were better adapted to dealing with increased levels of CO2.
Vincent Courtillot, director of the Paris Geophysical Institute in France,says that Wignall's idea is provocative. But he says it is incredibly hard todo these sorts of calculations. He points out that the killing power ofvolcanic eruptions depends on how long they lasted. And it is impossible totell whether the huge blasts lasted for thousands
or millions of years. He also adds that itis difficult to estimate how much lava prehistoric volcanoes produced, and thatlava volume may not necessarily correspond to carbon dioxide emissions.
Older volcanic eruptions did moredamage than more recent ones because A．older volcanoes were brighter.
B．carbon dioxide made the earth muchwarmer
C．older volcanoes were hotter
D．carbon dioxide killed off life moreeasily
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