填空题

Blasts from the Past
1 Volcanoes were destructive in ancient history. Not because they were bigger, but because the carbon dioxide they released wiped out life with greater ease.
2 Paul Wignall from the University of Leeds was investigating the link between volcanic eruptions and mass extinctions. Not all volcanic eruptions killed off large number of animals, but all the mass extinction cover the past 300 million years coincided with huge formations of volcanic rock. To his surprise, the older the massive volcanic eruptions were, the more damage they seemed to do.
3 Wignall calculated the "killing efficiency" for these volcanoes by comparing the proportion of life they killed off with the volume of lava that they produced. He found that size for size, older eruptions were at least 10 times as effective at wiping out life as their more recent rivals.
4 The Permian extinction, for example, which happened 250 million years ago, is marked by floods of volcanic rock in Siberia that cover an area roughly the size of western Europe. Those volcanoes are thought to have pumped out about 10 gigatonnes of carbon as carbon dioxide. The global warming that followed wiped out 80 percent of all marine genera at the time, and it took 5 million years for the planet to recover.
5 Yet 60 million years ago in the late Palaeocene there was another huge amount of volcanic activity and global warming but no mass extinction. Some animals did disappear but things returned to normal within ten thousands of years. "The most recent ones hardly have an effect at all," Wignall says. He ignored the extinction which wiped out the dinosaurs at the end of the Cretaceous,265 million years ago, because many scientists believe it was primarily caused by the impact of an asteroid.
6 Wignall thinks that older volcanoes had more killing power because more recent life forms were better adapted to dealing with increased levels of CO₂. Ocean chemistry may also have played a role. As the supercontinents broke up and exposed more coastline there may have been more weathering of silica rocks. This would have encouraged the growth of phytoplankton in the oceans, increasing the amount of CO₂ absorbed from the atmosphere.
7 Vincent Courtillot, director of the Paris Geophysical Institute in France, says that Wignall’s idea is provocative. But he says it is incredibly hard to do these sorts of calculations. He points out that the killing power of volcanic eruptions depends on how long they lasted. And it is impossible to tell whether the huge blasts lasted for thousands or millions of years.
8 Courtillot also adds that it is difficult to estimate how much lava prehistoric volcanoes produced, and that lava volume may not necessarily correspond to carbon dioxide or sulphur dioxide emissions.

• A．Killing Power of Ancient Volcanic Eruptions
• B．Association of Mass Extinctions with Volcanic
• C．Calculation of the Killing Power of Older Eruptions
• D．A Mass Extinction
• E．Volcanic Eruptions That Caused No Mass Extinction
• F．Accounting for the Killing Power of Older Eruptions

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