The extinction of the dinosaurs at the end of the Cretaceous period has long been attributed to a massive asteroid impact. However, recent evidence suggests that volcanic activity, specifically the Deccan Traps in India, was the primary driver. First, the Deccan Traps eruptions began well before the asteroid impact and lasted for nearly a million years, releasing vast amounts of sulfur dioxide and carbon dioxide. These gases would have caused severe global cooling and acid rain, stressing ecosystems long before the impact. Second, the timing of the main phase of volcanic activity coincides with the mass extinction horizon, whereas the Chicxulub impact occurred slightly earlier but did not immediately cause extinction. Third, models of volcanic gas emissions show that the Deccan Traps could have produced enough atmospheric change to trigger a collapse in photosynthesis and marine food chains, independent of any impact effects. Thus, volcanic activity, not the asteroid, was the decisive factor.
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The reading argues that volcanic eruptions caused dinosaur extinction, but this view is flawed. First, while the Deccan Traps erupted for a long time, the most intense phase occurred after the extinction event, not before. Paleomagnetic data indicate that the peak eruptions happened roughly 300,000 years after the iridium layer associated with the impact, meaning volcanism was too late to be the primary cause. Second, the alleged coincidence of volcanism with extinction is misleading. The fossil record shows a sudden die-off at the iridium layer, which marks the asteroid impact, while the volcanic strata show gradual faunal changes over hundreds of thousands of years. Third, the gas emission models ignore the role of the impact in triggering wildfires and a dust cloud that blocked sunlight globally, causing a rapid ‘impact winter.’ Sulfur from the impact was injected into the stratosphere, while volcanic gases were mostly tropospheric and quickly washed out. Therefore, the asteroid remains the only mechanism capable of producing the abrupt, worldwide extinction observed.
📝 写作提示 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
