The extinction of the dinosaurs at the end of the Cretaceous period has long been attributed to a massive asteroid impact. However, a growing body of evidence suggests that volcanic activity, specifically the Deccan Traps eruptions in present-day India, was the primary cause. First, the timing of the Deccan Traps eruptions coincides with the extinction event. Geochemical dating shows that the main phase of eruptions began about 66 million years ago, just before the extinction, and continued for several hundred thousand years. This prolonged volcanic activity would have released vast amounts of sulfur dioxide and carbon dioxide, leading to severe climate disruption, including acid rain and global cooling, which could have devastated ecosystems. Second, the fossil record indicates a gradual decline in dinosaur diversity well before the asteroid impact. Paleontologists have found that dinosaur species were already decreasing in number and variety in the last few million years of the Cretaceous, suggesting that environmental stress from volcanic emissions was already taking a toll. Third, the asteroid impact, while dramatic, was a short-term event. The immediate effects, such as fires and tsunamis, would have been catastrophic locally, but the climate changes from an impact would have been brief, lasting only a few years to decades. In contrast, volcanic eruptions released gases over hundreds of thousands of years, creating a sustained and more deadly environmental crisis. Therefore, the Deccan Traps volcanism, not the asteroid, was the decisive factor in the dinosaur extinction.
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The reading attributes dinosaur extinction to Deccan Traps volcanism, but this argument is flawed. First, while the Deccan Traps eruptions began around 66 million years ago, the most intense phase of volcanic activity actually occurred about 200,000 years before the extinction, and the eruptions had largely subsided by the time of the mass extinction. In contrast, the Chicxulub asteroid impact occurred precisely at the extinction boundary. The coincidence in time is far stronger for the asteroid than for the volcanism. Second, the claim of a gradual dinosaur decline is based on incomplete fossil sampling. Many regions that have yielded dinosaur fossils from the latest Cretaceous are not well represented, and when scientists account for sampling biases, they find that dinosaur diversity was actually stable until the very end. The apparent decline is an artifact of poor preservation. Third, the reading underestimates the global effects of the asteroid impact. The impact ejected huge amounts of dust and aerosols into the stratosphere, blocking sunlight for months or even years, causing a sudden and severe drop in temperature—a 'impact winter'—that would have halted photosynthesis on a global scale. This rapid, catastrophic change would have been far more lethal than the gradual volcanic emissions, which also allowed species time to adapt. Thus, the evidence strongly supports the asteroid impact as the primary cause.
📝 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.
