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 massive volcanic eruptions in the Deccan Traps of India were the primary cause. First, the timing of the Deccan eruptions overlaps with the extinction event. Radiometric dating shows that the main phase of volcanic activity began about 66 million years ago, just before the mass extinction, and continued for roughly 750,000 years. This temporal correlation is too precise to be coincidental, whereas the asteroid impact, dated to 66 million years ago, occurred after the onset of the extinction, according to some sediment records. Second, volcanic emissions of sulfur dioxide and carbon dioxide would have caused severe global cooling and acid rain, followed by long-term warming. Such climate disruptions could have stressed ecosystems worldwide, leading to the gradual decline of dinosaur populations over hundreds of thousands of years, as seen in the fossil record. Finally, the Deccan eruptions released vast amounts of toxic gases, including mercury, which would have poisoned soils and water bodies. High mercury concentrations found in sediments near the extinction boundary align with peak volcanic activity, providing a direct toxicological mechanism for the extinction.
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The reading’s argument for volcanic activity as the main cause of dinosaur extinction is not convincing. First, the claimed temporal overlap is misleading. High-resolution dating places the main phase of Deccan volcanism well before the extinction event, ending about 200,000 years before the boundary. In contrast, the Chicxulub asteroid impact occurred exactly at the boundary, with its global debris layer found in the same sediment layer as the extinction fossils. This exact coincidence strongly supports the impact as the trigger. Second, the volcanic climate effects, while severe, were not as extreme as the impact’s aftermath. The impact would have ejected dust and sulfur aerosols into the stratosphere, blocking sunlight for months to years, causing a sudden “impact winter” that halted photosynthesis. This abrupt catastrophe explains the rapid and selective extinction of large animals, whereas volcanic cooling would have been gradual and less severe, allowing species to adapt. Third, the mercury anomaly cited in the reading is not unique to volcanic activity. The asteroid impact itself could have released mercury from vaporized target rocks, and the highest mercury spike occurs precisely at the boundary, not during the main volcanic phase. Thus, the evidence points to the impact as the principal 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.
