The extinction of the dinosaurs at the end of the Cretaceous period has long been attributed to a single catastrophic asteroid impact. However, recent evidence suggests that massive volcanic eruptions in the Deccan Traps of India were the primary cause. Three key arguments support this volcanic hypothesis. First, the Deccan eruptions lasted for over a million years, releasing immense amounts of sulfur dioxide and carbon dioxide that would have caused severe global cooling and warming cycles, creating long-term climate instability that stressed ecosystems long before the asteroid struck. Second, sediment layers from the Cretaceous-Paleogene boundary show that the extinction occurred gradually over hundreds of thousands of years, consistent with prolonged volcanic activity rather than an instantaneous impact. Finally, marine fossil records indicate a slow decline in biodiversity in the millions of years preceding the boundary, aligning with the cumulative effects of volcanic emissions, whereas an asteroid impact would have caused sudden, simultaneous extinction across all species.
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The reading's volcanic hypothesis is flawed. While the Deccan eruptions were extensive, their timing does not match the extinction event. High-precision radiometric dating shows that the main phase of Deccan volcanism ended about 200,000 years before the impact layer. In contrast, the Chicxulub impact occurred exactly at the boundary, within the error margin of the extinction horizon. Second, the alleged gradual extinction pattern is misleading. Detailed studies of continuous cores from the boundary reveal that many species disappeared abruptly, with no significant decline in the preceding million years. The apparent gradual pattern in some sites is due to sediment mixing or incomplete records. Third, the marine fossil decline cited by the reading is not global; it is concentrated in regions near the Deccan eruptions, whereas unaffected areas show stable diversity until the impact. Furthermore, the impact itself would have triggered tsunamis, wildfires, and a dust cloud that blocked photosynthesis, causing a rapid collapse of food chains—a mechanism perfectly consistent with the sudden extinction observed in the fossil record.
📝 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.
