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 prolonged volcanic activity in the Deccan Traps of India was the primary cause. First, the timing of the Deccan eruptions overlaps with the extinction event. Radioisotopic dating places the main phase of volcanic activity at approximately 66 million years ago, coinciding with the Cretaceous-Paleogene boundary. This temporal correlation suggests that volcanic emissions could have triggered the extinction. Second, volcanic gases, particularly sulfur dioxide, would have caused severe global cooling. Sulfur aerosols reflect sunlight, leading to a 'volcanic winter' that could disrupt photosynthesis and collapse food chains, similar to the effects proposed for an asteroid impact. Third, the fossil record shows a gradual decline in dinosaur diversity in the millions of years before the boundary, which aligns with the long-term stress from volcanic activity rather than a sudden impact. This gradual pattern is inconsistent with an instantaneous catastrophe.
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The reading's argument for volcanic activity as the main cause is flawed. First, the overlap in timing is not precise. The most intense volcanic eruptions occurred about 300,000 years before the extinction boundary, and the eruptions were not continuous but pulsed. The extinction itself was sudden, occurring within a very short geological interval, which does not match the prolonged volcanic scenario. Second, while sulfur aerosols cause cooling, volcanic emissions also include carbon dioxide, a greenhouse gas. The net effect would be a complex climate perturbation, but the magnitude of cooling from the Deccan eruptions is estimated to be much smaller than that from an asteroid impact, which also injected dust and caused global wildfires. The impact's cooling would have been more severe and rapid, making it a more plausible kill mechanism. Third, the gradual decline in dinosaur fossils may be an artifact of sampling bias. Fossil preservation is incomplete, and many dinosaur-bearing rock layers from the latest Cretaceous are missing or poorly studied. When corrected for sampling, the decline disappears, and dinosaurs appear diverse until the very end. Thus, the gradual pattern is not evidence for volcanism but rather a gap 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.
