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 eruptions aligns closely with the extinction event. Radioisotopic dating of Deccan basalts indicates that the main phase of volcanism occurred approximately 66 million years ago, coinciding with the Cretaceous-Paleogene boundary. In contrast, the Chicxulub impact crater in Mexico is dated to slightly earlier, suggesting that the impact may have occurred before the main extinction pulse. Second, volcanic emissions of sulfur dioxide and carbon dioxide would have caused severe climate fluctuations, including rapid cooling and subsequent warming, which could have stressed ecosystems globally. Such environmental turmoil is consistent with the gradual decline in dinosaur diversity observed in the fossil record during the last few million years of the Cretaceous. Third, the Deccan eruptions released vast quantities of toxic gases, including mercury and halogen compounds, which would have poisoned soils and oceans. Geochemical signatures in sedimentary layers at the boundary show elevated mercury levels, a hallmark of volcanic activity, whereas impact-related deposits lack such consistent enrichment. Therefore, the Deccan Traps volcanism, not the asteroid, was the dominant driver of the end-Cretaceous mass extinction.
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Professor: The reading presents a compelling case for volcanism, but it overlooks key evidence that strongly supports the asteroid impact hypothesis. First, the claim about timing is misleading. While the main phase of Deccan volcanism began around 66 million years ago, detailed paleomagnetic and geochronological data show that the most intense eruptions occurred after the impact, not before. The impact itself is precisely dated to 66.04 million years ago, and the extinction horizon lies exactly at that boundary, whereas the peak volcanic activity postdates it by tens of thousands of years. So the sequence does not support volcanism as the trigger. Second, the reading argues that volcanic gases caused gradual climate change, but the fossil record shows a sudden extinction event, with over 75% of species disappearing in a geologically instantaneous moment. Volcanic emissions would have caused gradual stress, but dinosaur fossils remain abundant right up to the boundary and then vanish abruptly, which is more consistent with a catastrophic impact. Third, the mercury evidence is not unique to volcanism. Asteroid impacts can also release mercury from vaporized crustal rocks, and in fact, the boundary layer contains a distinct iridium anomaly, a rare element abundant in asteroids but scarce in volcanic rocks. This iridium spike, along with shocked quartz and tektites, provides unequivocal evidence of an impact, while volcanic deposits lack such markers. Thus, the asteroid impact remains the most plausible cause of the extinction.
📝 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.
