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 intense volcanic activity in the Deccan Traps of India was the primary cause. First, the timing of the Deccan eruptions aligns more closely with the extinction event. Radioisotopic dating of lava flows indicates that the main phase of eruptions began about 66.3 million years ago and lasted for roughly 750,000 years, which brackets the extinction horizon at 66 million years ago. In contrast, the Chicxulub impact occurred at 66.04 million years ago, slightly after the onset of the extinction. Second, volcanic emissions of sulfur dioxide and carbon dioxide would have caused severe climate swings, including rapid cooling from sulfate aerosols and subsequent warming from CO2, leading to ocean acidification and habitat disruption. Such effects are consistent with the selective extinction patterns, where marine species with calcium carbonate shells were disproportionately affected. Third, the Deccan eruptions released large quantities of mercury and other toxic metals, which have been found in sediments exactly at the Cretaceous-Paleogene boundary. These toxins would have poisoned ecosystems over a prolonged period, matching the gradual decline observed in fossil records prior to the impact.
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While the volcanic hypothesis seems plausible, it fails to account for several key observations. First, the precise timing of the extinction does not match volcanic activity. The most severe phase of Deccan volcanism occurred in pulses, with a major pulse around 66.1 million years ago, but the extinction event itself was extremely rapid, occurring within a few thousand years at 66.04 million years ago. This suddenness is inconsistent with the long-term, gradual effects of volcanism. Second, the selective extinction pattern actually supports an impact cause. The impact would have caused a global fireball and a dust cloud that blocked sunlight, leading to a collapse of photosynthesis and a sharp drop in temperature. This would have hit large-bodied animals and those at the top of food chains hardest, while organisms adapted to low-light or detrital feeding survived, which matches the fossil record. Volcanic climate swings would have been less severe and more gradual, allowing species to adapt. Third, mercury anomalies are not unique to impacts; they can also be produced by volcanic activity, but the concentration and isotopic signature of mercury at the boundary are consistent with a sudden, massive input, not a slow volcanic release. Moreover, the iridium layer, which is a hallmark of asteroid impacts, is globally distributed and has a composition matching that of chondritic meteorites, something volcanism cannot explain.
📝 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.
