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 volcanic activity, specifically the Deccan Traps eruptions in India, was the primary cause. Three key arguments support this volcanic hypothesis. First, the Deccan Traps eruptions began approximately 66 million years ago, well before the asteroid impact, and lasted for over a million years. This prolonged volcanic activity would have released vast amounts of sulfur dioxide and carbon dioxide, leading to severe climate change, including acid rain and global cooling, which could have stressed dinosaur populations over time. Second, the fossil record shows a gradual decline in dinosaur diversity in the millions of years leading up to the extinction event, not a sudden die-off consistent with an instantaneous asteroid strike. This gradual pattern aligns more closely with the long-term environmental stress caused by volcanism. Third, recent geochemical analyses of sediments from the Cretaceous-Paleogene boundary reveal elevated levels of mercury, a volcanic tracer, rather than solely the iridium spike typically associated with asteroids. This suggests that volcanic emissions played a more significant role in the environmental disruption than impact debris. Taken together, these points indicate that volcanic eruptions, not an asteroid, were the dominant force behind the dinosaur extinction.
🎧 听力讲座
🎯 模拟考试环境：先点「播放讲座」听完内容，再作答。听力原文默认隐藏。
The reading argues that volcanic activity was the primary cause of dinosaur extinction, but this view is flawed. While the Deccan Traps eruptions did begin before the impact, their climate effects were likely not severe enough to cause extinction. Geological evidence shows that the eruptions were intermittent, with long quiet periods that allowed ecosystems to recover. In contrast, the asteroid impact at Chicxulub caused a sudden and catastrophic global winter, darkness, and wildfires, which would have been far more lethal. Second, the claimed gradual decline in dinosaur diversity is not supported by recent high-resolution fossil studies. These studies show that dinosaur populations were stable and thriving right up to the last 100,000 years before the boundary, contradicting the idea of a long-term decline. The apparent decline in some older records is an artifact of incomplete sampling and poor preservation. Third, the mercury anomalies in the boundary sediments can be explained by the impact itself. The impact would have vaporized mercury-rich sedimentary rocks at the impact site, releasing mercury into the atmosphere. Additionally, the iridium spike, which is a hallmark of asteroids, is globally distributed at the boundary and is far more consistent with a single large impact than with prolonged volcanism. Therefore, the asteroid impact remains the most plausible and parsimonious explanation for the mass 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.
