阅读材料
Well the theories given in the reading may sound plausible, none of them is a good explanation for the massive extinction at the end of the Triassic period. First, sea level change. Well scientists agree that the sea level fluctuated at the end of the Triassic period often going down. This isn’t a good explanation for the extinctions. Coastal and shallow water ecosystems are usually capable of adapting to environmental changes that happen gradually. The falling sea level at the end of the Triassic period was quite gradual, taking place over several million years. The change would have to be much more sudden to have a widespread negative impact on the species in those ecosystems.Second, global cooling. It’s true that sulfur dioxide can lower global temperatures, but that can only happen during a relatively short period, when the sulfur dioxide that’s been released by volcanoes is actually still present in the atmosphere. In a matter of a few years, the excess SO2 is usually cleared out of the atmosphere. Basically, the SO2 combines with water in the atmosphere, and falls back on earth as rain. It doesn’t seem likely, therefore, that even if there was a lot of volcanic SO2 released at the end of the Triassic, it stayed in the atmosphere long enough to cause mass distinctions.Third, very few scientists believe the asteroid theory because we haven’t found any asteroid crater the side where the asteroid hit that can be dated to the time when the mass distinction occurred. We did find a crater, but it dates to about 12 million years before the extinction. That’s just too long before the extinction to have anything to do with it.
Summarize the points made in the lecture, being sure to explain how they challenge the specific theories presented in the reading passage.
This set of material focus on the debate over explanations about mass-extinction event. The author proposes three possible theories but the speaker retorts them one by one. First, the passage says that the decline of sea level will cause coast species to distinct and thus lead to the elimination of other species. However, the lecturer indicates that sea level indeed fluctuates during this period, but it actually goes down at a very low speed. This change takes millions of years. The period is so long that it is certainly enough time for earth species to adapt themselves to it. Therefore, the first idea of the essay is rebutted. Second, the author demonstrated that frequent volcanic activity will release too much sulfur dioxide which can lead to earth cooling. Nevertheless, the speaker argues that sulfur dioxide presents for few years in earth’s atmosphere, but it can combine with water and become rain. Consequently, the sulfur dioxide leaves the air so it is impossible for it to stay long enough in the atmosphere and cause apparent temperature drop. Third, the writer holds that asteroids collide with the earth produces large amounts of debris which block sunlight, finally leading to the death of plantations and the starvation of animals. While the speaker says that it is found the asteroid strike happened about 12 million years before the mass-extinction. It is too long before the sudden elimination of earth species so probably has nothing to do with this great collision. Thus overturns the third viewpoint in the essay. (254 words)
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