托福听力满分笔记该怎么做才好?

　　讲座文本及分析

　　Listen to part of a lecture in an earth science class.

　　Now, one of the things I like to do from time to time in this class is look at how knowledge we’ve gained from studying earth’s geology has been applied to questions outside our field. (理解为主，注意questions, 预判下文要解决的问题)Take the mass extinction that occurred around 13,000 years ago when most of the giant mammal species of North America vanished from the geological record. (举例)Creatures like wholly mammoths, saber-toothed cats, giant sloths, beavers, and camels. So, what caused these animals to suddenly disappear? (全文中心句：这些动物突然消失的原因是什么?)One possible answer lies at the start of a period of sudden climate change called the Younger Dryas.(第一个观点：与气候变化有关)

　　Let me back up a minute. Just before this extinction, Earth was coming out of a long ice age. Glaciers were beginning to recede, but then temperatures in North America suddenly plummeted again, setting off this frigid thousand year period known as the Younger Dryas. North America became so cold, that glaciers started expanding again and one theory is that this sudden change in climate would have made it difficult for these large beasts to survive. (解释：Younger Dryas时期很冷导致大型动物很难生存)Of course there’s been other climate changes, some even more extreme and longer lasting, yet with no evidence that they triggered any extinction events, so maybe not the strongest theory, which means we need to look elsewhere, maybe space.(对比：其他气候变化，但没有证据表明这些气候变化会引发动物灭绝，所以需要找其他证据，提出第二个观点——与太空有关)I mean, we’re all familiar with how impact events can affect life on earth, like it’s now generally agreed that a meteor triggered the extinction of the dinosaurs.(解释：提出撞击事件影响生命; 类比：流星导致恐龙灭绝) Of course, since that theory’s been widely accepted(态度：这个理论被广泛接受) it’s tempting to look to space to explain all extinction events. A large meteor crashing into earth would scatter cosmic debris and cause massive firestorms and that’s the theory proposed recently as an explanation for the large mammal extinction. (第二个观点：提出近期提出的流星撞击地球的理论可以解释大型哺乳动物灭绝)

　　Researchers who support this theory claim to have found evidence of a meteor impact at a site in the state of Arizona in a layer of sediment called the Younger Dryas Boundary. (支持第二个观点的证据)The Younger Dryas Boundary, or YDB，is a very thin layer of sediment that was laid down across North America at the beginning of the Younger Dryas period (YDB概念与解释)and what’s particularly significant about this 13,000 year old layer is that under it, we find lots of fossils of these large mammals, but above it, that is after the YDB was laid down, well, not a single one. (评价YDB的重要性，需要理解)So, what happened here?(引出下文的证据)

　　Although the researchers suspected a meteor event, they didn’t find any evidence of an impact crater. (没有找到撞击坑的证据)But they said they did find several types of particles commonly associated with meteor craters, including nano-diamonds and high concentrations of magnetic particles. (转折：提供证据 —— 纳米钻石和高密度的磁粒子)Nano-diamonds are small particles that can either originate in space or be formed in the extreme pressure of an extensive explosion, like the impact of a meteor for example. The magnetic particles can also come from space. And the ones found in YDB are very similar to particles associated with other meteor impacts. (两种粒子的解释与来源)

　　Recently however, a new group of researchers tried to replicate these findings. And while they also found nano-diamonds and high concentrations of magnetic particles at the Arizona site, these researchers wondered whether the presence of such particles might be accounted for in other ways. (转折：新研究及质疑) So, to test whether the magnetic particles were unique to the YDB, they analyzed dirt from the rooftop of a researcher’s house and in fact, they found magnetic particles in the rooftop sample too. Turns out that these particles are not just the result of impact events. They can have many origins, including the ash from nearby coal burning, electrical power plants or even cosmic dust that originates in space and then falls to earth. (发现：通过研究屋顶的尘土，也发现了粒子;结论：磁粒子不一定来撞击事件，质疑了第二个观点)

　　Ok, but then how to explain the elevated concentrations of the magnetic particles in the sample from the YDB?(转折+问题：如何解释YDB样本中发现的高密度磁粒子?)Well, these samples had come from a riverbed where rainwater would have carried and deposited the particles. And in fact, additional samples taken from outside the riverbed contained only normal concentrations of the magnetic particles in the YDB. (证据：高密度磁粒子来源于河床的样本，河床外的其他样本只有正常密度的磁粒子;反驳第二个观点) As for the nano-diamonds, while a group of meteorite or comet fragments colliding with earth would certainly provide the high temperatures and pressures needed to create such nano-diamonds, there might be other ways to explain their existence. It turns out, they’re also commonly found in cosmic dust, which could explain their presence not only in the YDB, but also in the rooftop sample. (证据：纳米钻石不止在YDB中，屋顶样本也有;反驳第二个观点)So, if there’s no strong evidence for a meteor impact or for a sudden climate change, then we need some other explanation for the disappearance of so many large, North American mammal species during the Younger Dryas. (结论：第一个观点和第二个观点都没有强力的证据解释北美大型动物灭亡，需要找其他解释说明)