2010年9月18日托福机经英文版原文(康老师)

　　2010年09月18日北美

　　3. 侏罗纪恐龙灭绝（内容大部分跟TPO08的阅读差不多）
　　第一段：长久以来，大家都很关注第一次物种大灭绝。但是现在中生代末期的灭绝吸引了越来越多学者的注意，原因是这次灭绝中的物种种类（考题：为什么吸引了注意力，我选的是因为灭绝的物种很特别）。
　　第二段：1980年之后，陨石撞地球的说法流行起来，因为地壳中发现了铱元素。
　　第三段：【插入题：段首】“发现铱元素并不是唯一的证据，而在于异常高的含量”，高含量的铱元素支援了陨石说法。同时，在河床沉积下发现的diameter－180的坑也支持了这一说法。
　　第四段：按照这种假设，撞击引起地球温度变化等等（考题：第四段的功能，我选了，描述了撞击后可能产生的影响）。
　　第五段：但是这个说法不能解释为什么物种灭绝是selective，比如跟恐龙很类似的蜥蜴等冷血动物都活下来了。而且，大陆和海洋的灭绝应该是同时发生的，但是现在还缺少证据证明这一点。
　　第五段：尽管科学家现在都concede（辞汇题，选admit）这种说法，但是它是有漏洞的，还有很多其他的可能。比如，那个时候印度火山活动频繁，可能引发气候异常等现象（有Except题：哪个现象不是可能的因素）。
　　前面给了一张表，是五大集群灭绝的名称及其发生时间。
　　正文开始讲的是几个灭绝事件，其中有问道为什么白垩纪Cretaceous period，比另外一个什么纪的受到更多关注（因为白垩纪的灭绝动物的种类多）。
　　之后提到集群灭绝事件发生的原因，有问到第二段和第三段之间的关系（我答：第二段提出一些因素证据，第三段是由此推出发生了什么）。说在地球表面发现了一种元素，但是在crust里面存在不多，认为是流星陨石所遗留的，这也是造成灭绝的主要原因。之后还说在一个地方发现了一些证据能证明是陨石造成的灭绝，好像是一个150米的大坑，还包括了三个证据，其中我记得有一个证据是什么被沉淀物覆盖了...还有一个是说有巨石的碎片散落...还有一个不记得了。题目有问是那哪个证据不能推出陨石造成集群灭绝。
　　但是有科学家怀疑陨石造成集群灭绝，因为恐龙和海洋生物的灭绝时间不同（有问为什么专家质疑陨石造成灭绝），而且灭绝也分种类，好像是自然选择的结果。最后专家提出可能有别的原因造成集群灭绝，也可能是这些生物已经开始进入灭绝过程，而陨石只是促进了灭绝的进行。
　　Controversy surrounds the extinction of the dinosaurs.
　　A traditional point of view held that dinosaurs were slowly driven to extinction by gradual and long-term environmental changes—changes that dinosaurs could not adapt to. Proponents of this theory postulated that dinosaurs dwindled in number and variety over several million years. Other causes once suggested for the end of the dinosaurs included epidemics or even egg-eating mammals.
　　Another group of theories have tied the extinction of the dinosaurs to more sudden, catastrophic events. The best known of these theories proposes that the impact of an asteroid caused catastrophic destruction of the environment, leading to the extinction of the dinosaurs. Evidence to support this theory includes the discovery of a buried impact crater (thought to be the result of a large comet striking the earth) that is 200 km (124 mi) in diameter in the Yucatan Peninsula of Mexico. A spray of debris, called an ejecta sheet, which was blown from the edge of the crater, has been found over vast regions of North America. Minerals found in asteroids and meteorites enriched material from the impact’s fiery explosion and were distributed all over the world. With radiometric dating (see Dating Methods: Radiometric Dating), scientists have used the decay rates of certain atoms to date the crater, ejecta sheet, and fireball layer. Using similar techniques to date the dramatic changes in the record of microscopic fossils, they have found that the impact and the dinosaur extinction occurred nearly simultaneously.
　　Although large amounts of ash suggest that vast areas of North and South America were devastated by fire from the impact, the longer-term planetwide environmental effects of the impact were ultimately more lethal to life than the fire. Dust blocked sunlight from Earth’s surface for many months. Scorched sulfur from the impact site, water vapor and chlorine from the oceans, and nitrogen from the air combined to produce a worldwide fallout of intensely acidic rain. Scientists postulate that darkness and acid rain caused plant growth to cease. As a result, both the herbivorous dinosaurs, which were dependent on plants for food, as well as the carnivorous dinosaurs, which fed on the herbivores, were exterminated. On the other hand, animals such as frogs, lizards, and small insect-eating turtles and mammals, which were dependent on organisms that fed on decaying plant material, were more likely to survive. Their survival indicates that, in most areas, the surface of Earth did not freeze.
　　An alternative theory links the extinction of the dinosaurs to giant volcanic eruptions in a region of India called the Deccan Traps. These massive eruptions occurred repeatedly over a period of thousands of years around 65 million years ago, releasing large quantities of carbon dioxide and other gases into the atmosphere. Some of the proposed effects of the eruptions would have been similar to those of an asteroid impact, creating a harsh environment in which dinosaurs struggled to survive.
　　Other scientists have suggested a combination of factors and events contributed to the demise of the dinosaurs. The asteroid impact may have been the final blow after climate change and volcanic eruptions had already reduced dinosaur numbers. Field work around the world continues to carefully document the sequence and scale of events that happened at the end of the Cretaceous period. Recent findings preserved in the sea off the coast of northern South America demonstrate that the extinctions did coincide with a single asteroid impact on Yucatan.
　　Extinction of the Dinosaurs (TPO-08)
　　Paleozoic Era 334 to 248 millions ago
　　Mesozoic Era 245 to 65 million years ago
　　- Trassic Period
　　- Jurassic Period
　　- Cretaceous Period
　　Cenozoic Era 65 million years ago to the present
　　Paleontologists have argued for a long time that the demise of the dinosaurs was caused by climatic alterations associated with slow changes in the positions of continents and seas resulting from plate tectonics. Off and on throughout the Cretaceous (the last period of the Mesozoic era, during which dinosaurs flourished), large shallow seas covered extensive areas of the continents. Data from diverse sources, including geochemical evidence preserved in seafloor sediments, indicate that the Late Cretaceous climate was milder than today’s. The days were not too hot, nor the nights too cold. The summers were not too warm, nor the winters too frigid. The shallow seas on the continents probably buffered the temperature of the nearby air, keeping it relatively constant
　　At the end of the Cretaceous, the geological record shows that these seaways retreated from the continents back into the major ocean basins. No one knows why. Over a period of about 100,000 years, while the seas pulled back, climates around the world became dramatically more extreme: warmer days, cooler nights; hotter summers, colder winters. Perhaps dinosaurs could not tolerate these extreme temperature changes and became extinct.
　　If true, though, why did cold-blooded animals such as snakes, lizards, turtles, and crocodiles survive the freezing winters and torrid summers? These animals are at the mercy of the climate to maintain a livable body temperature. It’s hard to understand why they would not be affected, whereas dinosaurs were left too crippled to cope, especially if, as some scientists believe, dinosaurs were warm-blooded. Critics also point out that the shallow seaways had retreated from and advanced on the continents numerous times during the Mesozoic, so why did the dinosaurs survive the climatic changes associated with the earlier fluctuations but not with this one? Although initially appealing, the hypothesis of a simple climatic change related to sea levels is insufficient to explain all the data.
　　Dissatisfaction with conventional explanations for dinosaur extinctions led to a surprising observation that, in turn, has suggested a new hypothesis. Many plants and animals disappear abruptly from the fossil record as one moves from layers of rock documenting the end of the Cretaceous up into rocks representing the beginning of the Cenozoic (the era after the Mesozoic). Between the last layer of Cretaceous rock and the first layer of Cenozoic rock, there is often a thin layer of clay. Scientists felt that they could get an idea of how long the extinctions took by determining how long it took to deposit this one centimeter of clay and they thought they could determine the time it took to deposit the clay by determining the amount of the element iridium (Ir) it contained.
　　Ir has not been common at Earth’s surface since the very beginning of the planet’s history. Because it usually exists in a metallic state, it was preferentially incorporated in Earth’s core as the planet cooled and consolidated. Iris found in high concentrations in some meteorites, in which the solar system’s original chemical composition is preserved. Even today, microscopic meteorites continually bombard Earth, falling on both land and sea. By measuring how many of these meteorites fall to Earth over a given period of time, scientists can estimate how long it might have taken to deposit the observed amount of lr in the boundary clay. ■ These calculations suggest that a period of about one million years would have been required. ■ However, other reliable evidence suggests that the deposition of the boundary clay could not have taken one million years. ■ So the unusually high concentration of lr seems to require a special explanation. ■
　　In view of these facts, scientists hypothesized that a single large asteroid, about 10 to 15 kilometers across, collided with Earth, and the resulting fallout created the boundary clay. Their calculations show that the impact kicked up a dust cloud that cut off sunlight for several months, inhibiting photosynthesis in plants; decreased surface temperatures on continents to below freezing; caused extreme episodes of acid rain; and significantly raised long-term global temperatures through the greenhouse effect. This disruption of food chain and climate would have eradicated the dinosaurs and other organisms in less than fifty years

　　以上机经是康老师2011年10月－12月托福预测范围里的。