雅思阅读材料：恐龙灭绝(中英)

　　EXTINCTION OF THE DINOSAURS

　　Paleontologistshave argued for a long time that the demise of the dinosaurs was caused byclimatic alterations associated with slow changes in the positions ofcontinents and seas resulting from plate tectonics. Off and on throughout theCretaceous (the last period of the Mesozoic era, during which dinosaursflourished), large shallow seas covered extensive areas of the continents. Datafrom diverse sources, including geochemical evidence preserved in seafloorsediments, 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 toowarm, nor the winters too frigid. The shallow seas on the continents probablybuffered the temperature of the nearby air, keeping it relatively constant.

　　At the end ofthe Cretaceous, the geological record shows that these seaways retreated fromthe continents back into the major ocean basins. No one knows why. Over aperiod of about 100,000 years, while the seas pulled back, climates around theworld became dramatically more extreme: warmer days, cooler nights; hottersummers, colder winters. Perhaps dinosaurs could not tolerate these extremetemperature changes and became extinct.

　　If true,though, why did cold-blooded animals such as snakes, lizards, turtles, andcrocodiles survive the freezing winters and torrid summers? These animals areat the mercy of the climate to maintain a livable body temperature. It’s hardto understand why they would not be affected, whereas dinosaurs were left toocrippled to cope, especially if, as some scientists believe, dinosaurs werewarm-blooded. Critics also point out that the shallow seaways had retreatedfrom and advanced on the continents numerous times during the Mesozoic, so whydid the dinosaurs survive the climatic changes associated with the earlierfluctuations but not with this one? Although initially appealing, thehypothesis of a simple climatic change related to sea levels is insufficient toexplain all the data.

　　Dissatisfactionwith conventional explanations for dinosaur extinctions led to a surprisingobservation that, in turn, has suggested a new hypothesis. Many plants andanimals disappear abruptly from the fossil record as one moves from layers ofrock documenting the end of the Cretaceous up into rocks representing thebeginning of the Cenozoic (the era after the Mesozoic). Between the last layer ofCretaceous rock and the first layer of Cenozoic rock, there is often a thinlayer of clay. Scientists felt that they could get an idea of how long theextinctions took by determining how long it took to deposit this one centimeterof clay and they thought they could determine the time it took to deposit theclay by determining the amount of the element iridium (lr) it contained.

　　It has notbeen common at Earth’s since the very beginning of the planet’s history.Because it usually exists in a metallic state, it was preferentiallyincorporated in Earth’s core as the planet cooled and consolidated. lr is foundin high concentrations in some meteorites, in which the solar system’s originalchemical composition is preserved. Even today, microscopic meteoritescontinually bombard Earth, falling on both land and sea. By measuring how manyof these meteorites fall to Earth over a given period of time, scientists canestimate how long it might have taken to deposit the observed amount of lr inthe boundary clay. These calculations suggest that a period of about onemillion years would have been required. However, other reliable evidencesuggests that the deposition of the boundary clay could not have taken onemillion years. So the unusually high concentration of lr seems to require aspecial explanation.

　　In view ofthese facts, scientists hypothesized that a single large asteroid, about 10 to15 kilometers across, collided with Earth, and the resulting fallout createdthe boundary clay. Their calculations show that the impact kicked up a dustcloud that cut off sunlight for several months, inhibiting photosynthesis inplants; decreased surface temperatures on continents to below freezing; causedextreme episodes of acid rain; and significantly raised long-term globaltemperatures through the greenhouse effect. This disruption of food chain andclimate would have eradicated the dinosaurs and other organisms in less thanfifty years.