托福阅读材料 行星和恐龙灭绝的关系

　　广泛的托福阅读背景知识积累，有助于新托福阅读速度和准确率的提升。托福阅读题材分为自然科学、人文科学、互联网等几大类，如果同学们平时能对这些种类的文章都有所涉猎，托福阅读考试中遇到相关的话题就会有非常熟悉的感觉。

　　在2014年5月24日的托福阅读考试中有这样一道题：行星对恐龙灭绝有没有影响。针对这道托福考试，帮大家普及一下相关的背景知识，这样有助于大家在面对这类题目时方便作答。

　　托福阅读真题再现：

　　小行星对恐龙灭绝的影响。一个科学家发现土层中里有很多Ir元素，而Ir元素在地球上少见，因此推断是小行星导致了恐龙灭绝。后面又说了小行星使得气温降低，空气化学组成改变等等也导致恐龙的灭绝，但是一些小的啮齿类动物则存活了下来。

　　托福阅读相关背景：

　　参考文章：Meteorite Impact and Dinosaur Extinction

　　Extinction of the Dinosaurs

　　Mass Extinctions

　　背景知识：

　　Impact event

　　Biospheric effects

　　The effect of impact events on the biosphere has been the subject of scientific debate. Several theories of impact related mass extinction have been developed. In the past 500 million years there have been five generally accepted, major mass extinctions that on average extinguished half of all species. One of the largest mass extinction to have affected life on Earth was in the Permian-Triassic, which ended the Permian period 250 million years ago and killed off 90% of all species; life on Earth took 30 million years to recover. The cause of the Permian-Triassic extinction is still matter of debate with the age and origin of proposed impact craters, i.e. the Bedout High structure, hypothesized to be associated with it are still controversial. The last such mass extinction led to the demise of the dinosaurs and coincided with a large meteorite impact; this is the Cretaceous–Paleogene extinction event (also known as the K–T or K–Pg extinction event); This occurred 66 million years ago. There is no definitive evidence of impacts leading to the three other major mass extinctions.

　　In 1980, physicist Luis Alvarez; his son, geologist Walter Alvarez; and nuclear chemists Frank Asaro and Helen V. Michael from the University of California, Berkeley discovered unusually high concentrations of iridium in a specific layer of rock strata in the Earth's crust. Iridium is an element that is rare on Earth but relatively abundant in many meteorites. From the amount and distribution of iridium present in the 65-million-year-old "iridium layer", the Alvarez team later estimated that an asteroid of 10 to 14 km (6 to 9 mi) must have collided with the earth. This iridium layer at the Cretaceous–Paleogene boundary has been found worldwide at 100 different sites. Multidirectionally shocked quartz (coesite), which is only known to form as the result of large impacts or atomic bomb explosions, has also been found in the same layer at more than 30 sites. Soot and ash at levels tens of thousands times normal levels were found with the above.

　　Anomalies in chromium isotopic ratios found within the K-T boundary layer strongly support the impact theory. Chromium isotopic ratios are homogeneous within the earth, therefore these isotopic anomalies exclude a volcanic origin which was also proposed as a cause for the iridium enrichment. Furthermore the chromium isotopic ratios measured in the K-T boundary are similar to the chromium isotopic ratios found in carbonaceous chondrites. Thus a probable candidate for the impactor is a carbonaceous asteroid but also a comet is possible because comets are assumed to consist of material similar to carbonaceous chondrites.

　　Probably the most convincing evidence for a worldwide catastrophe was the discovery of the crater which has since been named Chicxulub Crater. This crater is centered on the Yucatan Peninsula of Mexico and was discovered by Tony Camargo and Glen Pentfield while working as geophysicists for the Mexican oil companyPEMEX. What they reported as a circular feature later turned out to be a crater estimated to be 180 km (110 mi) in diameter. Other researchers would later find that the end-Cretaceous extinction event that wiped out the dinosaurs had lasted for thousands of years instead of millions of years as had previously been thought. This convinced the vast majority of scientists that this extinction resulted from a point event that is most probably an extraterrestrial impact and not from increased volcanism and climate change (which would spread its main effect over a much longer time period).

　　Recently, several proposed craters around the world have been dated to approximately the same age as Chicxulub — for example, the Silverpit crater in the United Kingdom, the Boltysh crater in Ukraine and the Shiva crater near India. This has led to the suggestion that the Chicxulub impact was one of several that occurred almost simultaneously, perhaps due to a disrupted comet impacting the Earth in a similar manner to the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994; however, the uncertain age and provenance of these structures leaves the hypothesis without widespread support.

　　It was the lack of high concentrations of iridium and shocked quartz which has prevented the acceptance of the idea that the Permian extinction was also caused by an impact. During the late Permian all the continents were combined into one supercontinent named Pangaea and all the oceans formed one superocean,Panthalassa. If an impact occurred in the ocean and not on land at all, then there would be little shocked quartz released (since oceanic crust has relatively little silica) and much less material.

　　Although there is now general agreement that there was a huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, remnants have been found of other, smaller impacts, some nearing half the size of the Chicxulub crater, which did not result in any mass extinctions, and there is no clear linkage between an impact and any other incident of mass extinction.

　　Paleontologists David M. Raup and Jack Sepkoski have proposed that an excess of extinction events occurs roughly every 26 million years (though many are relatively minor). This led physicist Richard A. Muller to suggest that these extinctions could be due to a hypothetical companion star to the Sun calledNemesis periodically disrupting the orbits of comets in the Oort cloud, and leading to a large increase in the number of comets reaching the inner solar system where they might hit Earth. Physicist Adrian Melott and paleontologist Richard Bambach have more recently verified the Raup and Sepkoski finding, but argue that it is not consistent with the characteristics expected of a Nemesis-style periodicity.