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草莓小菇凉:说的非常好,十分有道理,棒棒棒!

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TPO-15- Mass Extinctions

Cases in which many species become extinct within a geologically short interval of time are called mass extinctions. There was one such event at the end of the Cretaceous period (around 70 million years ago). There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago). The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar species perished at that time.

The fossil record shows at least five mass extinctions in which many families of marine organisms died out. The rates of extinction happening today are as great as the rates during these mass extinctions. Many scientists have therefore concluded that a sixth great mass extinction is currently in progress.

What could cause such high rates of extinction? There are several hypotheses, including warming or cooling of Earth, changes in seasonal fluctuations or ocean currents, and changing positions of the continents. Biological hypotheses include ecological changes brought about by the evolution of cooperation between insects and flowering plants or of bottom-feeding predators in the oceans. Some of the proposed mechanisms required a very brief period during which all extinctions suddenly took place; other mechanisms would be more likely to have taken place more gradually, over an extended period, or at different times on different continents. Some hypotheses fail to account for simultaneous extinctions on land and in the seas. Each mass extinction may have had a different cause. Evidence points to hunting by humans and habitat destruction as the likely causes for the current mass extinction.

American paleontologists David Raup and John Sepkoski, who have studied extinction rates in a number of fossil groups, suggest that episodes of increased extinction have recurred periodically, approximately every 26 million years since the mid-Cretaceous period. The late Cretaceous extinction of the dinosaurs and ammonoids was just one of the more drastic in a whole series of such recurrent extinction episodes. The possibility that mass extinctions may recur periodically has given rise to such hypotheses as that of a companion star with a long-period orbit deflecting other bodies from their normal orbits, making some of them fall to Earth as meteors and causing widespread devastation upon impact.

Of the various hypotheses attempting to account for the late Cretaceous extinctions, the one that has attracted the most attention in recent years is the asteroid-impact hypothesis first suggested by Luis and Walter Alvarez. According to this hypothesis, Earth collided with an asteroid with an estimated diameter of 10 kilometers, or with several asteroids, the combined mass of which was comparable. The force of collision spewed large amounts of debris into the atmosphere, darkening the skies for several years before the finer particles settled. The reduced level of photosynthesis led to a massive decline in plant life of all kinds, and this caused massive starvation first of herbivores and subsequently of carnivores. The mass extinction would have occurred very suddenly under this hypothesis.

One interesting test of the Alvarez hypothesis is based on the presence of the rare-earth element iridium (Ir). Earth’s crust contains very little of this element, but most asteroids contain a lot more. Debris thrown into the atmosphere by an asteroid collision would presumably contain large amounts of iridium, and atmospheric currents would carry this material all over the globe. A search of sedimentary deposits that span the boundary between the Cretaceous and Tertiary periods shows that there is a dramatic increase in the abundance of iridium briefly and precisely at this boundary. This iridium anomaly offers strong support for the Alvarez hypothesis even though no asteroid itself has ever been recovered.

An asteroid of this size would be expected to leave an immense crater, even if the asteroid itself was disintegrated by the impact. The intense heat of the impact would produce heat-shocked quartz in many types of rock. Also, large blocks thrown aside by the impact would form secondary craters surrounding the main crater. To date, several such secondary craters have been found along Mexico’s Yucatan Peninsula, and heat-shocked quartz has been found both in Mexico and in Haiti. A location called Chicxulub, along the Yucatan coast, has been suggested as the primary impact site.

许多物种在地质上很短的时间内灭绝的情况被称为大灭绝。白垩纪末(七千万年前)有一次这样的事件。二叠纪末(二亿五千万年前)还有另一场更大的灭绝事件。二叠纪事件比其他大规模灭绝少得多,因为当时大多数陌生的物种都灭绝了。

 

化石记录显示,至少有5个海洋生物家族灭绝的大规模灭绝。今天发生的灭绝的速度和这些大灭绝的速度一样大。许多科学家因此得出结论:目前正在进行第六次大规模灭绝。

 

什么会导致如此高的灭绝率呢?有几个假设,包括地球变暖或降温,季节性波动或洋流的变化,以及大陆的变化。生物假说包括由昆虫与开花植物或海洋底层捕食掠食者之间的合作进化带来的生态变化。一些提议的机制需要一个非常短暂的时间,在这段时间内突然发生所有的灭绝事件;其他机制更有可能在更长的时期内或在不同的时间在不同的大陆发生。有些假设不能说明陆地和海洋的同时灭绝。每一次大规模灭绝都可能有不同的原因。有证据表明,人类狩猎和栖息地破坏是当前大规模灭绝的可能原因。

 

已经在多个化石组中研究灭绝率的美国古生物学家大卫·劳普(David Raup)和约翰·塞普科斯基(John Sepkoski)表明,自白垩纪中期以来,大约每隔2600万年,定期复发的灭绝事件增加。恐龙和菊石化晚期的白垩纪灭绝只是这一系列反复灭绝事件中较为激烈的一次。大规模灭绝可能会周期性地发生的可能性已经引起了这样的假设,例如伴随着长周期轨道的伴星将其他正常轨道从其正常轨道偏转,使得其中一些作为流星落入地球,并在影响时造成广泛的破坏。

 

在试图解释白垩纪晚期灭绝的各种假设中,近年来引起最多关注的是由路易斯和瓦尔特·阿尔瓦雷斯首次提出的小行星撞击假说。根据这一假设,地球与估计直径为10公里的小行星相撞,或与几颗小行星相撞,其总体质量相当。碰撞的力量将大量的碎片喷入大气中,在更细的颗粒沉降之前使天空变暗数年。光合作用水平的降低导致各种植物生命的大量减少,这导致大量的食草动物首先是食草动物,然后是食肉动物。在这个假设下,大规模灭绝将会非常突然地发生。

 

阿尔瓦雷斯假设的一个有趣的测试是基于稀土元素铱(Ir)的存在。地壳含有很少的这种元素,但大多数小行星包含更多。通过小行星碰撞投入大气中的碎片可能含有大量的铱,而大气中的这种物质将会传播到全球各地。对跨越白垩纪和第三纪之间边界的沉积矿床的搜索表明,在这个边界处铱的丰度急剧增加。这种铱异常提供了强有力的支持阿尔瓦雷斯假说,即使没有小行星本身已经恢复。

 

即使小行星本身被撞击破坏,这样一个尺寸的小行星也可能会留下巨大的火山口。冲击的强烈热量会在许多类型的岩石中产生热震石英。另外,由于撞击而抛出的大块石块会形成围绕主火山口的二级陨石坑。迄今为止,在墨西哥尤卡坦半岛发现了几个这样的二级陨石坑,在墨西哥和海地都发现了热震石英。建议沿着尤卡坦半岛沿海的一个称为Chicxulub的地点作为主要的影响地点。


中文翻译

Paragraph 1 supports which of the following statements about mass extinctions?

  • Cases in which many species become extinct within a geologically short interval of time are called mass extinctions.  A They take place over a period of 70 million years. B They began during the Cretaceous period. C They eliminate many animal species that exist at the time they occur. D They occur every 250 million years.
正确答案: C

网友解析

  • 用户头像

    托儿  发表于 2018-10-21 22:02:53


    托福阅读答案

    1.以mass extinctions做关键词定位至第一句,说大量生物在短时间内灭绝的这种现象叫做大灭绝事件,C是原文的同义替换,所以是正确答案。70 million和250 million是两次大灭绝事件发生的时间,不是A说的七千万年一直在发生,也不是D说的每2500万年发生一次;B的began和原文的end是反的,错。

  • 用户头像

    joannazz  发表于 2018-01-16 11:32:05

    事实信息题
    解析:ABD没说。C可以根据第1句话得知

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