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有考到这题，但是不记得跟原文有没有差了，但是是Milankovitc这个人的理论无疑。这篇阅读跟JJ中的第一段应该是一致的，大家可以参考看看。
GMAT的阅读都是选自实际存在的一些科学论文，只不过是截取片段或者是删减内容等，所以这篇不妨可以当做背景资料看看。
呼唤木木来看看~~~~
OG-14
Milankovitch proposed in the early twentieth centurythat the ice ages were caused by variations in the Earth’s orbit around theSun. For some time this theory was considered untestable, largely because therewas no sufficiently precise chronology of the ice ages with which the orbitalvariations could be matched.
To establish such a chronology it is necessary todetermine the relative amounts of land ice that existed at various times in theEarth’s past. A recent discovery makes such a determination possible: relativeland-ice volume for a given period can be deduced from the ratio of two oxygenisotopes, 16 and 18, found in ocean sediments. Almost all the oxygen in wateris oxygen 16, but a few molecules out of every thousand incorporate the heavierisotope 18. When an ice age begins, the continental ice sheets grow, steadilyreducing the amount of water evaporated from the ocean that will eventuallyreturn to it. Because heavier isotopes tend to be left behind when waterevaporates from the ocean surfaces, the remaining ocean water becomesprogressively enriched in oxygen 18. The degree of enrichment can be determinedby analyzing ocean sediments of the period, because these sediments arecomposed of calcium carbonate shells of marine organisms, shells that wereconstructed with oxygen atoms drawn from the surrounding ocean. The higher theratio of oxygen 18 to oxygen 16 in a sedimentary specimen, the more land icethere was when the sediment was laid down.
As an indicator of shifts in the Earth’s climate, theisotope record has two advantages. First, it is a global record: there isremarkably little variation in isotope ratios in sedimentary specimens takenfrom different continental locations. Second, it is a more continuous recordthan that taken from rocks on land. Because of these advantages, sedimentary evidence can be dated with sufficient accuracy byradiometric methods to establish a precise chronology of the ice ages. Thedated isotope record shows that the fluctuations in global ice volume over thepast several hundred thousand years have a pattern: an ice age occurs roughlyonce every 100,000 years. These data have established a strong connectionbetween variations in the Earth’s orbit and the periodicity of the ice ages.However, it is important to note that other factors,such as volcanic particulates or variations in the amount of sunlight receivedby the Earth, could potentially have affected the climate. The advantage of theMilankovitch theory is that it is testable; changes in the Earth’s orbit can becalculated and dated by applying Newton’s laws of gravity to progressivelyearlier configurations of the bodies in the solar system. Yet the lack ofinformation about other possible factors affecting global climate does not makethem unimportant.
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发表于 2012-11-6 20:25:21
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