Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth’s orbit around the Sun. For sometime this theory was considered untestable, largely because there was no sufficiently precise chronology of the ice ages with which the orbital variations could be matched.
看到Milankovitch，出现人物，人物观点Point是 ice age were caused by variation in the earth's orbit around the sun. 意识到后面跟着illustration：this theory was untestable，because no chronology of the ice ages matched with orbital variation
To establish such a chronology it is necessary to determine the relative amounts of land ice that existed at various times in the Earth’s past. A recent discovery makes such a determination possible: relative land-ice volume for a given period can be deduced from the ratio of two oxygen isotopes, 16 and 18, found in ocean sediments. Almost all the oxygen in water is oxygen 16, but a few molecules out of every thousand incorporate the heavier isotope 18. When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces, the remaining ocean water becomes progressively enriched in oxygen 18. The degree of enrichment can be determined by analyzing ocean sediments of the period, because these sediments are composed of calcium carbonate (calcium carbonate: n.[化]碳酸钙) shells of marine organisms, shells that were constructed with oxygen atoms drawn from the surrounding ocean. The higher the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen, the more land ice there was when the sediment was laid down.
看到To do 意识到表示目的的Point 来了，to determine the relative amounts of land ice .
后面的是illustration：A recent discovery makes a determination：land-ice volume can be deduced from the ratio of two isotopes,16&18,found in ocean sediments
后面的是阐述的内容：1)in water is oxyen 16, but a few molecules incorporate the heavier isotope 18. 2) the continental ice sheets grow and reduce the amount of water evaporated .3) the degree of enrichment can be determined by analyzing ocean sediments, because these sediments are composed of calcium cardonate shells of marine organism,shells that were constructed with oxyen atoms drawn from the surrounding ocean. summary the higer the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen, the more land ice there was when the sediment was laid down.
As an indicator of shifts in the Earth’s climate, the isotope record has two advantages. First, it is a global record: there is remarkably little variation in isotope ratios in sedimentary specimens taken from different continental locations. Second, it is a more continuous record than that taken from rocks on land. Because of these advantages, sedimentary evidence can be dated with sufficient accuracy by radiometric methods to establish a precise chronology of the ice ages. The dated isotope record shows that the fluctuations in global ice volume over the past several hundred thousand years have a pattern: an ice age occurs roughly once every 100,000 years. These data have established a strong connection between variations in the Earth’s orbit and the periodicity of the ice ages.
The isotope record has two advantages. 观点来了！Point 来了
注意，first 和second： first 阐述 there is remarkably little variation in isotope ratios in sedimentary taken from different continental locations.
Second: it is a more continuous record than that taken from rocks on land.
Illustrations：sedimentary evidence can be dated to establish a precise chronology . the dated isotope shows that fluctuations have a pattern: an ice age occurs every 100000 years. these data have established a strong connection between variations 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 received by the Earth, could potentially have affected the climate. The advantage of the Milankovitch theory is that it is testable: changes in the Earth’s orbit can be calculated and dated by applying Newton’s laws of gravity to progressively earlier configurations of the bodies in the solar system. Yet the lack of information about other possible factors affecting global climate does not make them unimportant.
Conclusion:it is important to note that other factors, such as volcanic particulates could potentially have affected the climate.
the advantage is : changes can be calcuated and dated ;
最后一段也很简单，这也是GMAT文章很普遍的一个现象：为了表示其理论的严谨性，通常作者一会一杆子打死所有反驳，他会承认，的确是有例外的，我不是说没有例外，但是这些例外是可以解释的――不用我说，大家也该知道，S解释的内容才是本段的重点
文章读完，要化几秒钟回顾一下整个文章的架构： 第一段：ice age were caused by variation in the earth's orbit around the sun,this theory was untestable，because no chronology of the ice ages matched with orbital variation
第二段：To establish such a chronology is necessay to determine the relative amounts of land ice ，16和18 istope 第三段：The isotope record has two advantages. 观点来了！Point 来了
注意，first 和second： first 阐述 there is remarkably little variation in isotope ratios in sedimentary taken from different continental locations.
Second: it is a more continuous record than that taken from rocks on land.
第四段：it is important to note that other factors, such as volcanic particulates could potentially have affected the climate.
每一段的新名词，都要注意跟ice age之间的关系，因为文章的主题就是和ice age紧密相关。GMAT中，阅读也好，逻辑也好，主要看的，都是众多事物之间的关系。
DDBC BCEAB
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发表于 2014-4-2 22:59:34
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