GRE阅读综合辅导:63题网络课堂总结(二十二)

　　Passage 18 问题解决型特别套路

　　提出问题：冰期的计时未知Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earths orbit around the Sun. For sometime this theory was considered untestable, largely because there was no sufficiently precise 计时 of the ice ages with which the orbital variations could be matched.

　　提出解决方法：同位素方法To establish such a chronology it is necessary to determine the relative amounts of land ice that existed at various times in the Earths past. A recent discovery makes such a determination possible: relative land-ice volume for a given period 能从推出4C the ratio of two oxygen isotopes4, 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 由决定 analyzing ocean sediments of the period, because these sediments are composed of calcium carbonate 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. chronologylandicerationO18/O16

　　landicesedimentsCaCO3 ocean: O18/O16

　　方法的优点：地理连续性和时间连续性As an indicator of shifts in the Earths climate, the isotope record3 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 years6. These data have established a strong connection between variations in the Earths orbit and the periodicity of the ice ages.

　　延伸性内容：其他影响气候的因素有哪些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 Earths orbit can be calculated and dated by applying Newtons 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.

　　1. In the passage, the author is primarily interested in主题题

　　 suggesting an alternative to an outdated research method

　　 introducing a new research method that calls an accepted theory into question

　　 emphasizing the instability of data gathered from the application of a new scientific method

　　 presenting a theory and describing a new method to test that theory问题解决型，主题题不考虑延伸性内容

　　 initiating a debate about a widely accepted theory

　　2. The author of the passage would be most likely to agree with which of the following statements about the Milankovitch theory?看似信息题，实为态度题

　　 It is the only possible explanation for the ice ages.

　　 It is too limited to provide a plausible explanation for the ice ages, despite recent research findings.

　　 It cannot be tested and confirmed until further research on volcanic activity is done.

　　 It is one plausible explanation, though not the only one, for the ice ages.正评价，但不是唯一可行的理论

　　 It is not a plausible explanation for the ice ages, although it has opened up promising possibilities for future research.

　　3. It can be inferred from the passage that the isotope record3 taken from ocean sediments would be less useful to researchers if which of the following were true?改善题的变体

　　 It indicated that lighter isotopes of oxygen predominated at certain times.

　　 It had far more gaps in its sequence than the record taken from rocks on land.找优点，取非

　　 It indicated that climate shifts did not occur every 100,000 years.

　　 It indicated that the ratios of oxygen 16 and oxygen 18 in ocean water were not consistent with those found in fresh water.

　　 It stretched back for only a million years.

　　4. According to the passage, which of the following is true of the ratios of oxygen isotopes in ocean sediments4?信息题

　　 They indicate that sediments found during an ice age contain more calcium carbonate than sediments formed at other times.

　　 They are less reliable than the evidence from rocks on land in determining the volume of land ice.

　　 They can be used to deduce the relative volume of land ice4C that was present when the sediment was laid down.

　　 They are more unpredictable during an ice age than in other climatic conditions.

　　 They can be used to determine atmospheric conditions at various times in the past.

　　5. It can be inferred from the passage that precipitation formed from evaporated ocean water has 浅层理解题

　　 the same isotopic ratio as ocean water

　　 less oxygen 18 than does ocean water因为O18比较重，不太能蒸发出来

　　 less oxygen 18 than has the ice contained in continental ice sheets

　　 a different isotopic composition than has precipitation formed from water on land

　　 more oxygen 16 than has precipitation formed from fresh water

　　6. According to the passage, which of the following is true of the ice ages?

　　I. The last ice age occurred about 25,000 years ago.

　　II. Ice ages have lasted about 10,000 years for at least the last several hundred thousand years.

　　III. Ice ages have occurred about every 100,000 years6 for at least the last several hundred thousand years.

　　 I only

　　 II only

　　 III only

　　 I and only

　　 I, II and III

　　7. It can be inferred from the passage that calcium carbonate shells信息题

　　 are not as susceptible to deterioration as rocks

　　 are less common in sediments formed during an ice age

　　 are found only in areas that were once covered by land ice

　　 contain radioactive material that can be used to determine a sediments isotopic composition

　　 reflect the isotopic composition of the water at the time the shells were formed由海水决定

　　8. The purpose of the last paragraph of the passage is to

　　 offer a note of caution提出令人警戒的注意事项

　　 introduce new evidence

　　 present two recent discoveries

　　 summarize material in the preceding paragraphs

　　 offer two explanations for a phenomenon从全文的布局看，不是提供解释

　　9. According to the passage, one advantage of studying the isotope record of ocean sediments is that it

　　 corresponds with the record of ice volume taken from rocks on land

　　 shows little variation in isotope ratios when samples are taken from different continental locations地理连续性

　　 corresponds with predictions already made by climatologists and experts in other fields

　　 confirms the record of ice volume initially established by analyzing variations in volcanic emissions

　　 provides data that can be used to substantiate records concerning variations in the amount of sunlight received by the Earth