GMAT网络课堂阅读笔记-王昆嵩主讲[4]

文章作者 100test 发表时间 2007:03:10 10:22:30
来源 100Test.Com百考试题网

In choosing a method for determining climatic condi-
　　tions that existed in the past, paleoclimatologists invoke
　　four principal criteria. First, the material-rocks, lakes,
　　vegetation, etc.-on which the method relies must be
(5) widespread enough to provide plenty of information,
　　since analysis of material that is rarely encountered will
　　not permit correlation with other regions or with other
　　periods of geological history. Second, in the process of
　　formation, the material must have received an environ-
(10) mental signal that reflects a change in climate and that
　　can be deciphered by modern physical or chemical
　　means. Third, at least some of the material must have
　　retained the signal unaffected by subsequent changes in
　　the environment. Fourth, it must be possible to deter-
(15) mine the time at which the inferred climatic conditions
　　held. This last criterion is more easily met in dating
　　marine sediments, because dating of only a small
　　number of layers in a marine sequence allows the age of
　　other layers to be estimated fairly reliably by extrapola-
(20) tion and interpolation. By contrast, because sedimenta-
　　tion is much less continuous in continental regions, esti-
　　mating the age of a continental bed from the known
　　ages of beds above and below is more risky.
　　One very old method used in the investigation of past
(25) climatic conditions involves the measurement of water
　　levels in ancient lakes. In temperate regions, there are
　　enough lakes for correlations between them to give us a
　　reliable picture. In arid and semiarid regions, on the
　　other hand, the small number of lakes and the great
(30) distances between them reduce the possibilities for corre-
　　lation. Moreover, since lake levels are controlled by rates
　　of evaporation as well as by precipitation, the interpreta-
　　tion of such levels is ambiguous. For instance, the fact
　　that lake levels in the semiarid southwestern United
(35) States appear to have been higher during the last ice age
　　than they are now was at one time attributed to
　　increased precipitation. On the basis of snow-line eleva-
　　tions, however, it has been concluded that the climate
　　then was not necessarily wetter than it is now, but rather
(40) that both summers and winters were cooler, resulting in
　　reduced evaporation.
　　Another problematic method is to reconstruct former
　　climates on the basis of pollen profiles. The type of vege-
　　tation in a specific region is determined by identifying
(45) and counting the various pollen grains found there.
　　Although the relationship between vegetation and
　　climate is not as direct as the relationship between
　　climate and lake levels, the method often works well in
　　the temperate zones. In arid and semiarid regions in

(50) which there is not much vegetation, however, small
　　changes in one or a few plant types can change the
　　picture dramatically, making accurate correlations
　　between neighboring areas difficult to obtain.

1. Which of the following statements about the difference between marine and continental sedimentation is supported by information in the passage?
(A) Data provided by dating marine sedimentation is more consistent with researchers findings in other disciplines than is data provided by dating continental sedimentation.
(B) It is easier to estimate the age of a layer in a sequence of continental sedimentation than it is to estimate the age of a layer in a sequence of marine sedimentation.
(C) Marine sedimentation is much less widespread than continental sedimentation.
(D) Researchers are more often forced to rely on extrapolation when dating a layer of marine sedimentation than when dating a layer of continental sedimentation.
(E) Marine sedimentation is much more continuous than is continental sedimentation.

2. Which of the following statements best describes the organization of the passage as a whole?
(A) The author describes a method for determining past climatic conditions and then offers specific examples of situations in which it has been used.
(B) The author discusses the method of dating marine and continental sequences and then explains how dating is more difficult with lake levels than with pollen profiles.
(C) The author describes the common requirements of methods for determining past climatic conditions and then discusses examples of such methods.
(D) The author describes various ways of choosing a material for determining past climatic conditions and then discusses how two such methods have yielded contradictory data.
(E) The author describes how methods for determining past climatic conditions were first developed and then describes two of the earliest known methods.
3. It can be inferred from the passage that paleoclimatologists have concluded which of the following on the basis of their study of snow-line elevations in the southwestern United States?
(A) There is usually more precipitation during an ice age because of increased amounts of evaporation.
(B) There was less precipitation during the last ice age than there is today.
(C) Lake levels in the semiarid southwestern United States were lower during the last ice age than they are today.
(D) During the last ice age, cooler weather led to lower lake levels than paleoclimatologists had previously assumed.
(E) The high lake levels during the last ice age may have been a result of less evaporation rather than more precipitation.