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  3. In choosing a method for determining climatic conditions that existed in the past, paleoclimatologists invoke four principal cri

In choosing a method for determining climatic conditions that existed in the past, paleoclimatologists invoke four principal cri

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问题     In choosing a method for determining climatic conditions that existed in the past, paleoclimatologists invoke four principal criteria. First, the material— rocks, lakes, vegetation, etc—on which the method relies must be 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 environmental 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 determine 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 extrapolation and interpolation. By contrast, because sedimentation is much less continuous in continental regions, estimating 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 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 distances between them reduce the possibilities for correlation. Moreover, since lake levels are controlled by rates of evaporation as well as by precipitation, the interpretation of such levels is ambiguous. For instance, the fact that lake levels in the semiarid southwestern United 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 snowline elevations, however, it has been concluded that the climate then was not necessarily wetter than it is now, but rather 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 vegetation in a specific region is determined by identifying and counting the various pollen grains found there. Although the relationship between vegetation and cli- mate 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 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.
According to the passage, the material used to determine past climatic conditions must be widespread for which of the following reasons? I . Paleoclimatologists need to make comparisons between periods of geological history. II. Paleoclimatologists need to compare materials that have supported a wide variety of vegetation. III. Paleoclimatologists need to make comparisons with data collected in other regions.
选项 A、I only
B、II only
C、I and II only
D、I and III only
E、II and III only
答案D
解析 为什么测量古气候的材料分布要广泛?此原因见原文L8—11的解释，广泛材料能反映时空方面更大一些的范围。I．正确。古气候学家需对冰期不同时间作比较。Ⅱ．“vegetation”在此处未提。Ⅲ．正确。古气候学家需对其他地区的资料作比较，即“other regions”。∴D．I，Ⅲ正确。
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