gre考试阅读真题解析(A)

　　This passage is based on an article published in 1990.

　　Eight times within the past million years, something in the Earths climatic equation has changed, allowing snow in the mountains and the northern latitudes to accumulate from one season to the next instead of melting away. Each time, the enormous ice sheets resulting from this continual buildup lasted tens of thousands of years until the end of each particular glacial cycle brought a warmer climate. Scientists speculated that these glacial cycles were ultimately driven by astronomical factors: slow, cyclic changes in the eccentricity of the Earths orbit and in the tilt and orientation of its spin axis. But up until around 30 years ago, the lack of an independent record of ice-age timing made the hypothesis untestable.

　　Then in the early 1950s Emiliani produced the first complete record of the waxings and wanings of past glaciations. It came from a seemingly odd place, the seafloor. Single-cell marine organisms called foraminifera house themselves in shells made from calcium carbonate . When the foraminifera die, sink to the bottom, and become part of seafloor sediments, the carbonate of their shells preserves certain characteristics of the seawater they inhabited. In particular, the ratio of a heavy isotope of oxygen to ordinary oxygen in the carbonate preserves the ratio of the two oxygens in water molecules.

　　It is now understood that the ratio of oxygen isotopes in seawater closely reflects the proportion of the worlds water locked up in glaciers and ice sheets. A kind of meteorological distillation accounts for the link. Water molecules containing the heavier isotope tend to condense and fall as precipitation slightly sooner than molecules containing the lighter isotope. Hence, as water vapor evaporated from warm oceans moves away from its source, its oxygen-18 returns more quickly to the oceans than does its oxygen-16. What falls as snow on distant ice sheets and mountain glaciers is relatively depleted of oxygen-18. As the oxygen-18-poor ice builds up, the oceans become relatively enriched in the isotope. The larger the ice sheets grow, the higher the proportion of oxygen-18 becomes in seawaterand hence in the sediments.

　　Analyzing cores drilled from seafloor sediments, Emiliani found that the isotopic ratio rose and fell in rough accord with the Earths astronomical cycles. Since that pioneering observation, oxygen-isotope measurements have been made on hundreds of cores. A chronology for the combined record enables scientists to show that the record contains the very same periodicities as the orbital processes. Over the past 800,000 years, the global ice volume has peaked every 100,000 years, matching the period of the orbital eccentricity variation. In addition, wrinkles superposed on each cyclesmall decreases or surges in ice volumehave come at intervals of roughly 23,000 and 41,000 years, in keeping with the precession and tilt frequencies of the Earths spin axis.

　　17. Which of the following best expresses the main idea of the passage?

　　 Marine sediments have allowed scientists to amass evidence tending to confirm that astronomical cycles drive the Earths glacial cycles.

　　 The ratio between two different isotopes of oxygen in seawater correlates closely with the size of the Earths ice sheets.

　　 Surprisingly, single-cell marine organisms provide a record of the Earths ice ages.

　　 The Earths astronomical cycles have recently been revealed to have an unexpectedly large impact on the Earths climate.

　　 The earth has experienced eight periods of intense glaciation in the past million years, primarily as a result of substantial changes in its orbit.

　　18. The passage asserts that one reason that oceans become enriched in oxygen-18 as ice sheets grow is because

　　 water molecules containing oxygen-18 condense and fall as precipitation slightly sooner than those containing oxygen-16

　　 the ratio of oxygen-18 to oxygen-16 in water vapor evaporated from oceans is different from that of these isotopes in seawater

　　 growing ice sheets tend to lose their oxygen-18 as the temperature of the oceans near them gradually decreases

　　 less water vapor evaporates from oceans during glacial periods and therefore less oxygen-18 is removed from the seawater

　　 the freezing point of seawater rich in oxygen-18 is slightly lower than that of seawater poor in oxygen-18

　　19. According to the passage, the large ice sheets typical of glacial cycles are most directly caused by

　　 changes in the average temperatures in the tropics and over open oceans

　　 prolonged increases in the rate at which water evaporates from the oceans

　　 extreme seasonal variations in temperature in northern latitudes and in mountainous areas

　　 steadily increasing precipitation rates in northern latitudes and in mountainous areas

　　 the continual failure of snow to melt completely during the warmer seasons in northern latitudes and in mountainous areas

　　20. It can be inferred from the passage that which of the following is true of the water locked in glaciers and ice sheets today?

　　 It is richer in oxygen-18 than frozen water was during past glacial periods.

　　 It is primarily located in the northern latitudes of the Earth.

　　 Its ratio of oxygen isotopes is the same as that prevalent in seawater during the last ice age.

　　 It is steadily decreasing in amount due to increased thawing during summer months.

　　 In comparison with seawater, it is relatively poor in oxygen-18.

　　21. The discussion of the oxygen-isotope ratios in paragraph three of the passage suggests that which of the following must be assumed if the conclusions described in lines 49-58 are to be validly drawn?

　　 The Earths overall annual precipitation rates do not dramatically increase or decrease over time.

　　 The various chemicals dissolved in seawater have had the same concentrations over the past million years.

　　 Natural processes unrelated to ice formation do not result in the formation of large quantities of oxygen-18.

　　 Water molecules falling as precipitation usually fall on the open ocean rather than on continents or polar ice packs.

　　 Increases in global temperature do not increase the amount of water that evaporates from the oceans.

　　22. The passage suggests that the scientists who first constructed a coherent, continuous picture of past variations in marine-sediment isotope ratios did which of the following?

　　 Relied primarily on the data obtained from the analysis of Emilianis core samples.

　　 Combined data derived from the analysis of many different core samples.

　　 Matched the data obtained by geologists with that provided by astronomers.

　　 Evaluated the isotope-ratio data obtained in several areas in order to eliminate all but the most reliable data.

　　 Compared data obtained from core samples in many different marine environments with data samples derived from polar ice caps.

　　23. The passage suggests that the scientists mentioned in line 8 considered their reconstruction of past astronomical cycles to be

　　 unreliable because astronomical observations have been made and recorded for only a few thousand years

　　 adequate enough to allow that reconstructions use in explaining glacial cycles if a record of the latter could be found

　　 in need of confirmation through comparison with an independent source of information about astronomical phenomena

　　 incomplete and therefore unusable for the purposes of explaining the causes of ice ages

　　 adequate enough for scientists to support conclusively the idea that ice ages were caused by astronomical changes

　　Although Victor Turners writings have proved fruitful for fields beyond anthropology, his definition of ritual is overly restrictive. Ritual, he says, is prescribed formal behavior for occasions not given over to technological routine, having reference to beliefs in mystical beings or powers. Technological routine refers to the means by which a social group provides for its material needs. Turners differentiating ritual from technology helps us recognize that festivals and celebrations may have little purpose other than play, but it obscures the practical aims, such as making crops grow or healing patients, of other rituals. Further, Turners definition implies a necessary relationship between ritual and mystical beliefs. However, not all rituals are religious; some religions have no reference to mystical beings; and individuals may be required only to participate in, not necessarily believe in, a ritual. Turners assumption that ritual behavior follows belief thus limits the usefulness of his definition in studying ritual across cultures.

　　24. According to the passage, which of the following does Turner exclude from his conception of ritual?

　　 Behavior based on beliefs

　　 Behavior based on formal rules

　　 Celebrations whose purpose is play

　　 Routines directed toward practical ends

　　 Festivals honoring supernatural beings

　　25. The passage suggests that an assumption underlying Turners definition of ritual is that

　　 anthropological concepts apply to other fields

　　 festivals and ceremonies are related cultural phenomena

　　 there is a relationship between play and practical ends

　　 rituals refer only to belief in mystical beings or powers

　　 mystical beings and powers have certain common attributes across cultures

　　26. It can be inferred that the author of the passage believes each of the following concerning rituals EXCEPT:

　　 Some are unrelated to religious belief.

　　 Some are intended to have practical consequences.

　　 Some have no purpose other than play.

　　 They sometimes involve reference to mystical beings.

　　 They are predominantly focused on agricultural ends.

　　27. Which of the following best describes the organization of the passage?

　　 Factual data are presented and a hypothesis is proposed.

　　 A distinction is introduced then shown not to be a true distinction.

　　 A statement is quoted, and two assumptions on which it is based are clarified.

　　 A definition is challenged, and two reasons for the challenge are given.

　　 An opinion is offered and then placed within a historical framework.

答案：17-27：AAEECBBDDED