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托福阅读真题第241篇Pleistocene Climate Pleistocene Climate Paragraph 1: Even though glaciation of the Northern Hemisphere began earlier, we currently recognize 1.6 million years ago as the start of the Pleistocene epoch, corresponding to the southward advance of large ice sheets in North America and Europe. The final retreat (melting and backward movement) of these glaciers northward began roughly 13,000 years ago and didn’t actually end until the ice margins reached their present position about 8,000 years ago. 1. According to paragraph 1, what happened around the start of the Pleistocene epoch? A. Glaciers retreated from North America and Europe. B. Glaciation began in the Northern Hemisphere. C. Ice margins reached their present location. D. Glaciers moved southward into North America and Europe. Paragraph 2: Geologists studying Pleistocene glacial deposits (rocks and soil left behind by melting glaciers) recognized long ago that ice coverage was not continuous during the entire epoch. Soils containing plant debris are found between the layers of glacial sediments in many areas, recording times when the climate warmed and the glaciers retreated. Correlations (relationships) between different regions of glacial deposits and soils allowed geologists to develop an approximate chronology of glaciation. Initially, four principal stages of glacial advance were recognized; these were named (from oldest to youngest) Nebraskan, Kansan, Illinoian, and Wisconsin after the southernmost states of the United States in which the deposits of the advances were found. The warmer interglacial stages were named Aftonian, Yarmouth, and Sangamon after locations in which soils that formed following glacial retreats are particularly well developed. The chronicle of advances and retreats developed to describe the history of glaciation in Europe corresponds approximately to that of North America. 2. In stating that the history of glaciation in Europe “corresponds approximately to that of North America,” the author means that it A. matches roughly that of North America B. contrasts in part with that of North America C. predicts generally that of North America D. is based largely on that of North America 3. According to paragraph 2, what does plant debris found between layers of glacial sediment indicate? A. The climate was warmer during the entire epoch than previously thought. B. Glaciers must have advanced in some areas and retreated in other areas at the same time. C. Glaciers must have retreated at some points during the Pleistocene. D. Plants were able to form and develop underneath the glaciers. Paragraph 3: But correlating and dating glacial deposits is a notoriously tricky business: glaciers tend to destroy older deposits during advances, and the deposits themselves tend to be very disorganized. Deposits older than the age limit of radiocarbon dating (about 70,000 years) cannot be dated directly, although Analysis of the pollen deposited in the past and found now in surface layers can be useful to infer a record of past climate. Additionally, glacial advances did not always occur simultaneously in different regions. For example, geologists now recognize that the ice margin in the Great Lakes region experienced several mini advances and retreats during the late Wisconsin stage, but they have difficulty correlating this chronology to other regions. 4. According to paragraph 3, which of the following is true about the mini advances and retreats of the ice margin in the Great Lakes region during the late Wisconsin stage? A. Their occurrence is still doubted by some geologists. B. They are difficult to study because they occurred earlier than 70,000 years ago. C. They do not clearly match the pattern of changes in ice sheets in other locations. D. The retreats of the ice margin during the period were generally bigger than the advances. Paragraph 4: More recently, geologists have come to rely heavily on the composition of the shells of planktonic foraminifera (a type of single-celled shelled aquatic organism) for a record of glaciation. During evaporation, the lighter oxygen isotope O16 is preferentially removed from the ocean, leaving the heavier isotope O18 behind. Therefore, the continuous removal of ocean water containing the lighter isotope to form glaciers on land during cold intervals results in a higher proportion of the heavier isotope O18 in the remaining ocean water. Melting of the glaciers during warmer intervals returns the lighter isotope O16 to the sea, decreasing the proportion of the heavier isotope. This change in water composition, corresponding to periods of glacial advance and retreat, is recorded by certain foraminifera as they build their shells from dissolved calcium carbonate. 5. Paragraph 4 suggests which of the following about the shells of foraminifera? A. Shells formed during cold intervals had higher concentrations of O18 than shells from warm intervals. B. There were more shells produced during colder periods than during warmer periods. C. It was harder for foraminifera to build shells from dissolved calcium carbonate during periods of glacial advance. D. Foraminifera require equal amounts of O18 and O16 in order to form their shells. Paragraph 5: Analysis of these shells in sediment cores taken from the ocean floor has revealed a much more complex pattern of glacial advance and retreat than could be recognized from deposits on land. This record indicates that the growth and shrinkage of the ice sheets during the last million years has occurred cyclically, with a period of about 100,000 years (with many smaller fluctuations). Amazingly, this conclusion matches the hypothesis advanced in the 1930s by the Yugoslavian astronomer Milutin Milankovitch that glacial cyclicity is controlled by the variability of Earth’s orbit. Most scientists now accept the explanation that variations in the amount of sunlight reaching the Earth’s surface controlled glacial cyclicity during the Pleistocene. However, many questions about the mechanisms that trigger advances and retreats remain to be answered. 6. Paragraph 5 suggests that modern scientists believe which of the following about Milutin Milankovitch’s 1930s hypothesis? A. It accurately describes patterns of glacial movement from the last 100,000 years but not the growth and shrinkage of ice sheets during the last million years. B. It is not supported by evidence derived from the Analysis of shells in sediment cores. C. It explains the overall pattern of glaciers’ advancing and retreating but not all of the details. D. It needs to be replaced with a theory that explains variations in the amount of sunlight reaching Earth’s surface. Paragraph 6: The cyclicity described above clearly shows that the climate during the Pleistocene was not uniformly cold. Temperatures during the interglacial stages may have matched those we experience today; in fact, during the last interglacial stage (Sangamon), global temperatures were even warmer and sea level was several meters higher than today. But temperatures plummeted during the cooler glacial stages. Evidence from marine fossils indicated that although temperatures in the tropics were a modest few degrees cooler than today, higher-latitude regions (about 40-degree latitude) experienced truly polar climates. Interestingly, the isotope record tells us that the transitions from interglacial to glacial stages and back again did not occur at the same rate: cooling trends, marking the onset of glacial advances, were gradual events taking 10,000 to 15,000 years, but warmer intervals came on very abruptly, possibly in a few tens of years. 7. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. A. The isotope record reveals that cool glacial periods varied in length, with some lasting 10,000 years and others lasting as long as 15,000 years. B. The isotope record tells us that up until about 10,000 to 15,000 years ago, the climate was still transitioning from cool periods to warm periods and back again. C. Evidence shows that cooling trends associated with glacial advances lasted thousands of years, while warm interglacial periods came on quickly. D. It is interesting to note that the transition from an interglacial to a glacial stage and back again could take from a few tens of years to thousands of year. 8. According to paragraph 6, which of the following accurately describes temperatures during the Pleistocene? A. During interglacial periods, temperatures may have been very similar to temperatures today. B. During interglacial periods, the greatest increases in temperatures occurred in tropical regions. C. During glacial periods, temperatures in regions about 40-degree latitude were only a few degrees cooler than today. D. Temperatures during the Sangamon stage were generally lower than today’s temperatures. Paragraph 4：More recently, geologists have come to rely heavily on the composition of the shells of planktonic foraminifera (a type of single-celled shelled aquatic organism) for a record of glaciation. ■During evaporation, the lighter oxygen isotope 016 is preferentially removed from the ocean, leaving the heavier isotope 018 behind. ■Therefore, the continuous removal of ocean water containing the lighter isotope to form glaciers on land during cold intervals results in a higher proportion of the heavier isotope 018 in the remaining ocean water. ■Melting of the glaciers during warmer intervals returns the lighter isotope 016 to the sea, decreasing the proportion of the heavier isotope. ■This change in water composition, corresponding to periods of glacial advance the retreat, is recorded by certain foraminifera as they build their shells from dissolved calcium carbonate. 9. Look at the four squares ■ that indicate where the following sentence could be added to the passage. Where would the sentence best fit? But although much of the evaporated lighter isotope eventually returns to the ocean, some of it is locked up in glaciers in very cold regions. 10. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selected THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Pleistocene glaciation in North America and Europe lasted from 1.6 million years ago to 8,000 years ago. Answer Choices: A. Geological evidence indicates that most of North America experienced continuous glacial coverage during the Pleistocene, while only a few areas experienced warmer interglacial stages. B. The study of glacial deposits and soils in different regions led to the identification of four stages of glacial advance and three interglacial periods during the Pleistocene. C. Research into glaciation has been made difficult by the lack of agreement between North American and European geologists about which method of dating is reliable. D. Only a few, brief, glacial advances occurred at the beginning of the Pleistocene, but glacial stages had become much colder and last longer by the end of Pleistocene. E. Shells of single-celled organisms that change in composition depending on glaciation provide a better picture of glacial advance and retreat than do deposits on land. F. Some factors are still unknown, but Pleistocene glaciations probably occurred in cycles caused by Earth’s variable orbit and its effects on the amount of sunlight Earth received. 答案 TF241-Pleistocene Climate（答案文章最后） |
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