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  3. Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth’s orbit around the

Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth’s orbit around the

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问题     Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth’s orbit around the Sun. Eor sometime this the-  ory was considered untestable, largely because there was no sufficiently precise chronology of the ice ages with which the orbital variations could be matched.
    To establish such a chronology it is necessary to determine the relative amounts of land ice that existed at various times in the Earth’s past. A recent discovery makes such a determination possible: relative land-ice volume for a given period can be deduced from the ratio of two oxygen isotopes, 16 and 18, found in ocean sediments. Almost all the oxygen in water is oxygen 16, but a few molecules out of every thou- sand 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 wator becomes progressively enriched in oxygen 18. The degree of enrichment can be determined by analyzing ocean sediments of the period, because these sediments are composed of calcium carbonate shells of marine organisms, shells that were constructed with oxy- gen 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.
    As an indicator of shifts in the Earth’s climate, the isotope record 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 dat- ed 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 years. These data have established a strong connection between variations in the Earth’s orbit and the periodicity of the ice ages.
    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 Earth’s orbit can be calculated and dated by applying Newton’s 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.
According to the passage, one advantage of studying the isotope record of ocean sediments is that it
选项 A、corresponds with the record of ice volume taken from rocks on land.
B、shows little variation in isotope ratios when samples are taken from different continental locations.
C、corresponds with predictions already made by climatologists and experts in other fields.
D、confirms the record of ice volume initially established by analyzing variations in volcanic emissions.
E、provides data that can be used to substantiate records concerning variations in the amount of sunlight received by the Earth.
答案B
解析 文中指出下面哪一点是研究同位素的优点?原文提到两个优点L42—48，1、有全球性2、有延续性。∴B正确。从不同地区收集到的样品变化幅度小，即第二个优点。A．和陆上岩石中关于冰川的记录相符。未提。C．和气候学家及其他领域专家所做预测一致。未提。D．能证实当初通过分析火山爆发而得出的冰川总量的记录。无。E．未提此方法和“sunlight received by the Earth”有何联系。
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