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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.
It can be inferred from the passage that precipitation formed from evaporated ocean water has
选项 A、the same isotopic ratio as ocean water.
B、less oxygen 18 than does ocean water.
C、less oxygen 18 than has the ice contained in continental ice sheets.
D、a different isotopic composition than has precipitation formed from water on land.
E、more oxygen 16 than has precipitation formed from fresh water.
答案B
解析 蒸发形成的降水有何特点?这是文章的一个细节，出现在L21一29。B．正确。和海水比，O18些。原文讲：较重同位素蒸发后会留在海水中，因此O18在海水中要比在蒸发的水蒸气(以及由此形成的降水)中要多。A、C、D、E都不符合原文叙述。
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