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Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth’s orbit around the Sun. For some time this theory 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 thousand 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 water 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 oxygen 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 dated 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.
【OG19-P3401-482题】
It can be inferred from the passage that precipitation formed from evaporated ocean water has
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文中第二段（第19行）所在的句子谈到了蒸发的海水。根据后一句... the remaining ocean water... enriched in oxygen 18可以推测降水所含有的氧18比余下的海水中的含量要少
A错误，the same不对
B正确，符合文中意思
C错误，比较对象不对
D错误，precipitation formed from water on land不对
E错误，与氧16无关
这里ad说的是比例，bce直接用量比较，根本没有推算浓度。用排除法做才能稳健选出正确选项
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2019-02-14 10:01:35
定位次 evaporated ocean 定位在 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 water becomes progressively enriched in oxygen 18. 肯定跟O 18 有关系 排除E D定位错误 有比较 最后锁定BC The remaining ocean water 可见是一种水和另一种水比较， 排除C 2019-01-10 11:17:36
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2019-01-10 11:18:12
evaporated ocean water 指的是 蒸发掉的海水。18很重，难以被蒸发上去。
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2018-12-02 14:49:50
这里的precipitation是指的降水吧... 应该定位到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 water becomes progressively enriched in oxygen 18，既蒸发后的水会通过降水回到海里。重一点的同位素不会蒸发（会被留在海水里），因此剩下的海水中O18的浓度会非常高。因此降水中的O18浓度比海水中的低...
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2018-11-27 13:47:16
Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces, the remaining ocean water becomes progressively enriched in oxygen 18.一定要对应原文，不要想当然，注意这里说的是一部分水蒸发， 另一部分水里18很多，注意说的是水，不是说冰！！！
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2017-01-04 23:04:52
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