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Passage 18 问题解决型特别套路 提出问题：冰期的计时未知Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earths orbit around the Sun. For sometime this theory was considered untestable, largely because there was no sufficiently precise 计时 of the ice ages with which the orbital variations could be matched. 提出解决方法：同位素方法To establish such a chronology it is necessary to determine the relative amounts of land ice that existed at various times in the Earths past. A recent discovery makes such a determination possible: relative land-ice volume for a given period 能从推出4C the ratio of two oxygen isotopes4, 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 由决定 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. chronologylandicerationO18/O16 | 
