求大师指点，OG13阅读~~新手一枚

luy5451

文章（我只复制了前两段）：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.


问题：It can be inferred from the passage that precipitation formed from evaporated ocean water has
Athe same isotopic ratio as ocean water
Bless oxygen 18 than does ocean water
Cless oxygen 18 than has the ice contained in continental ice sheets
Da different isotopic composition than has precipitation formed from water on land
Emore oxygen 16 than has precipitation formed from fresh water
正确答案 B


想问下，文章不是说蒸发后，沉淀中O18  enrich吗，为什么更少呢。
我看OG解释说这里的沉淀应该指被蒸发出来的水，O18留在了remaining ocean water，所以就少了。
这样的话，我觉得D也是对的啊，remaining ocean water enriched O18 是用来证明M的那个理论的呀。

不知道哪里想错，还是哪里没搞懂，求大神指点

luy5451

自己顶一下，不要沉下去

luy5451

顶顶~~~求回复

smilecoco

我个人觉得文章中的水并没有淡水或咸水的区分，只是说水中的氧大都是16.而由于冰盖形成之后，海洋蒸发后再次通过降雨方式回到海洋的水分减少。而由于同位素18更重，所以留着了海洋中，使得海洋沉淀物中的18更为丰富。对应原文是l
most 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.
所以D是文章中未提及的，个人觉得ice age 后，陆地上的水大都以冰的形式存在，其实不管是陆地上以前湖泊的沉淀物或者是海洋的沉淀物 关于氧18的大多相同。
不知道这样你能理解么。

典座

smilecoco 发表于 2013-12-10 16:58
我个人觉得文章中的水并没有淡水或咸水的区分，只是说水中的氧大都是16.而由于冰盖形成之后，海洋蒸发后再 ...

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.
感觉这句话里面已经很明白的写了海洋里的同位素氧18很多。
同位素氧16轻，18重，所以水蒸气里的18比海洋里的少。B选项。
（虽然我也错了这道题，但是看到你标记出的这句话豁然开朗啦~~~谢谢）