请教 OG12 RC 74

ScentyGuy

OG12-RC74
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,

(5) 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

(10) 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

(15) 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

(20) 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

(25) 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

(30) 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

(35) 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,

(40) 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

(45) 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.

(50)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

(55) 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

(60) global climate does not make them unimportant.



It can be inferred from the passage that precipitation
formed from evaporated ocean water has

(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

答案是B。我能理解B的正确性，但想问为什么C错？我已参见了之前的帖子，但还是不明白。
C：由蒸发的海水而形成的降雨 比大陆冰床的冰 含有更少的O18。
蒸发的海水形成的降雨肯定是含有比较多的O16 少的018吧（OG上解答又说“heavier isotopes tend to be left behind when water evaporates from the ocean sufaces ”），那么C为什么不对呢？为什么会像解答里说的，两者的O18是similar的呢？

justabeginning

precipitation有沉淀物的意思，不止只有降水的意思哦

sevenmileser

原因在于这句话When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that 20) will eventually return to it.不知道你是否明白这句话的意思。。。就是说冰川时期的开始会使在大陆冰层（也就是常见的雪吧）开始增加，这种现象导致了重新流回海里的水减少。
地理知识告诉我们，大陆冰层实际都是来此于海蒸发的水。（这个你不了解也不要紧，毕竟文章也提示了，就是这句话evaporated from the ocean that 20) will eventually return to it.）。。。所以说大陆冰层的O16含量实际和降水中的是一样的。。。