OG12-RC74

LeannaChina

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

这个问题没看明白。那个precipitation 是降水的意思吧，那C选项我常见得也对啊，降水的时候，那些氧18不还都在Ocean里面嘛。那降水里含的氧18应该少于最后continental ice sheets里面有的吧？

十分的不解，还请大家帮忙。~~~

LeannaChina

顶一下，今天又看了一遍还是有点不明白。。。转不过来弯。。。
请NN们来帮我看看这道题呗~~小女子先谢过各位啦。。。

LeannaChina

是说蒸发的水会形成降水，蒸发的里面18没有那么多，因为大部分都留在ocean里面啦，所以由蒸发形成的降水里面也不会有那么多18，OG的意思是说，蒸发的水结冰帮助形成了大的ice sheet那这样的话ice sheet的成分应该和蒸发的水是一样滴，所以也没有那么多18.。。。。。

这是我看了OG后的理解，请NN们帮我指正一下是不是正确的思路呢？~~~

最近大家都忙啥呢？好像大家都好忙呢~~~还是这个问题太弱啦？我真的错了嘛

LeannaChina

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.看明白了，这里说的是，因为ice age开始的时候，大冰原开始长啦，然后就减少了已经蒸发的水里能再回到ocean的部分，潜在意思就是说这些蒸发的水都形成冰了。。。。

后面the remaining ocean
water becomes progressively enriched in oxygen 18说蒸发后，水又不能再回来的ocean里面18超多。。。

OK。。看来还是读文章时的理解不到位。。。。欢迎大家指点。。。。

trackofheart

lz自问自答似乎差不多把问题都已经解决了。这道题的依据是这句话：

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.

heavier isotopes指的就是oxygen 18。因为蒸发时就不包含o18，那么降雨时自然也不会包含，降雨后形成的冰层中也不会包含。

mandybabay

71. the anthor of the passage would be most likely to agree with which of the following statements about the Milankovitch theory?
A it is the only possible explanation for the ice ages.
B it is too limited to provide a plausible explanation for the ice ages, despite recent research findings.
C it cannot be tested and confirmed until further research on volcanic activity is done.
D it is one plausible explanation, though not the only one, for the ice ages.
E it is not a plausible explanation for the ice ages, although it has opened up promising possibilities for future research.
这五个答案，我怎么觉得都是错的，即使是正确答案D，文章中也没有说还有其他的解释though not the only one！

LeannaChina

文章最后一段，作者指出了有其他的factors的可能性，M能成立是因为现在找到了方法能test这个理论，
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.也可能有其他的因素
Yet the lack of information about other possible factors affecting
(60) global climate does not make them unimportant.缺少相关的信息不证明他们不重要
说明作者是同意M理论非唯一的。：）

mandybabay

不同意这种说法 ，文章中说的是other factors instead of other theories!

vernetta

这个题，我也纠结了很久，但想通了。其实挺简单的。
我们回想下 水结冰的过程，一般是surface结一层冰，然后底下是水，对吗，冰面可能很厚，但毕竟冰下的主要是水，做这题就是要清楚，冰的的形成是本来要被蒸发的水，后来steadily reducing the amount of water evaporated from the ocean that will eventually return to it，也就是说这部分水变成了冰。然后被封存在冰下的水，不断浓缩，18也就越来越多，这样解释就通了嘛。

Quadrifolia

可以解釋這句話?

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.