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
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。。。
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.
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！作者: LeannaChina 时间: 2012-5-4 06:37
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。：）作者: mandybabay 时间: 2012-5-4 21:54
不同意这种说法 ，文章中说的是other factors instead of other theories!作者: vernetta 时间: 2012-5-5 09:08
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。作者: Quadrifolia 时间: 2012-5-6 08:34
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.作者: annieview 时间: 2012-5-6 21:43
想问下75题，在d和e中不知道选哪个
75. It can be inferred from the passage that calcium carbonate shells
(A)are not as susceptible to deterioration as rocks
(B)are less common in sediments formed during an
(C)are found only in areas that were once covered by land ice
(D)contain radioactive material that can be used to determine a sediment’s isotopic composition
(E)reflect the isotopic composition of the water at the time the shells were formed
A ~e only mention of rocks in the passage is a comparison of “gappiness” of the rock and sedimentary specimen records in lines 38–39; this information does not allow any firm inference to be made with respect to relative susceptibility to deterioration, though a more continuous record might be the result of less susceptibility to deterioration.
B ~e passage does not make any reference to the relative abundance of these shells during ice ages; no such inference can be drawn.
C ~e only information in the passage that might support this statement is found in lines 29–32, but that information, about the correlation between oxygen ratios in sediment specimens and land ice, describes a relation that implies nothing about distributions of such specimens.
D ~ough the passage does indirectly indicate that the shells contained radioactive material, nothing in the passage suggests that radioactive material is used to determine isotopic composition.
E Correct. ~e passage explains that oxygen atoms in the surrounding water are one of the building blocks of calcium carbonate shells. ~e isotopic composition of the surrounding water changes during the ice age cycles, so it is logical that the isotopic composition of the shells will change depending on when they were formed.
~e correct answer is E.作者: elephantsmall 时间: 2012-5-8 21:55
74题，重点还是在这句话：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.
75. It can be inferred from the passage that calcium carbonate shells It can thus be inferred that the shells that make up those sediments must reflect the proportion of oxygen 18 found in the ocean water at the time they were formed.
(A) are not as susceptible to deterioration as rocks.文章没有针对susceptibility to deterioration做shell与rock的对比。无关。
(B) are less common in sediments formed during an ice age. NM，文章没有做关于ice age时期shell的情况的说明。
(C) are found only in areas that were once covered by land ice。文章没有就shells的地点分布做说明。虽然问斩个提到了，shells中氧的比例可以提供当时land ice情况的依据。
(D) contain radioactive material that can be used to determine a sediment’s isotopic composition. 文章间接的说shells中含有radioactive material（文章提到ratio可以用放射元素测定年代的方法测定。。。），但并没有说这种radioactive material可以被用来测定沉淀的元素组成——它只是可以用一种与放射性物质相关的方法测定氧的比例，也许测不出含有什么其他物质。
(E) reflect the isotopic composition of the water at the time the shells were formed
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