Passage 9
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 suffi-
ciently 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 Earths 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 isoTOPes2, 16 and 18, found in ocean sedi-
ments. Almost all the oxygen in water is oxygen 16, but
a few molecules3 out of every thousand incorporate the
heavier isoTOPe1 18. When an ice age begins, the conti-
nental ice sheets grow, steadily4 reducing the amount of
water evaporated from the ocean that will eventually
return to it. Because heavier isoTOPes tend to be left
behid when water evaporates from the ocean surfaces,
the remaining ocean water becomes progressively
enriched in oxygen 18. The degree of enrichment can
be determined5 by analyzing6 ocean sediments8 of the
period, because these sediments are composed of calcium9
carbonate shells of marine10 organisms, shells that were
constructed with oxygen atoms drawn11 from the sur-
rounding ocean. The higher the ratio of oxygen 18 to
oxygen 16 in a sedimentary specimen12, the more land ice
there was when the sediment7 was laid down.
As an indicator13 of shifts in the Earths climate, the
isoTOPe record has two advantages. First, it is a global
record: there is remarkably14 little variation in isoTOPe
ratios in sedimentary specimens15 taken from different
continental16 locations. Second, it is a more continuous
record than that taken from rocks on land. Because of
these advantages, 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 fluctuations17 in global
ice volume over the past several hundred 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 Earths orbit and
the periodicity of the ice ages.
However, it is important to note that other factors,
such as volcanic18 particulates19 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 is testable: changes in the Earths orbit
can be calculated and dated by applying Newtons laws
of gravity to progressively earlier configurations20 of the
bodies in the solar system. Yet the lack of information
about other possible factors affecting global climate does
not make them unimportant.
49. In the passage, the author is primarily interested in
suggesting an alternative to an outdated21 research method
introducing a new research method that calls an accepted theory into question
emphasizing the instability of data gathered from the application of a new scientific method
presenting a theory and describing a new method to test that theory（D）
initiating22 a debate about a widely accepted theory
50. The author of the passage would be most likely to agree with which of the following statements about the Milankovitch theory?
It is the only possible explanation for the ice ages.
It is too limited to provide a plausible23 explanation for the ice ages, despite recent research findings.
It cannot be tested and confirmed until further research on volcanic activity is done.
It is one plausible explanation, though not the only one, for the ice ages. （D）
It is not a plausible explanation for the ice ages, although it has opened up promising24 possibilities for future research.
51. It can be inferred from the passage that the isoTOPe record taken from ocean sediments would be less useful to researchers if which of the following were true?
It indicated that lighter25 isoTOPes of oxygen predominated at certain times.
It had far more gaps in its sequence than the record taken from rocks on land.
It indicated that climate shifts did not occur every 100,000 years.
It indicated that the ratios of oxygen 16 and oxygen 18 in ocean water were not consistent with those found in fresh water. （B）
It stretched back for only a million years.
52. According to the passage, which of the following is true of the ratios of oxygen isoTOPes in ocean sediments?
They indicate that sediments found during an ice age contain more calcium carbonate than sediments formed at other times.
They are less reliable than the evidence from rocks on land in determining the volume of land ice.
They can be used to deduce the relative volume of land ice that was present when the sediment was laid down.
They are more unpredictable during an ice age than in other climatic conditions. （C）
They can be used to determine atmospheric26 conditions at various times in the past.
53. It can be inferred from the passage that precipitation formed from evaporated ocean water has
the same isoTOPic27 ratio as ocean water
less oxygen 18 than does ocean water
less oxygen 18 than has the ice contained in continental ice sheets
a different isoTOPic composition than has precipitation formed from water on land（B）
more oxygen 16 than has precipitation formed from fresh water
54. It can be inferred from the passage that calcium carbonate shells
are not as susceptible to deterioration as rocks
are less common in sediments formed during an ice age
are found only in areas that were once covered by land ice
contain radioactive material that can be used to determine a sediments isoTOPic composition
reflect the isoTOPic composition of the water at the time the shells were formed（E）
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