Passage 9
Milankovitch proposed in the early twentieth century
that the ice ages were caused by variations in the Earth’s
orbit around the Sun. For sometime this theory was
considered untestable, largely because there was no suffi-
(5) 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 Earth’s past. A recent discovery
(10) 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
(15) 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,
(20) 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
(25) 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 Earth’s climate, the
(30) 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
(35) 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
(40) 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.
However, it is important to note that other factors,
(45) 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 Earth’s orbit
can be calculated and dated by applying Newton’s laws
(50) 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
(A)   suggesting an alternative to an outdated²¹ research method
(B)   introducing a new research method that calls an accepted theory into question
(C)   emphasizing the instability of data gathered from the application of a new scientific method
(D)  presenting a theory and describing a new method to test that theory（D）
(E)   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?
(A)   It is the only possible explanation for the ice ages.
(B)   It is too limited to provide a plausible²³ 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. （D）
(E)   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?
(A)   It indicated that lighter²⁵ isotopes of oxygen predominated at certain times.
(B)   It had far more gaps in its sequence than the record taken from rocks on land.
(C)   It indicated that climate shifts did not occur every 100,000 years.
(D)  It indicated that the ratios of oxygen 16 and oxygen 18 in ocean water were not consistent with those found in fresh water. （B）
(E)   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?
(A)   They indicate that sediments found during an ice age contain more calcium carbonate than sediments formed at other times.
(B)   They are less reliable than the evidence from rocks on land in determining the volume of land ice.
(C)   They can be used to deduce the relative volume of land ice that was present when the sediment was laid down.
(D)  They are more unpredictable during an ice age than in other climatic conditions. （C）
(E)   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
(A)   the same isotopic²⁷ 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（B）
(E)   more oxygen 16 than has precipitation formed from fresh water
 
54. 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 ice age
(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（E）

上一篇：GMAT考试RCOGWORD（八） 下一篇：GMAT考试RCOGWORD（十）