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 isotopes, 16 and 18, found in ocean sedi-
ments. Almost all the oxygen in water is oxygen 16, but
a few molecules out of every thousand incorporate the
(15) heavier isotope 18. When an ice age begins, the conti-
nental ice sheets grow, steadily 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 determined by analyzing ocean sediments of the
period, because these sediments are composed of calcium
carbonate shells of marine organisms, shells that were
(25) constructed with oxygen atoms drawn from the sur-
rounding ocean. The higher the ratio of 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
(30) isotope record has two advantages. First, it is 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
(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 fluctuations 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 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 is testable: changes in the Earth’s orbit
can be calculated and dated by applying Newton’s laws
(50) of gravity to progressively earlier configurations of the
bodies in the solar system. Yet the lack of information
about other possible factors affecting global climate does
not make them unimportant.
1. In the passage, the author is primarily interested in
(A) suggesting an alternative to an outdated research
(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
(E) initiating a debate about a widely accepted theory
2. 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 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.
3. 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 lighter 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
(D) It indicated that the ratios of oxygen 16 and oxygen
18 in ocean water were not consistent with those
found in fresh water.
(E) It stretched back for only a million years.
4. 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
(D) They are more unpredictable during an ice age
than in other climatic conditions.
(E) They can be used to determine atmospheric
conditions at various times in the past.
5. 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
6. According to the passage, which of the following is (are)
true of the ice ages?
Ⅰ. The last ice age occurred about 25,000 years ago.
Ⅱ. Ice ages have lasted about 10,000 years for at least
the last several hundred thousand years.
Ⅲ. Ice ages have occurred about every 100,000 years
for at least the last several hundred thousand years.
(A) Ⅰ only
(B) Ⅱ only
(C) Ⅲ only
(D) Ⅰand only
(E) Ⅰ,Ⅱ and Ⅲ
7. It can be inferred from the passage that calcium
(A) are not as susceptible to deterioration as rocks
(B) are less common in sediments formed during an ice
(C) are found only in areas that were once covered by
(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
8. The purpose of the last paragraph of the passage is to
(A) offer a note of caution
(B) introduce new evidence
(C) present two recent discoveries
(D) summarize material in the preceding paragraphs
(E) offer two explanations for a phenomenon
9. According to the passage, one advantage of studying the
isotope record of ocean sediments is that it
(A) corresponds with the record of ice volume taken
from rocks on land
(B) shows little variation in isotope ratios when samples
are taken from different continental locations
(C) corresponds with predictions already made by
climatologists and experts in other fields
(D) confirms the record of ice volume initially
established by analyzing variations in volcanic
(E) provides data that can be used to substantiate
records concerning variations in the amount
of sunlight received by the Earth