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
behind 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 LACE>NewtonLACE>’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.
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
54.
E is the best answer.
Lines 29-33 of the passage state that the calcium carbonate shells of marine organisms are
constructed with “oxygen atoms drawn from the surrounding ocean.”
This water contains varying proportions of oxygen 16 and oxygen 18 and, according to the
passage, “becomes progressively enriched in oxygen 18”with the onset of an ice age (lines 19-27).
The author states that “The degree of enrichment can be determined by analyzing ocean
sediments…composed of calcium carbonate shells of marine organisms (lines 27-31)” . Thus, it
can be inferred that the shells of marine organisms would reflect the isotopic composition of the
surrounding ocean water at the time when the shells were formed.
请 问 这 道 题 我 怎 么 感 觉 从 黄 色 部 分 就 能 得 到 答 案 了 。 还 是 必 须 结 合 下 面 提 到 的 两 处 才 能 解 。
为 什 么 og中 有 好 多 题 不 用 这 么 多 处 结 合 就 能 解 答 的 ， 是 不 是 我 的 理 解 有 问 题 了 ？ ？ ？
多 谢 | 
