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Milankovitch proposed in the early twentieth century that the ice ages

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Milankovitch proposed in the early twentieth century that the ice ages  [#permalink]

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New post 22 Oct 2017, 19:18
Line Milankovitch proposed in the early twentieth
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, largely
(5) 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.
(15) Almost 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
(25) determined 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
(30) 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 isotope record has two advantages. First, it is a
(35) 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, sedimentary
(40) 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
(45) 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.
However, it is important to note that other
(50) 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
is testable; changes in the Earth’s orbit can be
(55) 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
global climate does not make them unimportant.


(Book Question: 483)
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

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Re: Milankovitch proposed in the early twentieth century that the ice ages  [#permalink]

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New post 22 Oct 2017, 19:45
E

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Re: Milankovitch proposed in the early twentieth century that the ice ages &nbs [#permalink] 22 Oct 2017, 19:45
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