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 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
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. 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
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 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 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 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 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 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, 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 of gravity to progressively earlier configurations of the bodies in the solar system. Yetthe lack of information about other possible factors affecting
global climate does not make them unimportant.
https://image.slidesharecdn.com/gmatread ... 1360619804
and
https://image.slidesharecdn.com/gmatread ... 1360619804
These two show the passage.
The tricky question is the following question:
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
I thought D is better than A
Plz help!!!
in the Earth's orbit around the Sun. For some time this theory was considered untestable, largely 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
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. 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
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 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 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 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 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 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, 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 of gravity to progressively earlier configurations of the bodies in the solar system. Yetthe lack of information about other possible factors affecting
global climate does not make them unimportant.
https://image.slidesharecdn.com/gmatread ... 1360619804
and
https://image.slidesharecdn.com/gmatread ... 1360619804
These two show the passage.
The tricky question is the following question:
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
I thought D is better than A
Plz help!!!
