OG RC #72

[mundasingh123]

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. Yet the lack of
information about other possible factors affecting
global climate does not make them unimportant.

72. 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 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.
(E) It stretched back for only a million years.
How to discount # 72 D ?
First, it is
a global record: there is remarkably little variation
in isotope ratios in sedimentary specimens taken
from different continental locations.

This implies that if there were a variation in ratio of isotope in the specimen collected from different location , the usefulness of the recore would be undermined .

I dont want the OA Please LOL I already know it , so please refrain from stating IMO A/B/C/D/E
I post for Help not for Contest

[mundasingh123]

73. 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.
(E) They can be used to determine atmospheric
conditions at various times in the past.

How to Discount E ?
Isnt ice-age an atmospheric condition ?

[mundasingh123]

OG Explanation for #74 says that

Any inference about precipitation from
evaporated ocean water needs to be based on what
the passage says. Lines 20-22 show that heavier
isotopes tend to be left behind when water evaporates
from the ocean surfaces. Therefore, the evaporated
water would contain less oxygen 18 and the
remaining ocean water would contain more. It is
logical to infer that precipitation formed from this
evaporated water would also contain less oxygen
18.

But it is the formation of ice sheet that leads to less evaporation and not evaporation that leads to the formation of ice sheets

[Crystal W]

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. Yet the lack of
information about other possible factors affecting
global climate does not make them unimportant.

72. 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 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.
(E) It stretched back for only a million years.
How to discount # 72 D ?
First, it is
a global record: there is remarkably little variation
in isotope ratios in sedimentary specimens taken
from different continental locations.

This implies that if there were a variation in ratio of isotope in the specimen collected from different location , the usefulness of the recore would be undermined .

I dont want the OA Please LOL I already know it , so please refrain from stating IMO A/B/C/D/E
I post for Help not for Contest

I have a question about the Choice D, the explanation on OG is "This inconsistency would not affect the usefulness of the ocean-water record. Researchers would simply need to accommodate the fresh-water inconsistency." I cannot understand this explanation, but I think Choice D is wrong because the ratio in fresh water is different (more oxygen 16).
Thanks in advance.