Milankovitch proposed in the early twentieth century that the ice ages

Really... a tough one.. I was completely blown ......

here are my answers with explanations:

1. In the passage, the author is primarily interested in

Ans : (D) presenting a theory and describing a new method to test that theory
The author mentions the theory presented by Milankovitch and states that for some time this theory was untestable. It is also mentioned in next para that a recent discovery helps in testing the validity of the theory mentioned by Milankovitch.

2. The author of the passage would be most likely to agree with which of the following statements about the Milankovitch theory?

Ans : Answering this question consumed most of my time while solving this passage. I was confused between options A and D. My pick for this is
(D) It is one plausible explanation, though not the only one, for the ice ages.

As it has already been mentioned in the passage that Milankovitch's theory can be tested if chronology of ice ages is known. Now measuring the relative amount of land ice that existed during various times in the earth's past is helpful in determining the chronology of ice ages and the amount of land ice can be predicted using the findings of the recent discovery. So the answer B, C and E are out. Now between A and D we need to confirm if the theory is the only way to find the volume of land ice. The author mentions in last passage that there are some other factors as well such as volcanic particulates or variations in the amount of sunlight received by the earth which could potentially have affected the climate. The only reason for which the author chose the Milankovitch's theory is that it is testable. There are other factors but they are not considered just because of the lack of information about them.

3. Itcan be inferred from the passage that the isotope record taken from ocean sediments would be less usefulto researchers if which of the following were true?
Ans : (B) It had far more gaps in its sequence than the record taken from rocks on land.
It is clearly mentioned in lines 37-39 that records taken from ocean sediments are advantageous because they are more continuous in nature that those taken from rocks on land.

4. According to the passage, which of the following is true of the ratios of oxygen isotopes in ocean sediments?
Ans : (c) They can be used to deduce the relative volume of land ice that was present when the sediment was laid down.
It is clearly mentioned in lines 12 to 14 that 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.

5. It can be inferred from the passage that precipitation formed from evaporated ocean water has
Ans : (B) less oxygen 18 than does ocean water
It is mentioned in lines 20 to 24 that when water evaporates the heavier isotope remains in ocean water. This clearly causes the concentration of oxygen 18 to decrease in evaporated water. So precipitation formed from evaporated water will have less oxygen 18 than that in oceanic precipitations.

6. Itcan be inferred from the passage that calcium carbonate shells
Ans : (E) reflect the isotopic composition of the water at the time the shells were formed
From lines 24 to 29 it can be inferred that calcium carbonate shells were constructed using the oceanic water at the time of their composition. So the oxygen isotope's concentration found in their analysis will show the concentration of isotopes in oceanic water.

Carcass, It has taken a lot of time in formatting the question in my post but if some errors are still persisting, I am worried about them. Request you to review the answers and explanations. Its 3:40 am here...So i m gonna sleep with hope to get reply from you when i wake up .... It was really tough question. :done

Pressing +1 Kudos is the best way to say Thanks!

It's important to compare the content in the final paragraph to the rest of the passage. It introduces new considerations (volcanic ash, variations in sunlight) that might also explain ice ages. True, it reiterates that Milankovitch's theory is testable, but in no way does it summarize the passage. The paragraph is saying "Here are some other possible explanations. Milankovitch's explanation (orbital variations) is being tested, but that doesn't mean these other factors may not turn out to be important." Therefore, the author is sounding a note of caution. Despite the promising results so far, we can't be sure that this theory is the answer.
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I guess I can help.

The choice B has several issues:
Re-Stating the question and dissecting choices bit by bit.

18. 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.

No where it is claimed in the passage that theory presented is the only claim. In context of the last para it is clearly implied that it is one of the plausible theory explaining the ice-ages.

(B) It is too limited to provide a plausible explanation for the ice ages, despite recent research findings.

phrase "too limited" makes this option incorrect. Referring back to passage, in particular last line, " Yet the lack of information about other possible factors affecting global climate does not make them unimportant." - implies that author is kinda CONCERN about neglecting these factors even while author is identifying advantages of Milankovitch theory. This choice is too negative and changes the author stand point altogether.

(C) It cannot be tested and confirmed until further research on volcanic activity is done.
Directly contradicting author's view in the last para.

(D) It is one plausible explanation, though not the only one, for the ice ages.
Bingo!!! This basically implies that the presented theory by Milankovitch is one of the possible explanation but not the only one since there are other factors or theories(implied) which might have contributed to the ice age.
(E) It is not a plausible explanation for the ice ages, although it has opened up promising possibilities for future research.
Author concludes the passage indicating skepticism of ignoring factors like volcanic eruptions and duration of sunlight. He never recommends,explicitly, that an extended research is needed to prove something. Hence out!

Hope it helps
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19 and 21? I'm on it!

19. 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 oxygen18 in ocean water were not consistent with those found in fresh water.
(E) It stretched back for only a million years.

We're looking for something that would make this method less useful to researchers. B is a problem because we're taking away one of the stated advantages of the method. (Specifically, this knocks out the second advantage described in paragraph 3.)

A) The lighter isotope (16) always predominates, so this can't be a problem! (The passage states that only a few molecules out of every thousand have the heavier 18.)
C) This might be a problem for Milankovitch's theory, but not for researchers. The method is still useful, whether it supports the theory or casts doubt on it.
D) If fresh water samples gave us totally different indications of when the ice ages occurred, that would be a problem. However, just seeing variation between fresh and salt water doesn't tell us anything. Maybe the ratios in fresh water are supposed to be different, so we can't really interpret this without more information.
E) A million years is a long time! How do we know it's long enough for what we want? Ice ages appear to occur about once every hundred thousand years, and the author feel comfortable announcing this pattern after looking back a few hundred thousand years, or a few ice ages. By this standard, going back a million years would seem to be more than enough.
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21. 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 fresh water

The support for B is right here: "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."

Translation? When water evaporates, the heavier 18 gets left behind. When that evaporated water gets trapped in ice and doesn't return, the ocean gets steadily higher in oxygen 18. This can only happen if the missing water has less 16 than normal ocean water. (For a real-world connection, compare this to what happens with salt. When ocean water evaporates, the salt is left behind.)
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Its 2018 now, but i'll try to answer that for the benefit of those who might have gotten stuck here as well.

Milansovitch theory states that ice ages are caused by variations in the Earth's orbit around the sun (and nothing else). The passage then explains and confirms that such a connection does indeed exist. But when we read line (49-52), there could be other causes of ice age as well.

Hence the answer, "It is one plausible explanation, though not the only one, for the ice ages."

Why is C incorrect here? Doesn't the oxygen 18 enriched water eventually turn into ice?

bpdulog There's no indication that the 18-enriched water turns to ice. It's in the ocean, and the passage doesn't say that the ocean freezes.

However, that doesn't matter anyway. The question is asking us to compare rain from evaporated ocean water with continental ice sheets. That rain is exactly what forms the ice sheets, so they should have roughly the same concentration of oxygen 18 (a very low one, if any at all!).
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9. 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

Why is B or C wrong?
B: it does introduce 2 evidence of new possibilities?
C: It does present 2 discoveries?

This can't possibly be a 700 level passage. Seems more like sub 600 level

Akshit03 If you look carefully at the last paragraph, you'll see that no actual discoveries are cited. In fact, no evidence is presented at all. The author simply mentions some possible complicating factors (or even competing explanations) that we may need to consider. This is a common form of "disclaimer" in a science passage, since evidence in favor of one theory doesn't automatically rule out other possibilities.

However, there's another important point here. Even if the paragraph did cite discoveries or evidence, that wouldn't make B or C the right answer. The question is asking us for the purpose of the paragraph, not just a detail. The overall purpose of this passage is to warn us that while there is some support for the Milankovitch theory, we can't rule out other factors yet. This is a more important and larger overall point than simply looking at one or two pieces of evidence.
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comparitively easy got 1 incorrect out of 9 in 14 minutes

19 minutes with the result 6/9 right.

1) E D
presenting a theory and describing a new method to test that theory.

- I didn't pay attention to the sentense A recent discovery makes such a determination possible: (2nd paragraph)
so it meants that new way of finding solution became possible, so D is correct

3)C B - B correct
(B) It had far more gaps in its sequence than the record taken from rocks on land.

Second, it is a more continuous record than that taken from rocks on land. - in our case it means that if we can use rocks on land we don't need to spend time for learning
shells

4) E - C correct
(C) They can be used to deduce the relative volume of land ice that was present when the sediment was laid down.
I think that the explanation goes here: The dated isotope record shows that the fluctuations in global ice volume over the past several hundred thousand years have a pattern:

"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."

Can someone please provide translation for this one? karishma GMATNinja

Dear AjiteshArun IanStewart VeritasKarishma DmitryFarber GMATGuruNY VeritasPrepHailey GMATNinja

I don't understand the logic behind the passage

CASE 1 ↑ ice sheets ---> ↓ water evaporated ---> oxygen 16 is not evaporated ---> LOWER ratio of oxygen 18 : oxygen 16 in ocean water
CASE 1 ↓ ice sheets ---> ↑ water evaporated ---> oxygen 16 is evaporated and oxygen 18 is left in the water ---> HIGHER ratio of oxygen 18 : oxygen 16 in ocean water

Here the conclusion is backward.
The more land ice, the more oxygen 16 in the water, and hence the LOWER ratio of oxygen 18 to oxygen 16.

Dear AjiteshArun IanStewart VeritasKarishma DmitryFarber GMATGuruNY VeritasPrepHailey MartyTargetTestPrep GMATNinja

Why is Q2 choice B. wrong?

According to the passage:

Why is it wrong to say that the Milankovitch theory is "too limited"?

Why is Q2 choice D. right?

The implication is that the theory IS NOT the only one. However, the passage doesn't support this.
According to the passage:

The passage says that there MAY BE other possible explanations, not that there IS (definitely) other explanations.

Dear AjiteshArun IanStewart VeritasKarishma DmitryFarber GMATGuruNY VeritasPrepHailey MartyTargetTestPrep GMATNinja

Why is Q7 choice D. wrong?

According to the passage:

We know that oxygen in the shell is radioactive because it is used in RADIOMETRIC methods.

I also note that the "isotopic composition" part in choice D. also appears in the correct choice E. as well.

Nowhere does the passage tell you that it is possible to "determine a sediment's isotopic composition", using radioactivity or otherwise. All you learn from the passage is that radiometric methods can be used to date sediment "with sufficient accuracy".

Earth has a water cycle. Water evaporates from the ocean and falls as rain on land. It collects in the rivers and comes back to the ocean eventually.
When an ice age begins, continental ice sheets start expanding. From where does this ice come? It is formed by the water that evaporates from he ocean but does not return back to the ocean. After all, water is not created to make ice sheets. It is the same circulated water.

When an ice age begins, the continental ice sheets grow, - ice sheets grow
steadily reducing the amount of water evaporated from the ocean that will eventually return to it. - returning water is reduced
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 - Heavier water does not evaporate. O16 water does evaporate. Since water is not returning, O16 water concentration keeps dropping.

varotkorn - Hope this is clear now.