The information in the passage tells you that water vapor evaporated from the ocean contains a greater proportion of oxygen 16 and a smaller proportion of the heavier oxygen 18. We know that normally this doesn't effect the overall composition of the ocean because the evaporated seawater cycles back into the ocean through precipitation. However, during an ice age, the precipitation falls on ice caps where it is trapped. Therefore, during an ice age, the precipitation does not cycle back i.e. the overall composition of the ocean no longer balances. So, if a higher proportion of oxygen 16 is leaving and a smaller proportion of oxygen 18 is leaving, without balancing the ocean composition back, that means we are left with a smaller concentration of oxygen 16 in the seawater and a higher concentration of oxygen 18 than normal.
(A) is incorrect because the seawater vapor during an ice age does not cycle back into the seawater. Therefore, the the concentrations will no longer have the same proportions.
(B) is CORRECT because as we discussed above, since the vapor is not cycling back, the concentrations will not balance out in the seawater. Thus, the higher proportion of oxygen 16 found in seawater vapor is not cycling back in, leaving seawater with less concentration of oxygen 16, meaning there will be a higher concentration of oxygen 18.
Some basic numerical examples will clarify:
For vapor, let's pretend the ratio of o16 to o18 is 4 to 1 (i.e. 4 parts o16 for every 1 part o18). Now, for seawater, we need this ratio to be smaller, so let's pretend it is 1 to 1 (i.e. one part o16 to o18).
Normally, this difference has no effect on the composition of the ocean because this vapor returns to the ocean. During an ice age, however, a large amount of precipitation (i.e. the vapor returning) falls on ice caps, where it is trapped as ice. As such, during an ice age, we would be losing 4 parts of o16 for every 1 part of o18 (i.e. more o16 being removed than o18 but not returning to the ocean because it is falling on ice caps).
Therefore, we can conclude that in "a typical ice age" (language directly from the question stem), the concentration of o18 would increase.
(C) is incorrect because the concentration of oxygen 16 in the rain and snow is the same during ice ages as they are during normal periods. The difference is that the rain falls on ice caps during ice ages, and therefore the concentration of the seawater is thrown off.
(D) is incorrect because we do not necessarily know whether an ice cap is considered land or not because it was not discussed in the argument. Remember that the boundaries of the argument create the limit of our knowledge. Since the passage does not discuss precipitation falling over land, we cannot make a conclusion about it.
(E) is incorrect because we do not know anything about the composition of seawater changing in during a normal period. Thus, we cannot conclude answer choice (E) because we have no comparison.
Hope that was helpful! Let me know if you have any other questions!
YangDecember 17, 2015
How do you know that a higher proportion of o16 is leaving than the proportion of o18? There is no information that mentioning this, could be losing more o18 than o16.
YangDecember 17, 2015
Anyway I'm lost, I feel like at one moment I understand and the next moment I don't, no certainty....
