Premise: no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation
Premise: During an ice age, however, a large amount of precipitation falls on ice caps, where it is trapped as ice.
the premises tell us that although the overall composition of ocean does not change BUT during an ice age it does because a large portion of precipitation (more O-16) is trapped as ice on ice caps.
this must mean that the concentration of O-18 is now increased.
B is correct
Your explanation is acceptable with regard to the OA.
But I think, the text is a bit confusing to find the right answer. A, C, D, E are clearly out.
And let's consider the OA in the light of the main text.
The first premise is "Water vapor evaporated from the ocean contains a greater proportion of oxygen-16 and a smaller proportion of the heavier oxygen-18 than does seawater."
This premise separates the seawater from the ocean water. On this basis the second premise - "Normally, this phenomenon has no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation." - seems clearly ambiguous because of the use of the word "return".
Moreover, one may, because of the words " precipitation falls on ice caps", assume two cases, i.e. either water circulation from the ocean to the sea or vise versa. Herewith the reader can also be trapped in so far as the ocean water does usually not become icy (here the ice age is not an absolute evidence to disprove this fact and otherwise as in the GMAT-format the test-taker is not assumed to posses the knowledge of the special subject, as here the case relates to the geography), the same is true under conditions regarding the seawater.
Take my evaluations into consideration then even B is also not a proper ans. choice.
I would say it a bad question because of such internal contradictions in its construction and ambuqity of its content.