What PT/LR Section is this from? And what am I missing?

[ust2008]

Edit: Thanks JJ, realized this was actually from the "experimental" section of my PT - which, inconveniently, does not provide the PT #. I've update the thread title accordingly.

Regarding oxygen 16 & 18 contents of ocean water vs. seawater.

Here's how I read the stimulus:

Water vapor from the ocean has a greater proportion of O16 & a lesser proportion of O18 than seawater. However, because evaporated sea water (higher O18 and lower O16) returns to the ocean through precipitation, the overall composition of the ocean does not change. (My assumption is that sea and ocean water vapor sort of balance each other out - and therefore preserve the composition of the ocean).

During ice ages, a large amount of the precipitation falls on ice caps, where it is trapped as ice.

How is the correct answer choice b (that O18 in seawater will be increased)? I imagine its because somehow, during an ice age, this balancing process is interrupted. Therefore the composition of the ocean would be changed (likely in the direction of lower O16 and higher O18 levels). What I do not understand is why this requires any change w/respect to sea water. What am I missing?

Thanks for your help!

[JJDancer]

Not sure where this is from but it's not pT 59