# Water vapor evaporated from the ocean contains a greater proportion of oxygen–16 and a smaller proportion of the heav...

Xiang on July 18, 2017

Question

Can you explain why the answer is B? Thanks!

Mehran on July 23, 2017

@xiangzhou Thanks for your message. Let's breakdown the stimulus first:

Basically the ratio of o16 to o18 is higher in vapor than in seawater.

So let's use some basic numerical examples:

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.

Hope this helps! Let me know if you have any other questions.