Why does the ratio of oxygen 18 to oxygen 16 in the oceans increase during an ice age?

Question

Quizplus · Accepted Answer

During an ice age, cooler temperatures cause a larger portion of precipitation to fall as snow instead of rain. This results in less water flowing back into the oceans and instead staying on land in the form of glaciers and ice sheets. The oxygen-18 isotope is heavier than oxygen-16, and therefore it is less likely to evaporate and become part of the snow. As a result, the ratio of oxygen-18 to oxygen-16 in the oceans increases during an ice age.

Why Does the Ratio of Oxygen 18 to Oxygen 16