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  Normally, this phenomenon has no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation
During an ice age, however, a large amount of precipitation falls on ice caps, where it is trapped as ice
During the ice age, more of the Earth's precipitation falls over land than falls over the ocean.