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
The concentration of oxygen-18 in seawater is increased.