Why do glacial periods leave higher concentrations of oxygen-18 in ocean water?

During glacial periods, ocean water exhibits higher concentrations of oxygen-18 (${}^{18}$O) because of the way water molecules cycle between the oceans and land-based ice sheets.12

Water molecules containing the lighter isotope, oxygen-16 (${}^{16}$O), evaporate more readily than those containing the heavier oxygen-18 isotope. Under normal conditions, this water vapor eventually returns to the ocean through precipitation. However, during a major glaciation, this evaporated moisture is trapped on land as snow and ice. Because the ice sheets preferentially store the "lighter" water (enriched in ${}^{16}$O), the remaining ocean water becomes isotopically "heavier," meaning it contains a higher relative concentration of ${}^{18}$O.12

Consequently, the shells of marine organisms that form during these glacial periods incorporate this higher ratio of ${}^{18}$O, providing scientists with a reliable record of past global ice volume and climate change.12