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

Glacial periods result in higher concentrations of oxygen-18 (${}^{18}$O) in ocean sediments primarily due to the global redistribution of water isotopes during the formation of large ice sheets.12

The process functions as follows:

• Preferential Evaporation: Water molecules containing the lighter isotope, oxygen-16 (${}^{16}$O), evaporate from the ocean more readily than those containing the heavier oxygen-18 (${}^{18}$O).12
• Storage in Ice: As this water vapor moves toward polar regions and cools, it falls as precipitation (snow) and is stored on land as glacial ice. Because the lighter ${}^{16}$O is preferentially removed from the ocean through evaporation and trapped in these growing ice sheets, the remaining seawater becomes isotopically "heavier," meaning it is enriched in ${}^{18}$O.12
• Sedimentary Record: Marine microfossils that secrete their calcium carbonate shells during these glacial periods incorporate this higher ratio of ${}^{18}$O into their structures. When these organisms die, their shells accumulate on the seafloor, creating a continuous record that allows scientists to track past changes in global ice volume.12