Why do high oxygen-18 levels in ocean sediments indicate glacial periods?

High oxygen-18 (${}^{18}$O) levels in ocean sediments serve as a proxy for glacial periods due to the process of isotopic fractionation.

Water molecules containing the lighter isotope, oxygen-16 (${}^{16}$O), evaporate more readily than those containing the heavier oxygen-18 (${}^{18}$O). During glacial periods, this evaporated moisture travels toward the poles and falls as snow, eventually becoming trapped in large ice sheets on land. Because these ice sheets preferentially store the lighter ${}^{16}$O, the remaining seawater becomes enriched in the heavier ${}^{18}$O.12

Marine organisms, such as foraminiferans, incorporate this oxygen into their calcium carbonate shells as they grow. Consequently, shells formed during glacial periods contain higher concentrations of ${}^{18}$O compared to those formed during interglacial periods, when the ice sheets melt and return the lighter ${}^{16}$O to the ocean. By analyzing these microfossil shells in ocean sediment cores, scientists can reconstruct a continuous record of past global ice volume and climate fluctuations.12