How do oxygen isotope records in deep-sea sediments reveal glacial and interglacial periods?

Oxygen isotope records in deep-sea sediments serve as a primary tool for reconstructing past climate cycles by analyzing the ratio of oxygen-18 (${}^{18}$O) to oxygen-16 (${}^{16}$O) in the calcium carbonate shells of microfossils.1

This ratio is influenced by two main factors: water temperature and the isotopic composition of seawater. Because the heavier isotope, ${}^{18}$O, evaporates less readily and condenses more easily than ${}^{16}$O, water vapor becomes increasingly depleted in ${}^{18}$O as it moves toward colder regions and falls as precipitation. During glacial periods, large amounts of water—which is isotopically "light" (rich in ${}^{16}$O)—are trapped in continental ice sheets. Consequently, the remaining ocean water becomes enriched in the heavier ${}^{18}$O.1

Microfossils that formed during these glacial periods incorporate this higher concentration of ${}^{18}$O into their shells. Conversely, during warmer interglacial periods, ice sheets melt and return ${}^{16}$O to the oceans, resulting in shells with lower ${}^{18}$O levels. Because these deep-sea sediment records are continuous, they provide a standard global timeline of glaciations and interglaciations that can be correlated across the world's oceans.12