How do oxygen isotope records in ocean sediments reveal glacial and interglacial periods?

Oxygen isotope records in ocean sediments serve as a primary proxy for identifying past glacial and interglacial periods by tracking the ratio of oxygen-16 (${}^{16}$O) to oxygen-18 (${}^{18}$O) in the calcium carbonate shells of marine microfossils.1

Because the lighter ${}^{16}$O isotope evaporates more readily than the heavier ${}^{18}$O, water vapor becomes depleted in ${}^{18}$O as it moves toward the poles and falls as snow. During glacial periods, this "light" water is trapped on land in massive ice sheets, causing the remaining ocean water—and the shells formed within it—to become enriched in ${}^{18}$O. Conversely, during warmer interglacial periods like the Holocene Epoch, ice sheets melt and return ${}^{16}$O to the oceans, resulting in lower ${}^{18}$O concentrations in marine shells.12

By analyzing these continuous sediment cores, scientists can map climatic cycles, assigning odd numbers to warmer interglacials (such as the current stage 1) and even numbers to colder glacial periods. This marine record acts as the global standard for correlating geochronology across terrestrial and oceanic sites.1