How did ocean oxygen isotope records provide evidence for Pleistocene climate cycles?

Ocean oxygen isotope records provide a continuous, high-resolution history of Pleistocene climate cycles by analyzing the ratio of oxygen isotopes—specifically oxygen-16 (${}^{16}$O) and oxygen-18 (${}^{18}$O)—found in the calcium carbonate shells of marine microfossils preserved in seafloor sediments.1

This evidence relies on two primary factors:
* Global Ice Volume: Because the lighter isotope (${}^{16}$O) evaporates more readily than the heavier (${}^{18}$O), water vapor becomes depleted in ${}^{18}$O as it moves toward poles and falls as snow. During glacial periods, this "light" water is trapped in massive continental ice sheets, causing the remaining ocean water to become enriched in ${}^{18}$O. Consequently, shells formed during glaciations contain higher concentrations of ${}^{18}$O compared to those from warmer interglacial periods.12
* Temperature: The isotopic ratio in shells is also influenced by the temperature of the water in which the organism lived, as shells secreted in colder water incorporate more ${}^{18}$O.1

Because these records are continuous and global, they serve as the standard for correlating terrestrial climate events, revealing that the Pleistocene experienced significantly more glacial-interglacial cycles than previously identified in traditional land-based models.13