How do oxygen isotopes in marine fossils help scientists estimate Cretaceous ocean temperatures?

Scientists estimate Cretaceous ocean temperatures by analyzing the ratio of oxygen isotopes—specifically oxygen-18 (${}^{18}$O) and oxygen-16 (${}^{16}$O)—found in the calcitic remains of marine organisms.12

This method, known as the oxygen isotope geothermometer, relies on the principle of isotopic fractionation. When marine organisms secrete calcium carbonate shells, the ratio of oxygen isotopes incorporated into the shell is dependent on the temperature of the surrounding water. Because the precipitation of calcium carbonate is an equilibrium process, shells formed in colder water contain a higher proportion of the heavier isotope, ${}^{18}$O, relative to the lighter ${}^{16}$O, compared to those formed in warmer water.32

By measuring these isotopic ratios in fossilized remains, researchers have been able to calculate historical surface and bottom water temperatures. For instance, these measurements have provided evidence that mid-Cretaceous ocean temperatures were significantly warmer than today, with bottom temperatures potentially reaching 17 °C (63 °F) during the Albian Age before declining toward the end of the period.1