Isotopy

HYPERLINK "https://www.britannica.com/science/marine-oxygen-isotopic-record" Marine oxygen isotope record

The isotopic record is based on the ratio of two oxygen isotopes, oxygen-16 (16O) and oxygen-18 (18O), which is determined on calcium carbonate from shells of microfossils that accumulated year by year on the seafloor. The ratio depends on two factors, the temperature and the isotopic composition of the seawater from which the organism secreted its shell. Shells secreted from colder water contain more oxygen-18 relative to oxygen-16 than do shells secreted from warmer water. The isotopic composition of the oceans has proved to be related to the storage of water in large ice sheets on land. Because molecules of oxygen-18 evaporate less readily and condense more readily, an air mass with oceanic water vapour becomes depleted in the heavier isotope (oxygen-18) as the air mass is cooled and loses water by precipitation. When moistu