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How do oxygen isotopes in fossil shells reveal ancient ocean temperatures?

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Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

In geochronology, the ratio of oxygen isotopes—specifically oxygen-18 ( $^{18}$ O) and oxygen-16 ( $^{16}$ O)—found in the calcium carbonate shells of marine microfossils serves as a vital proxy for reconstructing ancient ocean temperatures.12

This method, known as the oxygen isotope geothermometer, relies on the principle of isotopic fractionation, where the precipitation of shell material is temperature-dependent. When organisms secrete their shells, the ratio of isotopes incorporated into the shell reflects the temperature of the surrounding water; specifically, shells formed in colder water contain a higher concentration of the heavier isotope, $^{18}$ O, compared to those formed in warmer water.21

However, this ratio is also influenced by the global volume of ice on land. During glacial periods, lighter $^{16}$ O is preferentially trapped in ice sheets, leaving the remaining ocean water enriched in $^{18}$ O. Consequently, the isotopic composition of fossil shells provides a dual record: it reflects both local water temperature and the global climate state, making it a standard tool for correlating stratigraphic records and identifying past glacial and interglacial cycles.1