During periods when glaciers are extensive,the concentration of ¹⁸O in seawater increases.

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During glaciations, more water is locked up in ice sheets, leading to a higher concentration of ¹⁸O in the remaining seawater. This is because the heavier isotope of oxygen (¹⁸O) is less likely to be evaporated and taken up into the ice, leaving a higher concentration in the seawater. Therefore, when studying isotopes in marine sediments, an increase in ¹⁸O is often used as a proxy for periods of glacial expansion.

During Periods When Glaciers Are Extensive,the Concentration of ¹⁸O in Seawater