δ18O

δ18O is the per-mil difference between a sample’s 18O/16O ratio and that of a reference standard. It records oxygen-isotope fractionation in materials such as water, carbonate, and ice.

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Oxygen isotope ratio: The relative abundance of oxygen isotopes in a sample, commonly expressed as 18O/16O. δ18O expresses this ratio as a deviation from a reference.
Oxygen isotope stage: A numbered interval of Earth's climate history identified from oxygen-isotope records. Marine δ18O curves define alternating glacial and interglacial stages.
Oxygen-18: A stable oxygen isotope with eight protons and ten neutrons. It is the heavy isotope in the ratio δ18O compares with oxygen-16.
δD: The per-mil deviation of a sample's deuterium-to-hydrogen ratio from a reference standard. It tracks hydrogen isotopes and is often paired with water δ18O.
Isotope fractionation: A change in isotope ratios caused by physical, chemical, or biological processes. Fractionation produces the δ18O differences measured between materials.
Paleothermometry: The reconstruction of past temperatures from physical or chemical indicators preserved in natural archives. Temperature-dependent fractionation lets carbonate δ18O constrain ancient ocean temperatures.
Oxygen-16: The most abundant stable oxygen isotope, with eight protons and eight neutrons. It is the denominator isotope in the conventional 18O/16O ratio.
δ13C: The per-mil deviation of a sample's carbon-13 to carbon-12 ratio from a reference standard. It measures carbon-isotope variation rather than oxygen-isotope variation.
Rayleigh fractionation: A model describing isotope changes as material is progressively removed from a reservoir. It predicts δ18O shifts during progressive condensation and water-vapor removal.
Speleothem: A cave mineral deposit, such as a stalagmite, formed by dripping or flowing water. Its δ18O can preserve changes in rainfall and moisture sources.