Paleothermometer

A natural proxy or measurement method used to estimate past temperatures from preserved physical, chemical, or biological evidence. Different paleothermometers record temperatures over different timescales and in different environments.

Oxygen isotope ratio: The relative abundance of oxygen isotopes in a sample, commonly expressed as δ¹⁸O. Temperature affects oxygen-isotope fractionation during carbonate formation and water evaporation.
Stable isotope geochemistry: The study of variations in stable isotope abundances and their use in tracing natural processes. Many chemical paleothermometers depend on isotope fractionation between substances.
Ice core: A cylindrical sample drilled from an ice sheet or glacier that preserves layered records of past environments. Ice-core water isotopes are used to reconstruct past polar temperatures.
Instrumental temperature record: A record of temperature measured directly by thermometers and other instruments. Instrumental observations are direct measurements, unlike inferences from preserved evidence.
Clumped isotope thermometry: A method that estimates formation temperature from the abundance of multiply substituted isotopologues in carbonate. It can estimate carbonate formation temperature without relying on the isotopic composition of source water.
Isotope fractionation: The separation of isotopes through physical, chemical, or biological processes that treat isotopes differently. Temperature-dependent fractionation is the basis of several isotope paleothermometers.
Foraminifera: Single-celled marine organisms that build mineral shells and are abundant in ocean sediments. Their preserved shells provide oxygen-isotope and Mg/Ca temperature records.
Paleobarometer: A proxy or method used to estimate atmospheric pressure in the geological past. It reconstructs past pressure rather than temperature, though some evidence can inform both.
Magnesium-to-calcium ratio: The proportion of magnesium to calcium in a material, used in some carbonates to infer formation conditions. Foraminiferal shell Mg/Ca generally rises with the temperature of seawater in which the shell formed.
Proxy (climate): A measurable natural feature used to infer a climate variable that was not directly observed. Paleothermometers are proxies whose relation to temperature must be established.

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