Oxygen Isotopes: Measuring Ancient Ocean Temperatures

Published: 2026-01-01

Oxygen isotope paleothermometry, pioneered by Harold Urey and Cesare Emiliani in the 1950s, exploits the temperature-dependent fractionation of oxygen isotopes during carbonate precipitation. Foraminifera building their shells in colder water incorporate relatively more oxygen-18, producing a measurable isotopic signal.

The oxygen isotope record from benthic (bottom-dwelling) foraminifera has become the standard reference for global climate change over the past 66 million years. The composite benthic δ¹⁸O stack reveals long-term cooling trends, the onset of Antarctic glaciation around 34 million years ago, and the rhythmic glacial-interglacial cycles of the Quaternary.

Interpreting these records requires careful consideration of the species used, the preservation state of the shells, and the competing influences of temperature and ice volume on seawater δ¹⁸O. Modern approaches combine oxygen isotopes with independent temperature proxies such as Mg/Ca ratios to disentangle these effects.