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How do oxygen isotopes in fossil shells help scientists estimate Permian ocean temperatures?

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Line graph showing the number of marine animal families from the Precambrian era to the present, with illustrations of key groups such as trilobites, ammonoids, marine reptiles, and diatoms. The graph highlights major extinction events and a rise to 1,900 living families today. — Permian extinction The diversity of marine animal families since late Precambrian time. The data for the curve comprise only those families that are reliably preserved in the fossil record; the 1,900 value for living families also includes those families rarely preserved as fossils. The several pronounced dips in the curve correspond to major mass-extinction events. The most catastrophic extinction took place at the end of the Permian Period.

Scientists estimate Permian ocean temperatures by analyzing the ratio of oxygen isotopes found in fossil shell materials. This method relies on the principles of isotopic fractionation, where the precipitation of calcium carbonate from water is influenced by temperature.123

Because the ratio of oxygen-18 to oxygen-16 in a shell depends on the temperature of the water in which the organism lived, these shells act as an "oxygen isotope geothermometer". Specifically, shells secreted in warmer waters contain different isotopic ratios than those formed in colder waters. By studying these ratios in Permian fossils, researchers have hypothesized that shallow marine surface temperatures reached approximately 25–28 °C (77–82 °F) during the transition between the Guadalupian and Lopingian epochs, potentially creating conditions too warm for many invertebrates to survive.4312