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How can coral skeletons reveal past seawater temperatures?

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coral reef The Great Barrier Reef, off the coast of Queensland, Australia, is the world's largest coral reef.

Coral skeletons serve as valuable natural archives for reconstructing past environmental conditions. Scientists can determine historical seawater temperature changes by analyzing the oxygen isotopic composition of the calcium carbonate that makes up coral skeletons.1

Because corals incorporate oxygen isotopes from the surrounding seawater as they grow, their skeletons preserve a chemical record of the water temperature at that time. Specifically, the ratio of oxygen-18 ( $^{18} O$ ) to oxygen-16 ( $^{16} O$ ) in the skeletal material is temperature-dependent; shells and skeletons formed in colder waters typically contain a higher proportion of the heavier oxygen-18 isotope compared to those formed in warmer waters.12

By examining these isotopic signatures, researchers can create high-resolution proxy measurements that provide insights into yearly or even seasonal climate variations over time. This method allows scientists to reconstruct surface temperature patterns and better understand long-term climatic shifts.3