Corals precipitate their calcareous skeletons (calcium carbonate) from seawater. Over thousands of years, vast coral reefs form due to the deposition of this calcium carbonate. During precipitation, corals prefer carbonate groups containing specific variants of oxygen (chemical symbol: O). For example, the lower the water temperature, the higher the abundance of a heavy oxygen variant, known as isotope 18O, within the precipitated carbonate. Unfortunately, the 18O abundance of the seawater also influences the abundance of 18O in the calcium carbonate—and the contribution of 18O from seawater cannot be resolved when determining temperatures based on carbonate 18O abundances alone.
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Source: Phys.org