A library of natural history
In the warm, clear waters of the Xisha Islands, also known as the Paracel Islands, lives a natural archivist: the massive reef-building coral Porites. These dome-shaped colonies are exceptionally long-lived, with some specimens in the Indo-Pacific estimated to be up to 1,000 years old. Like tree rings, these corals grow in distinct annual bands. As they build their calcium carbonate skeletons, they trap chemical evidence of the surrounding ocean conditions. By drilling long cores from these corals, scientists can access a library of climate data that extends back for centuries, long before modern instruments were available. One study on a remote reef in the South China Sea analyzed coral growth over the last 550 years.
The basis of this archive lies in geochemistry. Scientists analyze the ratio of different elements and isotopes within the coral's aragonite skeleton. The ratio of strontium to calcium (Sr/Ca), for example, is highly sensitive to the water temperature at the time of skeleton formation. Colder water allows for more strontium to be incorporated, so a higher Sr/Ca ratio indicates a lower sea surface temperature. By measuring this ratio millimeter by millimeter down a coral core, researchers can reconstruct past sea surface temperatures with remarkable precision, sometimes resolving changes on a monthly or even weekly basis. Other chemical tracers, like oxygen isotopes (δ¹⁸O), provide information on both temperature and seawater salinity, which can be linked to rainfall and river runoff. Barium-to-calcium ratios (Ba/Ca) can act as a proxy for nutrient upwelling and freshwater input.