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.
Reading the monsoon and typhoon record
The data extracted from Xisha Islands corals provides a detailed history of the East Asian Monsoon, a massive weather system that governs the climate for a huge portion of the world's population. Wind patterns from the monsoon influence ocean currents and sea surface temperatures, all of which are logged in the coral skeletons. For instance, stronger winter monsoons can cause more upwelling of cool, nutrient-rich water, a change that appears in the coral's chemistry. Studies of these coral cores have allowed scientists to reconstruct the strength and variability of the monsoon hundreds of years into the past.
This archive also holds clues to the history of extreme weather events. The South China Sea is a region frequently hit by powerful typhoons. These storms churn the ocean, bringing cooler deep water to the surface and causing immense rainfall. These sudden, dramatic changes are recorded in the coral's growth bands as distinct chemical anomalies. By identifying these signatures, scientists can build a timeline of past typhoon activity, noting periods of intense storms. Research on coral reef boulders on nearby Hainan Island has identified seven periods of intense typhoon activity over the last 4,000 years, a record that can be correlated with the high-resolution data from living corals. This long-term perspective is invaluable for understanding how storm frequency and intensity may be changing over time.