A Library of Ancient Weather
In the clear waters of the Andaman Sea, certain massive coral colonies act as unintentional historians. The species Porites lutea, a type of boulder coral common in the Similan Islands, grows in distinct layers, creating annual density bands similar to tree rings. These skeletal layers, made of calcium carbonate, chemically record the conditions of the surrounding seawater at the time of their formation. By drilling a narrow core from a large Porites colony—a process that does not kill the animal—scientists can access a continuous, high-resolution archive of ocean history stretching back centuries.
This field, known as paleoclimatology, uses such natural archives, or "proxies," to reconstruct past climates. The corals of the Andaman Sea are especially valuable because they record the powerful seasonal shifts of the Indian Ocean monsoon system. The chemical composition of each annual band holds precise data about sea surface temperature, salinity, and other environmental factors, providing a detailed logbook of weather patterns long before instrumental records began.
Deciphering the Chemical Code
The method for reading a coral core lies in its geochemistry. Scientists analyze several chemical tracers locked within the coral's aragonite skeleton. The ratio of strontium to calcium (Sr/Ca) and the ratio of stable oxygen isotopes (δ¹⁸O) are two of the most established proxies for sea surface temperature. As seawater temperature changes, the amount of strontium and the specific oxygen isotope incorporated into the growing coral skeleton changes in a predictable way.
By measuring these ratios millimeter by millimeter down a coral core, researchers can reconstruct a timeline of past ocean temperatures with near-monthly accuracy. Oxygen isotope ratios are also influenced by salinity, which in this region is heavily affected by monsoon rainfall and river runoff. By combining the Sr/Ca and δ¹⁸O records, scientists can disentangle the signals of temperature and salinity to build a detailed picture of past monsoon intensity and the behavior of climate patterns like the El Niño-Southern Oscillation (ENSO). Studies of these archives suggest that ENSO variability has intensified in recent decades, providing a long-term context for modern climate change.