Isotopic and geochemical time-series generated from massive coral skeletons are now being used to monitor environmental changes in the oceans and nearby coastal zones. They are also being used to reconstruct the past variations in tropical surface oceanographic conditions before any records were taken. Research has been focused on the skeletal oxygen and carbon isotopic composition (18O/16O, 13C/12C) as a tracer of past oceanographic conditions. The health, diversity and extent of corals, and geochemical makeup of their skeletons, record a variety of other changes in the ocean surface water. These include temperature, salinity, fertility, insolation, precipitation, sea levels, storm incidence, river runoff, and human inputs. Corals in coastal waters are susceptible to rapid changes in salinity and suspended matter concentrations which can help us to show the extent of effects of agriculture, urban, mining and industrial pollutants through river systems.
Coral produces annual growth bands, much like tree rings, which can record monthly environmental changes. The 18O/16O ratio varies as a function of temperature and salinity. In general, higher ratios indicate colder temperatures, and low ratios indicate warmer conditions. Also trace element (Sr, Ca, Ba, Mg) concentrations offer a wider range of climatic interpretation by recording site-specific features, such as upwelling of cold deep water to the surface, runoff of freshwater from rivers, and mixing of different water masses. Radiocarbon (14C) concentrations have been used to reconstruct ocean circulation, when deep water is brought to the surface the 14C concentration in the coral skeleton is decreased.
Corals are one of the richest natural environmental recorders and archives because of the abundance of geochemical tracers, near perfect dating capacity, continuous growth of most species, the deposition of annual time markers in the form of skeletal density bands, and the fact that the species can live for very long periods of time. A 30cm-diameter coral colony growing at an average rate of 1cm/yr will provide 20-25 years of baseline data, whereas massive colonies 3-6m high may provide historical data for extensive tracts of tropical ocean.