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Published 5 October 2012

Professor Cam Nelson explains how tropical limestone formation in warm seas involves coral reefs and chemical precipitation. The warm water becomes saturated with calcium carbonate, which chemically precipitates to form solid grains. Temperate limestone forms through a different mechanism. It is the smashed up shells of cool-water sea organisms that provides the material needed to form this type of limestone.

Points of interest

Look for this micrograph

  • Micrograph a: Thin section of ooids (x12.5), which are spherical balls of inorganically precipitated calcium carbonate.


The tropical limestone story in shallow water very much centres around coral reefs. Most warm water shallow seas where carbonates dominated have a lot of corals that build reefs. Moreover, shallow tropical seawater has a lot of calcium carbonate in solution in it – the warmth allows that calcium carbonate to be there and it can precipitate out of the actual seawater inorganically, chemically. And so you can get inorganic chemical precipitation of carbonate grains in warm tropical seas as well. Some of those are called oolites or ooids, for example, little spherical balls of calcium carbonate. They form in agitated water where there’s a lot of calcium carbonate held in solution, and you get that in warm waters. So the presence of coral reefs and the presence of inorganically chemically precipitated grains are common in the tropical story. 

When you come into the cooler waters, the temperate waters, cool water limestones, the degree of saturation in calcium carbonate is much, much less, and you do not get chemical precipitation to produce grains on the seafloor. All we have are shell fragments. The organisms can get the carbonate out of seawater by biochemically taking it into their shells, and then of course eventually they can get, they die and get smashed up and can contribute to the carbonate sediments, but we don’t have that chemical influence. Moreover, the cooler waters are such that we don’t have reef-forming corals living in these cooler temperate waters. They cut out with mean annual temperatures probably consistently above 20°C seawater temperatures rather than less than 20°C.

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