To work out how old coral skeletons are, we can examine their isotopes, which are atoms of the same element that differ in atomic mass. Isotopes can either be stable or unstable; stable isotopes have the same number of protons and neutrons and these remain the same over time, whereas unstable isotopes have too few or too many neutrons compared to protons.
A coral’s aragonitic skeleton is mainly composed of calcium, oxygen and carbon but it also contains other elements you may not even think of, such as uranium or magnesium. These elements have several isotopes, some stable and some unstable. Unstable isotopes are commonly referred to as radioactive isotopes, and are used to date geological features. As all the elements and their isotopes from the coral’ skeleton are, in theory, the same as the seawater in which the skeleton was formed, there is the same proportion of each element and their isotopes in the coral’s skeleton as in the seawater.
Unfortunately, things are a little bit more complicated than that. The growth of coral skeleton is also affected by the animal itself, as the animal can change the seawater properties during the process of skeleton construction, which is called biomineralisation. The biomineralisation process is still poorly understood and so we have to be careful when using cold-water corals as paleo-markers of past climate.