Seasonal high and low density bands in reef coral skeletons

THE skeletal aragonite deposited by scleractinian reef corals is characterised by alternating bands of high and low density carbonate that are concordant with the growth surface of the colony. These density variations, which are readily observed by X-radiographic examination of 5–10 mm thick sections of the corallum cut parallel to the axis of growth, are annual in periodicity, such that one high density band plus one adjacent low density band represent the skeletal increment formed over one calendar year 1–4 . Skeletal X radiography is a powerful tool for studying both modern and fossil corals, for in addition to providing a wealth of growth rate data from specimens that grew undisturbed for many years in their natural habitat on the reef, valuable information about the coral itself and about the environment in which the coral lived can be obtained. As Buddemeier 5 points out, density banding in reef corals can not only yield physiological and ecological information about these marine organisms, but also help to unlock the record of long term environmental conditions that is embodied in a coral skeleton. Reef corals, for example, are effective samplers of certain important radionuclides 6 .