Abstract
Ultraviolet irradiation of a sectional slice of the aragonitic skeleton cut along the growth axis of coral colonies reveals a series of alternating luminescent bands, the intensity of which appears to be related to environmental conditions at the former growth surface of the colony. Studies of the patterns of annual density banding in coral skeletons, as revealed by X-ray radiography, have been used in much the same way that growth rings in trees are used to provide indications of past environmental conditions. A second class of studies on coral skeletons attempts to quantify chemical and isotopic markers which are included within the calcium carbonate matrix to similar known climatic or environmental variables. The luminescence signal is generally believed to arise from the skeletal inclusion of varying amounts of naturally fluorescent dissolved organic matter (DOM) chromophores present in seawater. In particular, it has been argued that the fluorescent banding observed in corals found growing adjacent to coastal regions impacted by periodic discharge of terrestrially derived fluorescent humic and fulvic acids may thus be used as a proxy for past river discharge and, hence, past rainfall. It may be noted, however, that other components of calcium carbonate matrices, e.g., impurity Mn<sup>2+</sup> may contribute to some of the total observed luminescence signal.
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