SKELETAL CORRELATES OF CORAL DENSITY BANDING AND AN EVALUATION OF RADIOGRAPHY AS USED IN SCLEROCHRONOLOGY

The skeletal correlates of density band patterns seen on radiographs o f coral skeletons is investigated by theoretical analysis of radiograp hy, comparison of radiographs and skeleton, and modelling of skeletal slices of the massive Indo-Pacific coral Porites lutea and the branchi ng Caribbean coral Porites porites. Radiography resolves finer detail of the coral skeleton than has previously been recognised. Small chang es in path length of individual skeletal elements attenuating an X-ray beam, including dissepiments as thin as 3 to 10 mum, can generate con trast on radiographs as well as variations in skeletal bulk density. T he annual high/low density band pattern was found to be correlated wit h areas of relatively thick/thin skeletal elements respectively. Secon dary density variations seen on radiographs, 'fine' bands, were found to be correlated with the position of corallite walls oriented perpend icular to the the long axis of the skeletal slice or dissepiments. Mod elling of skeletal slices showed that errors in the density band recor d may be introduced by (1) skeletal slices not following the growth ax is of the colony, and (2) changes in corallite orientation which gener ate density variations that are not part of any growth record. The use of radiography as a tool for demonstrating growth records in coral sk eletons is critically reviewed. It is proposed that the skeletal growt h record consists of patterns of thickening/thinning and spacing of sk eletal elements. The illustrations of these patterns by analysis of th e actual skeleton, rather than by radiography, may prove to be a very accurate method of determining growth records in corals.