A STUDY OF MICROSCALE BANDING IN REEF CORAL SKELETON

Annual density bands are typical in the skeletons of massive reef corals and provide a means to estimate both age and growth rates of living and fossil colonies. Finer banding is also visible in petrographic thin sections and it has been suggested that these are formed daily. This offers the potential to estimate growth rate, measure skeletal chemistry and date episodic climate events on a daily scale. We investigated the structure and periodicity of microbanding in the skeleton of Stephanocoenia michilini, a colonial reef coral endemic to the Atlantic Ocean. Density banding revealed in x-ray indicates that the annual growth rate is approximately 2.8mm. Highly polished thin sections (15 microns thick) were used to examine fine banding in both transmitted and reflected light. The bands are most obvious at centers of calcification within the septae and corallite wall, oriented normal to the growth axis of the coral. On average, the distance between successive bands is 7-8 microns, so they are likely to be daily skeletal accretions. Moreover, there is significant variability in the daily extension rate which appears to correspond to the seasonal density changes. Fine bands in high-density regions are spaced 2-4 microns apart compared with 7-10 microns in low-density regions. These data can also be used to calculate calcification changes over the annual cycle.