Interference imaging of daily growth bands in massive corals

CORAL skeletons have been used to monitor a wide range of environmental parameters, including water temperature, insolation, siltation, river runoff and pollutant concentrations1–9. In most cases, interpretation of the coral record relies on a determination of the growth rate. Radiographic techniques9–13, which have been verified by a number of approaches8,14,15, are most commonly used for this purpose, but are restricted to the determination of yearly average growth rates. Interpolation procedures have been used to extract growth rates on shorter timescales, but their accuracy depends on various assumptions about relative growth rates of skeletal bands with different densities9,16. Here we report that reflectance optical microscopy can be used to measure daily growth rates of massive corals. This method should greatly expand the potential use of corals as environmental indicators.