A 4mm thick slab was cut longitudinally along the center of the core, and then the slab was x-rayed to reveal 270 annual density bands. These bands correspond to the age of the coral. Chips representing approximately one year were cut and subjected to isotopic analysis. Photosynthetic symbionts most likely determine the carbon isotope ratios in the skeleton of the coral. These symbionts compete with the coral for the lighter of the two carbon isotopes, carbon 12.
Oxygen isotopes are much easier to interpret than carbon isotopes.
The isotopic composition of the water fluctuates seasonally in the northeast Caribbean as a result of fresh water input from rivers mainly in South America. However, this effect on the isotopic composition of the water is small. Because the influx of fresh water occurs only for a few weeks, it should not affect the annual averages. The pinnacle coral colonies of Montastrea annularis are situated on the insular shelf platform in the northeastern Caribbean close to the marine laboratories of the Department of Marine Sciences, University of Puerto Rico at La Parguera on the southwestern side of the island. Isotopic ratios for oxygen and carbon from 1760 to 1991.