Separation of kinetic and metabolic isotope effects in carbon-13 records preserved in reef coral skeletons

The stable isotopic signatures of Jamaica and Zanzibar coral skeletons, collected over depth and light gradients, are strongly affected by kinetic isotope effects associated with the calcification process. These kinetic effects mask δ13C metabolic signals of light availability/photosynthesis. A simple data transformation, based on oxygen isotopic variation in the skeleton, has been developed to correct for the presence of these disequilibrium kinetic influences. The resulting transformed skeletal δ13C data show significant correlation with associated tissue δ13C and with depth/light attenuation. These relationships demonstrate the value of the transformation and suggest that transformed skeletal δ13C data faithfully record the carbon isotope effect associated with photosynthesis, in which increased photosynthesis leads to skeleton enriched in the heavy isotope of carbon. Possible applications of these transformations include improved modeling of carbon fluxes in corals and more accurate reconstruction of insolation, depth, water clarity and cloudiness. The 13C transformation presented will also allow for better identification of periods of metabolic stress in corals, when growth rate effects on δ13C might normally obscure the 13C metabolic signal.