HR: 17:30h
AN: B14B-07    [Abstracts]
TI: Stable Carbon Isotopes ($\delta^{13}$C) in Coral Skeletons: Experimental Approach and Applications for Paleoceanography
AU: * Grottoli, A G
EM: grottoli@sas.upenn.edu
AF: University of Pennsylvania, Department of Earth and Environmental Science, Philadelphia, PA 19104 United States
AB: Scleractinian corals obtain fixed carbon via photosynthesis by their endosymbiotic algae (zooxanthellae) and via hetertrophy (injestion of zooplankton, $\delta^{13}$C $\approx$ -17 to -22$\permil$). Carbon dioxide (CO$_{2}$) used for photosynthesis is obtained from seawater ($\delta^{13}$C $\approx$ 0%) or from respired CO$_{2}$ within the coral host. The $\delta^{13}$C of the carbon used in the formation of the underlying coral skeleton is fractionated as a result of both of these metabolic processes. Here I have pooled evidence from several field and tank experiments on the effect of photosynthesis and heterotrophy of coral skeletal $\delta^{13}$C. In the experiments, decreases in light levels due to shading or depth resulted in a significant decrease in skeletal $\delta^{13}$C in all species studied ({\it Pavona gigantea}, {\it Pavona clavus}, {\it Porites compressa}). Decreases in photosynthesis in bleached corals also resulted in a decrease in skeletal $\delta^{13}$C compared to non-bleached corals growing under the same conditions and at the same location. Skeletal $\delta^{13}$C also decreased at higher than normal light levels most likely due to photoinhibition. Thus, decreases in photosynthesis due to reduced light levels, due to bleaching-induced decreases in chlorophyll {\it a} concentrations, or due to photodamage-induced decreases in functional cholorphyll {\it a}, results in significant $\delta^{13}$C decreases. Comprehensive interpretation of all of the data showed that changes in photosynthesis itself can drive the changes in $\delta^{13}$C. In field experiments, the addition of natural concentrations of zooplankton to the diet resulted in decreases in skeletal $\delta^{13}$C. Such a decrease was more pronounced with depth and in {\it P. gigantea} compared to {\it P. clavus}. In situ feeding experiments have since confirmed these findings. However under tank conditions with unaturally high feeding rates, enhanced nitrogen supply in the diet can disrupt the coral-algal symbiosis, stimlate zooxanthellae growth and photosynthesis, and cause an incrase in skeletal $\delta^{13}$C. It is proposed that under natural field conditions corals feed on zooplankton below this `nutrient threshold' and that increases in heterotrophy should result in decreases skeletal $\delta^{13}$C values. Overall, changes in photosynthesis and heterotrophy have significant effects on coral skeletal $\delta^{13}$C. In shallower corals, photosynthesis drives the bulk of the variation in $\delta^{13}$C. In addition, boron isotope data indicate that pH levels do not vary with changes in photosynthesis or heterotrophy suggesting that metabolically driven $\delta^{13}$C fractionation during skeletogenesis is not pH driven. Thus the skeletal $\delta^{13}$C records from shallow corals in non-upwelling regions where zooplankton concentrations are relatively constant should represent a reliable proxy of light variability. Due to the complexity associated with nutrients and heterotrophy, $\delta^{13}$C records from upwelling regions or deep corals are still difficult to resolve.
DE: 4825 Geochemistry
DE: 4870 Stable isotopes
DE: 4267 Paleoceanography
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting