NOAA/WDS Paleoclimatology - Savusavu Bay, Fiji Coral d13C Data From 1781-1997

In the context of increasing anthropogenic CO2 emissions, determining the rate of oceanic CO2 uptake is of high interest. Centennial-scale changes in d13C of the surface water dissolved inorganic carbon (DIC) reservoir have been shown to be influenced by the carbon isotopic composition of atmospheric CO2. However, the availability of direct oceanic d13C measurements is limited and methods for reconstructing past d13C variability of the oceanic DIC are needed. Geochemical reconstructions of DIC variability can help in understanding how the ocean has reacted to historical changes in the carbon cycle. This study explores the potential of using temporal variations in d13C measured in five Fijian Porites corals for reconstructing oceanic d13C variability. A centennial-scale decreasing d13C trend is observed in these Fiji corals. Other studies have linked similar decreasing d13C trends to anthropogenic changes in the atmospheric carbon reservoir (the “13C Suess effect”). We conclude that solar irradiance is the factor influencing the d13C cycle on a seasonal scale, however it is not responsible for the centennial-scale decreasing d13C trend. In addition, variations in skeletal extension rate are not found to account for centennial-scale d13C variability in these corals. Rather, we found that water depth at which a Fijian Porites colony calcifies influences both d13C and extension rate mean values. The water depth-d13C relationship induces a dampening effect on the centennial-scale decreasing d13C trend. We removed this “water depth effect” from the d13C composite, resulting in a truer representation of d13C variability of the Fiji surface water DIC (d13CFiji-DIC). The centennial-scale trend in this Fiji coral composite d13CFiji-DIC time-series shares similarities with atmospheric d13CCO2, implicating the 13C Suess effect as the source of the this coral d13C trend. Additionally, our study finds that the d13C variability between the atmosphere and the ocean in this region is not synchronous; the coral d13C response is delayed by ~ 10 years. This agrees with the previously established model of isotopic disequilibrium between atmospheric d13CCO2 and oceanic surface water DIC.