The Suess effect in Fiji coral δ13C and its potential as a tracer of anthropogenlc CO2 uptake

In the context of increasing anthropogenic CO2 emissions, determining the rate of oceanic CO2 uptake is of high interest. Centennial-scale changes in delta C-13 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 delta C-13 measurements is limited and methods for reconstructing past delta C-13 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 delta C-13 measured in five Fijian Porites corals for reconstructing oceanic delta C-13 variability. A centennial-scale decreasing delta C-13 trend is observed in these Fiji corals. Other studies have linked similar decreasing delta C-13 trends to anthropogenic changes in the atmospheric carbon reservoir (the C-13 Suess effect). We conclude that solar irradiance is the factor influencing the delta C-13 cycle on a seasonal scale, however it is not responsible for the centennial-scale decreasing delta C-13 trend. In addition, variations in skeletal extension rate are not found to account for centennial-scale delta C-13 variability in these corals. Rather, we found that water depth at which a Fijian Porites colony calcifies influences both delta C-13 and extension rate mean values. The water depth-delta C-13 relationship induces a dampening effect on the centennial-scale decreasing delta C-13 trend. We removed this water depth effect from the 813C composite, resulting in a truer representation of delta C-13 variability of the Fiji surface water DIC (delta C-13(Fiji-DIC)). The centennial-scale trend in this Fiji coral composite delta C-13(Fiji-DIC) time-series shares similarities with atmospheric delta C-13(CO2), implicating the C-13 Suess effect as the source of the this coral delta C-13 trend. Additionally, our study finds that the delta C-13 variability between the atmosphere and the ocean in this region is not synchronous; the coral delta C-13 response is delayed by similar to 10 years. This agrees with the previously established model of isotopic disequilibrium between atmospheric delta C-13(CO2) and oceanic surface water DIC. (c) 2012 Elsevier B.V. All rights reserved.