Microanalysis of C and O isotopes of azooxanthellate and zooxanthellate corals by ion microprobe

We have determined the ¹⁸O and ¹³C values of azooxanthellate (Lophelia pertusa) and zooxanthellate (Porites lutea) corals at a micrometer scale using an ion microprobe (SIMS—secondary ion mass spectrometry). In P. lutea, centers of calcification are small (10 to 15 m) and difficult to locate during measurements. In L. pertusa, they are large (50 m) and arranged in lines of centers of calcification. Our results show that centers of calcification in L. pertusa have a restricted range of variation in ¹⁸O -2.8±0.3 (PDB)], and a larger range in ¹³C 14.3 to 10.9 (PDB)]. Surrounding skeletal fibers exhibit large isotopic variation both for C and O (up to 12), and ¹³C and ¹⁸O are positively correlated. The C and O isotopic compositions of the center of calcification deviate from this linear trend at the lightest ¹⁸O values of the surrounding fibers. Ion microprobe results on P. lutea demonstrate also a large range of variation for the ¹⁸O values (up to 10). No correlation is found with C isotopes that exhibit, in comparison with L. pertusa, a small range of variation (2). This variation of ¹⁸O at a micrometer scale is probably the result of two processes: (1) an isotopic equilibrium calcification with 1 pH unit variation in the calcification fluid as indicated by direct measurements of coelenteron pH in the coral Galaxea fascicularis (Al-Horani et al. 2003) and (2) a kinetic fractionation. The ¹³C apparent disequilibrium in P. lutea may be the result of mixing between metabolic CO₂ (respiration) and dissolved inorganic carbon (DIC) coming directly from seawater.