| Summary: | We have determined the d 18 O and d 13 C values of azooxanthellate corals ( Lophelia pertusa ) at a micrometre scale using an ion microprobe (SIMS-Secondary Ion Mass Spectrometry). Coral skeletons are composed of two different microstructures: Centres of calcification, and surrounding fibres. In Lophelia pertusa centres of calcification are large (50µm) and arranged in lines of centres of calcification. Our results show that centres of calcification in Lophelia pertusa have a restricted range of variation in d 18 O (-2.8 ± 0.3‰ (V-PDB)), and a larger range in d 13 C (14.3 to 10.9‰ (V-PDB)). Surrounding skeletal fibres exhibit large isotopic variation both for C and O (up to 12‰), and d 13 C and d 18 O are positively correlated. The C and O isotopic compositions of the centre of calcification deviate from this linear trend at the lightest d 18 O values of the surrounding fibres. The variation of d 18 O at a micrometre scale is probably the result of two processes: An isotopic equilibrium calcification with at least 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 a kinetic fractionation. The d 13 apparent disequilibrium in Lophelia pertusa may be the result of mixing between depleted d 13 C metabolic CO 2 (respiration) and DIC coming directly from seawater.
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