In situ measurements of oxygen isotopic composition in deep-sea coral, Lophelia pertusa: Re-examination of the current geochemical models of biomineralization

We present ion microprobe measurements of oxygen isotopic compositions in the deep-sea coral Lophelia pertusa. Compared to bulk skeletal aragonite fibres, the EMZ (early mineralization zone), near the inside of the calyx, was systematically depleted in ¹⁸O. Rayleigh fractionation from a semi-closed fluid reservoir does not explain this and other geochemical differences. Furthermore, pH values estimated from skeletal δ¹¹B data are inconsistent with the idea that EMZ (or centres of calcification) ¹⁸O depletion reflects a more alkaline calcification environment. Our data, combined with microstructural and geochemical observations, indicate that the aragonitic fibres and EMZ are formed by a compartmentalized mineralization calicoblastic ectoderm, which exerts strong biological control on the composition of the skeleton. Hence, we propose a new model whereby amorphous calcium carbonates (ACC) are precursors to the EMZ, whereas the fibre precipitation is probably governed by kinetic processes.