R4: Mineral sciences of the Earth surface

[R4P-01] Texture observation for the skeletons consisting of aragonite and calcite of culturing scleractinian corals

*Satoko MOTAI¹, Saki Harii², Naotaka Tomioka³, Motoo Ito³ (1. Yamagata University, 2. Univ. of Ryukyus, 3. KOCHI, JAMSTEC)

Keywords:reef-building coral, biomineralization, aragonite, calcite

Most scleractinian corals inhabit tropical and subtropical oceans with symbiont algae and have exoskeletons composed of calcium carbonate (CaCO₃). The exoskeletons are considered to be aggregates of aragonite grains. However, recently, calcite has been reported as a skeletal component in corals currently inhabiting the Southern Ocean with low temperatures without symbiont algae or in Cretaceous corals that inhabited seas with different chemical compositions. In this study, corals were cultured in conditions where calcite precipitation was induced and the skeletal texture were observed to reveal polymorph selection and calcification process in corals. To form calcite, reef-building coral larvae (Acropora sp.) isolated from symbiotic algae were cultured in seawater with a low Mg/Ca molar ratio (<2.0). An optical microscopic observation shows that the skeleton of cultured corals had a typical skeletal structure of reef-building coral with six-fold symmetrical septa and a basalplate. The cross section parallel to the skeletal growth direction was polished. Element mapping on the skeleton by an analytical scanning electron microscope (SEM-EDS) and analysis by a Raman microscope revealed that the Mg-poor region was consisted of aragonite and the Mg-rich region was consisted of calcite. A thin film including both aragonite and calcite regions located at the basalplate and the lower part of the septa was prepared by focused ion beams and observed by transmission electron microscopy (TEM). Elemental mapping by SEM-EDS and Raman analysis showed that calcite was concentrated in the basalplate, and aragonite was distributed in the upper part of the coral skeleton. These results show that the calcification condition changed from the environment in which calcite was likely to precipitate to the environment in which only aragonite precipitated. The TEM observation showed that granular grains with a size of about 100 nm composed the lower part of the basalplate, and granular and needle-like grains with a size of several µm in the lower part of the septa, which is the upper part of the basalplate. The boundary between fine grains and coarse grains does not coincide with the concentration boundary of Mg. Therefore, there is no relationship between grain size and mineral phases.