Fluoride in non-symbiotic coral associated with seawater carbonate

In order to elucidate factors controlling incorporation of fluoride from seawater into non-symbiotic coral skeletons, the mole ratio of fluorine to calcium (F/Ca) in Flabellum corals was determined. The samples were collected from water depths between 185 and 350 m in Tosa Bay, Japan. The coral F/Ca ratio increased from 6.01 to 8.04 mmol/mol with increasing water depth. The factor controlling F/Ca ratio is discussed based on the proposed coprecipitation process of F in coral aragonite as the ion-exchange reaction: CaCO3 (s) + 2F− (aq) = CaF2 (s) + CO32 − (aq). Based on the above reaction, F/Ca in coral is represented as: F/Ca = 0.5 KF´[F−]2 / [CO32 −]. Using the carbonate equilibrium system in seawater, [CO32 −] in sampling depth was calculated. A strong correlation was observed between F/Ca in Flabellum corals and the calculated [CO32 −] in seawater, while salinity, temperature and pressure have little or negligible effects on F/Ca in coral aragonite. Our results indicate that F/Ca in non-symbiotic coral aragonite possibly serves as a proxy of [CO32 −] in seawater where the corals grew.