Abstract
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 350m in Tosa Bay, Japan. The coral F/Ca ratio increased from 6.01 to 8.04mmol/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.
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