Fluoride Content in Coral Aragonite Associated with Carbonate Concentration in Seawater

Abstract

In order to elucidate the controllinng factors for fluorine incorporation into corala ragonite (CaCO₃), using the ion-chromatograph method, we determined the fluoride content in coral samples collected from Pohnpei (Micronesia), Khang Khao (Thailand), Cebu (The Philippines), Sakai Port (Wakayama), Rukan-sho and Mizugama (Okinawa). Fluorine in coral was highest in Sakai Port (Wakayama) with an average F content of 955 ppm, whereas coral collected from Okinawa, Cebu, Pohnpei and Khang Khao exhibited F contents of 815 ppm, 725 ppm, 713 ppm and 677 ppm, respectively. This observation indicates a temperature dependence of fluorine incorporation into coral aragonite, in which F content increases with decreasing seawater temperature.
This is based on the proposed coprecipitation process of F in coral aragonite as an ion-exchange reaction: CaCO₃(s)+2F-(aq)=CaF₂(s)+CO₃^2-(aq). The equilibrium constant for the above equation can be expressed as: K_F=[CaF₂][CO₃^2-]/[CaCO₃][F^-]². The equilibrium constant K_F and seawater [CO₃^2-] change with temperature. In this paper we discuss whether the effect of change of seawater temperature on fluorine incorporation in coral aragonite is partly due to the corresponding change in carbonate ion concentration.