Fluoride in coral aragonite related to seawater carbonate

Abstract

In order to elucidate the controlling factors for fluoride incorporation into coral aragonite (CaCO₃), we determined the ratio of fluorine to calcium (F/Ca) in coral skeletons collected from Pohnpei (Micronesia), Khang Khao (Thailand), Cebu (The Philippines), Tosa Bay (Kochi, Japan), Sakai Port (Wakayama, Japan), Tateyama (Chiba, Japan), Sesoko, Rukan-sho and Mizugama (Okinawa, Japan) using the ion-chromatograph method. The F/Ca (mmol/mol) ratio was highest in Tateyama with an average ratio of 6.04, whereas coral collected from Sakai Port, Tosa Bay, Okinawa, Cebu, Pohnpei and Khang Khao exhibited F/Ca ratios of 5.57, 5.02, 4.49, 4.24, 4.00 and 3.75, respectively. The factor controlling F/Ca ratio is discussed based on the proposed coprecipitation process of F in coral aragonite as the ion-exchange reaction: CaCO_3(s) + 2F^-_(aq) = CaF_2(s) + CO₃^2-_(aq). The equilibrium constant for the above equation can be expressed as: K_F´ = [CaF₂][CO₃^2-]/[CaCO₃][F^-]². In this paper, analyzed F/Ca ratio in coral shows a proportional change to calculated seawater [F^-]²/[CO₃^2-]. This finding suggests that F/Ca in coral skeletons is possibly controlled by carbonate ion concentration in seawater.