Abstract
Sulfate can be incorporated into coral aragonite (CaCO3) by means of an ion-exchange reaction: CaCO3(s)+SO42-(aq)=CaSO4(s)+CO32-(aq). The equilibrium constant for this equation can be expressed as: Kso4=[CaSO4] [CO32-]/[CaCO3] [SO42-]. The equilibrium constant Kso4 and seawater [CO32-] change with temperature. From the above equation, it is expected that the sulfate content in a coral skeleton is inversely proportional to the carbonate concentration in seawater.
In order to understand the coprecipitation process of sulfate in a coral skeleton by evaluating the correlation between the sulfate content and the carbonate concentration or temperature of seawater, we determined the sulfate content in coral samples from Pohnpei (Micronesia), Khang Khao (Thailand), Cebu (The Philippines), Sakai Port (Wakayama), Rukan-sho and Mizugama (Okinawa). The sulfate content was approximately constant (0.566±0.064wt%), with the exception of the sample from Khang Khao (0.404±0.050wt%). There is no correlation between the sulfate content and the carbonate concentration or temperature of seawater. Therefore, the controlling factors for incorporating sulfate into coral skeleton is not only the carbonate concentration, but also other trace elements in the skeleton and the temperature dependence of the equilibrium constant.
