Calcium carbonate precipitation on the Bahama Banks

Several parameters of water samples collected from Grand Bahama Bank in June 1962 and in June 1963 were measured. They include the partial pressure of CO2, the total dissolved inorganic CO2, the C14/C12 ratio in the inorganic CO2, and the CaCO3 saturation (by the Weyl saturometer). From these results absolute residence times of water on the bank up to 250 days have been computed. An average CaCO3 precipitation rate of 50 mg/cm2 yr is estimated. The rate of CaCO3 deposition is proportional to the degree of supersaturation. By elimination of 0.6 mole of CO2 for each mole of CaCO3 precipitated, the bank water maintains a nearly constant CO2 partial pressure. Combining measurements made with the Weyl saturometer with estimates of the observed activity product made it possible to estimate an activity product for aragonite of 0.80×10−8. This agrees satisfactorily with the value calculated from the free energies of the CO32− ion, Ca2+ ion, and aragonite solid. The waters come onto the bank with an activity product of 1.68×10−8. The value falls to about 0.9×10−8 for those samples residing longest on the bank. C14 measurements on material centrifuged from ‘whitings’ indicate that their turbidity results from resuspension of sediment rather than in situ precipitation. The methods used here should have broad application to problems involving shallow-water CaCO3 deposition.