THE ROLE OF PH IN PHOSPHATE INHIBITION OF CALCITE AND ARAGONITE PRECIPITATION RATES IN SEAWATER

Seeded growth experiments were used to investigate carbonate precipitation kinetics in natural seawater spiked with 5 to 100μmol/kg dissolved inorganic phosphate. Precipitation rates of calcite and aragonite were measured using a constant disequilibrium technique (pH-stat) (S = 35, 25°C, and 1 atm total pressure) at three CO2 partial pressures (10-1, 10-2, and 10-3.5 atm). Over solution pH values from 6.9 to 8.6, aragonite growth rates decrease with increasing PO43- concentration, whereas decreases in calcite rates are dependent on increasing concentrations of both HPO42- and PO43-. Linear least-squares minimization modeling of rate data indicates that calcite and aragonite precipitation rates depend on both the calcium carbonate ion molal product (mCa2+mCO32-) (saturation state), and phosphate species concentrations. In phosphate-free seawater, aragonite grows two to three times faster than calcite regardless of solution pH and carbonate ion concentration. In seawater containing phosphate, the relative rates of calcite and aragonite precipitation are determined by the ratio of PO43- to HPO42- concentrations, and are essentially independent of total dissolved phosphate and carbonate ion concentrations. These results suggest that pH fluctuations in phosphate-enriched marine pore waters may be responsible for controlling microscale variations in cement mineralogies. © 1990.