The influence of P-CO2 on the partitioning of magnesium in calcite overgrowths precipitated from artificial seawater at 25 degrees and 1 atm total pressure

The influence of carbon dioxide partial pressure(P-CO2) on the composition of magnesian calcite overgrowths precipitated from artificial seawater at 25 degrees C on synthetic, rhombohedral calcite seeds was investigated using a chemo-stat precipitation technique. Powder X-ray diffraction (XRD) analyses of the overgrowths indicate that the amount of MgCO3 incorporated during the precipitation is independent of the P-CO2 over four orders of magnitude (10(-0.5) to 10(-4.6) atm). The magnesium content of the overgrowths estimated from mass balance calculations of the carbonate alkalinity of the reacting solutions (Delta A(c)) and flame atomic absorption spectrophotometric (AA) analyses of the reacted solids yield similar results. Values determined by the latter method are compromised by calculation errors arising from gas phase disequilibrium when the alkalinity is computed from the steady state pH and P-CO2 of the gas phase. The concentration of sodium in the overgrowths, however, appears to correlate with variations in P-CO2. The lack of correlation between magnesium partitioning in calcite overgrowths and P-CO2 observed in this study contrasts with results obtained by previous investigators but does not preclude that the composition of calcite cements could be affected by variations in growth mechanism induced by changes in solution variables or the nature of the substrate. Based on the results of this study and recent laboratory and field observations, we propose that a change in growth mechanism and differential partitioning of magnesium on nonequivalent faces of a growing calcite crystal in seawater could explain some of the compositional variability of modern, marine calcite cements. Tn view of these latter considerations, we feel that it is premature to speculate on conditions that led to cyclic secular variations in the composition and mineralogy of marine carbonate cements precipitated during the Phanerozoic.