Diagenesis and preservation of calcium carbonate

Sedimentary calcium carbonates are formed as the shells of marine plants and animals. Biologically produced CaCOs consists primarily of two minerals aragonite and calcite. Shallow-water carbonates, primarily corals and shells of benthic algae (e.g. Halimeda) are heterogeneous in their mineralogy and chemical composition but are composed mainly of aragonite and magnesium-rich calcite (see Morse and Mackenzie (1990) for a discussion). Carbonate tests of microscopic plants and animals, most of which hve in the surface ocean (there are also benthic animals that produce carbonate shells), are primarily made of the mineral calcite, which composes the bulk of the CaCOs 

The solubility of CaCOs in seawater has been studied extensively because of its great abundance in sedimentary rocks and the ocean. The equation for dissolution of pure calcium carbonate  

Chapter 3). Apparent constants are usually used in seawater because the constants are determined in this medium in the laboratory. The saturation state of seawater with respect to the solid is sometimes denoted by the Greek letter omega, fi  

The numerator of the right side is the product of measured total concentrations of calcium and carbonate in the water, or the ion 

The apparent solubility products of calcite and aragonite have been determined repeatedly in seawater solutions. We adopt the values of Mucci (1983) fC sp for calcite and aragonite = 4.35 0.20 x 10 and 6.65 0.12 X lO moPkg, respectively, at 25 °C, 35ppt and one atmosphere. These data agree within error to measurements determined previously and represent many repetitions to give a clear estimate of the reproducibility (c. 5%). 

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