Carbonate minerals seawater saturation state

One of the primary concerns in a study of the geochemistry of carbonates in marine waters is the calculation of the saturation state of the seawater with respect to carbonate minerals. The saturation state of a solution with respect to a given mineral is simply the ratio of the ion activity or concentration product to the thermodynamic or stoichiometric solubility product. In seawater the latter is generally used and Qmjneral is the symbol used to represent the ratio. For example ... 

In this chapter, we introduced the reader to some basic principles of solution chemistry with emphasis on the C02-carbonate acid system. An array of equations necessary for making calculations in this system was developed, which emphasized the relationships between concentrations and activity and the bridging concept of activity coefficients. Because most carbonate sediments and rocks are initially deposited in the marine environment and are bathed by seawater or modified seawater solutions for some or much of their history, the carbonic acid system in seawater was discussed in more detail. An example calculation for seawater saturation state was provided to illustrate how such calculations are made, and to prepare the reader, in particular, for material in Chapter 4. We now investigate the relationships between solutions and sedimentary carbonate minerals in Chapters 2 and 3. 

A classic example of metastability is surface-seawater supersaturation with respect to calcite and other carbonate minerals (Morse and Mackenzie 1990 Millero and Sohl 1992). The degree of calcite supersaturation in surface seawater varies from 2.8- to 6.5-fold between 0 and 25 °C (Morse and Mackenzie 1990). In Fig. 3.18, experimental calcite solubility (metastable state) is approaching model calcite solubility (stable state) at subzero temperatures. In Table 5.1, the difference in seawater pH, assuring saturation or allowing supersaturation with respect to calcite, is 0.38 units. Moreover, in running these calculations, it was necessary to remove magnesite and dolomite from the minerals database (Table 3.1) because the latter minerals are more stable than calcite in seawater. But calcite is clearly the form that precipitates... 

Calculation of the saturation state of seawater with respect to carbonate minerals... 

CALCULATION OF THE SATURATION STATE OF SEAWATER WITH RESPECT TO CARBONATE MINERALS... 

The equations and methods given in this chapter can be used to calculate the distribution of carbonic acid system components and the saturation state of a solution with respect to a carbonate mineral under varying temperature, pressure, and composition. To illustrate the type of changes that occur, a calculation has been done for seawater, and the results summarized for nine different cases in Table 1.12. Case 1 is used as a reference typical of surface, subtropical, Atlantic seawater in equilibrium with the atmosphere. In all other cases the salinity and total... 

The applicability of scanning Auger spectroscopy to the analysis of carbonate mineral surface reactions was demonstrated by Mucci and Morse (1985), who carried out an investigation of Mg2+ adsorption on calcite, aragonite, magnesite, and dolomite surfaces from synthetic seawater at two saturation states. Results are summarized in Table 2.5. 

Aragonite is the only one of the four carbonate minerals examined that does not have a calcite-type rhombohedral crystal structure. For all the minerals examined, with the exception of aragonite, the two solution saturation states studied represent supersaturated conditions, because at a saturation state of 1.2 with respect to calcite, the seawater solution is undersaturated (0.8) with respect to aragonite. 

Table 2.5. The Mg to Ca concentration ratio on the surface of four carbonate mineral crystals after extended exposure to synthetic seawater at two different saturation states. (After Mucci and Morse, 1985.)...

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

The numerator of the right side is the product of measured total concentrations of calcium and carbonate in the water—the ion concentration product (ICP). If n = 1 then the system is in equilibrium and should be stable. If O > 1, the waters are supersaturated, and the laws of thermodynamics would predict that the mineral should precipitate removing ions from solution until n returned to one. If O < 1, the waters are undersaturated and the solid CaCOa should dissolve until the solution concentrations increase to the point where 0=1. In practice it has been observed that CaCOa precipitation from supersaturated waters is rare probably because of the presence of the high concentrations of magnesium in seawater blocks nucleation sites on the surface of the mineral (e.g., Morse and Arvidson, 2002). Supersaturated conditions thus tend to persist. Dissolution of CaCOa, however, does occur when O < 1 and the rate is readily measurable in laboratory experiments and inferred from pore-water studies of marine sediments. Since calcium concentrations are nearly conservative in the ocean, varying by only a few percent, it is the apparent solubility product, and the carbonate ion concentration that largely determine the saturation state of the carbonate minerals. 

THE CARBONATE MINERAL SATURATION STATE OF SOME REPRESENTATIVE GROUNDWATERS AND SEAWATER... 

Chemical analyses of 10 groundwaters from springs and wells in carbonate rocks are shown in Table 6.7, along with their apparent CO2 pressures and saturation indices with respect to calcite and dolomite, which have been calculated using the computer model SOLMINEQ.88 (Kharaka et al. 1988). The composition of seawater and its modeled carbonate-mineral saturation state is also shown. SOLMINEQ.88 calculates the concentrations of ion pairs, such as CaHCO and MgSO, and uses the Truesdell-Jones equation to compute ion activity coefficients. (See Chap. 4.)... 

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