Calcium saturation state

Langdon, C., Takahashi, T., Sweeney, C., Chipman, D., Goddard, J., Marubini, F., Aceves, H., Barnett, H., and Atkinson, M.J., Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef, Global Biogeochem. Cy., 14, 639-654,2000. 

Like the climate system described in Chapter 7, this diagenetic system consists of a chain of identical reservoirs that are coupled only to adjacent reservoirs. Elements of the sleq array are nonzero close to the diagonal only. Unnecessary work can be avoided and computational speed increased by limiting the calculation to the nonzero elements. The climate system, however, has only one dependent variable, temperature, to be calculated in each reservoir. The band of nonzero elements in the sleq array is only three elements wide, corresponding to the connection between temperatures in the reservoir being calculated and in the two adjacent reservoirs. The diagenetic system here contains two dependent variables, total dissolved carbon and calcium ions, in each reservoir. The species are coupled to one another in each reservoir by carbonate dissolution and its dependence on the saturation state. They also are coupled by diffusion to their own concentrations in adjacent reservoirs. The method of solution that I shall develop in this section can be applied to any number of interacting species in a one-dimensional chain of identical reservoirs. 

The saturation state of seawater can be used to predict whether detrital calcite and aragonite are thermodynamically favored to survive the trip to the seafloor and accumulate in surfece sediments. Any PIC or sedimentary calcium carbonate exposed to undersaturated waters should spontaneously dissolve. Conversely, PIC and sedimentary calcium carbonate in contact with saturated or supersaturated waters will not spontaneously dissolve. Typical vertical trends in the degree of saturation of seawater with respect to calcite and aragonite are shown in Figure 15.11 for two sites, one... 

NMR can provide detail regarding the types of NOM structures that are preferentially sorbed to mineral surfaces in soils and sediments. Simpson et al. (2006b) used H liquid-state and HR-MAS NMR methods to study the sorption of model compound mixtures to calcium-saturated montmorillonite. The model compound mixture included one representative compound from each of the following structural classes sugars, lignin, peptides, and long-chain aliphatics. After sorption, the supernatant was analyzed by liquid-state NMR and the organo-mineral complex... 

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. 

As previously mentioned, the primary processes responsible for variations in the deep sea C02-carbonic acid system are oxidative degradation of organic matter, dissolution of calcium carbonate, the chemistry of source waters and oceanic circulation patterns. Temperature and salinity variations in deep seawaters are small and of secondary importance compared to the major variations in pressure with depth. Our primary interest is in how these processes influence the saturation state of seawater and, consequently, the accumulation of CaC03 in deep sea sediments. Variations of alkalinity in deep sea waters are relatively small and contribute little to differences in the saturation state of deep seawater. 

One of the most controversial areas of carbonate geochemistry has been the relation between calcium carbonate accumulation in deep sea sediments and the saturation state of the overlying water. The CCD, FL, R0, and ACD have been carefully mapped in many areas. However, with the exception of complete dissolution at the CCD and ACD, the extent of dissolution that has occurred in most sediments is difficult to determine. Consequently, it is generally not possible to make reasonably precise plots of percent dissolution versus depth. In addition, the analytical chemistry of the carbonate system (e.g., GEOSECS data) and constants used to calculate the saturation states of seawater have been a source of almost constant contention (see earlier discussions). Even our own calculations have resulted in differences for the saturation depth in the Atlantic of close to 1 km (e.g., Morse and Berner, 1979 this book). 

More recent calculations such as those in this book indicate substantially lower saturation depths. Those calculated here are plotted in Figure 4.21. The SD is generally about 1 km deeper than that presented by Berger (1977). Clearly the new SD is much deeper than the R0 and appears only loosely related to the FL. Indeed, in the equatorial eastern Atlantic Ocean, the FL is about 600 m shallower than the SD. If these new calculations are even close to correct, the long cherished idea of a "tight" relation between seawater chemistry and carbonate depositional facies must be reconsidered. However, the major control of calcium carbonate accumulation in deep sea sediments, with the exceptions of high latitude and continental slope sediments, generally remains the chemistry of the water. This fact is clearly shown by the differences between the accumulation of calcium carbonate in Atlantic and Pacific ocean sediments, and the major differences in the saturation states of their deep waters. 

Figure 6.4. Histograms of the number of samples found within different ranges of saturation state with respect to aragonite for different shallow water calcium carbonate-rich sediments. Values for Bermuda, Florida Bay and the Everglades were calculated from the data of Berner (1966). (After Morse et al., 1985.)...

Here we will use a simplified example to illustrate some basic aspects of the mass transport process for carbonates that avoids most of the more complex relationships. In this example, the calcium and carbonate ion concentrations are set equal, and values of the activity coefficients, temperature, and pressure are held constant. The carbonate ion concentration is considered to be independent of the carbonic acid system. The resulting simple (and approximate) relation between the change in saturation state of a solution and volume of calcite that can be dissolved or precipitated (Vc) is given by equation 7.4, where v is the molar volume of calcite. 

The initial raw material should contain the minimum impurities and admixtures of sulphates, calcium and magnesium salts. A sodium chloride solution is applied in a saturated state potassium chloride solutions are generally unsaturated to prevent the danger of the pipes becomming clogged at an unexpected cooling of the solution as the solubility of this salt depends to a considerable degree upon the temperature. 

In order to understand the chemistry of calcium carbonate accumulation in the deep oceans, the sources of calcium carbonate, its distribution in recent pelagic sediments, the saturation state of seawater overlying deep-ocean sediments with respect to calcite and aragonite, and the relation between saturation state and dissolution rate must be known. These aspects of calcium carbonate chemistry are examined in this paper. 

General Considerations. In order to facilitate the discussion of methods for calculating the saturation state of seawater with respect to calcium carbonate, initial consideration will be given to pure calcium carbonate phases. The method most frequently used expressing the saturation state of a solution with respect to solid phase is as the ratio (Q) of the ion activity (a) product to the thermodynamic solubility constant (K). For the calcium carbonate phase calcite, the expression for the saturation state is defined as (e.g., 13) ... 

Because of difficulties in precisely calculating the total ion activity coefficient (y) of calcium and carbonate ions in seawater, and the effects of temperature and pressure on the activity coefficients, a semi-empirical approach has been generally adopted by chemical oceanographers for calculating saturation states. This approach utilizes the apparent (stoichiometric) solubility constant (K ), which is the equilibrium ion molal (m) product. Values of K are directly determined in seawater (as ionic medium) at various temperatures, pressures and salinities. In this approach ... 

Sample Calculation of Calcium Carbonate Saturation State in Seawater (After Morse et al. (35))... 

The Distribution of Calcium Carbonate Saturation States and Their Relation to Sediment Marker Levels... 

Calcium carbonate is accumulating in deep ocean sediments, in which the overlying water is undersaturated with respect to both aragonite and calcite, and sediment marker levels closely correspond to unique saturation states. This indicates that dissolution kinetics play an important role in determining the relation between seawater chemistry and calcium carbonate accumulation in deep ocean basins. It is, therefore, necessary to have knowledge of the dissolution kinetics of calcium carbonate in seawater if the accumulation of calcium carbonate is to be understood. 

The accumulation of calcium carbonate in deep ocean sediments is a complex process. It is primarily governed by the interplay between biological production of calcium carbonate in the nearsurface ocean and the chemistry of deep ocean waters. After over 100 years of study, the major problem of determining the saturation state of deep ocean water remains largely unresolved. It is currently possible, using recent laboratory measurements, to arrive at saturation states that differ by as much as a factor of 2. Both laboratory and water column experiments indicate that calcium carbonate dissolution kinetics are not simply related to saturation state. It is our opinion that the saturation state problem must be resolved and considerably more detail added to our present knowledge of calcium carbonate dissolution kinetics and accumulation patterns before attempts to model the accumulation of calcium carbonate in deep ocean sediments can be truly successful. 

Morse, J.W., de Kanel, J., and Craig, H.L., Jr. A literature review of the saturation state of seawater with respect to calcium carbonate and its possible significance for scale formation on OTEC heat exchangers. Ocean Engineering (in press). 

Berner, R.A. and Wilde, P. Dissolution kinetics of calcium carbonate in seawater I. Saturation state parameters for kinetic calculations, Amer. Jour. Sci. 272, 826-839 (1972). 

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. 

Jahnke R. J. and Jahnke D. B. (2002) Calcium carbonate dissolution in deep-sea sediments implications of bottom water saturation state and sediment composition. Geochim. Cosmochim. Acta (in press). 

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