Saturation state undersaturation

As Fig. 6.13 illustrates growth and dissolution are not symmetric with respect to the saturation state. At very high undersaturation, the rate of dissolution becomes independent of S and converges to the value of the apparent rate constant. This is why studies of dissolution far from equilibrium allow to study the influence of inhibition/ catalysis on the apparent rate constant, independently from the effect of S. The same is not true for crystal growth. 

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... 

Fig. 14. Change in ealcile saturation state during single-step adiabatic boiling (vapour saturation pressure) of aquifer water from four wet-steam wells in three areas Krafla. Iceland Momotnmho. Nicaragua and Zunil. Guatemala. A positive Sl-value corresponds to oversaluralion and a negative value to undersaturation. An SI of zero corresponds with equilibrium. SI is on a log Scale so an Sl-value of +1 indicates tenfold oversaturation. The numbers indicate well numbers. The calculated Sl-values for the aquifer waters (dots) depart a little from equilibrium. In view of all errors involved in the calculation of the Sl-values. the departure front equilibrium is. however, not significant. Nine that variations in Sl-values are more accurately calculated than absolute values.

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. 

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. 

An approximate relationship between the degree of undersaturation of seawater with respect to calcite and the extent of dissolution can be established by comparing the saturation state at the various sediment marker levels with estimates of the amount of dissolution required to produce these levels. In Figure 9 the "distance from equilibrium (1 - 2) has been plotted against the estimated percent dissolution of the calcitic sediment fraction. Within the large uncertainties that exist in the amount of dissolution required to produce the FL and Rq levels, a linear relation between the degree of undersaturation and extent of dissolution can be established. The intercept of the linear plot with the FL and Rq levels indicates that approximately 15 percent more material has been lost than Berger s (12) minimum loss estimate of 50% and 10%, respectively. 

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 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. 

One of the primary aims in the 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 (Equation (3)). In seawater the latter is generally used and Hmingjai is the symbol used to represent the ratio. If H = 1, the solid and solution are in equilibrium if H < 1, the solution is undersaturated and mineral dissolution can occur, and if H > 1, the solution is supersaturated and precipitation should... 

Figure 6.7 Schematic plot showing the general effects of various processes on the saturation state of natural waters with respect to calcite. Waters are supersaturated with respect to calcite above and to the left of the calcite line and undersaturated with respect to calcite below and to the right of the calcite line.

Primary saturation is the first state reached during the congruent dissolution of a solid-solution, for which the aqueous-solution is saturated with respect to a secondary solid-phase (JJ., J 4, Glynn and Reardon, Am. J. ScL, in press). This secondary solid will usually have a composition different from that of the dissolving solid. At primary saturation, the aqueous phase is at thermodynamic equilibrium with respect to this secondary solid but remains undersaturated with respect to the primary dissolving solid. The series of possible primary-saturation states for a given SSAS system is represented by the solutus curve on a Lippmann diagram. 

Distinct, end-member phosphate phases that are likely to form in anoxic marine sediments include Ca5(P04)j0H (Hydroxyapatite), Ca3(P04)z (Whitlockite), Fe3(P04)2 8H2O (Vivianite), and Mnj(P04)2 3H2O (Red-dingite). Struvite, mentioned earlier, may also occur in inshore LIS sediments, but is highly undersaturated at the stations examined in this study (Martens et al., 1978). The presence of a pure phase is the exception rather than the rule in low-temperature disgenesis (Suess, 1979), but in the absence of information on the solid-phase composition, comparison of pore-water saturation states with respect to only the end member will be treated here. 

The solubility products and reactions used here as a guideline to saturation states are given in Table VI. The results of the calculations for phosphate compounds are plotted as -log lAP (lAP is the ion activity product) as a function of depth at each station (Figs. 49 and 50). Only data from box cores collected during 1975-1976 and some selected horizons from the gravity cores are shown. Hydroxyapatite was supersaturated by a factor of lO -lO at all stations and is not plotted precipitation of this phase is known to be kinetically hindered in seawater (Martens and Harriss, 1970). Bray (1973) and Norvell (1974) inferred likely equilibrium of pore waters with whitlockite [Ca3(P04)2] in Chesapeake Bay and anoxic lake sediments, respectively. Long Island Sound pore waters also tend to have activity products close to those predicted for saturation with respect to whitlockite, although distinct undersaturation is found for most sediment intervals at NWC. 

A zone of possible saturation with vivianite occurs first beneath the interface at all stations during the summer and at FOAM and DEEP during winter (Fig. 49). Otherwise pore water is undersaturated with respect to this phase. A similar depth distribution of saturation states is found for reddingite at FOAM and NWC. At DEEP, saturation can occur well below 4 cm in summer and winter box cores. The near-interface bands of sat-... 

There are several reasons why undersaturation with respect to pure Fe and Mn phosphates could occur (1) solid-solution formation (Tessenow, 1974) (2) inaccurate values for K p (3) the pore waters are not in equilibrium with an Fe and Mn phosphate and (4) the average pore-water concentrations cannot be used to calculate saturation states. This last reason applies only to the bioturbated zone and comes about because a wide range of concentrations actually exists in any sediment interval of this region (see Fig. 42, for example). Production and consumption rates of HP04 would be required to check this possibility in general. In the present case, the continued undersaturation of pore waters below the bioturbated zone argues for a reason, or reasons, other than Eq. (6.4) to explain the discrepancy. 

In theory, the lysocline records the sedimentary expression of the saturation horizon, that is the depth-dependent transition from waters oversaturated to waters undersaturated with respect to carbonate solubility (Figure 4). The lysocline thus marks the top of a depth zone, bounded at the bottom by the CCD, over which the bulk of carbonate dissolution in the ocean is expected to occur in response to saturation state-driven chemistry. The thickness of this sublysocline zone, as indicated by the vertical separation between the lysocline and CCD, is variable and is governed by the rate of carbonate supply, the actual dissolution gradient, and... 

For evaluating the direction of a reaction are used various values log(na./]C.°) -saturation index, SI., log(K7Hfl ) - disequilibrium index or Yla /K = Cl. -saturation state. At saturation state greater than 1 the solution is considered oversaturated, and less than 1, undersaturated relative to the products of the reaction. 

Mass transfer is always directed towards the medium with lower fugacity or partial pressure. In connection with this disequilibrium of the underground water-gas system is determined either from the degree of water saturation by component i relative to the rmderground gas, i.e., from the ratio pjp., or using disequilibrium index log p./p.)-equilibrium the saturation state is equal 1 and the index, zero. If the saturation state is greater than I (index less than 0), water is oversaturated relative the subsurface gas, if it is less (the index greater than 0), water is undersaturated. 

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