Seawater calcite saturation

Table 1.12. Variation of carbonic acid components and calcite saturation in seawater under different conditions.

The Geochemical Ocean Section Program (GEOSECS) has produced data from which it is possible to profile the saturation state of seawater with respect to calcite and aragonite in the Atlantic and Pacific oceans. Representative north-south calcite saturation profiles for the Western Atlantic and Central Pacific oceans are presented in Figures 5 and 6 (based on 39). It was observed that the saturation state of seawater with respect to calcite at the CCD was close to constant ( 2 = 0.70 I" 0,05) except in the southern extremes (39). Broecker and Takahashi (31) have recently found that the carbonate ion concentration is close to constant at the FL, when appropriate corrections are made for pressure. The saturation state of seawater at the FL, calculated by the method presented in this paper, is 0.80 0.05. Berger (40) has presented profiles for Rq, FL, CCD and CSL (calcite saturation level) in the eastern and western Atlantic ocean (see... 

Calcite saturation indices plotted as a function of percent seawater mixing with a carbonate aquifer water with contours representing different levels of Pqo (source Plummer, 1975). 

Figure 5 Results of in situ dissolution experiments. Peterson (1966) re-weighed polished calcite spheres after a 250 d deployment on a mooring in the North Pacific. Honjo and Erez (1978) observed the weight loss for calcitic samples (coccoliths, foraminifera and reagent calcite) and an aragonitic sample (pteropods) held at depth for a period of 79 d. While Peterson hung his spheres directly in seawater, the Honjo-Erez samples were held in containers through which water was pumped. The results suggest that the calcite saturation horizon lies at 4,800 200 m in the North Atlantic and at about 3,800 200 m in the North Pacific. For aragonite, which is 1.4 times more soluble than calcite, the saturation horizon in the North Atlantic is estimated to be in the range 3,400 200 m.

Table 1 The calcite-saturation carbonate ion concentration in cold seawater and the slope of this solubility as a function of water depth based on a 1 atm solubility of 45 pimol CO kg and a AT of 40 cm mol . ...

Calculations such as those by Morse and Mackenzie (1990) indicate that the calcite saturation depth is generally —1 km greater than proposed by Berger (1977) and that it is much greater than R. It appears only loosely related to the FL. In the equatorial eastern Atlantic Ocean, FL is —600 m shallower than the saturation depth. If these observations are close to correct, the long cherished idea of a tight relation between seawater chemistry and carbonate depositional facies must be reconsidered (Mekik et al., 2002). The influence of near interfacial diagenetic processes on these relationships is discussed in the next section. 

An additional argument for considering the effect of pressure on ionization equilibria is presented in Figure 8. This figure plots calculated temperature and pressure conditions at which the modified seawater becomes saturated with calcite or anhydrite. At a constant pressure, the predicted temperature of mineral saturation is significantly different (higher for calcite and lower for anhydrite) when the effect of pressure on ionization equilibria are taken into account. This observation has important consequences for calculated geochemical models of water-rock interactions and ore deposition (23,24). 

Figure 3 Bathymetric profiles of calcium carbonate (calcite) saturation for hydrographic stations in the Atlantic and Pacific Oceans (data from Takahashi etai 1980). Carbonate saturation here is expressed as ACOa ", defined as the difference between the in situ carbonate ion concentration and the saturation carbonate ion concentration at each depth ACOa " = [C03 ]seawater - [COa Jsaturation)-The saturation horizon corresponds to the transition from waters oversaturated to waters undersaturated with respect to calcite (A 003 = 0). This level is deeper in the Atlantic than in the Pacific because Pacific waters are COa-enriched and [C03 ]-depleted as a result of thermohaline circulation patterns and their longer isolation from the surface. The Atlantic data are from GEOSECS Station 59 (30°12 S, 39°18 W) Pacific data come from GEOSECS Station 235 (16°45 N,161°23 W).

Broecker and Takahashi, 1978). zs is the depth in km and carbonate concentrations are in mol kg" . Since seawater Ca2+ concentration is allowed to change as a result of calcite precipitation and river input, it is more general to state that the saturation product Ks changes as... 

All of these measures of saturation are very useful in predicting the geographic distribution of sedimentary carbonates. For example, sediments lying below waters that are undersaturated with respect to calcite should be devoid of calcareous oozes. Since direct measurement of [COj ] observed difficult, its concentration is usually computed from two more easily measured parameters, the carbonate alkalinity and pH of a seawater sample. 

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... 

Saturation state of seawater, Cl, with respeot to (a) calcite and (b) aragonite as a function of depth. The dashed vertical line marks the saturation horizon. North Pacific profile is from 27.5°N 179.0°E (July 1993) and North Atlantio profile is from 24.5°N 66.0°W (August 1982) from CDIAC/WOCE database http //cdiac.esd.oml.gov/oceans/CDIACmap.html) Section P14N, Stn 70 and Section A05, Stn 84. Source From Zeebe, R.E. and D. Wolf-Gladrow (2001) Elsevier Oceanography Series, 65, Elsevier, p. 26. 

As with the calcareous tests, BSi dissolution rates depend on (1) the susceptibility of a particular shell type to dissolution and (2) the degree to which a water mass is undersaturated with respect to opaline silica. Susceptibility to dissolution is related to chemical and physical factors. For example, various trace metals lower the solubility of BSi. (See Table 11.6 for the trace metal composition of siliceous shells.) From the physical perspective, denser shells sink fester. They also tend to have thicker walls and lower surface-area-to-volume ratios, all of which contribute to slower dissolution rates. As with calcivun carbonate, the degree of saturation of seawater with respect to BSi decreases with depth. The greater the thermodynamic driving force for dissolution, the fester the dissolution rate. As shown in Table 16.1, vertical and horizontal segregation of DSi does not significantly coimter the effect of pressure in increasing the saturation concentration DSi. Thus, unlike calcite, there is no deep water that is more thermodynamically favorable for BSi preservation they are all corrosive to BSi. 

The model provided in Figure 20.1 is for an ocean basin whose abyssal plains all lie below the CCD. This most closely resembles the conditions in the North Pacific, whereas the rest of the ocean basins have a significant portion of their abyssal plains lying above the CCD, and, hence, contain some calcareous oozes. From a global perspective, calcareous oozes are more abundant than siliceous oozes. This is caused by two phenomena (1) all seawater is undersaturated with respect to opal, whereas all surface waters and 20% of the deep waters are saturated with respect to calcite, and (2) siliceous plankton are dominant only in upwelling areas. 

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... 

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. 

At the higher saturation state, the seawater solution is more than 5 times supersaturated with respect to aragonite so that aragonite would be expected to precipitate on the aragonite seed crystal. Results indicated that Mg2+ is adsorbed between 25 to 40 times less on aragonite than on calcite from solutions supersaturated with respect to both minerals. 

The Mg to Ca surface ratios for calcite in both supersaturated seawater solutions were nearly identical. The lower Mg to Ca surface ratio obtained in the less supersaturated solution may be the result of incomplete coverage of the pure calcite crystal by the magnesian calcite overgrowth. The Mg to Ca surface ratio on calcite exposed to both saturation state solutions is in close agreement with the value of 1 obtained in a solution with a Mg2+ to Ca2+ ratio of 5 by Moller and his associates. 

Figure 2.13. Log of the rate of recrystallization plotted as a function of log (Q - 1) -1/2 log Z for artificial seawater systems in which calcite seeds are suspended in seawater, and the saturation state and coating thickness (Z) calculated from measurements of pH and total alkalinity. The solid line represents the data for the majority of experiments, whereas the dashed line is for a system containing a great deal of calcite in which the pH was measured in the sediment rather than in the supernatant seawater. (After Schoonmaker,1981.)...

Figure 4.2. The variation of total carbon dioxide (ICO2) and the saturation state of seawater with respect to calcite (Qc) with temperature for seawater with a total alkalinity of 2400 peq kg- seawater and in equilibrium with atmospheric CO2...

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. 

Figure 4.6. Variation in the saturation depth of seawater with respect to calcite as a function of potential Pc02 f°r seawater at 2°C, S=35, and At=2400 ieq kg-1 seawater.

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