Seawater calcium carbonate saturation

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

Chave, K.E. and Suess, E., 1970. Calcium carbonate saturation in seawater Effects of dissolved organic matter. Limnol. Oceanogr., 15 633—637. 

Dilute seawater is undersaturated in carbonates, which reduces the likelihood of forming protective ctdcareous films on a metal surface. In deep ocean waters, the calcareous deposits are not spontaneously formed in an ambient environment and are often not precipitated under cathodic protection conditions [6]. In the cold waters of the deep ocean environmental zones, the precipitation and/or dissolution of the calcareous deposits is mainly controlled by the calcium carbonate saturation level, II [25]. 

Figure 8-2 shows the depth profiles of the saturation index omegadel), the solution rate, and the respiration rate. At the shallowest depths, the saturation index changes rapidly from its supersaturated value at the sediment-water interface, corresponding to seawater values of total dissolved carbon and alkalinity, to undersaturation in the top layer of sediment. Corresponding to this change in the saturation index is a rapid and unresolved variation in the dissolution rate. Calcium carbonate is precipitating... 

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 horizon The depth range over which seawater is saturated with respect to calcium carbonate, i.e., D = 1. At depths below the saturation horizon (D < 1), calcium carbonate will spontaneously dissolve if exposed to the seawater for a sufficient period of time. 

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. 

Assoc, of Scientific Hydrology, General Assembly of Helsinki, pp. 618-634 (1960). Edmond, J. M., Gieskes, J. M. T. M. On the calculation of the degree of saturation of seawater with respect to calcium carbonate under in situ conditions. Geochim. et Cosmochim. Acta 34, 1261-1291 (1970). 

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) ... 

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. 

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). 

Figure 6.10 Saturation index of calcite in mixtures of seawater and freshwater in equilibrium with calcite at 25°C and different CO2 pressures. From L. N. Plummer, Mixing of seawater with calcium carbonate water, Geol. Soc. Am. MetruAr 142. 1975 by The Geological Society of America. Used by permission.

When equilibrium with the atmosphere is reached, approximately 87% of ionic carbonate is present as bicarbonate ion, the remainder being carbonate. In many places, especially close to the surface, seawater is saturated with respect to calcium carbonate, which will precipitate slowly from solution, thus regulating the amount of carbonate in solution. This process is perhaps the most important of all the geological systems since it regulates the amount of carbon dioxide in the atmosphere. 

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).

Let us consider the dissolution-precipitation process in seawater in the following example. The normal concentrations of calcium and of carbonate in the near-surface oceanic waters are about [Ca2+] = 0.01 and [C032-] 2 x lO"4 M. The CaC03 in solution is metastable and roughly 2U0% saturated (1). Should precipitation occur due to an abundance of nuclei, TC032-] will drop to 10-4 M but [Ca2+] will change by no more than 2%. Therefore, the ionic strength of the ionic medium seawater will remain essentially constant at 0.7 M. The major ion composition will also remain constant. We shall see later what the implications are for equilibrium constants. 

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