Saturation state of seawater

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. 

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

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

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...

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). 

It is also important to keep in mind that the relation between the saturation state of seawater and carbonate dissolution kinetics is not a simple first order dependency. Instead it is an exponential of about third to fourth order (e.g., Berner and Morse, 1974). Thus dissolution rates are very sensitive to saturation state. This type of behavior has not only been demonstrated in the laboratory (see Chapter 2), but also has been observed in numerous in situ experiments in which carbonate materials and tests have been suspended in the oceanic water column. The depth at which a rapid increase in dissolution rate with increasing water depth is observed usually has been referred to as the chemical or hydrographic lysocline. In some areas of the ocean it is close to coincident with the FL (e.g., Morse and Berner, 1972). 

Figure 6.11. A. Saturation state of seawater with respect to aragonite as a function of sulfate reduction. Based on general model of Ben-Yaakov (1973), updated by using pK a values and total ion activity coefficients of Millero (1982). B. Observed saturation state of Mangrove Lake, Bermuda pore waters with respect to calcite.

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) ... 

From the above considerations all calculations necessary to determine the situ saturation state of seawater can be carried out. A sample calculation using the methods described is presented in Table II. 

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... 

Figure 7). His results indicate that Rq and CSL are close to coincident with the probable uncertainty of their determination. A detailed profile showing the relations among the sediment marker levels and the saturation state of seawater with respect to calcite and aragonite in the Northwest Atlantic Ocean is presented in Figure 8.

Both Peterson (41) and Berger (42) found that dissolution started at approximately 0.5 km water depth and the rate of dissolution increased slowly with increasing water depth until a depth of approximately 3.8 km was reached. Below this depth the rate of dissolution rapidly increased with increasing water depth. The change in the saturation state of seawater, with respect to calcite, in the deep water of this region is close to linear with depth (43). Consequently, the results of these experiments indicated that the rate of dissolution was not simply related to saturation state. Edmond (44) proposed that the rapid increase in dissolution rate could be attributed to a change in water velocity. Morse and Berner (45) pointed out that this could be true only if the rate of dissolution was transport controlled. Their calculations indicated that the rate of dissolution measured by Peterson (41) was over 20 times too slow for diffusion controlled dissolution, this being the slowest transport process. 

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). 

Chapter 3). Apparent constants are usually used in seawater because the constants are determined in this medium in the laboratory. The saturation state of seawater with respect to the solid is sometimes denoted by the Greek letter omega, fi ... 

Continuous p(C02) systems are widely used in marine CO2 research. They provide important information about the saturation state of seawater at the air-sea interface when operated on board research vessels with a continuous flow of seawater, usually obtained by means of a shipbome pumping system. This more frequently applied continuous or underway mode is described here in detail. Three different designs of discrete p(C02) systems can be found in Wanninkhof and Thorting (1993), DOE (1994) and Neill et al (1997). 

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