Seawater constituents salinity

The distribution of salinity in surface waters of the ocean is presented in Fig. 1.1. Because the concentrations for many major seawater constituents are unaffected by chemical reaction on the time scale of ocean circulation, local salinity distributions are controlled by a balance between two physical processes, evaporation and precipitation. This balance is reflected by low salinities in equatorial regions that result from extensive rain due to rising atmospheric circulation (atmospheric lows) and high salinities in hot diy subtropical g5Tes that flank the equator to the north and south (20-35 degrees of latitude) where the atmospheric circulation cells descend (atmospheric highs). 

Table 1.4. 1 Major ions° in surface seawater at salinity S = 35, and their role in the calculation of alkalinity s Major ions are defined here as those charged constituents with concentrations greater than 10 [jmol kg , excluding the nutrient nitrate, which varies in concentration. ...

TABLE 1—Major seawater constituents at (35 parts per thousand (%o) salinity) and [/]. 

Table 1. Artificial seawater constituents for a salinity of 35 after Kester et al. (1967).

The mathematical models used to infer rates of water motion from the conservative properties and biogeochemical rates from nonconservative ones were flrst developed in the 1960s. Although they require acceptance of several assumptions, these models represent an elegant approach to obtaining rate information from easily measured constituents in seawater, such as salinity and the concentrations of the nonconservative chemical of interest. These models use an Eulerian approach. That is, they look at how a conservative property, such as the concentration of a conservative solute C, varies over time in an infinitesimally small volume of the ocean. Since C is conservative, its concentrations can only be altered by water transport, either via advection and/or turbulent mixing. Both processes can move water through any or all of the three dimensions... 

The relative constancy in the chemistry of the main saline components of seawater does not extend to minor and trace constituents, which are somewhat variable. 

Table 8.30 shows the chemistry of seawater compiled by Turekian (1969) for major, minor, and trace constituents, expressed in parts per billion (ppb) at a mean salinity of 35. The listed values are estimates of mean amounts in solution, whereas elemental concentrations actually vary with depth. The most conspicuous variations are observed in the first 200 m from the surface, where photosynthetic processes are dominant and phosphorus and nitrogen are fixed by plankton and benthos, as well as silica and calcium, which constitute, respectively, the skeletons of planktonic algae (diatom) and the shells of foraminifera and mollusks. 

The major constituents in seawater are conventionally taken to be those elements present in typical oceanic water of salinity 35 that have a concentration greater than 1 mg kg excluding Si, which is an important nutrient in the marine environment. The concentrations and main species of these elements are presented in Table 1. One of the most significant observations from the Challenger expedition of 1872-1876 was that these major components existed in constant relative amounts. As already explained, this feature was exploited for salinity determinations. Inter-element ratios are generally constant, and often expressed as a ratio to Cl%o as shown in Table 1. This implies conservative behaviour, with concentrations depending solely upon mixing processes, and indeed, salinity itself is a conservative index. 

A component can undergo considerable physico-chemical speciation alterations in an estuary. With respect to dissolved constituents, the composition and concentration of available ligands changes. Depending upon the initial pH of the riverine water, OH may become markedly more important down the estuary. Similarly, chlorocomplexes for metals such as Cd, Hg and Zn become more prevalent as the salinity increases. Conversely, the competitive influence of seawater derived Ca and Mg for organic material decreases the relative importance of humic complexation for Mn and Zn. 

One advantage of the ion interaction theory is that it can be applied to solutions of different salinities, that is, to brines, seawaters with different salinities, and estuarine waters. While the ionic medium method provides a very simple solution to many problems, especially for the speciation of constituents in the open ocean, it cannot readily be applied to solutions of different salinity that is, brines, seawater, and estuarine waters must be treated as separate solvents (Pabalan and Pitzer, 1988). 

Table I. Concentrations of the Major Nonvolatile Constituents of 35%o Salinity Seawater...

Table II. Approximate Concentrations of the Minor Nonvolatile Constituents of 35%o Salinity Seawater"...

Before launching into a detailed discussion of individual constituents, we would like to introduce the total quantity of dissolved material in seawater, salinity, and the processes that determine it. 

Classification of the chemical constituents of seawater into conservative, bioactive and adsorbed (Chapter 1) revealed much about the processes that control concentration distributions in seawater of the latter two categories, but little about the conservative elements. Concentrations of the elements that make up most of the salinity of the oceans provide clues to the mechanisms that control their sources and sinks. Thus, the chemical perspective of oceanography revealed by conservative element concentrations is about processes that occur at the ocean boundaries weathering reactions on land, authigenic mineral formation in marine sediments and reactions with the crust at hydrothermal areas. The amount of time some of the dissolved constituents remain in solution before they are removed chemically is very long, suggesting the possibility for chemical equilibrium between seawater and the minerals in the ocean... 

In addition to the dependence on temperature and pressure, the physical properties of seawater vary with the concentration of the dissolved constituents. A convenient parameter for describing the composition is the salinity, S, which is defined in terms of the electrical conductivity of the seawater sample. The defining equation for the practical salinity is ... 

The first table below gives several properties of seawater as a function of temperature for a salinity of 35. The second and third give density and electrical conductivity as a function of salinity at several temperatures, and the fourth lists typical concentrations of the main constituents of seawater as a function of salinity. The final table gives the freezing point as a function of salinity and pressure. 

The oceans of the world by no means have uniform physical or chemical properties. There are major differences in near-surface salinity and temperature gradients among the major oceans, which depend on local meteorological conditions, ocean current regimes, and nearness to land or ice masses. However, a combination of tides, currents, waves, and upwellings tend to keep the waters of the oceans more or less well mixed, so that on the whole the ratios of the major dissolved ionic constituents of seawater are nearly constant (Table 1.8). To a crude approximation, seawater is a solution 0.5 M in NaCl and 0.04 M in MgS04. The mean pH of the ocean is about 8.2. 

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