Major ions in seawater

The salinity of seawater is defined as the grams of dissolved salt per kg of seawater. Using good technique, salinity can be reported to 0.001%o or 1 ppm(m). By tradition the major ions have been defined as those that make a significant contribution to the salinity. Thus, major ions are those with concentrations greater than 1 mg/kg or 1 ppm(m). By this defirution there are 11 major ions in seawater (Table 10-9). 

Equation 1.2 assumes that the concentration of C is constant throughout the ocean, i.e., that the rate of water mixing is much fester than the combined effects of any reaction rates. For chemicals that exhibit this behavior, the ocean can be treated as one well-mixed reservoir. This is generally only true for the six most abundant (major) ions in seawater. For the rest of the chemicals, the open ocean is better modeled as a two-reservoir system (surface and deep water) in which the rate of water exchange between these two boxes is explicitly accoimted for. 

Conservative ions Ions that exhibit conservative behavior. These include the major ions in seawater. 

Elements considered in seawater speciation calculations can be separated into major and minor components. Such a separation is possible because the vast majority of seawater constituents have concentrations so low that they do not significantly influence the activities of the major cations and anions in seawater. As such, the equilibrium behaviour of the major ions in seawater can be understood (calculated) independently of the numerous minor constituents and these results can then be applied to calculations involving individual minor constituents. 

For the major ions in seawater, the input from rivers is generally the dominant source. The historical approach to estimate the river flux of... 

Because the relative proportions of the major ions in seawater are constant (see Table 15.2), a consistent set of seawater constants can be given as a function of temperature (0-45 °C) and salinity (0-45). Millero (1995) has given sum-... 

The relative proportions of the major ions in seawater and average river water are quite different. Concentration of river water does not produce ocean water. If average river water were concentrated by evaporation, a variety of minerals would precipitate and the ratio among the elements would change. As... 

We define the major ions in seawater (Table 1.4) as those with concentrations greater than 10 pmol kg. Most of the major ions are conservative (exceptions are Sr +, HCO3 and COi ) and these ions make up more than 99.4% of the mass of dissolved solids in seawater. Na+ and Cl account for 86% and Na+, Cfr, SO4, and Mg + make up 97%. Conservative elements with concentrations less than 10 pmolkg are found in rows 5 and 6 of the periodic table where elements with lower crustal abundances occur. 

Bar graphs showing the relative concentrations of the major ions in seawater and river water along with a comparison of some of the major ion ratios in the ocean and river water. The concentrations in the two solutions are listed in Table 2.3. 

Table 2.4. I (a) A summary of the inventory of major ions in seawater after the normative removal from the ocean by formation of the main sedimentary minerals and (b) the mineral formation reactions... 

Because reactions among ionic species in solution are rapid, thermo-d5mamic calculations are used to constrain the activities of dissolved chemical species at equilibrium. Garrels and Thompson (1962) were the first to calculate the speciation of the major ions in seawater by determining the extent to which each species is involved in ion pairing with each counter-ion. This information is necessary to establish the percentages of free major ions available in chemical equilibrium calculations. This section presents an example of how such multiple equilibrium systems can be constrained. 

Garrels and Thompson (1962) performed essentially the same calculation for the major ions in seawater. Their calculations were based on the assumption that the major ions in seawater associate only into ion pairs consisting of 1 1 cation anion complexes. The activity coefficients used in this calculation and the resulting percentages of ion pairing (Table 3.5) have largely survived the test of time and can be broadly apphed in thermodynamic descriptions of the major seawater ions and their reactions. 

The first assumption is probably valid, since the other sources listed in Table 6.2 do not greatly alter the results derived by considering rivers alone. The issue of steady state cannot be verified for very long (millions of years) timescales, but the geological evidence does suggest that the concentration of major ions in seawater has remained broadly constant over very long time periods (Box 6.2). As an example of the residence time calculation, consider sodium (Na+) ... 

Table 6.2 Simplified budget for major ions in seawater. All values are in 1012 molyr Data from Berner and Berner (1987).

Despite these complications, the main removal mechanisms of major ions from seawater are known (Table 6.2). Quantifying the importance of each mechanism is less easy and the uncertainty of data in Table 6.2 should not be forgotten. The amount of removal is compared with the riverine inputs resulting in a geochemical budget that helps constrain the quality of the data. In the following section we outline the important removal processes for major ions in seawater. 

Table 1 Percentage of each major ion in seawater present in various ion pairs.

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