Salinity, of 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). 

Salinity of seawater captured by various sampling devices in the CEPEX enclosure indicates problems not revealed in the usual oceanographic sampling situation. Relative to peristaltic pumping, all samples exhibited some salinity anomalies. Inadequate flushing to rinse the sampler of any concentrated brine or entrapped seawater is thought to be a problem. 

Neve et al. [547] digested the sample with nitric acid. After digestion the sample is reacted selectively with an aromatic o-diamine, and the reaction product is detected by flameless atomic absorption spectrometry after the addition of nickel (III) ions. The detection limit is 20mg/l, and both selenium (IV) and total selenium can be determined. There was no significant interference in a saline environment with three times the salinity of seawater. 

Early scientists recognized that standards were needed to determine reliable values of the chlorinity and salinity of seawater. The IAPSO Standard Sea Water Service (originally based in Copenhagen) collected and distributed seawater from the North Atlantic with a known, measured chlorinity. This sample was supplied to oceanographers to standardize the AgNOg solutions used to determine chlorinity in various laboratories. 

The concept of salinity was introduced by Georg Forchhammer in 1865. From extensive analyses of seawater samples, he was able to demonstrate the validity of Marcet s principle for the most abundant of the salt ions chloride, sodium, calcium, potassium, magnesium, and sulfete. Thus, he recognized that the salinity of seawater could be inferred from the easily measurable chloride concentration or chlorinity. The details of this relationship were worked out by Martin Knudsen, Carl Forch, and S. E L. Sorenson between 1899 and 1902. With the international acceptance of their equation relating salinity to chlorinity (S%o = 1.805 Cl%o + 0.030), the standardization necessary for hydro-graphic research was provided. A slight revision in this equation (S%o = 1.80655 Cl%o) was made in 1962 by international agreement. 

Since the mass ratio is multiplied by 1000, the units of salinity are parts per thousand, symbolized by %o. The average salinity of seawater is 35%o, which is eqifivalent to a 3.5% salt solution. As shown in Figure 3 3, 99% of the seawater in the ocean has a salinity between 33%o and 37%o. Note that the temperature of seawater is far more variable, ranging from -2° to 30°C with an average of 3 5°C. Fifty percent of the water has a salinity between 34.6%o and 34.8%o and a temperature between 1.3°C and 3.8°C. 

Because the major ions are present in nearly constant proportions, the salinity of seawater can be inferred from any of their individual concentrations. The easiest concentration to measure is that of the chloride ion, which is also the most abundant. In practice, this concentration is determined by titrating a sample of seawater with a standardized solution of silver nitrate. The reactions that take place are ... 

Table 3.8 Physical Processes That Can Alter the Salinity of Seawater without Significantly Affecting Ion Ratios.

In seawater, physical processes that transport water can also cause mass fluxes and, hence, are another means by which the salinity of seawater can be conservatively altered. The physical processes responsible for water movement within the ocean are turbulent mixing and water-mass advection. Turbulent mixing has been observed to follow Pick s first law and, hence, is also known as eddy diffusion. The rate at which solutes are transported by turbulent mixing and advection is usually much faster than that of molecular diffusion. Exceptions to this occur in locations where water motion is relatively slow, such as the pore waters of marine sediments. The effects of advection and turbulent mixing on the transport of chemicals are discussed further in Chapter 4. 

The global heat cycle drives the hydrological cycle, which in turn controls the salinity of seawater. The most important contributor of heat to the crustal-ocean-fectory is solar radiation. The flux of solar radiation that reaches Earth is termed insolation. Only a fraction of the incoming solar radiation reaches Earth s surfece, because a large portion is either reflected or absorbed by the atmosphere. That which reaches Earth s surface is also either reflected or absorbed. In the end, about half of the incoming radiation is absorbed by the rocks and water on Earth s surfece. (A detailed heat budget is provided... 

Marine chemists have developed two approaches to handling nonspecific effects. The easiest one, which we will adopt in this book, is to use equilibrium constants appropriate for the temperature, pressure, and salinity of seawater. (Since most of the ionic... 

Because of their role as an elemental sink, the formation and weathering of evaporites has the potential to affect the salinity of seawater. This can in turn alter climate, because the heat capacity of seawater is a function of its salt content. Changes in the salt content of seawater also have the potential to affect survival of marine biota, particularly the calcifiers. 

The mineralogy, distribution, and formation of evaporites are the subjects of this chapter. The role of evaporite formation and dissolution in determining the salinity of seawater is discussed in Chapter 21. 

This is why the salinity of seawater is nearly the same throughout the open ocean, varying by only a few parts per thousand. (As per Figure 3.3, 75% of seawater has a salinity between 34 and 35 %o.) The small degree of spatial variability is a consequence of geographic variations in the balance of evaporation versus precipitation in the surface waters. Recall that these surface waters are the source waters for intermediate and deep water masses. Since shifts in the relative rates of evaporation versus precipitation involve only addition or removal of water, the major ion ratios are unaltered. This is why the major ion ratios do not exhibit little if any spatial differences within the open ocean. 

The other reason why the average salinity of seawater is 35%o lies in the fundamental chemistry of major ions. For example, the sevenfold increase in the Na /K ratio between river water and seawater (Table 21.8) reflects the lower affinity of marine rocks for sodium as compared to potassium. In other words, the sodium sink is not as effective as the one for potassium. Thus, more sodium remains in seawater, with its upper limit, in theory, being controlled by the solubility of halite. Likewise, the Ca /Mg ° ratio in seawater is 12-fold lower than that of river water due to the highly effective removal of calcium through the formation of biogenic calcite. 

Rule of Constant Proportions The relative abundances of the six major cations (Na, K, Ca, Mg CF, and SO ) is constant regardless of the salinity of seawater. 

Salinometer The device used to measure the salinity of seawater by deterrmning its electrical conductivity. 

The salinity of seawater ranges from 33 to 37. Its value can be derived directly from chlorinity through the relationship... 

However, salinity values are easily obtained with a salinometer (which measures electrical conductivity and is appropriately calibrated with standard solutions and adjusted to account for T effects). The salinity of seawater increases if the loss of H2O (evaporation, formation of ice) exceeds the atmospheric input (rain plus rivers), and diminishes near deltas and lagoons. Salinity and temperature concur antithetically to define the density of seawater. The surface temperature of the sea reflects primarily the latitude and season of sampling. The vertical thermal profile defines three zones surface (10-100 m), where T is practically constant thermoclinal (100-1000 m), where T diminishes regularly with depth and abyssal... 

The definition of the sea boundary of the mouth area is related to the term mouth-mixing zone. Water salinity within this zone increases from the salinity inherent in river water (usually 0.2-0.5%o) to the salinity of seawater (usually 10-40%o in different seas). The salt composition of water radically changes within the mixing zone river water of hydrocarbonate class and calcium group transforms into seawater of chloride class and sodium group. 

Ion exchange also occurs in estuaries (i.e., where rivers discharge) rich in clay-suspended particles saturated with adsorbed Ca2+ ions. As river water mixes with the high salinity of seawater (rich in Na+), ion exchange takes place and Ca2+ is liberated while Na+, K+, and Mg2+ are removed. Phosphates and Fe ions may also be removed. This is why the Na+/Ca2+ proportion in estuaries is sometimes inverted with respect to that in the oceans. 

The concentration of uranium in the sea is relatively constant and is equal to 3 pg/1 at a salinity of 35%. A slight increase of uranium concentration has been observed to occur upon increase of the general salinity of seawater in numerous deep-water studies [169]. 

Ideas about variations in sodium ion (Na+) and chloride ion (Cl ) concentrations are based on ancient halite inventories. The total volume of known halite deposits amounts to about 30% of the NaCI content of the present oceans. If all of this salt were added to the present oceans, the salinity of seawater would increase by about 30%, setting an upper limit. However, the ages of major halite deposits are reasonably well dispersed through geological time, suggesting that there was never a time when all of these ions were dissolved in seawater. 

As Cl has a very long oceanic residence time, the sporadic distribution of evaporate-forming episodes (Box 6.2), integrated over million-year timescales, results in only quite small fluctuations in the salinity of seawater. However, the lack of major evaporate-forming environments today suggests that both Cl and sulphate (S04 ) are gradually accumulating in the oceans until the next episode of removal by evaporite formation. 

In Table 20.4, the abbreviation psu, short for practical salinity unit, indicates salinity expressed in the Practical Salinity Scale of 1978 (PSS-78) as a dimensionless quantity. The term psu is not an official unit (Unesco, 1985 Siedler, 1998) but is in widespread use and is particularly helpful to distinguish, say, a given salinity value from absolute salinity in g/kg. Before 1978, salinity was computed from chlorinity, CL by the Cox scale, 5= 1,80655 xCZ (Mamayev et al, 1991), The recommended numerical conversion factor between the PSS-78 salinity and the Cox salinity is 1, Cox salinity is usually expressed in parts per thousand, ppt, %o, or g/kg. None the less, it is lower by about 0.5% than the absolute salinity of seawater in grams of dissolved substance per kilogram of seawater, which in turn is not exactly known but can be estimated sufficiently well (Millero et al., 2008), see Section 20.2.1. 

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