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Lecture 17: SALINITY AND COMPOSITION OF SEA WATER

Water's solvent properties- review
Salinity
Composition of dissolved salts
"Constant proportions"
"Conservative" and "nonconservative" species
Salinity distribution in surface ocean waters
-- controlling factors

Powerpoint Lecture Slides


Review: WATER IS AN EXCELLENT SOLVENT -- dissolves many substances
Polar H2O molecule interacts with other polar substances (solids)
negative H2O end -- (positive) cations
positive H2O end -- (negative) anions
overcomes ionic bonding in solids --> dissolution

SALINITY (revisited)
ave. S = 35 g/kg; range = 30-37 g/kg
Variations due to gains vs. losses of H2O ("water budget")

COMPOSITION OF DISSOLVED SALTS in Ocean water
Dissolved substances are ions: cations (+) & anions (-)
99.7% of salinity is made up of only 7 ionic species ("majors")

 Cations
(g/kg)

 Anions
(g/kg)

 Na+ 10.8  Cl- 19.4
 Mg2+ 1.3  SO42- 2.7
 Ca2+ 0.4  HCO3- 0.1
 K+ 0.4  

Other elements -- "minor" or "trace" elements (less abundant, but some are vitally important)

Major dissolved species (& a few minors) are always present in "constant proportions"

[Na+] / [Cl-] = a constant value everywhere
[Na+] / Salinity = (ditto)

Why does this occur?
Mixing time of oceans (<2,000 yr) much less that rates of addition and removal of those species.

Dissolved constituents (ions, species) present in constant proportions
= "conservative" constituents (conserved; no loss or gain)

Most minor and trace species....
....not present in constant proportions -- "non-conservative"
Removed rapidly from sea water (life processes)

DISTRIBUTION OF SALINITY IN SURFACE OCEAN WATER
Highest S...
center of oceans at ~ 25-30° lat.
enclosed seas at same lat.
Lowest S...
temperate lats. (~40-50°)
near coasts
equatorial regions

Controlling process: variations in the "water budget" at any given location.

Global hydrologic cycle -- evap., ppt. (rain), runoff
Oceans: E>P --> water vapor to land (as rain)
Land: P>E --> runoff returned to oceans (fresh water)

Salinity variations with latitude -- E/P ratio variations in oceans
Temperate and equatorial: E<P ---> low S
Subtropics: E>P ---> high S

Salinity near continents
Runoff from major rivers --> low S
Semi-enclosed seas of high E --> high S

Effect of currents -- move water (and S) across latitudes
Gulf Stream: warm saline water transported N and W
Eastern Pacific: cool, "dilute" water transported
S --> N
E --> W



(Detailed notes start here)

Water's solvent properties

As noted in a previous lecture, water is an excellent solvent. That is, it is able to dissolve a range of solids, gases, and even other liquids. The reason for its solvent properties is that the polar H2O molecule interacts strongly with other polar substances. For example, most common minerals are composed of oppositely charged ions, e.g., (Na+) and (Cl-) in halite, (Ca2+) and (CO32-) in calcite. When minerals are immersed in water, the negative end of H2O coordinates to positive cation, and the positive end of H2O coordinates to negative anion. The combined effect of these strong interactions overcomes ionic bonding in the minerals and leads to dissolution.

Salinity of sea water

Recall that the average salinity of sea water is 35 gm dissolved salt / kg sea water (= 35 p.p.t. = 35 o/oo), and that 99% of sea water is in the range 30 to 37 g/kg. Variations in salinity are due to gains (rainfall, runoff from land) and losses (evaporation) of water.

Composition of dissolved salts

Almost all of the dissolved substances in sea water are cations (positively charged) and anions (negatively charged). Cations and anions must be present in equal proportions to maintain electrical neutrality. In addition, more than 99.7 % of dissolved salt are made up of only 7 ionic species. These are called "major" elements (ions, species) in sea water.

 Cations
(g/kg)

 Anions
(g/kg)

 Na+ 10.8  Cl- 19.4
 Mg2+ 1.3  SO42- 2.7
 Ca2+ 0.4  HCO3- 0.1
 K+ 0.4  

Almost all of the other chemical elements are present in sea water, but at lower concentrations. These are called "minor " or "trace" elements.

Although the salinity of sea water varies, major dissolved species are always present in constant proportions. That is, the ratio of major species to one another (e.g., Cl-/Na+) or major species to total salinity (e.g., Na+/S,
Cl-/S) is constant regardless of salinity. Major ions that occur in constant proportions in sea water are called conservative constituents (ions, species). Their concentrations change only as salinity changes. Why do proportions of conservative species remain constant? Almost certainly, it is related to the fact that the oceans are comparatively "well stirred." That is, the mixing time for the entire ocean (< 2,000 years) is faster than the rates of addition and removal of conservative species.

Most minor and trace species are non-conservative constituents. Their concentrations vary independently of salinity. Many of these chemical species are removed rapidly by biological processes.

Distribution of salinity in surface ocean water

When you examine a map of surface salinities, you should see that:
* Highest salinities occur in the center of ocean at about 25-30 deg. latitude in both hemispheres and in enclosed seas at about the same latitude.
* Lowest salinities occur in temperate latitudes (40 - 50 deg) in both hemispheres, near coasts, and in equatorial regions.
This pattern of surface salinity variations is controlled primarily by variations in the "water budget" (gains vs. losses) at any locality.

The cycling of H2O between the sea surface, atmosphere, and land is described by the global hydrologic cycle. The cycle is driven by evaporation (from the sea surface and land), condensation and precipitation (as rain and snow), and runoff (from land back to the oceans).
* In oceanic areas, evaporation exceeds precipitation (E > P); excess water vapor is transported to land by the circulation of the atmosphere.
* In continental areas, evaporation is less than precipitation (E < P); excess rainfall returns to the oceans as river runoff and groudwater flow.

Variations in salinity with latitude are governed latitudinal variations in the E/P ratio (loss vs. gain of water) in the oceans.
* In temperate and equatorial latitudes, E < P (or E/P < 1); hence, salinity is low in those regions.
* In subtropical latitudes, E > P (or E/P > 1); hence, salinity is high in those regions.

Proximity to continents can also play a role in controlling salinity.
* In coastal regions close to major rivers, runoff decreases salinity.
* On the other hand, salinity is high in semi-enclosed seas where evaporation is high, such as the Caribbean Sea, the Gulf of Mexico, the Mediterranean Sea, and the Red Sea.

Currents modify the latitudinal bands of salinity controlled by E vs. P by transporting surface waters (and salinities) across latitude belts.
* For example, the Gulf Stream transports warm, saline waters transported north and west.
* In the eastern Pacific, south-to-north "boundary" currents and east-to-west equatorial currents transport cool, dilute waters.

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