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Water's solvent properties- review
Salinity
Composition of dissolved salts
"Constant proportions"
"Conservative" and "nonconservative" species
Salinity distribution in surface ocean waters
-- controlling factors
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 |
| 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.
Salinity variations with latitude -- E/P ratio variations
in oceans
Temperate and equatorial: E<P ---> low S
Subtropics: E>P ---> 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
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.
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.
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 |
| 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.
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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