1. Salinity is related to the concentration of dissolved salts in
seawater.
In the past, salinity of seawater was measured by evaporating the
water and weighing the amount of salt remaining.Since that approach
is difficult and inaccurate, electrical conductivity of seawater is
now used to measure salinity.

Conductivity increases as salt content of the water increases.
Conductivity gives very accurate salinity data: 35.0000X.
Conductivity (and temperature and depth) are measured by instruments
called CTDs (Conductivity Temperature Depth). These instruments can
make thousands of measurements/hour.
Salinity, temperature, and depth (pressure) can be used to calculate
density, which is important to understanding vertical circulation of
the water.
Salinity is greatest in warm, tropical surface waters, where there is
more evaporation than precipitation. It is lowest where there are
large inputs of freshwater from rivers.
Salinity has no units. (The PSU or "practical salinity unit" is
incorrect, although frequently used.)

Salinity is approximately equal to the weight, in grams, of salt
dissolved in 1000 g of seawater. This would be the salt concentration
in parts per thousand (‰).
Average ocean water has a salinity of 35.0.
This means that 1000 g of average seawater contains 965 g of water
and 35 g of salts.



Notes Index

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2. Salts consist of ions.
Cations have a positive electrical charge. Anions have a negative
electrical charge. Salts are electrically neutral because the cation
and anion charges are opposite and equal.

When salts dissolve in water, they break apart into their cations and
anions. Examples are:

Sodium chloride, NaCl, dissociates to Na+ and Cl-.
Magnesium sulfate, MgSO4, dissociates to Mg2+ and SO42-.

3. Six major ions make up >99% of the total dissolved in seawater.
They are sodium ion (Na+), chloride (Cl-), sulfate (SO42-), magnesium
ion (Mg2+), calcium ion (Ca2+), and potassium ion (K+).

4. Every naturally-occurring element has been found in seawater.
Some, however, have minuscule dissolved concentrations:

Iron, 0.06 parts per billion (ppb)
Lead, 0.002 ppb.
Gold, 0.005 ppb.
Protactinium, 0.00000005 ppb.

5. The major ions are conservative. This means that they have
constant ratios, to one another and to salinity, in almost all ocean
water.
Another way of saying this is that sea salts have constant
composition. They almost always consist of 55% sodium ion, 31%
chloride, 8% sulfate, 4% magnesium ion, 1% calcium ion, and 1%
potassium ion.

The main exception is where freshwater is mixing with seawater. River
water has a different composition than seawater, for example, it
contains more calcium ion.

6. Sea salts mostly came from the weathering of rocks on land (the
cations) and from the interior of the earth (anions).
The weathering of rock on land is a slow processes of breakdown by
water, with dissolved carbon dioxide, which makes it slightly acidic.

Igneous (volcanic) rocks do not contain enough anions to be the
source to the oceans. Now, sedimentary rocks are the source. In the
past, volcanoes and, probably, an initial rapid release when the
earth melted were the source.

Rivers carry the dissolved ions to the ocean.
Weathering may have been somewhat faster on the early earth, but even
at the present rate it would take only about 8 to 260 million years
to replace all the salts in seawater with those in the river inflow.
The time to replace the total amount of an ion in seawater with the
ions in the river inflow is called the residence time.
Since this is much less than the age of the Earth and the oceans,
some processes must remove the salts from seawater to keep them from
building up to even higher concentration.

7. Ocean salt composition and concentration is in "steady state".
This means that it does not change significantly over time.
Evidence indicates that sea salt concentration and composition has
been about the same for 1.5 billion years at least. The tolerances of
bacteria that probably lived 3.8 bybp indicate that sea salt
concentration and composition were not too different, even that long
ago.

The "steady state" results from the removal rate of salts from the
ocean being equal to the input rate.

This balance holds because the removal rate of salts is related to
their concentration, and increases when their concentration
increases.
Removal processes include:
formation of evaporites (salt deposits left behind when seawater
evaporates)
burial of sediment porewater (the water between sediment grains)
sediments, especially biogenic sediments, for Ca2+ (calcium ion) as
calcium carbonate.
hydrothermal vents, especially formation of the mineral chlorite
within the cracks and fissures of the vents, which removes Mg2+
(magnesium ion).

8. Most of the other substances in seawater are not conservative.
Their concentrations vary geographically and with depth, most often
due to uptake and release by organisms.
9. Several important gases are not conservative. These include oxygen
and carbon dioxide.
Oxygen dissolves in ocean surface water from the atmosphere.
Photosynthesis is also a source of oxygen to ocean surface waters.

Oxygen is consumed by respiration. Rarely, animals and bacteria use
all of the oxygen in sub-surface waters, which become anoxic. This
can only happen if the waters are isolated from the atmosphere in
some way.

Carbon dioxide is consumed during photosynthesis and released during
respiration. It can also be exchanged with (dissolved from and
released to) the atmosphere.

Carbon dioxide can react with water to form bicarbonate and carbonate
ions.

CO2 + H2O –› HCO3- + H+ –› CO32- + 2H+

These reactions control the acidity (pH) of seawater.

Organisms use carbonate ion and calcium ion to make calcium carbonate
shells, which sink after the organisms die to form calcareous
sediments.

10. Another important group of nonconservative substances dissolved
in seawater are the nutrients.
These are fertilizers essential for the growth of plants, including
algae.

Major nutrients include nitrate, phosphate, and silicate (the latter
required only by siliceous organisms).

Nutrients are depleted in surface waters, where plants grow, and are
found in higher concentrations in deep waters, where the plant and
animal remains that sink from surface waters decay.


SOURCE: WEB PAGE http://www.sfos.uaf.edu/msl111/notes/chem.html#1

Claudio Acuña
claudioacuna4yahoo.com