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Salinity in the Worlds Oceans and
the Chemical Composition ofWater
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Garry & Shannon
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Salinity is defined as the total quantity of
dissolved salt in seawater measured in PSU.
Measured in grams per kilogram (g/kg)
The average salinity of seawater is 33-37ppt
Six ions make up more than 99% of the salts
dissolved in seawater: sodium (Na+),
magnesium (Mg+
), calcium (Ca2+
), Potassium(K+), chloride (Cl-), and sulfate (SO4
2-)
When saltwater and freshwater meet an
estuary is formed.
Salinity and Chemical Composition of
Water
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Chemical composition of seawater
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Salinity in the worlds bodies of water
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Latitude versus salinity
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The distribution of sea-surface salinity also tends to be
zonal.
The saltiest waters are at mid-latitudes where evaporation
is high. Less salty waters are near the equator where rain freshens
the surface water, and at high latitudes where melted sea
ice freshens the surface waters.
The zonal (east-west) average of salinity shows a close
correlation between salinity and evaporation minusprecipitation plus river input.
Salinity distribution
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Mean sea-surface salinity
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Factors that affect the chemical composition of the sea
1. Exchange with the atmosphere (eg, nitrogen cycle)
2. Solubility of different compounds
3. Reduction by anaerobic bacteria
4. Precipitation and exchange with the ocean bottom5. Inflow of fresh water
6. Freezing and melting of sea ice
7. Chemical reactions that control or influence the concentrations
of different elements
8. Hydrothermal discharges from deep sea vents
9. Biological processes eg, life processes and decomposition
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The Effects of Adding Salt to Water
The specific heat (the amount of heat necessary to
raise the temperature of one gram of water one
degree) decreases with increasing salinityThe density increases as salinity increases
Freezing point decreases when salinity increases
The vapour pressure is lowered as salinity
increases
Osmotic pressure increases as salinity increases
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The density increases as salinity
increases
As salinity increases and the
temperature decreases the density
increasesFreshwater has the highest density at 4
degrees Celsius, Saltwater of ~ -1.8
degrees celsius with a ppt of 35.
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Depth and salinity
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Ways to measure salinity
Salinometer
Electrical conductivity (CTD can measure this)
Boiling of saltwater to measure the amount of salt left per litre
of saltwater
Chemical method using ion ratios (possible due to commonratios)
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Local examples and information
Average salinity is 30 in
Labrador current due to
high precipitation versusevaporation ratio, highest
at deep-water upwelling
(~35)
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