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Class 17 -- The Oceans
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
How and Why Do Various Elements
Dissolve in Water?
WATER IS AN EXCELLENT SOLVENT -Dissolves many substances
H2O molecule is “polar”
•Negative H2O end -- attracts + ions (cations)
•Positive H2O end -- attracts - ions (anions)
•Many ions are “happy” in water because of this
•The attraction of the H2O’s overcomes ionic bonding
in solids --> solids dissolve
Which ions are found in seawater
and how much of each is there?
SALINITY (review)
•Ave. S = 35 g/kg; range = 30-37 g/kg
•Variations due to gains vs. losses of H2O
COMPOSITION OF DISSOLVED SALTS
•Dissolved substances are ions
– cations (+) & anions (-)
•99.7% of salinity is made up of only 7 ions
–"major elements”...
Cations
Na+
Mg 2+
Ca 2+
K+
g/kg
10.8
1.3
0.4
0.4
Anions
Cl SO 4 2HCO 3 -
g/kg
19.4
2.7
0.1
•These are the Major Elements in Seawater
–Others -- "minor" or "trace”elements
•Major elements: "constant proportions"
–[Na+] / [Cl-] = a constant value everywhere
–[Na+] / Salinity = “
“
“
Why does this occur?
•Oceans are “well stirred”
•i.e., Mixing time (<2,000 yr) is much less
than the time for much addition/removal
•Most minor and trace species are NOT
present in constant proportions -–“non-conservative”species
–removed rapidly from sea water
–some removed by living organisms
Where is salinity high? Where is it low?
Highest S...
•center of oceans at ~ 25˚ lat.
•enclosed seas at same lat.
Lowest S...
•temperate latitudes. (~40-50˚)
•equatorial regions
•near coasts
Sea Surface Salinities (Fig. 5-21)
What causes these variations in salinity?
•
Is salt added in certain areas and not in
others?
• No…
• It must be addition or loss of water.
– Variations in the "water budget" from
place to place
The Global Hydrologic Cycle
Evaporation/ Precipitation Imbalance and Salinity
Overview of salinity-controlling processes:
•Salinity variations with latitude
–E vs. P balance is the key…
–Temperate + equatorial: E < P ---> low S
–Subtropics (Dry!): E > P ---> high S
•Salinity near continents
–Runoff from major rivers --> low S
–Semi-enclosed seas of high E --> high S
Sea Surface Salinities (Fig. 5-21)
Sea Surface Salinities
•Ocean currents modify the salinity pattern -•Ex. #1: Gulf Stream: warm, saline water
transported North and West
•Ex. #2: SE Pacific: cool, less saline water
transported Northward, then Westward
Sea Surface Salinities (Fig. 5-21)
Why is salinity so important?
High S water is more dense than low S water.
Convection is driven by the sinking of cold,
high salinity water masses.
Deep Ocean currents driven by convection
These currents carry heat from low to high
latitudes and influence global climate strongly
From Before..... Convection in the Oceans-
Salinity is critical too.
North Atlantic
Sea Ice
Cooling
Equator
Evap.
Heating