Sea Water Chemistry
Sea Water Chemistry
determine many important
oceanographic phenomena including :
Global patterns of oceanic and
atmospheric circulation, and the growth
and distribution of marine organisms.
Life on earth probably evolved in
Most animals are 50-65% water
water exists in all three physical
states of matter: solid, liquid, and
71% of the Earth Surface - Sea
regulates the climate, dilute waste
major habitat for living creatures
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polarity of water molecules results
in hydrogen bonding
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Characteristics of H20
1. has cohesive behavior
2. resists changes in temperature
3. has a high heat of vaporization
and cools surfaces as it evaporates
4. expands when it freezes
5. is a versatile solvent
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1. Surface tension
measure of how difficult it is to
stretch or break the surface of a
water has a greater surface
tension than most liquids
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2-3. Water's high heat of
moderates the earth's climate.
solar heat absorbed by tropical
seas dissipates when surface water
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4. Oceans and lakes don't freeze
because of hydrogen bonding,
water is less dense as a solid than
it is as a liquid.
consequently, ice floats.
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5. water is a versatile solvent
owing it to the polarity of the
ionic compounds dissolve in water
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Major Components of Seawater
dissolved salts - hydrated anions and
cations (Table 7.1;, f. 7.3)
dissolved gases - nitrogen, oxygen,
organic and inorganic - dissolved
organic materials suspended particulate
Major Ions in Typical Seawater
Ion Parts per thousand by weight ‰ (ppt)(Table
On average, concentration of dissolved salts,
i.e., the salinity, in seawater is 3.5% or
The relative abundances of the ions listed
above does not change, even though salinity
does; are said to be conservative.
Relative abundances of minor and trace
constituents do vary
Evaporation of water to weight the salt is an
Because of the constancy of composition if
we measure one component we can get a
more precise measurement
Salinity ppt = 1.80655 x Chlorinity in ppt
If chlorinity is 19.2 ppt, what is the salinity of
34.7 ppt = 35 ppt
Sources of Salt
Rivers (winds and glaciers are a less
important, indirect source)
Weathering of oceanic crust
Hydrothermal Vents associated with
Mid-ocean ridges and other submarine
Interaction with Particulate matter: clays
and organic matter absorb dissolved
Hydrothermal Activity: (f. 7.4)
Reaction between seawater and new
Minerals like magnesium is incorporated
Calcium is added to sea water
Physical and Chemical
Properties of Water
Heat Capacity - energy added to raise
temperature of 1 gram of substance by
adding energy breaks H-bonds,
increases fraction of free water
important in thermal buffering and heat
transport to higher latitudes
Latent Heats and Evaporation
heat input or release associated with
phase changes (ice - liquid, liquid -
changes in water structure, H-bonding
with phase changes
important in thermal buffering, heat
transport and heat exchange with
Density - mass per unit volume
density of water phases (ice, liquid,
vapor) due to structural changes at
density maximum at 4°C in pure water
Major role in deep ocean circulation and
water column structure and stability
transparent in visible part of spectrum
Absorbed as is goes deeper in the
strongly absorbs infrared (heat) and
ultraviolet (prevents damage to DNA)
hydration of solutes - interactions
between solutes and free water
decreases H-bonding, increases order
of free water, increases density
exclusion of solutes on freezing and
other effects of solutes: freezing point
depression, boiling point elevation
pH (acidity or alkalinity)
measure of the dissociation of water
into ions (H+, OH), (f. 7.9)
pH = - log [H+]
pH effects on biological and
Conservative vs. Non
Conservative Properties of Seawater
those properties that can only be altered
at the sea surface: temperature, salinity,
properties not altered by biological or
importance in water mass identification,
tracing and mixing
Nonconservative Properties of
those properties that can be altered
anywhere in the water column
properties altered by biological and
The proportions of gases in the atmosphere
is not the same as their proportions in
seawater, (Table 7.4)
There is less N2 (nitrogen gas) in the ocean
than in the atmosphere,
much more oxygen,
and even more CO2.
All this CO2 in the oceans keeps CO2 from
being in the atmosphere and causing global
The colder the water, the more gas
can dissolve in it.
When you leave your can of pop in the
car in the sun, then open it, what
Pop sprays all over you.
That's because the gas has exsolved
(come out of solution); a lot has
accumulated in the little space at the top
of the can.
Very active fish, such as trout and salmon,
require very cold water to live in because they
have high oxygen requirements.
They literally suffocate when the water gets
too warm, and the oxygen levels drop.
This explain why these fish don't live down
'Thermal pollution' occurs when electric
plants put warm water into streams, lowering
the oxygen level.
CO2 is important because it is needed by
plants so they can photosynthesize.
O2 is important because animals need it for
CO2 + H2O + energy [from the sun] O2 +
sugar (organic matter)
respiration (the reverse of
O2 + sugar CO2 + H2O + energy
seawater\atmosphere exchange at air
water interface only (f.7.8)
biological processes that affect O2
concentration: photosynthesis and
typical distribution of O2 in water column
and processes that control this
inorganic sources of N, P, S and other
atoms required for phytoplankton
photosynthesis and respiration
contributes in nutrient distribution
Especially important, because so much is
needed, are N (nitrogen) and P
Si (silica) is also important for all the
siliceous organisms we've discussed:
diatoms, radiolarians, and siliceous sponges.
N is necessary to make proteins.
P is necessary to make new cells (it's part of
the cell wall), and also genetic material, DNA
N is useful for plants only in these forms:
N2, the gas, is not usable by most plants.
Only a few bacteria can break this very strong
molecule apart and turn it into nitrate.
These are 'nitrogen-fixing bacteria'.
P is useful in the form of phosphate, PO43-
Thus weathering, sedimentation, and
ocean chemistry are all closely linked.
Other ions in seawater, such as Cl – and
SO4 , are not derived from weathering,
but from volcanic degassing.
The Carbonate System in
CO2 in seawater is controlled by: ( f.7-
Exchange with the atmosphere
6CO2 + 6H2 O C6 H12 O6 + 6O2
The Carbonate Buffer
Carbon Dioxide: CO2 ,
Carbonic Acid: H2CO3 ,
Bicarbonate: HCO3 - ,
CO2 + H2O H2CO3 .
H2CO3 HCO3 - + H+2.
HCO3 CO32- + H +3.
Another important reaction is the dissolution
and precipitation of calcium carbonate:
CO3 + Ca+2 CaCO34..
Importance of these reactions:
Maintain constant pH (seawater is said to be
Few marine organisms can tolerate a pH very
different from 8.
In general, shells of organisms are likely
to be preserved where their production
rate is high,
Siliceous shells are preserved only
where the production rate is high.
Siliceous oozes occur where
productivity rate is high and terrigenous
sedimentation rate low.
Carbonate shells: the oceans are
supersaturated with respect to CaCO3 at the
surface and become increasingly
undersaturated with depth.
Shells more likely to be preserved at shallow
Lysocline: depth at which rapid dissolution of
This is deeper than the depth where ocean
Carbonate Compensation Depth (or
depth where dissolution>supply of
below which CaCO3 shells are not
preserved in sediment.