Oceanography

Course No: CDM 301

Tasnim Jerin
Lecturer
Department of Coastal Studies and Disaster Management
University of Barisal
tasnimjerintusliha@gmail.com
Physical and Chemical Properties of Sea
Water
 Seawater
• Sea water is a mixture of
96.5% pure water and 3.5%
other material, such as salts,
dissolved gases, organic
substances, and undissolved
particles.
• Its physical properties are
mainly determined by the
96.5% pure water.
• Pure water, when compared
with fluids of similar
composition, displays most
uncommon properties. This is
the result of the particular
structure of the water
molecule H2O: The hydrogen
atoms carry one positive Left diagram: Arrangement of the oxygen atom
(O) and the two hydrogen atoms (H) in the
charge, the oxygen atom two water molecule. The angle between the
negative charges, but the positively charged hydrogen atoms is 105°,
atom arrangement in the which is very close to the angles in a regular
water molecule is such that tetrahedron (109° 28').
Right diagram: Interaction of two water
the charges are not molecules in the tetrahedral arrangement of the
neutralized (the charges hydrogen bond. The hydrogen atoms of the blue
would be neutralized if the water molecule attach to the red water
angle were 180° rather than molecule in such a way that
the four hydrogen atoms form a tetrahedron.
105°).
Physical states of water
 Physical Properties of Water
• Water molecules are asymmetrical,
and this has important consequences.
1. The electric charge is asymmetrical,
causing strong attraction between
molecules, resulting in:
• High melting temperature.
• High boiling point. Hydrogen bonds are attractions of
electrostatic force caused by the
• High heat of vaporization. difference in charge between slightly
positive hydrogen ions and other, slightly
• High surface tension. negative ions. In the case of water,
hydrogen bonds form between
neighboring hydrogen and oxygen atoms
of adjacent water molecules
2. The molecule has a large dipole moment, resulting in:
• High dielectric constant.
• Great power for dissolving inorganic chemicals, which leads to high
salinity and conductivity of sea water. It splits dissolved material into
electrically charged ions. As a consequence, dissolved material greatly
increases the electrical conductivity of water.

Fig: Sodium chloride Na+ Cl- is the main component of salinity in the ocean. The diagram shows how in
hydrated form it causes water molecules to attach themselves with the positive hydrogen charges to the chloride and
with the negative oxygen charge to the natrium.
3. The high conductivity causes:
• Rapid electrolysis of metals in sea water, causing rapid corrosion.
• The motion of sea water in Earth’s magnetic field creates a potential.
Measurements of the potential can be used for measuring the velocity
of oceanic currents.
4. The angle 105° is close to the angle of a tetrahedron, i.e. a structure
with four arms emanating from a centre at equal angles (109° 28´). As a
result, oxygen atoms in water try to have four hydrogen atoms
attached to them in a tetrahedral arrangement. This is called a
"hydrogen bond", in contrast to the (ionic) molecular bond and
covalent bonding. Hydrogen bonds need a bonding energy 10 to 100
times smaller than molecular bonds, so water is very flexible in its
reaction to changing chemical conditions.
5. Water molecules pack together either in tetrahedral structures or in
spherical, close-packing, structures of ice.
• The properties of the tetrahedral structure, which is more common
at higher temperatures, is superimposed on the properties of
the ice structure, which is more common at lower temperatures.
• The conflict between these two structures leads to the parabolic
shape of water properties as a function of temperature

Fig: The shape of the water and ice molecules determines the density. The ice molecule is
packed in a lattice that takes more volume than water molecules
6. Tetrahedral packing is
denser than the spherical
close packing of ice .
• The maximum density of
pure water is above the
freezing point.
• Ice is less dense than
water.
• The maximum density of
sea water, however, is at
freezing
When freezing, all water molecules form tetrahedrons. This leads to a sudden
expansion in volume, ie a decrease in density. The solid phase of water is therefore
lighter than the liquid phase, which is a rare property. Some important
consequences are:
1. Ice floats. This is important for life in freshwater lakes, since the ice acts as an
insulator against further heat loss, preventing the water to freeze from the surface
to the bottom.
2. Density shows a rapid decrease as the freezing point is approached. The
resulting
expansion during freezing is a major cause for the weathering of rocks.
3. The freezing point decreases under pressure. As a consequence, melting occurs
at the base of glaciers, which facilitates glacier flow.
4. Hydrogen bonds give way under pressure, i.e. ice under pressure becomes
plastic. As a consequence, the inland ice of the Antarctic and the Arctic flows,
shedding icebergs at the outer rims. Without this process all water would
eventually end up as ice in the polar regions.
 Element classification of Sea Water
• Chemicals in sea water can be classified into four groups based
mainly the shapes of their dissolved concentration distributions with
depth of water. The four categories chemicals of the sea water are:
i. Conservative elements
ii. Bioactive elements
iii. Absorbed or scavenged elements
iv. Gases
•
i. Conservative elements:
• Non-reactive
• Concentration is constant
• Their concentrations are not greatly affected by processes other than
precipitation and evaporation
• Remain in the ocean in long period
• Major components of sea water belong to conservative elements
• Examples: Na, K, S, Cl, B
•
iii. Absorbed elements:
• Have depth profiles that are reversed from those in the bioactive
category
• Concentrations are higher in surface waters and decrease with depth
as the elements are absorbed to particles that fall through the ocean
• Absorbed elements are exclusively low in concentration, meaning
that this mechanism is not pervasive enough to alter the
concentrations of elements with higher concentrations.
• Examples of metals that have concentration profiles influenced by
this process are Mn and Al.
•
 • The elements found in the sea water include most of those in the
periodic table, of these, only 14 elements (O, H, Cl, Na, Mg, S, Ca, K, Br,
C, Sr, B, Si and F) have concentrations greater than 1 ppm.
• Most of these elements (except Si) are generally unreactive (both
chemically and biologically)
• Many of the remaining elements, called minor elements are involved in
inorganic and biological reactions in the marine environment.
• Bruland conveniently divided the elements into three classes based on
concentration:
i. Major elements (0.05 to 740 mM)
ii. Minor elements (0.05 to 50 µM)
iii. Trace elements (0.05 to 50 nM)
 Salinity
• Sea water contains 3.5% salts, dissolved gasses, organic substances
and undissolved particulate matter.
• The presence of salts influences most physical properties of sea water
(density, compressibility, freezing point, temperature of the density
maximum) to some degree but does not determine them.
• Two properties which are determined by the amount of salt in the
sea are conductivity and osmotic pressure.
• At the simplest level, salinity is the total amount of dissolved material
in grams in one kilogram of sea water.
• It has no units.
• In practice, this is difficult to measure.
• Salinity was defined in 1902 as the total amount in grams of dissolved
substances contained in one kilogram of sea water if all carbonates
are converted into oxides, all bromides and iodides into chlorides,
and all organic substances oxidized.
• The relationship between salinity and chloride was determined
through a series of fundamental laboratory measurements based on
sea water samples from all regions of the world ocean and was given
as

• The symbol o/oo stands for "parts per thousand" or "per mil"; a salt
content of 3.5% is equivalent to 35 o/oo, or 35 grams of salt per
kilogram of sea water.
• The fact that the equation of 1902 gives a salinity of 0.03 o/oo for
zero chlorinity is a cause for concern. It indicates a problem in the
water samples used for the laboratory measurements.
• The United Nations Scientific, Education and Cultural Organization
(UNESCO) decided to repeat the base determination of the relation
between chlorinity and salinity and introduced a new definition,
known as absolute salinity,

• The definitions of 1902 and 1969 give identical results at a salinity of

35 o/oo and do not differ significantly for most applications.
• The definition of salinity was reviewed again when techniques to determine
salinity from measurements of conductivity, temperature and pressure were
developed
• Since 1978, the "Practical Salinity Scale" defines salinity in terms of a
conductivity ratio:
" The practical salinity, symbol S, of a sample of sea water, is defined in terms
of the ratio K of the electrical conductivity of a sea water sample of 15°C and
the pressure of one standard atmosphere, to that of a potassium chloride
(KCl) solution, in which the mass fraction of KCl is 0.0324356, at the same
temperature and pressure. The K value exactly equal to one corresponds, by
definition, to a practical salinity equal to 35." The corresponding formula is:

• Note that in this definition, salinity is a ratio and (o/oo) is therefore no

longer used. As the practical salinity is a ratio and therefore does not have
units, the unit "psu" is rather meaningless and strongly discouraged.
 Geographical Distribution of Surface
Temperature and Salinity
• Many physical processes depend on temperature
• The unit of T is the kelvin, which has the symbol K.
• The distribution of temperature at the sea surface tends to be zonal,
that is, it is independent of longitude
• Warmest water is near the equator, coldest water is near the poles.
• The annual range of sea-surface temperature is highest at
mid-latitudes, especially on the western side of the ocean
• 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
• Because many large rivers drain into the Atlantic and the Arctic Sea,
why is the Atlantic saltier than the Pacific?
• Broecker (1997) showed that 0.32 Sv of the water evaporated from
the Atlantic does not fall as rain on land. Instead, it is carried by winds
into the Pacific
• The mean temperature of the ocean’s waters is: T = 3.5◦C
• The mean salinity is S = 34.7
 The Oceanic Mixed Layer
• A mixed layer of constant
temperature and salinity is usually
found in the top 1–100 meters of
the ocean.
• The depth is determined by wind
speed and the flux of heat
through the sea surface.
• The mixed layer is roughly over
most of the tropical and mid
latitude belts
• The mixed layer also tends to be
saltier than the deeper layers
except at high latitudes
• Below the mixed layer, water
temperature rapidly decreases
with depth
 Electrical Conductivity
• The conductivity of sea water depends on the number of dissolved
ions per volume (i.e. salinity) and the mobility of the ions (ie
temperature and pressure).
• Its units are mS/cm (milli-Siemens per centimetre).
• Conductivity increases by the same amount with a salinity increase of
0.01, a temperature increase of 0.01°C, and a depth (ie pressure)
increase of 20 m.
• In most practical oceanographic applications the change of
conductivity is dominated by temperature.
 Density
• Density is one of the most important parameters in the study of the oceans'
dynamics.
• Small horizontal density differences can produce very strong currents.
• The density of sea water depends on temperature T, salinity S and pressure p.
This dependence is known as the Equation of State of Sea Water.
• The equation of state for an ideal gas was is given by

where R is the gas constant. Seawater is not an ideal gas, but over small
temperature ranges it comes very close to one. The exact equation for the entire
range of temperatures, salinities and pressures encountered in the ocean

(where S is salinity) is the result of many careful laboratory determinations.

• The first fundamental determinations to establish the equation were
made in 1902 by Knudsen and Ekman. Their equation expressed in g
cm-3.
• New fundamental determinations, based on data over a larger
pressure and salinity range, resulted in a new density equation,
known as the "International Equation of State (1980) ". This equation
uses temperature in °C, salinity from the Practical Salinity Scale and
pressure in dbar (1 dbar = 10,000 pascal = 10,000 N m-2) and gives
density in kg m-3.
• Thus, a density of 1.025 g cm-3 in the old formula corresponds to a
density of 1025 kg m-3 in the International Equation of State.
• Oceanographers usually use the symbol (the Greek letter sigma
with a subscript t) for density, which they pronounce "sigma-t".
• Density increases with an increase in salinity and a decrease in
temperature
• The density maximum is above the freezing point for salinities below
24.7 but below the freezing point for salinities above 24.7. This
affects the thermal convection:
• S < 24.7: The water cools until it reaches maximum density; then,
when the surface water becomes lighter (ie after the density
maximum has been passed) cooling is restricted to the wind-mixed
layer, which eventually freezes over. The deep basins are filled with
water of maximum density.
• S > 24.7: Convection always reaches the entire water body. Cooling
is slowed down because a large amount of heat is stored in the water
body. This is because the water reaches freezing point before the
maximum density is attained.