Seas and oceans are very huge bodies of saline waters. Their distribution and dynamics are very influential in several ways. Understanding the properties of seawater is inevitable in oceanographic studies. Seawater is one of the most fascinating and plentiful substances on the planet. The basic properties of seawater and their distribution, the interchange of properties between sea and atmosphere or land, the transmission of energy within the sea, and the geochemical laws which are governing the composition of seawater and marine sediments, are the fundamental aspects studied in the subject oceanography.
Pests of wheat_Identification, Bionomics, Damage symptoms, IPM_Dr.UPR.pdf
Properties of sea water
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Topic of the lesson
Properties of Seawater
Properties of SeawaterProperties of Seawater
By
Prof.A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore, India
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The chemical composition of seawater is an essential
topic in oceanography.
After attending this module, the user would be able
to know about the properties of seawater, their
variations and distribution in the oceans.
The role of seawater in controlling the marine
ecosystems and life are also highlighted in this
module.
Objectives
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All water existing at or near the surface of the Earth
belongs to the hydrosphere. It includes atmospheric
water vapor, groundwater, lakes, rivers, polar icecaps
and the oceans.
The waters of the oceans and seas cover more than 70
percent of the Earth’s surface. The water that is most
often found in nature is the seawater. It is about 98%,
existing on the globe as seas and oceans. The rest is
distributed as ice, water vapor, and fresh water on
land.
Introduction
(…Contd)
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Oceanic waters play a very important role in
controlling the global weather and climate, interactions
between atmosphere and the hydrosphere and
maintaining water balance of the globe.
Oceans also provide enormous living and non-living
resources for many life forms to survive.
Seawater has a very unique chemistry, physics and
biology.
(…Contd)
Introduction
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Seawater is one of the most fascinating and plentiful
substances on the planet.
The basic properties of Seawater and their
distribution, the interchange of properties between
sea and atmosphere or land, the transmission of
energy within the sea, and the geochemical laws
which are governing the composition of seawater and
marine sediments, are the fundamental aspects
studied in the subject of oceanography.
Introduction
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The key roles of the oceans are:
a) They absorb and reflect sun light
b) They store the heat
c) They transport the stored heat
d) They cause major changes in the climate system
e) They are the main source of atmospheric water vapour
f) They exchange gases (e.g. CO2) with the atmosphere.
Key Roles of the OceansKey Roles of the Oceans
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To know the role of seawater in the earth’s
hydrosphere, the following aspects are to be
understood first:
1. Physical properties of seawater
2. Chemical composition of seawater
3. Biological Conditions of seawater
4. Temperature distribution in oceans
5. Salinity and density of oceans.
Seawater in hydrosphereSeawater in hydrosphere
(…Contd)
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Seas and oceans are very huge bodies of saline
waters.
Their distribution and dynamics are very influential
in several ways.
Understanding the properties of seawater is
inevitable in oceanographic studies.
Seawater in hydrosphereSeawater in hydrosphere
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The waters of the seas and oceans has formed over
millions of years. Most people do not realize the
complex nature of seawater.
In fact seawater cannot be duplicated in any lab
anywhere in the world.
Seawater has its own physical, chemical and biological
properties.
Physical Properties of SeawaterPhysical Properties of Seawater
(…Contd)
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Due to its huge volume and thickness, it has certain
unique characteristics in the distribution of
temperature, pressure and density.
Most of these properties vary horizontally and
vertically.
Physical properties also act as limiting factors in
marine ecosystems.
Physical Properties of SeawaterPhysical Properties of Seawater
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Water, in general, is a good solvent. Seawater is an
effective solvent.
Seawater is also at the receiving end to dissolve all
the sediments derived from land.
Rivers carry much of the dissolved organic and
inorganic substances towards the sea.
These loads increase the salts of the oceans every
year. It has many conservative and non-conservative
properties.
An Efficient SolventAn Efficient Solvent
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Seawater has several unique properties like
high heat capacity
latent heat of fusion (LHF)
latent heat of vaporization
latent heat of melting
thermal expansion
density
viscosity and turbidity.
High Heat CapacityHigh Heat Capacity
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Water has the capacity to store heat, conduct heat
and release heat.
The heat capacity of seawater is the highest of all
solids and liquids except liquid ammonia. The heat
transfer in oceanic currents is large.
The latent heat of fusion (LHF) is also the highest in
seawater except ammonia. Hence, it acts as a
thermostat at freezing point owing to uptake or
release of latent heat.
High LHFHigh LHF
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Latent Heat of EvaporationLatent Heat of Evaporation
The latent heat of evaporation is yet another
property, which is also the highest in seawater than
other substances. It is important in heat and water
transfers to the atmosphere.
Thermal expansion is another important property of
seawater. The temperature of maximum density
decreases with increasing salinity. For pure water it is
at 4 deg. C.
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Seawater is characterized by its surface tension.
It is the highest among all liquids.
Seawater is colorless in small volumes.
Due to the presence of organic life and sediment
loads near the coasts, it may look greenish blue or
turbid in some places.
High Surface TensionHigh Surface Tension
(…Contd)
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Blue is the longest wavelength of the colors of the
spectrum. Since it is the last one to be absorbed by
the ocean, it is the most dominant color reflected.
When descending into the sea, the colors of the
spectrum begin to be filtered out. The first color
to disappear is red.
High Surface TensionHigh Surface Tension
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In addition to these, seawater also transmits sound.
The speed of sound travelling in Seawater is also a
special feature.
It is about 1500 m per second and some low
frequencies travel for long distances also. Hence, it
is possible to analyze the depth of the seas and
oceans using sound waves.
Transmitting Sound WavesTransmitting Sound Waves
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Seawater is a complex mixture of water, salts and
many other organic and inorganic substances.
Seawater contains more dissolved ions than all other
types of water like river water, rainwater, lake water
and groundwater.
It contains 96.5 percent water, 2.5 percent salts, and
smaller amounts of other substances, including
dissolved inorganic and organic materials,
particulates, and a few atmospheric gases.
Chemical Composition of SeawaterChemical Composition of Seawater
(…Contd)
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The chemical composition and ratios of the
minerals and naturally occurring elements are too
complex to accurately replicate.
The chemical constituents of seawater include major
ions and minor trace elements. In addition, Seawater
contains the suspended solids, organic substances,
and dissolved gases.
Chemical Composition of SeawaterChemical Composition of Seawater
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Seawater chemistry shows 96 percent water and only
4 percent other elemental composition.
Oxygen alone is 85.84%
Hydrogen is 10.82%
Chloride is 1.94%
Sodium is 1.08%
Magnesium is 0.12%.
(…Contd)
Elemental CompositionElemental Composition
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Sulfur is 0.09 %
Calcium is 0.04%
Potassium is 0.04 %
Bromine is 0.0067%
Carbon is 0.0028%.
Elemental CompositionElemental Composition
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Because of these, seawater is dominated by six
most abundant ions like chloride (Cl-
), sodium
(Na+
), sulfate (SO2
4
-
), magnesium (Mg2+
), calcium
(Ca2+
), and potassium (K+
).
By weight these ions make up to about 99 percent
of all sea salts.
Six Abundant IonsSix Abundant Ions
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When we analyse seawater, the major ion
composition of seawater will be invariably showing
the following composition in mg/L.
Ions Concentration in seawater in mg/L
Chloride (Cl-
) 18980 mg/L
Sodium (Na+
) 10556 mg/L
Sulfate (SO4
2-
) 2649 mg/L
Magnesium (Mg2+
) 1262 mg/L
Overall ChemistryOverall Chemistry
(…Contd)
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Fluoride (F-
) 1 mg/L
Silicate (SiO3
2-
) 1 mg/L
Iodide (I-
) <1 mg/L
Total dissolved solids (TDS) of seawater is 34483
mg/L.
It is also expressed in parts per million (ppm).
Overall ChemistryOverall Chemistry
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Minor ElementsMinor Elements
With reference to the other elements,
Bromine 67.3 mg/l
Inorganic carbon is 28 mg/L
Nitrogen 15.5 mg/L
Fluoride is 13 mg/L
Strontium 8.1 mg/L
(…Contd)
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Boron 4.45 mg/l
Silicon 2.9 mg/L
Iodine 0.064mg/L
Rubidium 0.12 mg/L
Barium 0.021 mg/L and
Uranium is 0.0033 mg/L in seawater.
Minor ElementsMinor Elements
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Sea salt is produced by evaporating seawater, but this
is not a straight-forward method.
Between 100% and 50%, first the calcium carbonate
(CaCO3= limestone) gets precipitated and it is
followed by the crystallization of gypsum between
50% and 20% water.
Between 20% and 1%, sea salt precipitates (NaCl) but
going further, the bitter potassium and magnesium
chlorides and sulfates precipitates.
Sea SaltSea Salt
(…Contd)
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The amount of these salts in a volume of seawater
varies because of the addition or removal of water
locally (e.g., through precipitation and evaporation).
The salt content in seawater is indicated by salinity
(S), which is defined as the amount of salt in grams
dissolved in one kilogram of seawater and expressed
in parts per thousand(ppt).
Sea SaltSea Salt
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Main saltsMain salts
Main salts/elements present in oceans are in parts per trillion:
1. NaCl (halite) 23.48 (about 2.35%)
2. MgCl 2 4.98 (about 0.50%)
3. Na2SO4 3.92
4. CaCl2 1.10
5. KCl 0.66
6. NaHCO3 (Sodium bicarbonate) 0.192
7. KBr 0.096
8. H3BO3 (Hydrogen borate) 0.026.
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Salinities in the open ocean have been observed to
range from about 34 to 37 parts per thousand.
It is denoted as 3.5%.
All major ions of seawater are conservative.
Though their salt contents vary from one place to
the other, their ratios do not change.
Residence TimeResidence Time
(…Contd)
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All dissolved materials have residence times varying
from hours to millions of years.
Nitrate has the lowest residence time and sodium
has the highest residence time in seawater.
Residence TimeResidence Time
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Weathering of rocks and minerals and salts introduced
into the streams feed into the oceans as contributors.
The total quantity of this is on the order of 2.5 billion
tons per year.
The minerals and salts are also derived from volcanic
eruptions, known as outgassing from Earth's interior.
Major ContributorsMajor Contributors
(…Contd)
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We know that this is true because certain
elements like Chlorine, Bromine Sulphur and
Boron are more abundant in oceans than in
Earth's crust.
Major ContributorsMajor Contributors
(…Contd)
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The Hard parts of marine organisms also
contribute some salts to accumulate. (i.e., shell
material). Of the many minor dissolved chemical
constituents, inorganic phosphorus and inorganic
nitrogen are among the most notable, since they
are important for the growth of organisms that
inhabit the oceans and seas.
Major ContributorsMajor Contributors
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At the ocean-atmosphere interface, gases are added to
Seawater by diffusion and wave action.
Hence, Seawater contains various dissolved
atmospheric gases, chiefly nitrogen, oxygen, argon, and
carbon dioxide.
Gas composition at the ocean surface is in equilibrium
with atmosphere. Gas composition changes with time
and depth in the oceans.
Addition Of GasesAddition Of Gases
(…Contd)
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As salinity increases, the amount of gas dissolved
decreases because more water molecules are
immobilsed by the salt ion.
Gases like oxygen and CO2 are influenced by the
sea life.
Addition Of GasesAddition Of Gases
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Plants reduce the concentration of CO2 in the
presence of sunlight, whereas, animals do the
opposite in wither light or darkness.
The solubility of gases in seawater is controlled by the
temperature, salinity and pressure.
All gases are less soluble as temperature increases.
When water is warmed, most of the gas bubbles leave
the liquid.
(…Contd)
Solubility of Gases in SeawaterSolubility of Gases in Seawater
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Solubility of Gases in SeawaterSolubility of Gases in Seawater
As water temperature increases, the increased mobility of gas
molecules makes them escape from the water, thereby
reducing the amount of gas involved. This is shown in the
following table.
Gas
molecule
% in
atmosphere
% in surface
Seawater
ml/litre
Seawater
mg/kg (ppm)
in Seawater
Nitrogen N2 78% 47.5% 10 12.5
Oxygen O2 21% 36.0% 5 7
Carbondioxi
de CO2
0.03% 15.1% 40 90
Argon 1% 1.4% . 0.4
(…Contd)
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One kg of fresh water contains 55.6 mol H2O.
Nitrogen and argon are the inert gases which do not
take part in the processes of life. Hence, they are not
affected by plant and animal life.
But non-conservative gases like oxygen and
carbondioxide are influenced by sea life.
(…Contd)
Solubility of Gases in SeawaterSolubility of Gases in Seawater
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Plants reduce the concentration of carbondioxide in
the presence of sunlight, whereas animals do the
opposite in either light or darkness.
In the above table, the conservative gases nitrogen
and argon do not contribute to life processes, even
though nitrogen gas can be converted by some
bacteria into fertilizing nitrogen compounds (NO3,
NH4).
Solubility of Gases in SeawaterSolubility of Gases in Seawater
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All gases are less soluble as temperature increases,
particularly nitrogen, oxygen and carbondioxide which
become about 40-50% less soluble with an increase of
25ºC.
When water is warmed, it becomes more saturated,
eventually resulting in bubbles leaving the liquid.
Less soluble gasesLess soluble gases
(…Contd)
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Some other components of seawater are the dissolved
organic substances, such as carbohydrates and amino
acids, and organic-rich particulates.
These materials originate primarily in the upper 100 m
(300 feet) of the ocean, where dissolved inorganic
carbon is photosynthetically transformed into organic
matter.
Less soluble gasesLess soluble gases
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pH is another important property of seawater.
It is reflected in the form of acidity and alkalinity.
This distribution of ocean acidity shows that ocean
pH varies from about 7.90 to 8.20.
pH of SeawaterpH of Seawater
(…Contd)
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The Seawater pH is the lowest in most productive
regions where upwelling occurs.
It is also thought that the average acidity of the oceans
decreased from 8.25 to 8.14 since the advent of fossil
fuel in the world according to scientific observations.
pH of SeawaterpH of Seawater
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Biological Conditions of SeawaterBiological Conditions of Seawater
Biological oceanographers study all forms of life in
the oceans, from microscopic plants and animals to
fish and whales.
In addition, biological oceanographers examine all
forms of oceanic processes that involve living
organisms.
(…Contd)
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Biological Conditions of SeawaterBiological Conditions of Seawater
These include processes that occur at molecular
scales, such as photosynthesis, respiration, and
cycling of essential nutrients, to large scale
processes such as effects of ocean currents on
marine productivity.
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Many biological properties and processes control the
abundances and distributions of life in oceans.
In oceans the sun’s energy is transformed into organic
matter and is also used by living organisms.
The abundances of various constituents affect the life
and abundances as well.
Marine organisms exchange matter and energy with
each other and with the waters around them.
Primary ProductivityPrimary Productivity
(…Contd)
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Photosynthesis, chemosynthesis, primary productivity
and nutrient cycling are the major processes helpful in
marine life survival and maintain the ecology of
oceans.
The availability of light controls the plant growth and
phytoplankton distributions in the ocean.
Primary ProductivityPrimary Productivity
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Phosphorous and nitrogen compounds are
necessary for phytoplankton growth.
These two compounds are more abundant in deep
waters than the near surface waters, as they are
removed by plant growth near the surface.
Nitrogen and PhosphorousNitrogen and Phosphorous
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Seawater is dominated by much amount of dissolved
organic matter.
Dissolved organic matter remains in the ocean for
very long periods of time.
May be many thousands of years. It is roughly equal
to the abundance of living matter on earth.
Dissolved Organic MatterDissolved Organic Matter
(…Contd)
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There are many sources of organic matter.
Decomposition of dead plant and animal is one
source.
Secretion of organic compounds by living plants is
also another source.
Dissolved Organic MatterDissolved Organic Matter
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The relative abundance of various forms of organic
matter in seawater are:
Dissolved organic matter 95%
Particulate organic matter 5 %
Phytoplankton 0.1%
Zooplankton 0.01 %
Fishes 0.0001 %.
Dissolved organic matter enters food webs primarily
through tiny bacteria.
Forms of Organic MatterForms of Organic Matter
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Particles dispersed in seawater also influence the
chemical and biological behavior of Seawater.
The total amount of particles in the oceans is about
10000 million tons.
Biological particles are relatively large ranging from
1 micrometer to 1 mm.
These constitute upto 70% of the particulate matter
in the ocean.
Particles Dispersed In SeawaterParticles Dispersed In Seawater
(…Contd)
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Particles are also destroyed either mechanically or
chemically.
These dissolved particles release nutrients, silica and
metals.
Hence, the chemistry and biology of seawater is fully
influenced.
Depositions of sediments are also controlled by these
masses.
Particles Dispersed In SeawaterParticles Dispersed In Seawater
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Oxygen Dissolved In SeawaterOxygen Dissolved In Seawater
Oxygen dissolved in seawater participates in both
biological and chemical processes of oceans. It comes
from the atmosphere.
When nutrients concentrations are high, dissolved
oxygen concentrations are low.
The oxygen dissolves by diffusion from the surrounding
air; aeration of water that has tumbled over falls and
rapids; and as a waste product of photosynthesis.
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Fish and aquatic animals cannot split oxygen from water
(H2O) or other oxygen-containing compounds.
How much Dissolved Oxygen an aquatic organism needs
depends upon its species, its physical state, water
temperature, pollutants present, and more.
Numerous scientific studies suggest that 4-5 parts per
million (ppm) of Dissolved Oxygen is the minimum amount
that will support a large, diverse fish population.
Dissolved OxygenDissolved Oxygen
(…Contd)
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The Dissolved Oxygen level in good fishing waters
generally averages about 9.0 parts per million (ppm).
Adequate dissolved oxygen is necessary for good
water quality.
Oxygen is a necessary element to all forms of life.
Total dissolved gas concentrations in water should not
exceed 110 percent.
Dissolved OxygenDissolved Oxygen
(…Contd)
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Concentrations above this level can be harmful to
aquatic life.
Fish in waters containing excessive dissolved gases
may suffer from "gas bubble disease".
When an organism dies and decomposes, most of its
organic molecules end up in solution as Dissolved
Organic Carbon (DOC), molecules that are very
much smaller than the smallest of organisms which
are viruses.
Dissolved OxygenDissolved Oxygen
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Oceans are considered to be the carbon reservoirs.
It is evident based on the following statistics:
Carbon ReservoirsCarbon Reservoirs
Carbon reservoir Percentage
CO2 dissolved in oceans 87.5
Dissolved Organic Carbon
(DOC) in oceans
7.1
Biosphere, all living organisms 4.0
Atmospheric CO2 1.4
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Ocean is the major recipient of the sun’s radiant energy.
It has the capacity to store heat.
Many physical processes depend on temperature of
waters. In oceanographic studies, platinum-resistance
thermometers are used to measure the temperature of
waters.
Temperature Distribution in OceansTemperature Distribution in Oceans
(…Contd)
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The distribution of temperature in the surface of the
oceans follows a zonation pattern. It is independent
of longitude.
The warmest water is near the equator and the
coldest water is near the polar regions.
Heat fluxes, evaporation, rain, river water inflow,
freezing and melting of ice caps, all influence the
distribution of temperature in the oceans.
Temperature Distribution in OceansTemperature Distribution in Oceans
(…Contd)
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Changes in temperature and salinity can increase or
decrease the density of ocean waters at the surface
which can lead to convection.
The temperature of the world's ocean is highly
variable over the surface of the ocean, ranging from
less than 0°C near the poles to more than 29°C in the
tropics.
Temperature Distribution in OceansTemperature Distribution in Oceans
(…Contd)
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The maximum surface temperature of course depends
on many factors, like latitude and season.
It is heated from the surface downward by sunlight,
but at depth most of the ocean is very cold.
Seventy-five percent of the water in the ocean falls
within the temperature range of 1 to +6°C and the−
salinity range of 34 to 35.
Temperature Distribution in OceansTemperature Distribution in Oceans
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Under the enormous pressures of the deep ocean,
seawater can reach very high temperatures without
boiling.
A water temperature of 400 degrees C has been
measured at one hydrothermal vent.
The average temperature of all ocean water is about
3.5° C.
Extreme TemperatureExtreme Temperature
(…Contd)
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The variations in total salinity and in temperature cause
variations in the density of seawater also.
Several biotic and abiotic factors can cause the salinity to
deviate from the common value of 35.
The inflow of river water and rainwater decreases the
salinity.
Excess evaporation or formation of pack ice causes the
salinity to increase.
Extreme TemperatureExtreme Temperature
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Three general layers are present, except in Polar
Regions where only one or two layers are present
because of coldness:
1. Shallow surface mixed zone (2%): this is the warmest
zone made from solar energy, mixed by waves,
around 500m in thickness and the most saline zone.
Thermal LayersThermal Layers
(…Contd)
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2. Transition zone (18%): this zone includes thermocline,
which is the point of great drop-off in temperature
existing below 3000m and halocline, which is the point of
salinity drop off, which is roughly corresponding to the
thermocline.
3. Deep zone (80%): located just above or below
freezing point. This is not a saline zone.
Thermal LayersThermal Layers
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Salinity and Density of OceansSalinity and Density of Oceans
Ionic Concentration is the amount (by weight) of salt
present in water and can be expressed in parts per million
(ppm). The classification of water based on total salt content
is:
Fresh water - less than 1,000 ppm
Slightly saline water - From 1,000 ppm to 3,000 ppm
Moderately saline water - From 3,000 ppm to 10,000 ppm
Highly saline water - From 10,000 ppm to 35,000 ppm
Ocean water has a salinity that is approximately 35,000
ppm.
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Salinity is the saltiness or dissolved salt content of a body
of water.
It is a general term used to describe the levels of
different salts such as sodium chloride, magnesium and
calcium sulfates, and bicarbonates. Salinity in the ocean
refers to the water's "saltiness".
In oceanography, it has been traditional to express
salinity not as percent, but as parts per thousand (‰),
which is approximately grams of salt per kilogram of
solution.
Salinity is the SaltinessSalinity is the Saltiness
(…Contd)
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Salinity is the SaltinessSalinity is the Saltiness
In the other disciplines, people use the chemical
analyses of solutions directly, and thus salinity is
frequently reported in mg/L or ppm (parts per
million).
All over the globe and from the top of the ocean all
the way to the bottom of the ocean, salinity is
between 33-37 parts per trillion.
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In 1978, oceanographers redefined salinity with a new
property known as Practical Salinity Scale (PSS).
It is the conductivity ratio of a Seawater sample to a
standard KCl solution.
As PSS is a ratio, it has no units. It is not the case that
a salinity of 35 exactly equals 35 grams of salt per litre
of a salt solution.
Practical Salinity ScalePractical Salinity Scale
(…Contd)
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Some scientists estimate that the oceans contain as
much as 50 quadrillion tons of dissolved solids.
If the salt in the ocean could be removed and spread
evenly over the Earth’s land surface it would form a
layer more than 166 m thick, about the height of a
40-story office building.
Practical Salinity ScalePractical Salinity Scale
(…Contd)
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Salinity affects marine organisms because the process
of osmosis transports water towards a higher
concentration through cell walls.
A fish with a cellular salinity of 1.8% will swell in fresh
water and dehydrate in salt water. So, saltwater
fishes drink water copiously while excreting excess
salts through their gills.
Practical Salinity ScalePractical Salinity Scale
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Freshwater fish do the opposite by not drinking but
excreting copious amounts of urine while losing little
of their body salts.
Marine plants (seaweeds) and many lower organisms
have no mechanism to control osmosis, which makes
them very sensitive to the salinity of the water in
which they live.
Life in SeawaterLife in Seawater
(…Contd)
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The main nutrients for plant growth are nitrogen (N
as in nitrate NO3-
, nitrite NO2-
, ammonia NH4+
),
phosphorus (P as phosphate PO43-
) and potassium (K)
followed by Sulfur (S), Magnesium (Mg) and Calcium
(Ca).
Iron (Fe) is an essential component of enzymes and is
copiously available in soil, but not in seawater
(0.0034ppm).
Life in SeawaterLife in Seawater
(…Contd)
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This makes iron an essential nutrient for plankton
growth.
Plankton organisms (like diatoms) that make shells of
silicon compounds furthermore need dissolved
silicon salts (SiO2) which at 3ppm can be rather
limiting.
Life in SeawaterLife in Seawater
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Zones of SalinityZones of Salinity
Based on the salinity, the oceanic water masses are
classified into various zones:
Thalassic series
>300 hyperhaline
60 – 80 metahaline
40 mixoeuhaline
30 polyhaline
18 mesohaline
5 oligohaline
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Marine waters are those of the ocean, another term for
which is euhaline seas.
The salinity of euhaline seas is 30 to 35. Brackish seas or
waters have salinity in the range of 0.5 to 29 and
metahaline seas from 36 to 40.
These waters are all regarded as thalassic because their
salinity is derived from the ocean and defined as
homohaline if salinity does not vary much over time
(essentially constant).
Types of SeasTypes of Seas
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Salinity is an Ecological FactorSalinity is an Ecological Factor
The ocean salinity at the surface is high and then salinity
decreases until a depth of about 1,000 meters. Salinity
then increases again slightly with increasing depth.
The halocline is a layer of water where the salinity changes
rapidly with depth.
Salinity is an ecological factor of considerable importance,
influencing the types of organisms that live in a body of
water.
(…Contd)
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Salinity is an Ecological FactorSalinity is an Ecological Factor
As well, salinity influences the kinds of plants that will
grow either in a water body, or on land fed by water (or
by a groundwater).
A plant adapted to saline conditions is called a halophyte.
Organisms (mostly bacteria) that can live in very salty
conditions are classified as extremophiles, halophiles
specifically.
An organism that can withstand a wide range of salinities is
euryhaline.
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The degree of salinity in oceans is a driver of the
world's ocean circulation, where density changes due
to both salinity changes and temperature changes at
the surface of the ocean produce changes in
buoyancy, which cause the sinking and rising of water
masses.
Water CirculationWater Circulation
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Changes in the salinity of the oceans are thought to
contribute to global changes in carbon dioxide as more
saline waters are less soluble to carbon dioxide.
Salinity affects ocean organisms because the process of
osmosis transports water towards a higher
concentration through cell walls.
Fish with a cellular salinity of 1.8% will swell in fresh
water and dehydrate in salt water.
(…Contd)
Changes in SalinityChanges in Salinity
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Saltwater fish drink water copiously while excreting
excess salts through their gills.
Freshwater fish do the opposite by not drinking but
excreting copious amounts of urine while losing little
of their body salts.
Marine plant life (seaweeds) and many lower
organisms have no mechanism to control osmosis,
which makes them very sensitive to the salinity of the
water in which they live.
Changes in SalinityChanges in Salinity
(…Contd)
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This world map shows how the salinity of the oceans
changes slightly from around 32ppt (3.2%) to 40ppt
(4.0%).
Low salinity is found in cold seas, particularly during
the summer season when ice melts.
Changes in SalinityChanges in Salinity
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High salinity is found in the ocean 'deserts' in a band
coinciding with the continental deserts.
Lowest salinity is found in the upper reaches of the
Baltic Sea (0.5%).
The Dead Sea is 24% saline, containing mainly
magnesium chloride MgCl2.
Shallow coastal areas are 2.6-3.0% saline and estuaries
0-3%.
High SalinityHigh Salinity
(…Contd)
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The density of a water sample is a measure of the
total mass in a given unit volume.
The density of fresh water is 1.00 (gram/ml or
kg/liter) but added salts can increase this.
The saltier the water, the higher the density.
When water warms, it expands and becomes less
dense.
High SalinityHigh Salinity
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The colder the water, the denser it becomes. So it is
possible that warm salty water remains on top of
cold, less salty water.
The density of 35ppt saline seawater at 15ºC is about
1.0255, or s (sigma)= 25.5. Another word for density
is specific gravity.
The deep ocean is layered with the densest water on
bottom and the lightest water on top.
DensityDensity
(…Contd)
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Circulation in the depths of the ocean is horizontal.
That is, water moves along the layers with the same
density.
The density of ocean water is rarely measured
directly.
DensityDensity
(…Contd)
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Salinity increases the density because the dissolved
salts are contained in the same volume as the water.
Cold seawater is denser than warm seawater.
There are several areas at the ocean surface where
the surface water becomes very cold.
DensityDensity
(…Contd)
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Density differences among different water masses
allow physical oceanographers to calculate the
movements of water in the oceans.
Water molecules cluster more closely around positive
and negative ions in solution in a process called
electrostriction, which also serves to increase sea-
water density.
DensityDensity
(…Contd)
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Density of water in the ocean, reported as
sigma t (σ t) is calculated from temperature,
salinity and pressure by using the equation of
state for seawater: σ t = ( 1) × 1,000.σ −
DensityDensity
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Properties of SeawaterProperties of Seawater
At 4°C and with the salinity of 35, the density ofσ
seawater is 1.02781 gram per cubic centimeter.
At depth, pressure from the overlying ocean water
becomes very high (pressure at 4,000 meters is about
400 atmospheres), but water is only slightly
compressible, so that there is only a minor pressure
effect on density.
(…Contd)
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At a depth of 4,000 meters, water decreases in
volume only by 1.8 percent.
Although the high pressure at depth has only a slight
effect on the water, it has a much greater effect on
easily compressible materials.
Properties of SeawaterProperties of Seawater
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The relationship between temperature, salinity and
density is shown by the blue isopycnal (of same density)
curves in this diagram.
In red, green and blue the waters of the major oceans of
the planet is shown for depths below -200 metre.
The Pacific has most of the lightest water with densities
below 26.0, whereas the Atlantic has most of the densest
water between 27.5 and 28.0.
Interdependent PropertiesInterdependent Properties
(…Contd)
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Antarctic bottom water is indeed densest for Pacific and
Indian oceans but not for the Atlantic which has a lot of
similarly dense water.
The density of ocean water varies. It becomes more dense
as it becomes colder, right down to its freezing point of
-1.9 degrees C.
The density of pure water is 1000 kg/m3
. Ocean water is
more dense because of the salt in it. Density of ocean
water at the sea surface is about 1027 kg/m3
.
Interdependent PropertiesInterdependent Properties
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The composition of seawater is affected by many
different chemical and physical transport
mechanisms.
The Dissolved substances and particulates are
regularly added to the oceans by rivers.
The particulates may be transported by the wind to
mid-ocean regions. Many chemical substances are
also added to deep ocean waters by hydrothermal
solutions.
ConclusionConclusion
(…Contd)
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Seawater is a rich source of various commercially
important chemical elements. Much of the world’s
magnesium is recovered from seawater.
In certain parts of the world, sodium chloride (table
salt) is still obtained by evaporating seawater. The
waters of the seas and oceans have formed over
millions of years.
ConclusionConclusion
(…Contd)
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Most people do not realize the complex nature of
seawater. In fact, the seawater cannot be easily
duplicated in any lab in any manner in the world.
The chemical composition and ratios of the minerals
and naturally occurring elements of seawater are too
complex to accurately replicate.
ConclusionConclusion