properties of sea water

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Topic of the lessonProperties of Seawater

Properties of SeawaterProperties of SeawaterByProf.A. BalasubramanianCentre for Advanced Studies in Earth ScienceUniversity 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

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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 lightb) They store the heatc) They transport the stored heatd) They cause major changes in the climate systeme) They are the main source of atmospheric water vapourf) 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 seawater2. Chemical composition of seawater3. Biological Conditions of seawater4. Temperature distribution in oceans5. Salinity and density of oceans.

Seawater in hydrosphere Seawater in hydrosphere

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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 hydrosphere Seawater 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

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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 Capacity High 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

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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 Waves Transmitting 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

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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 Ions Six 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 (SO42-) 2649 mg/L

Magnesium (Mg2+) 1262 mg/L

Overall Chemistry Overall Chemistry

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Calcium (Ca2+) 400 mg/LPotassium (K+) 380 mg/LBicarbonate (HCO3

-) 140 mg/LStrontium (Sr2+) 13 mg/LBromide (Br-) 65 mg/L Borate (BO3

3-) 26 mg/L.


(…Contd)

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Fluoride (F-) 1 mg/LSilicate (SiO3

2-) 1 mg/L Iodide (I-) <1 mg/LTotal dissolved solids (TDS) of seawater is 34483

mg/L. It is also expressed in parts per million (ppm).


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Minor Elements Minor Elements

With reference to the other elements, Bromine 67.3 mg/lInorganic carbon is 28 mg/L Nitrogen 15.5 mg/LFluoride is 13 mg/LStrontium 8.1 mg/L

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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 Elements Minor 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 Salt Sea Salt

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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 salts Main 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 0.666. NaHCO3 (Sodium bicarbonate) 0.192

7. KBr 0.0968. 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 Time Residence Time

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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 Time Residence 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 Contributors Major Contributors

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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 Contributors Major Contributors

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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 Gases Addition Of Gases

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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.

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Solubility of Gases in Seawater Solubility of Gases in Seawater

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Solubility of Gases in Seawater Solubility 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

Carbondioxide CO2

0.03% 15.1% 40 90

Argon 1% 1.4% . 0.4

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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.

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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).


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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 gases Less soluble gases

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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 gases Less 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 Seawater pH of Seawater

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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 Seawater pH 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.

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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 Productivity Primary Productivity

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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 Productivity Primary 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 Phosphorous Nitrogen 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

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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

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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 Oxygen Dissolved Oxygen

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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.


(…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.


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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

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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.


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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.


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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.


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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 Temperature Extreme Temperature

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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 Temperature Extreme 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 Layers Thermal Layers

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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 Layers Thermal 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

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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

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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.


(…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.


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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 Seawater Life in Seawater

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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).


(…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.


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Zones of Salinity Zones of Salinity Based on the salinity, the oceanic water masses are classified into various zones: Thalassic series>300 hyperhaline60 – 80 metahaline40 mixoeuhaline30 polyhaline18 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 Seas Types 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.

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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 Circulation Water 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 Salinity Changes 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.


(…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.


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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 Salinity High Salinity

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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 Salinity High 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.

Density Density

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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.

Density Density

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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.

Density Density

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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.

Density Density

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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 −

Density Density

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Properties of Seawater Properties of Seawater

At 4°C and with the salinity of 35, the density of σseawater 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.

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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 Seawater Properties 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 Properties Interdependent Properties

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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 Properties Interdependent 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

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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.


(…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.


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