© 2006 Jones and Bartlett Publishers Chapter 5 The Properties of Seawater

© 2006 Jones and Bartlett Publishers

Chapter 5

The Properties of Seawater

• Atoms are the smallest unit which display all of the properties of the material.

• Atoms are composed of:– Nucleus - the center of the atom

consisting of:• positively charged particles (protons)• neutrally charged particles (neutrons)

– Electrons - negatively charged particles that orbit the nucleus in discrete electron shells.

5-1 Basic Chemical Notations

Figure 5-1 Atomic Structure

• Electrically stable atoms have the same number of electrons as protons.

• Ions are atoms with either more or fewer electrons than protons.

• Ions are electrically charged.


• Isotopes are atoms containing the same number of protons, but different numbers of neutrons.– Therefore, they have different atomic weights.

• Molecules are chemically-combined compounds formed by two or more atoms.


• Heat results from the vibrations of atoms (kinetic energy).

– It can be measured with a thermometer.

5-1 Basic Physical Notations

• In solids, the atoms or molecules:– vibrate weakly – are rigidly held in place

• In liquids, the atoms or molecules:– vibrate more rapidly– move farther apart – are free to move relative to each other

• In gases, the atoms or molecules:– are highly energetic– move far apart – are largely independent



Figure 5-2a Solid Water

• Melting is the transition from solid to liquid; freezing is the reverse.

• Evaporation (vaporization) is the transition from liquid to gas; condensation is the reverse.

• Temperature controls density. – As temperature increases:

• atoms or molecules move farther apart

• density (mass/volume) decreases because there is less mass (fewer atoms) in the same volume.


The water molecule is unique in structure and properties.

• H2O is the chemical formula for water.

• Unique properties of water include:– Higher melting and boiling point than other hydrogen

compounds.

– High heat capacity (amount of heat needed to raise the temperature of one gram of water 1oC).

– Greater solvent power than an other substance.

• Water molecules are asymmetrical is shape:– the two hydrogen molecules are at one end, separated by 105o

when in the gaseous or liquid phase and 109.5o when ice.

5-3 The Water Molecule

• Asymmetry of a water molecule and distribution of electrons result in a dipole structure.

• The oxygen end of the molecule is negatively charged.

• The hydrogen end of the molecule positively charged.

• Dipole structure of water molecule produces an electrostatic bond (hydrogen bond) between water molecules, which cluster together.


• The dipole structure of water makes it an exceptional solvent.

• Na+ and Cl- ions are removed from the halite crystal by the negative and positive ends of the water dipole.

5-3 The Water MoleculeFigure 5-4 Halite (Rock Salt)

• Ice floats in water because all of the molecules in ice are held in hexagons.– The center of the hexagon is open space,

making ice 8% less dense than water.

Figure 5-5a Hexagonal Crystal Structure of Ice Figure 5-5b Density of Water


• Water reaches its maximum density at 3.98oC.– Below this temperature increasing numbers of water

molecules form hexagonal polymers and decrease the density of the water.

– Above this temperature water molecules are increasingly energetic and move farther apart, thereby decreasing density.

• Hydrogen bonding is responsible for many of the unique properties of water.

• This is because energy is required to break the hydrogen bonds and separate the water molecules.


• Water dissolves salts by:– surrounding the atoms in the salt molecule – neutralizing the ionic bond holding the

molecule together.

• Dissolved salts form cations (positively charged ions) and anions (negatively charged ions).– The process of water surrounding an ion is

called hydration.


Seawater consists predominantly of water with various materials dissolved within it.

• The solvent is the material doing the dissolving.– In seawater it is the water.

• The solute is the material being dissolved.

• Salinity is the total amount of salts dissolved in the water.– It is measured in parts of salt per thousand parts of salt

water and is expressed as ppt (parts per thousand) or abbreviated as o/oo.

• Average salinity of the ocean is about 35 o/oo.


• 99% of all the salt ions in the sea are:– sodium (Na+) – chloride (Cl-) – sulfate (SO4

-2) – magnesium (Mg+2) – calcium (Ca+2) – potassium (K+)

• Sodium and chloride alone comprise about 86% of the salt in the sea.

• The major chemical constituents of seawater display little variation over time and are a conservative property of sea water.


Nutrients are chemicals essential for life.• Major nutrients in the sea are compounds of:

– phosphorus 0.07 ppm– nitrogen 0.5 ppm– Silicon 3 ppm

• Because of usage, nutrients are scarce at the surface and their concentrations are measured in parts per million (ppm).

• Concentration of nutrients vary greatly over time and because of this they are considered a nonconservative property of the sea.


• In order of decreasing abundance the major gases in the sea are:– nitrogen (N2)

– oxygen (O2)

– carbon dioxide (CO2)

– the noble gases:• argon (Ar)

• neon (Ne)

• helium (He)

• Nitrogen and the noble gases are considered to be inert because they are chemically non-reactive.


• Trace elements occur in minute quantities.– They are usually

measured in parts per million (ppm) or parts per billion (ppb).

• Even in small quantities they can be important for either promoting or killing life.


• Marine organic compounds occur in low concentrations and consist of large complex molecules, such as:– Fat– Proteins– Carbohydrates– Hormones – Vitamins

• These molecules are produced by organisms or through decay.


• Salinity is the total mass (in grams) of all substances dissolved in one kilogram of sea water when:

– all carbonate has been converted to oxide– all bromine and iodine has been replaced by

chlorine – all organic compounds have been oxidized at a

temperature of 480oC

5-4 Salinity

• The Principle of constant proportion states that:

The absolute amount of salt in sea water varies, but the relative proportions of the ions is constant.

• Because of this principle, it is necessary to test for only one salt ion, usually chloride, to determine the total amount of salt present.

5-4 Salinity

• Chlorinity is the amount of halogens in seawater and is expressed as grams/kilogram or o/oo.

• Halogens are:– chlorine– bromine– iodine– Fluorine

• Salinity is equal to 1.8065 times chlorinity.

• Salinometers determine salinity from the electrical conductivity produced by the dissolved salts.

5-4 Salinity

• Salinity in the ocean is in a steady-state condition.

• This is because the amount of salt added to the ocean (input from source) equals the amount removed (output into sinks).

5-4 Salinity

• Salt sources include:– weathering of rocks on land – the reaction of lava with seawater

• Weathering mainly involves the chemical reaction between rock and acidic rainwater.– Acid rain is produced by the interaction of carbon dioxide and rainwater,

which forms carbonic acid.

5-4 SalinityFigure 5-6 Sedimentary Cycle - Weathering

• Salt sinks include the following:– Wind-blown spray carries minute droplets of saltwater

inland.

– Adsorption of ions onto clays and some hydrogenous minerals.

– Shell formation by organisms.

• Evaporation removes only water molecules.– Remaining water becomes increasingly saline,

eventually producing a salty brine.

– If enough water evaporates, the brine becomes supersaturate and salt deposits begin to precipitate forming evaporite minerals.

5-4 Salinity

• Lack of similarity between relative composition of river water and the ocean is explained by residence time.– Residence time is average length of time that an ion

remains in solution in the ocean.

• Ions with long residence times tend to accumulate in the sea.

• Ions with short residence times are removed.

• Rapid mixing and long residence times explain constant composition of sea water.

5-4 Salinity

Addition of salt modifies the properties of water.• Pure water freezes at 0oC.

• Adding salt increasingly lowers the freezing point.– This is because salt ions interfere with the formation of the

hexagonal structure of ice.

• Density of water increases as salinity increases.

• Vapor pressure is the pressure exerted by the gaseous phase on the liquid phase of a material.

• It is proportional to the amount of material in the gaseous phase.

• Vapor pressure decreases as salinity increases because salt ions reduce the evaporation of water molecules.

5-4 Salinity

Effects of Salinity on the Maximum-Density and Freezing-Point Temperatures of Seawater

5-4 SalinityFigure 5-8

• Ocean surface temperature strongly correlates with latitude.– This is because insolation – the amount of sunlight

striking Earth’s surface – is directly related to latitude.

• Ocean isotherms – lines of equal temperature – generally trend east-west except where deflected by currents.

• Ocean currents carry warm water poleward on the western side of ocean basins.

• They carry cooler water equatorward on the eastern side of the ocean.

5-5 Chemical and Physical Structure of the Oceans

Sea Surface Temperatures• Insolation and ocean-surface water temperature vary with the season.

• Ocean temperature is highest in the tropics (25oC) and decreases poleward.

Figure 5-9a Sea-Surface Temperature in August


• Tropical and subtropical oceans are permanently layered with warm, less dense surface water separated from cold, dense deep water by a thermocline.– The thermocline is a layer in which water temperature

and density change rapidly.

• Temperate regions have a seasonal thermocline and polar regions have none.


Salinity changes with latitude due to variations in precipitation and evaporation

• Highest ocean salinity is between 20-30o north and south or the equator, because evaporation exceeds precipitation.

• Salinity at the equator and poleward of 30o is low because evaporation is less than precipitation.

Figure 5-12b Latitudinal Variations in Salinity and “Dryness”


• In some places surface water and deep water are separated by a halocline.

– The halocline is a zone of rapid change of salinity with water depth.

• Water stratification (layering) within the ocean is more pronounced between 40oN and 40oS.


Density of sea water is a function of temperature, salinity and pressure

• Density increases as temperature decreases and as salinity and pressure increase.

• Pressure increases regularly with depth, but temperature and salinity are more variable.


• Higher salinity water can rest above lower salinity water if:– the higher salinity water is sufficiently warm

– the lower salinity water is sufficiently cold

• Pycnocline is a layer within the water column where water density changes rapidly with depth.

Figure 5-14a Seawater Density


Thermocline, Halocline, and Pycnocline

Figure 5-14b


• The water column in the ocean can be divided into the:– surface layer

– pycnocline

– deep layer

Figure 5-14c Density Structure of the Oceans


The Surface Layer

• About 100m thick

• Comprises about 2% of the ocean volume

• Is the most variable part of the ocean because it is in contact with the atmosphere.

• Is less dense than the layers below because of its lower salinity or higher temperature.


The Pycnocline

• Is transitional between the surface and deep layers.• Comprises 18% of the ocean basin.

• In the low latitudes, the pycnocline coincides with the thermocline.

• In the mid-latitudes it coincides with the halocline.


The Deep Layer

• Represents 80% of the ocean volume.

• Water in the deep layer originates at the surface in high latitudes, where it:– cools– becomes dense– sinks to the sea floor– flows equatorward across the ocean basin


• The solubility and saturation value of gases in sea water increase as:– temperature decreases– salinity decreases– pressure increases

• Solubility is a measure of a substance's tendency to dissolve and go into solution.

5-6 Gases in SeawaterFigure 5-15a Solubility of Oxygen

• Saturation value is the equilibrium amount of gas dissolved in water at an existing temperature, salinity and pressure.

– Water is undersaturated when it has the capacity to dissolve more gas.

• This means that its gas content is below the saturation value.

– Water is saturated when it contains as much dissolved gas as it can hold in equilibrium.

• Gas content is at its saturation value.

– Water is supersaturated when it contains more dissolved gas than it can hold in equilibrium.

• Gas content is then above its saturation value and excess gas will come out of solution.

5-6 Gases in Seawater

• The surface layer is usually saturated in atmospheric gases because of direct gas exchange with the atmosphere.

• Below the surface layer, gas content reflects the relative importance of:– Respiration– Photosynthesis– Decay– Infusion from volcanic vents


Sources and Sinks of Gases

5-6 Gases in SeawaterFigure 5-15 Sources and Sinks of Gases

Table 5-9

Helium-3

Figure 5-16 Helium-35-6 Gases in Seawater

Volcanic emissions from the East Pacific Rise cause this large plume of helium-3.

• Oxygen tends to be abundant in the water of the surface layer and deep layer, and lowest in the pycnocline.

• Surface layer is rich in oxygen because of photosynthesis and diffusion from the atmosphere layer.

• Oxygen minimum layer occurs at about 150 to 1500m below the surface and coincides with the pycnocline.

5-6 Gases in SeawaterFigure 5-17 a Vertical O2 Profiles in the Atlantic Ocean

The Oxygen Minimum Layer

• Sinking food particles settle into this layer and are slowed down by the sharp density gradient.

• The food draws large numbers of organisms which respire, consuming oxygen.

• Decay of uneaten material consumes additional oxygen.

• Density difference prevents mixing downward of oxygen-rich water from the surface or upwards from the deep layer.


• The deep layer is rich in oxygen because its water is derived from cold surface waters that sank to the bottom.

• Oxygen consumption by respiration is low because there are fewer organisms in the deep layer.

• Anoxic waters contain no oxygen and are inhabited by anaerobic organisms (bacteria).


Carbon dioxide is of major importance in controlling the acidity of seawater

• Major sources of carbon dioxide are respiration and decay.

• Major sinks are photosynthesis and construction of carbonate shells.

5-6 Gases in SeawaterFigure 5-19b Photosynthesis and Respiration

Carbon dioxide controls the acidity of sea water

• A solution is acidic if it has excess H+ (hydrogen) ions

• A solution is basic if it has excess OH- (hydroxyl) ions.

• pH measures how acid or base water is.– - pH of 0 to 7 is acidic.

– - pH of 7 is neutral.

– - pH of 7 to 14 is basic.


The pH Scale

Figure 5-18 The pH Scale 5-6 Gases in Seawater

Carbon Species and the CO2 system in the oceans

• Dissolved carbon appears in water in 3 forms:– Carbon dioxide (CO2)

– Bicarbonate (HC03-)

– Carbonate (CO32-)

• Basic solutions are dominated by carbonate.

• Acidic solutions are dominated by carbon dioxide.


Distribution of Carbon Species in Water

Figure 5-19a Distribution of Carbon Species in Water5-6 Gases in Seawater

The CO2 System

Figure 5-19c The CO2 System5-6 Gases in Seawater

• pH is related to the amount of CO2 dissolved in water.

• CO2 combines with the water to produce carbonic acid, which releases H+ ions.

CO2 + H2O H2CO3 H+ + HCO3- H+ + CO3

-2

– H2CO3 is carbonic acid

– HCO3- is the bicarbonate ion

– CO3-2 is the carbonate ion


CO2 + H2O H2CO3 H+ + HCO3- H+ + CO3

-2

• Changing the amount of CO2 shifts the reaction to either the right or left of the equation.

– Adding CO2 shifts the reaction to the right and produces more H+ ions.

• This makes the water more acidic.

– Removing CO2 shifts the reaction to the left, combining H+ ions with carbonate and bicarbonate ions.

• This makes the water less acidic.


• Dissolved CO2 in water acts as a buffer

– A buffer is a substance that prevents large shifts in pH.


Figure 5-19d Carbonate Buffer

• Dissolution of carbonate shells in deep water occurs because:

– cold water under great pressure has a high saturation value for CO2

– the additional CO2 releases more H+ ions making the water acidic

• Carbonate sediments are stable and do not dissolve in warm, shallow water because the water:

– is under low pressure

– contains less dissolved CO2

– is less acidic than the deep water.


Water is recycled continually between the ocean and the land

• The reservoirs of water include:

– Oceans: • cover 60% of the northern hemisphere

• cover 80% of the southern hemisphere

• contain 97% of Earth’s water

– Rivers, lakes and glaciers

– Groundwater • contains a larger volume of water than all of the combined water in

lakes and rivers

5-7 The Ocean as a Physical Chemical System

• The hydrologic cycle describes the exchange of water between ocean, land and atmosphere.– On land precipitation exceeds evaporation.– In the ocean evaporation exceeds precipitation.

Figure 5-20 Hydrologic Cycle5-7 The Ocean as a Physical Chemical System

Recycling of Water

Figure 5-21 Recycling of Water5-7 The Ocean as a Physical Chemical System

The ocean is part of a vast biogeochemical system

• Rocks on land undergo weathering.

• The products are transported to the sea.

• They may be deposited directly or used by organisms and later deposited as organic remains or organic wastes.

• These sedimentary deposits are buried, lithified and recycled by plate tectonics into new land

• The new land then undergoes weathering repeating the cycle.


Biochemical Recycling of Matter

Figure 5-22 Biogeochemical Recycling of Matter


• Water samples must be collected in inert containers and isolated as they are recovered so as to prevent contamination.

• The Niskin bottle has valves at each end.– They automatically close when a weight (called

a messenger), is sent down the cable and causes the bottle to flip over and seal itself.

• Sample depth can be determined from cable inclination and length or with a pulsating sound source.

The Ocean Sciences: Chemical Techniques

Desalinization is the process of producing potable (drinkable) water from seawater using

one of the following methods

• Distillation is the evaporation of seawater and the condensation of the vapor.

• Freezing produces salt-free ice which can be melted for water.

The Ocean Sciences: DesalinizationFigure B5-4 Distillation of Seawater

• Reverse osmosis is placing seawater under pressure and forcing water molecules through a semi-permeable membrane, leaving a brine behind.

• Electrodialysis is using electrically charged surfaces to attract cations and anions, leaving behind freshwater.

• Salt absorption is using resins and charcoal to absorb ions from seawater.

The Ocean Sciences: Desalinization

• Sea ice is ice that forms by the freezing of sea water.– Icebergs are detached parts of glaciers.

• As ice forms, the salt remains in solution:

– increasing salinity

– further lowering the freezing point of the water

• Depending upon how quickly the ice freezes, some salt may be trapped within the ice mass, but is gradually released.

The Ocean Sciences: Other Physical Properties of Water

As sea water freezes, needles of ice form

The needles grow into platelets

The platelets produce a slush at the sea surface

• Pancake ice are rounded sheets of flexible sea ice that become abraded as they collide.

• Pressure ridges are the buckled edges of sea ice masses that have collided.


• Sea ice thickens with time as snow freezes to its surface and water to its bottom.

• Sheets of ice are broken by waves, currents and wind into irregular, mobile masses, called ice floes.


• The amount of light entering the ocean depends upon:– the height of the sun above the horizon – the smoothness of the sea surface

• 65% of light entering the ocean is absorbed within the first meter and converted into heat.

• Only 1% of light entering the ocean reaches 100m.

• Water displays the selective absorption of light:– long wavelengths are absorbed first – short wavelengths are absorbed last

• In the open ocean, blue light penetrates the deepest.


Light Absorption


Figure B5-6a Light Absorption in Nearshore Waters

• In turbid coastal waters light rarely penetrates deeper than 20m.– The water appears

yellow to green because particles reflect these wavelengths.

Figure B5-7 Yangtze River

• The photic zone is the part of the water column penetrated by sunlight.

• The aphotic zone is the part of the water column below light penetration and permanently dark.


• The speed of sound in water increases as:– Salinity increases– Temperature increases– Pressure increases

• In the ocean, the speed of sound is mainly a function of temperature and pressure.


• Above the pycnocline, increasing pressure with depth increases the speed of sound, despite the gradual decrease in temperature.

• Within the pycnocline, the speed of sound decreases rapidly because of the rapid decrease in temperature and only slight increase in pressure.

• Below the pycnocline the speed of sound gradually increases because pressure continues to increase, but temperature only declines slightly.

• SOFAR Channel is located where sound speed is at a minimum.

• Refraction of sound waves within the channel prevents dispersion of the sound energy.

• Sound waves travel for 1000s of kilometers within the channel.


• The sea surface microlayer is the water surface to a depth of a few hundred micrometers.

• It is critical for the exchange of gases, liquids, and solids between the atmosphere and the ocean.

• The neuston layer is the habitat of the sea surface microlayer and is inhabited by neuston.

The Ocean Sciences: Sea Surface Microlayer

• Processes that transport matter to the surface layer from below are:

– Diffusion - random movement of molecules.

– Convection - vertical circulation resulting in the transfer of heat and matter.

– Bubbles - the most important process because bubbles absorb material and inject it into the air as they bursts.


• Processes within the microlayer can be divided into the:

– Biological - bacteria and plankton are more densely concentrated in the neuston layer than below.

– Photochemical effect - the interaction of ultraviolet light and organic compounds.


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© 2006 Jones and Bartlett Publishers Chapter 5 The Properties of Seawater