Chapter 5 The Properties of Seawater ©2003 Jones and Bartlett Publishers

Chapter 5

The Properties of Seawater

©2003 Jones and Bartlett Publishers

Types of Oceanography

So far – Geology

Today – Chemistry (geochemistry)

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

– Subatomic particles are smaller, but do not retain materials characteristics.

The Atom and Electron Shells

• Composed of

– Nucleus

• center; positive charge• protons and neutrons

– Electrons – outside in shells; negative charge; orbit the nucleus in discrete shells

• Atomic numbers (number of protons)

– Hydrogen – 1

– Oxygen - 8

Notation

168O

Atomic number (# of protons in nucleus)

Mass number = #protons + # neutrons

Element symbol (oxygen)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

1H He

1 2

2Li Be

B C N O F Ne

3 4 5 6 7 8 9 10

3Na Mg Al Si P S Cl Ar

11 12 13 14 15 16 17 18

4K Ca

ScTi V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

5Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

6Cs Ba * Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

55 56 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

7Fr Ra ** Rf Db Sg Bh Hs Mt Uun Uuu Uub

87 88 104 105 106 107 108 109 110 111 112

*La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

**Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

89 90 91 92 93 94 95 96 97 98 99 100 101 02 103

Element Groups (Families)

Alkali Earth

Alkaline Earth

Transition Metals

Rare Earth

Other Metals

Metalloids

Non-Metals

HalogensNoble Gases

Periodic Table of the Elements

Element: pure substances that cannot be separated into constituent substances with unique properties.

•Electrically stable atoms: same number of electrons as protons, no charge.

– Examples: H, N, O

• Ions: atoms with either more or fewer electrons than protons and are therefore electrically charged.

– Example: H+, Na+ (cations); Cl-, Br- (anions).

Basic Chemical Notions

• Isotope: atoms of the same element that differ in weight due to variable numbers of neutrons. (all work almost the same way chemically).

– Example: 3H, 12C, 13C, 14C, 16O, 17O, 18O.

– Unstable (radioactive, e.g., 14C) and stable (e.g., 13C) isotopes.



• Compound: a pure substance that can be resolved into two or more elements.

• Molecule : small group of atoms held together by relatively strong forces called chemical bonds.

– Example: CaCO3, H2O, CO32-, proteins, nucleic acids,

carbohydrates, etc.

Heat results from the vibrations of atoms (kinetic energy) and can be measured with a thermometer.

• Three states of matter:

– Solids: atoms or molecules vibrate weakly and are rigidly held in place.

– Liquids: atoms or molecules vibrate more rapidly, move farther apart and are free to move relative to each other.

– Gases: atoms or molecules are highly energetic, move far apart and are largely independent.

Heat and State

Changes of State

solid liquid

Gas

melting

freezing

evaporation

condensation

• Temperature controls density. As temperature increases, atoms or molecules move farther apart and 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 any other substance.

The Water Molecule

The Water Molecule

• Ice floats in water because all of the molecules in ice are held in hexagons and the center of the hexagon is open space, making ice 8% less dense than water.

The Water Molecule

Hexagonal Crystal Structure of Ice

The Water Molecule

The Water Molecule

Dissolution• Solvent is material doing

dissolving

• Solute is material being dissolved

• Water is universal solvent, dissolves many substances

• Dipole structure of water helps it to dissolve substances with charges

• NaCl = Sodium Chloride = table salt

• Dissolved NaCl = Na+ + Cl-

Seawater consists predominantly of water with various materials dissolved within it.• Salinity is the total mass, expressed 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 and all organic compounds have been oxidized at a temperature of 480oC.

• ppt (parts per thousand) or o/oo

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

• Analytical methods:

– Weight of salts

– Chemical titration

– conductivity

Seawater

Five Groups of Solutes in Seawater

• Major Constituents (the salts)

• Nutrients

• Gases

• Trace elements

• Organic compounds

Major Constituents

Salt Ion Ions in Seawater %

Chloride (Cl-) 18.980 55.04

Sodium (Na+) 10.556 30.61

Sulfate (So42-) 2.649 7.68

Magnesium ( Mg2+) 1.272 3.69

Calcium (Ca2+) 0.400 1.16

Potassium (K+) 0.480 1.10

Bicarbonate (HCO3-) 0.140 0.41

Bromide (Br-) 0.065 0.19

Boric acid (H3BO3-) 0.026 0.07

Strontium (Sr2+) 0.013 0.04

Floride (F-) 0.001 <0.01

Other ions <0.001 <0.01%

Nutrients are chemicals essential for life.

• Major nutrients in the sea are compounds of nitrogen, phosphorus and silicon.

• 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.

Nutrient element

Nutrient compound

Concentration [ppm]

Phosphorus (P) Phosphate(PO43-) 0.07

Nitrogen (N) Nitrate (NO3-) 0.5

Silicon (Si) Silicate (SiO44-) 3

•In order of decreasing abundance the major gases in the sea are:

-Nitrogen (N2)-Oxygen (O2)-Carbon dioxide (CO2)-Noble gases, argon (Ar), neon (Ne) &

helium (He).

The noble gases are considered to be inert because they are chemically non-reactive.

Nitrogen (N2) is inert with the exception of nitrogen fixing cyanobacteria.

Oxygen (O2) and Carbon dioxide (CO2) are biologically active.

Gases

Trace elements occur in minute quantities and are usually measured in parts per billion (ppb).

-even in small quantities they can be important for either promoting or killing life.

Trace Elements

Trace element Concentration [ppb]Lithium (Li) 170

Iodine (I) 60

Molybdenum (Mo) 10

Zinc (Zn) 10

Iron (Fe) 10

Aluminum (Al) 10

Copper (Cu) 3

Manganese (Mn) 2

Cobalt (Co) 0.1

Lead (Pb) 0.03

Mercury (Hg) 0.03

Gold (Au) 0.004

•Marine organic compounds occur in low concentrations and consist of large complex molecules produced by organisms, such as: -

-lipids (fats)-proteins-carbohydrates-nucleic acids (DNA, RNA)-hormones-vitamins

Organic Compounds

• Principle of constant proportion –applies to salts that exhibit conservative properties in seawater (Cl-, Na+).

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

Principle of Constant Proportion

Salinity (o/oo) = 1.80655 x chlorinity (o/oo)

Chlorinity: concentration of chloride ion (Cl-), plusall other halide ions (Br-, Fl-).

Major Constituents

Salt Ion Ions in Seawater %

Chloride (Cl-) 18.980 55.04

Sodium (Na+) 10.556 30.61

Sulfate (So42-) 2.649 7.68

Magnesium ( Mg2+) 1.272 3.69

Calcium (Ca2+) 0.400 1.16

Potassium (K+) 0.480 1.10

Bicarbonate (HCO3-) 0.140 0.41

Bromide (Br-) 0.065 0.19

Boric acid (H3BO3-) 0.026 0.07

Strontium (Sr2+) 0.013 0.04

Floride (F-) 0.001 <0.01

Other ions <0.001 <0.01%

Salinity in the ocean is in a steady-state condition because the amount of salt added to the ocean (input from source) equals the amount removed (output into sinks).

Steady-state Equilibrium

Ocean(salts constant)

source sink

Salt Sources and Sinks

•SourcesWeathering of rocks on land (acid rainwater)

Reaction of lava with seawater

•SinksPrecipitation due to evaporation (brine, evaporite minerals)

Adsorption on clays

Authigenic sedimentation- (i.e., ferromanganese nodules)

Wind-blown spray

Shell formation by organisms (CaCO3)

• Lack of similarity between relative composition of river water and the ocean is explained by residence time, average length of time that an ion remains in solution in the ocean.

– Ions with long residence times (Cl- , Na+) have low chemical reactivity and tend to accumulate in the sea

– Ions with with short residence times (Ca2+, K+) have higher chemical reactivity, and are removed.

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

Residence time

Salt Ion Residence Time in sea [106 y]

Concentration in seawater [%]

Concentration in river water [%]

Chloride (Cl-) Inf. 55.04 6.5

Sodium (Na+) 260 30.61 5.2

Calcium (Ca2+) 8 1.16 12.4

Potassium (K+)

11 1.10 1.9

Residence time

Ocean takes about 1000 y to completely mix.

Addition of salt modifies physical properties of water.

• Freezing point

– Increase salt content lowers temperature to freeze.

• Density

– Increase salt content makes water more dense.

• Vapor pressure

– Increase salt content lowers the vapor pressure above the water.

Chemical and Physical Structure of the Oceans

•How do temperature and salinity vary in the ocean?

•Daily (diurnal)•Seasonally•Vertically with depth•Horizontally with latitude and longitude

Ocean surface temperature strongly correlates with latitude 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



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

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

Temperature Profiles

• Tropical and subtropical oceans are permanently layered with warm, less dense surface water separated from cold, dense deep water by a thermocline.

• -clines are depth intervals where there is rapid vertical changes in properties.

Salinity changes with latitude due to variations in precipitation and evaporation.

Salinity changes with latitude due to variations in precipitation and evaporation.

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 and the lower salinity water sufficiently cold.

Thermocline, Halocline, Pynocline

The water column in the ocean can be divided into the surface layer, pycnocline and deep layer.

• The surface layer is about 100m thick, comprises about 2% of the ocean volume and is the most variable part of the ocean because it is in contact with the atmosphere.

• The surface layer 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 and comprises 18% of the ocean basin.

• 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 and flows equatorward across the ocean basin.

Density Structure of the Oceans

Gases in Seawater

Gas In dry air

(%)

In surface ocean water (%)

Nitrogen (N2) 78.03 47.5

Oxygen (O2) 20.99 36.0

Carbon Dioxide (CO2)

0.03 15.1

Ar, H2 ,Ne & He 0.95 1.4

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

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

• 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 and infusion from volcanic vents.

Gas solubility and saturation

Gas solubility and saturation

•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.

Oxygen distribution

Oxygen distribution

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.

• Carbon dioxide controls the acidity of sea water.

– A solution is acidic if it has excess H+ (hydrogen) ions and 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.

Carbon Dioxide

The pH scale

Carbon dioxide species

Carbon Dioxide

•Dissolved CO2 in water acts as a buffer, a substance that prevents large shifts in pH.

•Dissolution of carbonate shells in deep water results because cold water under great pressure has a high saturation value for CO2 and the additional CO2 releases more H+ ions making the water acid.

•Warm, shallow water is under low pressure, contains less dissolved CO2 and is less acidic than the deep water. Carbonate sediments are stable and do not dissolve.

5-6 Gases in Seawater

Carbon dioxide species

Water is recycled continually between the ocean and the land.• The reservoirs of water include:

– Oceans - cover 60% of the northern hemisphere and 80% of the southern hemisphere and 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.

• The hydrologic cycle describes the exchange of water between ocean, land and atmosphere.

– On land precipitation exceeds evaporation.

– In the ocean evaporation exceeds precipitation.

5-7The Ocean as a Physical System

The Hydrologic Cycle

•The ocean is part of a vast biogeochemical system. Rocks on land undergo weathering and these weathered products are transported to the sea where 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, which undergoes weathering repeating the cycle.

5-7The Ocean as a Physical System

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 which are automatically closed 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 can produce salt-free ice which can be melted for water.

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

The Ocean Sciences: Desalinization

•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 sea water.

The Ocean Sciences: Desalinization

Sea ice is ice that forms by the freezing of sea water; icebergs are detached parts of glaciers.

• As sea water freezes, needles of ice form and grow into platelets which gradually produce a slush at the sea surface.

• As ice forms, the salt remains in solution, increasing salinity and 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.

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

The Ocean Sciences: Other Physical Properties of Water

•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.


Amount of light entering the ocean depends upon the height of the sun above the horizon and 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 with long wavelengths absorbed first and short wavelengths absorbed last.

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


• In turbid coastal waters light rarely penetrates deeper than 20m and the water appears yellow to green because particles reflect these wavelengths.

•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, temperature and pressure increase, but 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 and 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 Ocean Sciences: Sea Surface Microlayer

Neuston layer is the habitat of the sea surface microlayer and is inhabited by the neuston, all organisms of the microlayer.


Model of the Sea-Surface Microlayer

Profile of the Ocean

•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.


Documents

Chapter 5 The Properties of Seawater ©2003 Jones and Bartlett Publishers