Chapter 5 - Sea Water

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Chapter 5 - Sea Water
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The Atom and Electron Shells
Atoms are the smallest unit in matter that
display the properties of a material
(-)
Ions have a charge because there is a different
number of protons and electrons giving the atom a
charge. Molecules are made of 2 or more atoms.
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Basic Chemical Notions
5-1

• Isotopes atoms protons are same but of
  neutrons are different and therefore have
  different atomic weights.

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5-2
Basic Physical Notions

• 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.

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The water molecule is unique in structure and
properties.
5-3
Water Molecule

• 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.

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The water molecule is unique in structure and
properties.
5-3
Water Molecule
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5-3
Water Molecule

• Asymmetry of a water molecule and distribution of
  electrons result in a dipole structure with the
  oxygen end of the molecule negatively charged and
  the hydrogen end of the molecule positively
  charged.

This dipole structure produces an electrostatic
bond between water molecules hydrogen bonding
makes water a great solvent.
Dipole Structure
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Water Molecule
5-3

• Ice, the solid state of water floats in water
  because the hexagons of ice make it 8 less dense
  than water. Maximum density of water is reached
  at 3.98oC

Hexagonal Crystal Structure of Ice
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5-3
Water Molecule

• Hydrogen bonding is responsible for many of the
  unique properties of water because energy is
  required to break the hydrogen bonds and separate
  the water molecules.

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5-3
Water Molecule

• Water dissolves salts by surrounding the atoms in
  the salt molecule and breaking apart or
  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.

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Seawater consists predominantly of water with
various materials dissolved within it.
5-3
Water Molecule

• Salinity is the total amount of salts dissolved
  in the water. 99 of the salts are sodium (Na),
  chloride (Cl-), sulfate (SO4-2), magnesium
  (Mg2), calcium (Ca2) and potassium (K).
• 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 35 o/oo.

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Composition of Sea Water
Water Molecule
5-3

• 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.

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Nutrients are chemicals essential for life.
5-3
Water Molecule

• Major nutrients in the sea are compounds of
  nitrogen, phosphorus and silicon.
• Nutrients are used up and scarce at the surface
  (measured in parts per million- ppm).
• Concentration of nutrients vary greatly over time
  and are a nonconservative property of the sea.

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Major gases in the sea are nitrogen, oxygen,
carbon dioxide and the noble gases, argon (Ar),
neon (Ne) and helium (He).
5-3
Water Molecule

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

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Trace elements occur in minute quantities and are
usually measured in parts per million (ppm) or
parts per billion (ppb).
5-3
Water Molecule

• Even in small quantities they can be important
  for either promoting or killing life. (Fe, Al)

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Marine organic compounds occur in low
concentrations and consist of large complex
molecules, such as fat, proteins, carbohydrates,
hormones and vitamins, produced by organisms or
through decay.
5-3
Water Molecule
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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.
5-4
Salinity
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5-4
Salinity

• 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.

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5-4
Salinity

• Chlorinity is the amount of halogens (chlorinity,
  bromine, iodine and fluorine) in seawater and is
  expressed as grams/kilogram or o/oo.
• Salinity is equal to 1.8065 times chlorinity.
• Salinometers determine salinity from the
  electrical conductivity produced by the dissolved
  salts.

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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).
Salinity
5-4

• Salt sources include weathering of rocks on land
  and the reaction of lava with sea water.
• Weathering mainly involves the chemical reaction
  between rock and acidic rainwater, produced by
  the interaction of carbon dioxide and rainwater
  forming carbonic acid.

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INPUTS EROSION, CHEMICAL REACTIONSOUTPUTS
EVAPORATION, PRECIPITATION
Salinity
5-4
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5-4
Salinity
SPRAY

• Salt sinks
• Evaporation removes only water molecules.
• Wind-blown spray carries minute droplets of
  saltwater inland.
• Adsorption of ions onto clays and some authigenic
  minerals.
• Shell formation by organisms.

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5-4
Salinity

• 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 tend to accumulate
  in the sea, whereas those with short residence
  times are removed.
• Rapid mixing and long residence times explain
  constant composition of sea water.

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Addition of salt modifies the properties of water.
5-4
Salinity

• Pure water freezes at 0oC. Adding salt
  increasingly lowers the freezing point because
  salt ions interfere with the formation of the
  hexagonal structure of ice.
• Density of water increases as salinity increases.

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Ocean surface temperature strongly correlates
with latitude because insolation, the amount of
sunlight striking Earths surface, is directly
related to latitude.
Chemical and Physical Structure of the Oceans
5-5

• 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 and cooler water
  equatorward on the eastern side of the ocean.

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Chemical and Physical Structure of the Oceans
5-5

• Tropical and subtropical oceans are permanently
  layered with warm, less dense surface water
  separated from cold, dense deep water by a
  thermocline, a layer in which water temperature
  and density change rapidly.
• Temperate regions have a seasonal thermocline and
  polar regions have none.

Temperature Profiles
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Growth of Seasonal Thermocline
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Salinity changes with latitude due to variations
in precipitation and evaporation.
Chemical and Physical Structure of the Oceans
5-5

• Highest ocean salinity is between 20-30o north
  and south or the equator, because evaporation
  exceeds precipitation.
• Low salinity at the equator and poleward of 30o
  results from evaporation being less than
  precipitation.
• In some places surface water and deep water are
  separated by a halocline, a zone of rapid change
  of salinity with water depth.
• Water stratification (layering) within the ocean
  is more pronounced between 40oN and 40oS.

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Density of sea water is a function of
temperature, salinity and pressure.
Chemical and Physical Structure of the Oceans
5-5

• Density increases as temperature decreases and as
  salinity and pressure increase.
• Pressure increases regularly with depth, but
  temperature and salinity are more variable.

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Chemical and Physical Structure of the Oceans
5-5

• Higher salinity water can rest above lower
  salinity water if the higher salinity water is
  sufficiently warm and the lower salinity water
  sufficiently cold.
• Pycnocline is a layer within the water column
  where water density changes rapidly with depth.

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Thermocline, Halocline, Pynocline
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The water column in the ocean can be divided into
the surface layer, pycnocline and deep layer.
Chemical and Physical Structure of the Oceans
5-5

• 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.
• In the low latitudes, the pycnocline coincides
  with the thermocline, but in the mid-latitudes it
  icoincides with the halocline.

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Chemical and Physical Structure of the Oceans
5-5

• 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
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The solubility and saturation value of gases in
sea water increase as temperature and salinity
decrease and as pressure increases.
5-6
Gases in Seawater

• 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.
• Water is undersaturated when, under existing
  conditions, it has the capacity to dissolve more
  gas. This means that its gas content is below the
  saturation value.

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5-6
Gases in Seawater

• Water is saturated when under existing conditions
  it contains as much dissolved gas as it can hold
  in equilibrium. Gas content is at its saturation
  value.
• Water is supersaturated when under existing
  conditions 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.
• 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.

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Oxygen tends to be abundant in the water of the
surface layer and deep layer, and lowest in the
pycnocline.
5-6
Gases in Seawater

• 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.
• 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.

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5-6
Gases in Seawater

• 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 the cold surface waters
  which sank to the bottom. Oxygen consumption by
  respiration is low because there are fewer
  organisms in the deep layer of the ocean than in
  the water above.
• Anoxic waters contain no oxygen and are inhabited
  by anaerobic organisms (bacteria).

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Oxygen Advection Pattern in the Atlantic Ocean
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Carbon dioxide is of major importance in
controlling the acidity of seawater.
5-6
Gases in 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.

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5-6
Gases in Seawater

• pH is related to the amount of CO2 dissolved in
  water because it 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 and CO3-2 is the carbonate ion.

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5-6
Gases in Seawater

• 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 making the water more acid.
• Removing CO2 shifts the reaction to the left,
  combining H ions with carbonate and bicarbonate
  ions reducing the acidity.

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5-6
Gases in Seawater

• 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.

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Water is recycled continually between the ocean
and the land.
The Ocean as a Physical System
5-7

• The reservoirs of water include
• Oceans - cover 60 of the northern hemisphere and
  80 of the southern hemisphere and contain 97 of
  Earths 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.

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What happens to sea level as glaciers wax and
wane?
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The Hydrologic Cycle
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The Ocean as a Physical System
5-7

• 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.

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Water samples must be collected in inert
containers and isolated as they are recovered so
as to prevent contamination.
The Ocean Sciences Chemical Techniques

• 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.

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Desalinization is the process of producing
potable (drinkable) water from seawater using one
of the following methods.
The Ocean Sciences Desalinization

• 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.

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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.

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Sea ice is ice that forms by the freezing of sea
water icebergs are detached parts of glaciers.
The Ocean Sciences Other Physical Properties of
Water

• 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.

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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.

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Amount of light entering the ocean depends upon
the height of the sun above the horizon and the
smoothness of the sea surface.
The Ocean Sciences Other Physical Properties of
Water

• 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.

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The Ocean Sciences Other Physical Properties of
Water

• 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.

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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.
The Ocean Sciences Other Physical Properties of
Water

• 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.

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The Ocean Sciences Other Physical Properties of
Water

• 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.

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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
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Neuston layer is the habitat of the sea surface
microlayer and is inhabited by the neuston, all
organisms of the microlayer.
The Ocean Sciences Sea Surface Microlayer
Model of the Sea-Surface Microlayer
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Profile of the Ocean
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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.

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The Ocean Sciences Sea Surface Microlayer

• 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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