Salinity

1
Salinity

• Salinity total amount of solid material
  dissolved in water
• Can be determined by measuring water conductivity
  
• Typically expressed in parts per thousand ()

Figure 5-15
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Constituents of ocean salinity

• Average seawater salinity 35
• Main constituents of ocean salinity
• Chloride (Cl)
• Sodium (Na)
• Sulfate (SO42)
• Magnesium (Mg2)

Figure 5-13
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Salinity variations
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How to change salinity

• Add water
• Remove water
• Add dissolved substances
• Remove dissolved substances

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Processes affecting seawater salinity

• Processes that decrease seawater salinity
• Precipitation
• Runoff
• Icebergs melting
• Sea ice melting
• Processes that increase seawater salinity
• Sea ice forming
• Evaporation

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Processes that add/subtract water
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Hydrologic cycle describes recycling of water
near Earths surface
Fig. 5.15
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The hydrologic cycle

• 97.2 oceans
• 2.15 glaciers and ice caps
• 0.62 groundwater
• 0.02 streams and lakes
• 0.001 vapor

Figure 5-19
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Processes that add/subtract dissolved substances
Salinity increases through
Salinity decreases through

• Salt spray
• Chemical reactions at seawater seafloor interface
• Biologic interactions
• Evaporite formation
• Adsorption

• River flow
• Volcanic eruptions
• Atmosphere
• Biologic interactions

10
Residence time

• Average length of time a substance remains
  dissolved in seawater
• Ions with long residence time are in high
  concentration in seawater
• Ions with short residence time are in low
  concentration in seawater
• Steady state condition

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Residence time and steady state
Fig. 5.16
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RESIDENCE TIME OF DISSOLVED SUBSTANCES IN SEA
WATER Definition "Average time substance spends
in ocean before removal." Representative
values Residence times, years Cl- 80
million Na 60 million Mg2 10
million SO42- 9 million Ca2 1
million PO43- 0.7 million Mn 7,000 Fe
100 Al 100
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• Importance
• Indication of "reactivity" in sea water.
• Long residence time (Cl, Na) not very reactive,
  removed slowly
• Short residence time (Mn, Fe, Al) reactive,
  removed rapidly
• Helps describe the cycling of these species
  (e.g., Nutrients (PO43- and Fe) cycled rapidly
  between biological pool and dissolved pool vs.
  Cl- and Na

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Acidity and alkalinity

• Acid releases H when dissolved in water
• Alkaline (or base) releases OH-
• pH scale measures acidity/alkalinity
• Low pH value, acid
• High pH value, alkaline (basic)
• pH 7 neutral

15
pH

• Water (H20) contains both hydrogen (H) and
  hydroxyl (OH-) ions. The pH of water is a
  measurement of the concentration of H ions,
  using a scale that ranges from 0 to 14. A pH of 7
  is considered "neutral", since concentrations of
  H and OH- ions are equal.
• For every one unit change in pH, there is
  approximately a ten-fold change in acidity or
  alkalinity. Therefore, a pH of 4 is 10 times more
  acidic than a pH of 5.

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pH and Life

• Most aquatic plants and animals are adapted to a
  specific pH range, and natural populations may be
  harmed by water that is too acidic or alkaline.
• In very acidic waters, metals which are normally
  bound to organic matter and sediment are released
  into the water. Many of these metals can be toxic
  to fish and humans.
• Below a pH of about 4.5, all fish die.

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Carbonate buffering

• Keeps ocean pH about same (8.1)
• pH too high, carbonic acid releases H
• pH too low, bicarbonate combines with H
• Precipitation/dissolution of calcium carbonate
  CaCO3 buffers ocean pH
• Oceans can absorb CO2 from atmosphere without
  much change in pH

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Ocean buffering

• Ocean pH 8.1 (slightly basic)
• Buffering protects the ocean from experiencing
  large pH changes

Figure 5-18
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Carbonate buffering
Fig. 5.18
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Hydrologic Cycle Links

• NASAs Observatorium
• The Weather World 2010 Project