Seawater

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					Seawater

  15.2
                  Seawater
• Seawater is 96.5% water and 3.5% salt (NaCl).

• Most elements on Earth are present in
  seawater.
  – EX: chlorides, magnesium, potassium, and calcium
  Chemical Properties of Seawater
• Salinity is a measure of the amount of
  dissolved salts in seawater.

• Oceanographers express salinity as grams of
  salt per kilogram of water, or parts per
  thousand (ppt).

• On average the total salt content of seawater
  is 35 ppt, or 3.5%.
 Chemical Properties of Seawater
• Seawater also contains dissolved gases and
  nutrients.
  – Oxygen, nitrogen, carbon dioxide, nitrates,
    phosphates, and silicates
          Variations in Salinity
• At the equator where rain is abundant,
  salinities are lower.

• Polar regions have low salinities because of
  the melting freshwater sea ice.

• The lowest salinities occur where large rivers
  empty into the oceans.
           Variations in Salinity
• Salinities vary from place to place.

• In subtropical regions the waters salinity
  would be higher due to high rates of
  evaporation.
            Sources of Sea Salt
• Based on the evidence of calcium-carbonate
  shells of some marine organisms, scientists
  believe that the salinity of ancient seas are the
  same as today.
         Removal of Sea Salts
• Although salt is continuously added to
  seawater, the salinity of seawater does not
  increase because salts are removed from the
  ocean at the same rate as they are added.
          Removal of Sea Salts
• Sometimes salt will precipitate from seawater
  near arid, coastal regions which will remove
  immense quantities of salt.

• Small salty spray droplets from breaking waves
  are picked up by winds and deposited inland.

• Marine organisms also remove salt to build their
  shells, bones, and teeth.
  Physical Properties of Seawater
• Because salt ions are heavier than water
  molecules, saltwater has a higher density.

• Cold water is denser than warm water.

• Variations in salinity cause the freezing point
  of seawater to be lower than that of
  freshwater.
           Absorption of Light
• Water absorbs light, which is why it gets
  darker the deeper a body of water is.

• Light penetrates only the upper 100 meters of
  seawater.
             Ocean Layering
• Ocean water temperatures decrease with
  depth.

• A temperature profile plots changing water
  temperatures with depth, which can vary on
  location and season.
               Ocean Layering
• The ocean can be divided into 3 layers.

• The first is a relatively warm, sunlit, surface
  layer some 100 m thick.

• Under this is a transitional layer known as the
  thermocline, which has rapidly decreasing
  temperatures with depth.
             Ocean Layering
• The bottom layer is cold and dark with
  temperatures near freezing.

• Both the thermocline and the warm surface
  layer are absent in polar seas, where water
  temperatures are cold from top to bottom.

• Ocean layering is caused by density
  differences.
               Water Masses
• When seawater freezes during the arctic winter,
  sea ice forms.

• The cold water beneath the ice becomes saltier
  and denser than the surround seawater and
  starts to sink.

• This salty water then migrates toward the
  equator as a cold, deep water mass along the
  ocean floor.
              Water Masses
• Three water masses account for most of the
  deep water in the Atlantic Ocean:
       –Antarctic Bottom Water
       –North Atlantic Deep Water
       –Antarctic Intermediate Water