Oceanography Lecture Notes Outline

Seawater Chemistry and Ocean Salinity

I. Contents -  Topics Covered

Water the Universal Solvent

Salinity of Seawater

Salts and Dissolved Ions

Sources of Seawater Ions

Principle of Constant Proportion

Dissolved Gases in Seawater

Buffering of Seawater’s pH (acid-base balance)


II. Water as a Solvent

         A.  Water is a Very Good Solvent

                  1.  Water’s universal solvency is due to its

·       Relatively low liquid viscosity

·       Strongly polar nature

·       Unique hydrogen bonding character.


2.  Water dissolves rocks and atmospheric gases

·       The dissolved minerals from rocks is a primary

                                         source of many salts found in seawater


·       Gases from the atmosphere are dissolved into seawater

   at the ocean’s surface


                  3. Water can hold a tremendous amount of ions and gases

                        in solution


                  4. Water’s ability to dissolve and retain mineral ions and

                        gases makes it a readily available storehouse of

                       essential substances to marine


·       Nutrients

·       Building Materials

·       Carbon dioxide (for plants)

·       Oxygen (for animals)


III. Salinity of SeaWater

         A.  Concentration of Dissolved Solids in Seawater

1. Salinity Defined: The total quantity (concentration) of

         dissolved inorganic solids in water.


2. Salinity is approximately equal to the weight, in grams, of

     salt dissolved in 1000 g of seawater.

·       This would be the salt concentration in parts per thousand (%o).


3. Salinity of ocean water varies from 3.3% to 3.7%

                           or 33%o  to 37.0%o

4. The average ocean salinity is 35.0 %o.  

5. Salinity has no units. (The PSU or "practical salinity unit" is incorrect, although frequently used.)

·       This means that 1000 g of average seawater contains 965 g of water and 35 g of salts.


         B. Measuring Salinity

1.    In the past, salinity of seawater was measured by evaporating the water and weighing the amount of salt remaining.


               2. Since that approach is difficult and inaccurate, electrical

                 conductivity of seawater is now used to measure salinity.

·       Measuring of the Chlorinity (Total halogen ions)

·       Salinity (in %o) = 1.80655 x Chlorinity (in %o)


3.    Conductivity increases as salt content of the water



4.    Conductivity gives very accurate salinity data: 35.0000%o. 


     5. Conductivity (and temperature and depth) are measured by

        instruments called CTDs (Conductivity Temperature Depth)


·       These instruments can make thousands of measurements/hour.



         B. Salinity Modifies the Physical Properties of Water

                  1. Heat Capacity decreases with increasing salinity.


                  2. Freezing Point of water drops with increasing salinity.


                  3. Evaporation Rate of water slows with increasing salinity


                  4. Osmotic pressure rises with increasing salinity


                  5. These properties are termed colligative properties


         C. Salinity and Seawater Density

1. Salinity, temperature, and depth (pressure) can be used to

                        calculate density, which is important to understanding

                      vertical circulation of the water.


2. Salinity is greatest in warm, tropical surface waters,

   where there is more evaporation than precipitation and

    polar regions where large amounts of sea ice form.


3. Salinity is lowest where there are large inputs of

         freshwater from rivers or melting glaciers


D. The Major Dissolved Solids in Seawater

1.  When salts dissolve in water, they break apart into two types of ions:

Ø    Cations - positive electrical charge

Ø    Anions  - negative electrical charge


  2. Salts are electrically neutral because the cation and

        anion charges are opposite and equal.


·       Examples are: Sodium chloride, NaCl, dissociates to Na+ and Cl-.


·       Magnesium sulfate, MgSO4, dissociates to Mg2+

          and SO42-.


  3.  Six major ions make up >99% of the total dissolved in

        seawater.  They are:


·      sodium ion (Na+),

·      chloride (Cl-),

·       sulfate (SO42-),

·      magnesium ion (Mg2+),

·      calcium ion (Ca2+), and

·      potassium ion (K+).


·       See Figure 7.3 and Table 7.1 (page 172) in the text


4.   The major ions are conservative. This means that they

          have constant ratios, to one another and to salinity, in

          almost all ocean water.


·       Another way of saying this is that sea salts have

          constant composition.


·        They almost always consist of 55% sodium ion,

          31% chloride, 8% sulfate, 4% magnesium ion, 1%

           calcium ion, and 1% potassium ion.


·          The main exception is where freshwater is mixing

           with seawater.


Ø    River water has a different composition than seawater, for example, it contains more calcium ion.


E. Minor and Trace Dissolved Ions in Seawater

  1. Every naturally-occurring element has been found in



2. The minor and trace dissolved ions account for only about

              1.5% of the total dissolved solids in seawater


3. See Figure 7.3 and Table 7.2 (page 173) in the text


4. Some, however, have minuscule dissolved concentrations:

·      Iron, 0.06 parts per billion (ppb)

·      Lead, 0.002 ppb.

·      Gold, 0.005 ppb.


     5. Many of the minor ions in seawater are Nonconservative


·     Their concentrations vary geographically and with depth, most often due to uptake and release by organisms.


IV. Sources of the Ocean’s Sea Salts

A. There are Two Primary Sources for Sea Salts 

1.  Weathering of rocks on land (the cations)

2.  Outgassing from the interior of the earth (anions)


B. The Weathering of Rock on Land is a Very Slow


1. Breakdown by water, with dissolved carbon dioxide,

            which makes it slightly acidic.


2. Rivers carry the dissolved cations to the ocean.


3. Weathering may have been somewhat faster on the early Earth, but even at the present rate it would take only about 8 to 260 million years to replace all the salts in seawater with those in the river inflow.


 C. Outgassing from Mantle is an Equally Slow Process

1. The halogen- and sulfur-based anions are mainly derived from the continuous, long-term outgassing of the mantle via volcanic venting – mainly from the mid-ocean ridge system


D. The Time It Takes to Replace the Total Amount of an  Ion in Seawater with Ions from the Source

 Reservoirs is called the Residence Time


1. Residence time = ____Amount of ion in ocean____

                                    The rate at which the ion is added to

                                         (or removed from) the ocean


2.  See Table 7.3 (page 178) for residence times


3. Residence times vary greatly for various dissolved solids

·       Chlorine (Cl-) = 100,000,000 years (greatest time)

·       Iron (Fe) = 200 years


4. Since the residence times for all the ions in seawater is much less than the age of the Earth and the oceans, some processes must remove the ions from seawater to keep them from building up to even higher concentration.


5. Both organic and inorganic processes at work



V. Principle of Constant Proportion

A. Ocean’s Salt Composition and Concentration is Stable

               1. This means that there is no significantly change over time

·       The term for this quality is “Steady State


2. The "steady state" results from the removal rate of salts

      from the ocean being equal to the input rate.


·       This balance holds because the removal rate of salts is

         related to their concentration, and increases when

         their concentration increases.


B. Salt Removal Processes Include:

1. Formation of evaporites (salt deposits left behind when

       seawater evaporates)


2. Burial of sediment porewater (the water between

     sediment grains)  sediments, especially biogenic

    sediments, for Ca2+ (calcium ion) as calcium carbonate.


3. Hydrothermal vents, especially formation of the mineral

   chlorite within the cracks and fissures of the vents, which

   removes Mg2+ (magnesium ion).


C. Evidence Indicates that Sea Salt Concentration and

Composition has been about the Same for at Least

 the Last 1.5 Billion Years


1.    The tolerances of bacteria that probably lived 3.8   bybp indicate that sea salt concentration and composition were not too different, even that long ago.


D. Another Important Group of Nonconservative

    Substances Dissolved in Seawater are the Nutrients.

1. These are fertilizers essential for the growth of plants,

         including algae.


2. Major nutrients include nitrate, phosphate, and silicate

      (the latter required only by siliceous organisms).


3. Nutrients are depleted in surface waters, where plants

      grow, and are found in higher concentrations in deep

      waters, where the plant and animal remains that sink

       from surface waters decay.


VI. Dissolved Gasses in Seawater

         A. Most of the Gasses Found in the Earth’s Atmosphere

Readily Dissolve in Seawater

1. Major ones include nitrogen, oxygen, and carbon dioxide


2. The amount of gasses able to dissolve in seawater

      increases with decreasing temperature


3. See Table 7.4 (page 179)


B. Several Important Gases are Nonconservative,

     Including Oxygen and Carbon Dioxide

1. Oxygen

·       Oxygen dissolves in ocean surface water from the atmosphere.


·        Photosynthesis is also a source of oxygen to ocean surface waters.


·       Oxygen is consumed by respiration. Rarely, animals and bacteria use all of the oxygen in sub-surface waters, which become anoxic. This can only happen if the waters are isolated from the atmosphere in some way.


2. Carbon Dioxide

·       Carbon dioxide is consumed during photosynthesis and released during respiration


·       It can also be exchanged with (dissolved from and released to) the atmosphere.


·       Carbon dioxide can react with water to form bicarbonate and carbonate ions.


CO2 + H2O HCO3- + H+ CO32- + 2H+


·       These reactions control the acidity (pH) of seawater.


·       Organisms use carbonate ion and calcium ion to make calcium carbonate shells, which sink after the organisms die to form calcareous sediments.


VII. Acid-Base Balance in Seawater


VIII. Vocabulary Terms