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Oceanography
An Invitation to Marine Science, 7th
Tom Garrison
Chapter 7
Ocean Chemistry
Chapter 7 Study Plan
• Water Is a Powerful Solvent
• Seawater Consists of Water and Dissolved
Solids
• Gases Dissolve in Seawater
• The Ocean’s Acid-Base Balance Varies with
Dissolved Components and Depth
Chapter 7 Main Concepts
• Water is a powerful solvent. The total quantity (or
concentration) of dissolved inorganic solids in water
is its salinity.
• Though salinity may vary with location, the ratio of
dissolved solids in seawater is a constant.
• Gases dissolve in seawater. Cold water can hold
more gas in solution than warm water.
• The ocean is a vast reservoir of carbon. The
dynamics of carbon exchange between ocean and
atmosphere affect Earth’s climate.
• The ocean’s acid-base (pH) balance varies with
depth and dissolved components.
• Carbonate chemistry serves to moderate (buffer)
wide swings in oceanic pH.
Water Is a Powerful Solvent
A simplified hydrologic cycle. Water moves from ocean to air, onto land, to lakes
and streams and groundwater, back to the sky and ocean, in a continuous cycle.
The numbers indicate the approximate volumes of water in cubic kilometers per
year (km3/yr). Water is also stored in the ocean, ice, groundwater, lakes, and the
atmosphere.
Water Is a Powerful Solvent
• What are solutions and mixtures?
• A solution is made of two components, with
uniform molecular properties throughout:
– The solvent, which is usually a liquid, and is the
more abundant component.
– The solute, often a solid or gas, is the less
abundant component.
• A mixture is different from a solution. In a
mixture the components retain separate
identities, so it is NOT uniform throughout.
Water Is a Powerful Solvent
Salt in solution. When a salt such as NaCl is put in water, the positively charged
hydrogen end of the polar water molecule is attracted to the negatively charged Clion, and the negatively charged oxygen end is attracted to the positively charged
Na+ ion. The ions are surrounded by water molecules that are attracted to them
and become solute ions in the solvent.
Salinity
• Salinity is the total quantity of dissolved
inorganic solids in water.
• Water’s colligative properties are:
– The heat capacity of water decreases with
increasing salinity
– As salinity increases, freezing point decreases
– As salinity increases, evaporation slows
– Osmotic pressure increases as salinity increases
A Few Ions Account for Most of
the Ocean’s Salinity
A representation of the most abundant components of a
kilogram of seawater at 35‰ salinity. Note that the specific
ions are represented in grams per kilogram, equivalent to
parts per thousand (‰).
Components of Ocean Salinity Came
From Earth’s Crust
Processes that regulate the major constituents in seawater. Ions are added to
seawater by rivers running off crustal rocks, volcanic activity, groundwater,
hydrothermal vents and cold springs, and the decay of once-living organisms. Ions
are removed from the ocean by chemical entrapment as water percolates through
the mid-ocean ridge systems and seamounts, sea spray, uptake by living
organisms, incorporation into sediments, and ultimately by subduction.
The Ratio of Dissolved Solids in
the Ocean is Constant
• Forchhammer’s principle, also known as the
principle of constant proportions states that although
the salinity of various samples of seawater may vary,
the ratio of major salts is constant.
• How do scientists determine the salinity of seawater?
• Salinity can be determined by measuring the chlorinity
of the sample. Since the chlorinity is easy to measure,
and the principle of constant proportions applies to all
seawater, scientists can use the following formula to
determine salinity:
Salinity in ‰ = 1.80655  Chlorinity in ‰
The Ocean Is in Chemical
Equilibrium
• Is the ocean becoming progressively saltier with age?
• No, the ocean is in chemical equilibrium. The
proportion and amounts of dissolved solids remain
constant and is known as the “steady state ocean.“
• Ions are being added to and removed from the ocean at
the same rate.
• Residence time is the average length of time an
element spends in the ocean and can be calculated as:
Residence Time =
___Amount of element in the ocean____
The rate at which the element is added
to (or removed from) the ocean
Seawater’s Constituents May Be
Conservative or Non-conservative
• Conservative constituents of seawater are
those constituents that occur in constant
proportions. Conservative elements have
long residence times and are the most
abundant dissolved material in the ocean.
• Non-conservative constituents have short
residence times, and are usually associated
with seasonal, biological or short geological
cycles.
Gas Concentrations Vary with Depth
How concentrations of oxygen and
carbon dioxide vary with depth. Oxygen
is abundant near the surface because of
the photosynthetic activity of marine
plants.
Oxygen concentration decreases below
the sunlit layer because of the
respiration of marine animals and
bacteria, and because of the oxygen
consumed by the decay of tiny dead
organisms slowly sinking through the
area.
In contrast, because plants use carbon
dioxide during photosynthesis, surface
levels of CO2 are low.
Because photosynthesis cannot take
place in the dark, CO2 given off by
animals and bacteria tends to build up at
depths below the sunlit layer. CO2 also
increases with depth because its
solubility increases as pressure
increases and temperature decreases.
The Ocean’s Acid-Base Balance Varies
with Dissolved Components and Depth
• What are acids and bases?
• An acid is a substance that releases a
hydrogen ion in solution.
• A base is a substance that combines with a
hydrogen ion in solution.
• A solution containing a base is called an
alkaline solution.
• Acidity or alkalinity is measured on the pH
scale.
The Ocean’s Acid-Base Balance Varies
with Dissolved Components and Depth
(right) The pH scale.
A solution at pH 7 is
neutral; higher numbers
represent bases, and
lower numbers represent
acids.
The Ocean’s Acid-Base Balance Varies
with Dissolved Components and Depth
Carbon dioxide (CO2) combines readily with seawater to form carbonic acid (H2CO3). Carbonic acid
can then lose a H+ ion to become a bicarbonate ion (HCO3-), or two H+ ions to become a carbonate
ion (CO32-). Some bicarbonate ions dissociate to form carbonate ions, which combine with calcium
ions in seawater to form calcium carbonate (CaCO3), used by some organisms to form hard shells
and skeletons. When their builders die, these structures may fall to the seabed as carbonate
sediments, eventually to be re-dissolved. As the double arrows indicate, all these reactions may move
in either direction.
Chapter 7 in Perspective
In this chapter you learned that water has the remarkable ability to dissolve more substances
than any other natural solvent. Though most solids and gases are soluble in water, the ocean is
in chemical equilibrium and neither the proportion nor amount of most dissolved substances
changes significantly through time. Most of the properties of seawater are different from those
of pure water because of the substances dissolved in the seawater.
About 3.5% (35‰) of seawater consists of dissolved substances. These almost always exist as
ions – “salts” do not exist in the ocean. The most abundant ions dissolved in seawater are
chloride, sodium, and sulfate. Seawater is not concentrated river water or rain water – its
chemical composition has been altered by circulation through the crust at oceanic spreading
centers and by other chemical and biological processes.
Most gases in the air dissolve readily in seawater at the ocean’s surface. Plants and animals
living in the ocean require these dissolved gases to survive. In order of their relative
abundance, the major gases found in seawater are nitrogen, oxygen, and carbon dioxide. The
proportions of dissolved gases in the ocean are very different from the proportions of the same
gases in the atmosphere because of differences in their solubility in water and air.
In the next chapter you will learn how atmosphere and ocean work together to shape Earth’s
weather, climate, and natural history. Our past discussions of heat, temperature, convection,
dissolved gases, and geological cycles will be put to use. You will find surprising oceanic
connections among storms, tropical organisms, deserts, and balloon races!