GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 9: Salt Water
POWERPOINT SLIDE SHOW NOTES
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Topic 9: Salt Water
What is in Seawater?
Salinity. Salinity is a measure of the amount of dissolved solids in seawater. We often
refer to the dissolved solids simply as “salt” because the ocean is salty. But there is more
than just true salt (table salt) dissolved in sea water. Also, here we are talking about
dissolved solids. We’ll talk about gases (such as oxygen) a little later.
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Note the fancy “‰” symbol. Like a percent sign with an extra 0 on the bottom. This is
the symbol for “per mille” meaning per thousand. Average seawater salinity is 35 parts
per thousand, or 35‰. Thus, a kilogram of seawater (one thousand of the grams, or
cubic centimeters of water), contains 35 grams of dissolved solids.
All naturally occurring solids can be found dissolved in seawater, but only six elements
are found in abundance everywhere. These are called the major constituents.
Dominating the major constituents are sodium and chlorine which together make up
table salt. The six major constituents make up 99.36% of everything dissolved in
seawater. The rest are called trace elements.
The constituents of seawater. Note that the major constituents are shown in some
different forms here than in the previous slide, but the basic elements are the same.
The actual amounts of the major constituents that are found in one kilogram of
seawater. The major constituents add up to 34.91 grams out of the 35 grams of solids
dissolved in seawater. Note that chlorine and sodium (NaCl—table salt) add up to
almost 86% of the major constituents.
Another way to look at the major constituents. Note that again, the forms of the
elements are a little different than the way they were introduced. For example, chloride
is just chlorine with an extra electron; sulfate is just sulfur with oxygen bonded to it.
Table salt
Just like solid water had a definite crystal structure, so does table salt when it is in crystal
form. Table salt has the chemical name of sodium chloride. The negative chlorine atoms
bond to the positive sodium atoms.
Halite dissolves easily in water. The molecules break apart, or dissociate. The sodium
and chlorine break apart and are surrounded by water molecules.
Because of its polar nature, water dissolves everything given enough time. That is why
water is called the universal solvent.
Also, the dissolving ability of water means that minerals dissolved by rivers and ground
water on land eventually make their way to the ocean. These dissolved solids
accumulate in the ocean to become the salt in seawater. Remember, we use the term
“salt” in the ocean to mean all dissolved solids, not just the mineral halite.
The oceans haven’t gotten saltier for billions of years because processes that add salt to
the ocean (inputs) are balanced by processes that remove salt (outputs).
Salt inputs and outputs.
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Principle of Constant Proportions. Applies only to major constituents.
Also known as Forchhammer’s principle. Salinity varies, meaning total amount of salt
varies, 34‰, 35‰, 36‰, etc. But the ratios stay the same. There is always 55.07%
chlorine, 30.62% sodium, and so forth.
Constant proportions makes it much easier to measure salinity. Instead of having to
obtain a measured sample of seawater (1 kg, for example), letting the water evaporate,
and weighing the solids left behind, we only need to measure the amount of one of the
major constituents and then we can calculate the total amount of dissolved solids. Most
salinometers simply measure the amount of dissolved chlorine.
Gases are also dissolved in seawater. They do not contribute to salinity, which is only
dissolved solids. We’ll look at only three common gases.
Open a can of soda and you’ll release pressure and carbon dioxide (the carbonation) will
fizz out of the soda. Releasing pressure means the soda can’t keep as much carbon
dioxide dissolved. Also, try it with a warm soda versus a cold one. The cold one fizzes a
lot less than the warm one. Warm water can’t hold as much carbon dioxide dissolved in
it.
Oxygen in the ocean
Carbon dioxide in the ocean
Carbon dioxide and oxygen graphed versus depth in the ocean. Note how different the
two curves are.
Oxygen concentration in the ocean varies with depth.
Why does oxygen concentration increase below 800 meters? It is only formed at the
surface, so how can it increase?
Carbon dioxide concentration in the ocean varies with depth.
Salt as a resource
Desalinization is also known as desalination.
Why desalinate?
Solar still
A solar still example
Osmosis
Reverse osmosis can be used to desalinate water.
Reverse osmosis desalinization. We supply pressure and push salt water through a semipermeable membrane which filters out the salt and provides fresh water.
These canisters hold the filters in a reverse osmosis operation.
The canisters must be cleaned occasionally to remove material from the filters.