Page 218 - NS-2 Textbook

OCEANOGRAPHY 213

Water affects sOlmd and light in important ways, too. remain in the ocean water. Insoluble materials fall to the
The speed of sOlmd in -water, for example, is very much bottom and form sediments and clays that may eventu-
greater than in an' and increases with temperature, pres- ally turn into sedimentary rocks. Though the process
sure, and salinity (salt content). Of these factors, temper- continues, much of the material that runs into the ocean
ature is by far the most important in affecting the veloc- now is from sedimentary rocks that have gone through
ity of sound. The optical properties (ability to transmit the cycle before. For this reason, the concentration of
light) of seawater are of fWldamental importance to life salts in the sea is fairly stable now, having changed very
in the oceans. little for millions of years.
There are many other fascinating facts about water. During all this time, the water of the oceans has been
Besides being essential to all animal and plant life, it is passing through continuous cycles of evaporation and
also widely used in science and industry as a solvent, as condensation. Every year about SO,OOO cubic miles of sea-
a blending agent, and even as a standard for certain water are drawn off by evaporation. Of this huge quan-
physical properties. The reference points of most ther- tity of water, about 24,000 cubic miles retilrn to the conti-
mometers, for example, are the freezing and boiling nents as rain, sleet, and snow. Most of the rest returns
points of V\Tater. Water is also used as a coolant, a dilu- directly to the ocean as rain, but 1 or 2 percent remains in
tant, a cleansing medium, and in the production of heat the atmosphere as water vapor. Gusty surface winds
andpmver. carry aloft salt from ocean spray, dust, volcanic ash, and
even smokestack pollutants that become nuclei for rain
SALTS OF THE SEA or snow. TIle moisture in the atmosphere is attracted to
these foreign bodies. Droplets form and gradually grow
Chemically, seawater is a very pure substance. It is more
Wltil they become so heavy that they fall to the surface as
than 95 percent water, that is, hydrogen and oxygen.
raindrops during the warm months and snowflakes dur-
About eighty elements are fOWld in solution or suspen-
ing the cold months.
sion in the remaining 5 percent. TIle two basic elements
Water that has evaporated from the surface of the
in this remaining portion are sodiunl and chlorine, which
ocean finally returns to it carrying a microscopic pollu-
combine to become common table salt. The most signifi-
tant or mineral. This rotmd trip of evaporation, conden-
cant of the other elements in seawater in concentrations
sation, and return travel to the sea by way of precipita-
greater than one part per million, or one milligram per
tion is called the hydrologic cycle (water cycle). (See
litel~ are sulfate, magnesium, calcitun, and potassium.
diagram.) Plants on land also add to the amount of water
The remaining elements are present in extremely small
vapor entering the air by the process called trallspiratioll.
amounts. TIlis is a special term used to identify the evaporation
The total salt in seawater is expressed in parts per
process through plants and trees.
thousand. Ocean salinity varies between 32 and 37 parts
There are nearly 329 million cubic miles of seawater
per thousand (3 to 4 percent by volume), with open
on our globe. The dissolved minerals carried to the ocean
ocean waters usually about 35. (TIlat is, if a seawater
in the hydrologic cycle represent fantastic amounts of
droplet were divided into 1,000 tiny parts, there would
every known element. In only 1 cubic mile of semvater, it
be 965 parts of water and 35 parts of salt.) The enclosed
is estimated that there are nearly 165 million tons of dis-
basins and seas have higher salt concentrations. For ex-
solved minerals, as shown in the following list:
ample, the Mediterranean Sea has about 3S.5, and some
areas in the Red Sea, particularly during the summer Sodium chloride (conunon salt) 128,000,000 tons
months, have salinities as high as 41, the highest salinity Magnesium chloride 17,900,000 tons
values in the world ocean. Landlocked lakes that serve as Magnesium sulfate 7,900,000 tons
basins for water running off surro,mding land, like the Calcium sulfate 5,900,000 tOilS
Great Salt Lake of Utah or the Dead Sea of Israel, with Potassium sulfate 4,000,000 tons
Calcium carbonate (lime) 57S,832 tons
salinities of 250 and 350, have the highest salt content of
Magnesiunl bromide 350,000 tons
any bodies of water on Earth.
Bromine 300,000 tons
How did the ocean water get salty? The early world Strontium 60,000 tons
ocean probably was much less salty than toda)"s ocean, Boron 21,000 tons
since most of the water came from rains caused by the Fluorine 6,400 tons
condensation of steanl from escaping water vapors of the Barium 900 tons
developing Earth. But for millions of years, rain and Iodine 100--1,200 tons
Arsenic 50-350 tons
melted snow have been running over the land, dissolv-
Rubidium 200 tons
ing various minerals and carrying them dmvn to the sea.
Silver up to 45 tons
In fact, the salts of the ocean are the result of over 2 Copper, lead, manganese, zinc 10-30 tons
billion years of wearing away of the rocks of Earth's Gold up to 25 tons
crust. Those materials that are soluble (can be dissolved) Uranium 7 tons