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FIGURE 24.12 Distribution of the oceans and major seas on Earth’s surface. There is really only one ocean; for example, where is the
boundary between the Pacific, Atlantic, and Indian oceans in the Southern Hemisphere?

patterns. The Arctic Sea, which is also sometimes called the Arctic sediments that settle to the ocean floor. Hard parts of organisms
Ocean, is a part of the North Atlantic Ocean that is less salty. Thus, and solid deposits are cycled to Earth’s interior along with sus-
the terms ocean and sea are really arbitrary terms that are used to pended sediments that have settled out of the seawater. Studies
describe different parts of Earth’s one continuous ocean. of fossils and rocks indicate that the composition of seawater has
changed little over the past 600 million years.
The dissolved materials of seawater are present in the form
THE NATURE OF SEAWATER
of ions because of the strong dissolving ability of water molecules.
According to one theory, the ocean is an ancient feature of Earth’s Almost all of the chemical elements are present, but only six ions
surface, formed at least 3 billion years ago as Earth cooled from its make up more than 99 percent of any given sample of seawater. As
early molten state. The seawater and much of the dissolved ma- shown in Table 24.1, chlorine and sodium are the most abundant
terials are believed to have formed from the degassing of water ions. These are the elements of sodium chloride, or common table
vapor and other gases from molten rock materials. The degassed salt. As a sample of seawater evaporates, the positive metal ions
water vapor soon condensed, and over a period of time, it began join with the different negative ions to form a complex mixture
collecting as a liquid in the depression of the early ocean basin. of ionic compounds known as sea salt. Sea salt is mostly sodium
Ever since, seawater has continuously cycled through the hydro-
logic cycle, returning water to the ocean through the world’s rivers.
For millions of years, these rivers have carried large amounts of TABLE 24.1
suspended and dissolved materials to the ocean. These dissolved Major dissolved materials in seawater
materials, including salts, stay behind in the seawater as the wa-
ter again evaporates, condenses, falls on the land, and then brings Ion Percent (by weight)
more dissolved materials much like a continuous conveyor belt. Chloride (Cl ) 55.05
–
You might wonder why the ocean basin has not become filled +
in by the continuous supply of sediments and dissolved materials Sodium (Na ) 30.61
2–
that would accumulate over millions of years. The basin has not Sulfate (SO 4 ) 2+ 7.68
filled in because (1) accumulated sediments have been recycled to Magnesium (Mg ) 3.69
2+
Earth’s interior through plate tectonics and (2) dissolved materials Calcium (Ca ) 1.16
+
are removed by natural processes just as fast as they are supplied Potassium (K ) 1.10
–
by the rivers. Some of the dissolved materials, such as calcium and Bicarbonate (HCO 3 ) 0.41
–
silicon, are removed by organisms to make solid shells, bones, and Bromine (Br ) 0.19
other hard parts. Other dissolved materials, such as iron, magne- Total 99.89
sium, and phosphorus, form solid deposits directly and also make
608 CHAPTER 24 Earth’s Waters 24-12