deglaciation heralding the end of the last glacial episode 18,000 years ago, the sea
level has risen 120 meters and the ocean has covered much of the continental
shelves of the continents.
Ocean water is salty and these impuritiesreflect the nature of the continental
crust and ocean bottom. Water is capable of dissolving any rock (though at widely
varying rates), and precipitation flow to streams carries dissolved rock components
to the ocean basins. In addition, the contact of the ocean bottom waters with the
oceanic crust is responsible for further solution of rocks. Not surprisingly, ocean
water contains most all known minerals in solution and some of these can be
reclaimed for use. Several minerals are economically extracted. Yet, there are lim-
itations to the use of the mineral content of seawater. A major consideration is that
most minerals are present in trace amounts. It is technologically possible to extract
gold, yet there is only a milligram of gold in every ton of seawater. The cost to
concentrate the gold into usable amounts is far from being economically possible.
Although the variousclimatesandlocationsassociated with the ocean mean
that there are some geographic differences in the waters, ocean water is impres-
sively similar from place to place because the world ocean is subject to slow mix-
ing. By weight, seawater is approximately 96.5 percent water and 3.5 percent
dissolved minerals that are called “salts.” The most common salt is sodium chlo-
ride or table salt. Other salts contain potassium, sulfur, magnesium, and calcium.
The salinity varies geographically. Places that are sunny, warm, and arid have
high rates of evaporation leaving an increased concentration of salts at the surface.
In the narrow Red Sea, which has limited mixing with the rest of the ocean, the
concentration of salts is about 4 percent. Rainy places tend to be a bit less salty
than average. In places where there islittle evaporation the water tends to be a
bit less salty; a prime example is polar ocean water. So, too, there are local varia-
tions in saltiness resulting from the debouching of freshwater streams to the ocean.
The Amazon River was discovered by the Spanish who found potable water out-
side sight of the coastline.
The ocean is vertically zoned. The top waters of the ocean are subject to the
vagaries of climate and vary in temperature and circulation as forced by the atmos-
phere above. Deeper than the first hundred meters exists the thermocline, a pre-
cipitous shift to the colder and more uniform characteristics of the deeper
waters. In this same few hundred meters of transition, density and salinity greatly
increase. Underneath, the waters are impressive for their relative homogeneity of
characteristics.
In general, except for limited areasof upwelling of immense importance to
ocean energy balance and fisheries, the ocean is thermally stable. That is, the deep
waters are cold and near freezing while the top waters are far above freezing over
most of the planet. This provides a lack of impetus for the ocean to circulate via
Oceans 249