2. Composition of Seawater
Seawater consists of about 3.5 percent (by weight)
dissolved mineral substances that are collectively
termed “salts.”
Salinity is the total amount of solid material
dissolved in water; commonly expressed in percent
(%), or parts per thousand (‰).
average salinity of seawater is 3.5%, or 35‰.
5. What are the primary sources for the vast quantities
of dissolved substances in the ocean?
1. Chemical weathering of rocks on the continents These dissolved
materials are delivered to the oceans by streams at an estimated rate of
more than 2.3 billion metric tons (2.5 billion short tons) annually
2. Earth’s interior Through volcanic eruptions, large quantities of
water vapor and other gases have been emitted during much of
geologic time.
Certain elements—notably chlorine, bromine, sulfur, and boron—were
outgassed along with water and exist in the ocean in much greater abundance
than could be explained by weathering of rocks alone.
6. Why doesn’t the sea get saltier?
material is being removed just as rapidly as it is added
8. Salinity
Surface salinity variation in the open ocean normally
ranges from 33‰ to 38‰.
Some marginal seas, however, demonstrate
extraordinary extremes.
For example, in the restricted waters of the Middle East’s
Persian Gulf and Red Sea—where evaporation far
exceeds precipitation—salinity may exceed 42‰.
Conversely, very low salinities occur where large quantities
of freshwater are supplied by rivers and precipitation. Such
is the case for northern Europe’s Baltic Sea, where salinity
is often below 10‰.
9. Concept Check
What is salinity and how is it usually expressed?
What is the average salinity of the ocean?
What are the six most abundant elements dissolved in seawater?
What is produced when the two most abundant elements combine?
What are the two primary sources for the elements that comprise the
dissolved components in seawater?
List several factors that cause salinity to vary from place to place and
from time to time.
10. Ocean Temperature Variation
The ocean’s surface
water temperature varies
with the amount of solar
radiation received, which
is primarily a function of
latitude
average temperatures
are highest near the
equator and get colder
moving poleward
11. Temperature Variation with Depth
Certain high-latitude regions
experience an extremely weak
seasonal thermocline.
Midlatitude waters experience
a more dramatic seasonal
thermocline.
12. Ocean Temperature Change over Time
Seawater has many unique thermal properties that
make it resistant to changes in temperature
Studies on the effect of global warming on the ocean
indicate that atmospheric warming will eventually be
transferred to the ocean
13. How can scientists determine if the average temperature of the global
ocean is actually changing?
The speed of sound in seawater increases as temperature increases
Scientists have used a worldwide sound channel—called the SOFAR
channel(SOund Fixing And Ranging)— to transmit low-frequency sound
across an ocean basin to a distant receiver.
The experiment—which is called ATOC (Acoustic Thermometry of
Ocean Climate)—is designed to accurately measure the travel time of
similar sound signals through the SOFAR channel now and in the future.
Scripps Institution of Oceanography successfully tested the experiment
by transmitting signals across the Pacific Ocean in 1991 and 1995; (led by
Walter Munk)
14. Concept check
What one factor is primarily responsible for influencing
seawater temperature?
Describe temperature variation with depth in both high and
low latitudes.
Why do high-latitude waters generally lack a thermocline?
15. Ocean Density variation
Density is defined as mass per unit volume but can be
thought of as a measure of how heavy something is for its
size.
Density determines the water’s vertical position in the ocean.
Density differences cause large areas of ocean water to sink
or float. (ex: mixing of freshwater with saltwater).
16. Factors Affecting Seawater Density
1. salinity
An increase in salinity adds dissolved substances and results
in an increase in seawater density
2. temperature
As the ocean temperature decreases, its density increases.
Cold water that also has high salinity is some of the highest-
density water in the world.
17. The Dead Sea, which has
a salinity of 330‰, has
high density.
As a result, it also has high
buoyancy that allows
swimmers to float easily.
(Photo by Peter
Guttman/CORBIS/Bettmann)
18. Density Variation with Depth
Pynocline =
layer of rapidly
changing density
Isopycnal =
constant density
19. Ocean Layering
Three main layers in the ocean based on water density:
1. The warm surface mixed layer accounts for only 2 percent of ocean water;
2. the transition zone includes the thermocline and pycnocline and accounts for 18 percent of ocean
water;
3. the deep zone contains cold, high-density water that accounts for 80 percent of ocean water.
20. Ocean Layering
In high latitudes, the three-layer structure of
ocean layering does not exist because the
water column is isothermal and isopycnal, which
means that there is no rapid change in
temperature or density with depth.
21. Concept check
Which two factors influence seawater density? Which one has
the greater influence on surface seawater density?
Describe density variation with depth in both high and low
latitudes. Why do high-latitude waters generally lack a
pycnocline?
Describe the ocean’s layered structure. Why does the three
layer structure not exist in high latitudes?
23. A. Sea star.
(Photo by David
Hall/Photo
Researchers, Inc.);
B. Yellow tube
sponge. (Photo
by Andrew
Martinez/Photo
Researchers, Inc.);
C. Green sea
urchin. (Photo by
Andrew
Martinez/Photo
Researchers, Inc.);
D. Coral crab.
(Photo by Images &
Stories/Alamy)
