Ocean water

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‰.

Relative proportions of water and dissolved components in seawater

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.

Why doesn’t the sea get saltier?

material is being removed just as rapidly as it is added

Processes Affecting Seawater Salinity

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‰.

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.

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

Temperature Variation with Depth

Certain high-latitude regions experience an extremely weak seasonal thermocline.

Midlatitude waters experience a more dramatic seasonal thermocline.

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

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)

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?

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).

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.

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)

Density Variation with Depth

Pynocline = layer of rapidly changing density

Isopycnal = constant density

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.

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.

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?

A. Gray reef shark, Bikini Atoll. (Photo by Yann Hubert/BIOSphoto/Specialist Stock); B. California market squid. (Photo by Tom McHugh/Photo Researchers, Inc.); C. School of grunts, Florida Keys. (Photo by Georgie Holland/age footstock); D. Yellow-head moray eel. (© by David B. Fleetham/Seapics.com)

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)