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1Earth’s OceansKey Concept The characteristics of ocean water, such as temperature and salinity, affect the circulation of the ocean.

Earth is unique in our solar system because 71% of its surface is covered with liquid water. Most of Earth’s water is in the globalocean. The global ocean is divided by the continents into five main oceans. These five main oceans are shown in Figure 1. The global ocean has characteristics that play an important role in regulating Earth’s climate.

Divisions of the Global OceanThe largest ocean is the Pacific Ocean. It lies between Asia

and the Americas. The volume of the Atlantic Ocean, the second-largest ocean, is about half the volume of the Pacific. The Indian Ocean is the third-largest ocean. It is located between Africa and Australia. The Arctic Ocean is the smallest ocean. This ocean is unique because much of its surface is covered by ice. The Southern Ocean extends from the coast of Antarctica to 60° south latitude.

What You Will Learn• Ocean water contains dissolved

solids that make the water salty.

• The temperature of ocean water varies with depth, latitude, and movement of the water.

Why It MattersThe global ocean’s temperature and salinity affect climate and life on Earth.

Vocabulary• salinity• thermocline

Prediction Guide Before reading this section, write each heading from this section in your Science Journal.Below each heading, write what you think you will learn.

Atlantic Ocean

Pacific Ocean Indian Ocean

Arctic Ocean

Southern Ocean

Parts of the Global OceanFigure 1

Chlorine55.0%

Sodium30.6%

Magnesium7.7%

Sulfur3.7%

Calcium1.2%

Potassium1.1%

Other0.7%

Section 1 Earth’s Oceans 401

Characteristics of Ocean WaterOcean water is different from the water that flows from

your sink at home. For one thing, you can’t drink ocean water.It is too salty and doesn’t taste very good. But there are otherthings that make ocean water special.

Ocean Water Is SaltyHave you ever swallowed water while swimming in the

ocean? It tasted really salty, didn’t it? Most of the salt in theocean is the same kind of salt that we sprinkle on our food.This salt is called sodium chloride. This compound consists ofthe elements sodium, Na, and chlorine, Cl. There are manyother dissolved solids in ocean water, such as magnesium andcalcium. Figure 2 shows the relative amounts of the dissolvedsolids in ocean water.

Salts have collected in the ocean for billions of years. Asrivers and streams flow toward the ocean, they dissolve miner-als from the land. The running water carries these dissolvedminerals to the ocean. At the same time, water is evaporatingfrom the ocean. As the water evaporates, it leaves the dissolvedsolids behind. The most abundant dissolved solid in the oceanis sodium chloride.

SalinityA measure of the amount of dissolved solids in a given

amount of liquid is called salinity.salinity. Salinity is usually measuredas grams of dissolved solids per kilogram of water. Every 1 kg(1,000 g) of ocean water has 35 g of dissolved solids in it.Therefore, if you evaporated 1 kg of ocean water, 965 g of freshwater would be removed and 35 g of solids would remain.

Figure 2 This pie graph showsthe relative percentages ofdissolved solids (by mass) inocean water.

salinitysalinity (suh LIN uh tee) a measureof the amount of dissolved salts in agiven amount of liquid

6.3.c Students know heat fl ows in solids byconduction (which involves no fl ow of matter) andin fl uids by conduction and by convection (whichinvolves fl ow of matter).6.4.d Students know convection currents distributeheat in the atmosphere and oceans.

Chapter 12 Exploring the Oceans402

Climate Affects SalinitySome parts of the ocean are saltier than others, as shown

in Figure 3. In places that have hotter, drier climates, salinityis usually higher. In these areas, the evaporation rate is highbecause the temperatures are high. Evaporation removes waterbut leaves salts and other dissolved solids behind. The salin-ity of the Red Sea is very high because the hot, dry climatearound the Red Sea causes a high rate of evaporation.

Coastal waters commonly have lower salinity than watersfarther offshore. Fresh water from streams and rivers on landruns into the ocean in these areas. As the fresh water mixeswith the ocean water, the concentration of salts in the oceanwater decreases. The mouth of the Amazon is one such loca-tion, as shown in Figure 3.

Water Movement Affects SalinityAnother factor that affects ocean salinity is water move-

ment. Slower-moving areas of water develop higher salinity.Some parts of the ocean, such as bays, gulfs, and seas, moveless than other parts do. Parts of the open ocean that do nothave currents running through them can also be slow mov-ing. In Figure 3, identify the areas that are most likely to haveslower-moving water.

Figure 3 Salinity variesin different parts ofthe ocean because ofvariations in evaporation,circulation, and freshwater inflow. Does thearea of the oceannear the mouth of theAmazon have a highor low salinity? Explainyour answer.

HudsonBay

Gulf ofMexico Mediterranean

Sea

Mouthof the

Amazon

RedSea

Section 1 Earth’s Oceans 403

Temperature of Ocean WaterThe temperature of ocean water decreases as depth increases.

However, this temperature change is not uniform. Water inthe ocean can be divided into three layers by temperature.Those three layers are the surface zone, the thermocline, andthe deep zone, as shown in Figure 4.

Surface ZoneThe surface zone, or top layer of ocean water, is heated by

the sun’s energy. Heated ocean water becomes less dense andrises above the denser, cool water. This movement of watercaused by differences in density forms convection currents.The convection currents distribute heat in the surface zonedown to a depth of about 100 m to 300 m. The convectioncurrents distribute the heat until the temperature is fairly uni-form throughout the surface zone.

How is heat distributed in the surface zone of

the ocean? 6.4.d

ThermoclineThe sun cannot directly heat ocean water below the surface

zone. And the heated, less dense water of the surface zonecannot easily mix with the cold, dense water below. Therefore,below the surface zone, the temperature of the water decreasessharply as depth increases. The layer of the ocean in whichtemperature drops with increased depth faster than it does inother layers is called the thermocline.thermocline. The thermocline’s depthvaries in different parts of the ocean. However, it may extendbetween 100 m and 1000 m below the ocean’s surface.

Temperature Zones in the OceanFigure 4

thermoclinethermocline (THUHR moh KLIEN)a layer in a body of water in whichwater temperature drops withincreased depth faster than it does inother layersWordwise The root therm- means“heat.” The root clino- means “slope.”Other examples are incline, decline,and thermometer.

How Deep Is It?One area of the ocean is4,000 m deep. Its surfacezone extends to about 300 mbelow the surface of theocean. What percentage ofthe total depth of the oceanin this particular area is thesurface zone? Record yourwork in your Science Journal.

Surface zone The surface zone is the warm, top layer ofocean water. It can extend to 300 m below sea level. Sunlightheats the top 100 m of the surface zone. Convection currentsmix the heated water with cooler water below.

Thermocline The thermocline is the second layer of oceanwater. In the thermocline, temperature drops with increaseddepth faster than it does in the other two zones.

Deep zone The deep zone is the bottom layer that extendsfrom the base of the thermocline to the bottom of the ocean.The temperature in this zone can range from 1°C to 3°C.

Try It!

Think About It!

Quick Lab

Chapter 12 Exploring the Oceans404

Deep ZoneThe layer of the ocean directly below the thermocline is

called the deep zone. In the deep zone of the ocean, the tem-perature of the water is usually about 2°C. The colder the wateris, the denser it is. The density of cold, deep water controlsthe slow movement of deep ocean currents. This movementbegins when the cold, dense water at the poles sinks and flowsbeneath warm water.

Surface Temperature ChangesSurface temperatures of different parts of the ocean are

different depending on the latitude. Water along the equa-tor is warmer because it receives more direct sunlight thanwater closer to the poles does. At low latitudes, ocean surfacetemperatures can be as high as 30°C. But in the polar oceans,temperatures of the ocean surface can be as low as –1.9°C!

The temperature of surface water also changes dependingon the time of the year. The ocean surface at higher latitudesreceives more direct sunlight during the summer season thanduring the winter season. Therefore, during summer, the surfacewater is warmer. For this reason, the Maine beach shown inFigure 5 is crowded during the summer.

Figure 5 People in Maine enjoyswimming in the ocean duringthe summer because the water iswarmer during the summer.

6.3.cDensity FactorsIn this lab you will learn how salinity andtemperature affect the density of ocean water.

20 min

6. Describe what happened when you addedthe red salt water to the fresh water. Whichis denser: fresh water or salt water?

7. What happened when you added the coldwater to the room-temperature water?Which is denser: cold water or room-temperature water?

8. Explain how differences in density mayaffect the movement of ocean water.

1. Fill a deep, clear plastic container half fullwith room-temperature water.

2. In a 1 L beaker, mix 30 g (1/8 cup) oftable salt, three drops of red food color-ing, and 1 L of room-temperature water(about 21°C). Stir the mixture until the salt isdissolved.

3. Slowly add the red saltwater mixture to thewater in the clear plastic container. Recordyour observations. Then, dump out thewater and refill the container half full withroom-temperature water.

4. In the 1 L beaker, mix a three drops of bluefood coloring with water that is 8°C.

5. Slowly add the cold, blue water to the clearplastic container. Record your observations.

Internet Resources

Review

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For a variety of links related to thischapter, go to www.scilinks.org

Summary

DensitySalinity and temperature affect the density of ocean water.

The large amount of dissolved solids in ocean water makesocean water denser than pure fresh water. Ocean water alsobecomes denser as it becomes colder. Water temperature affectsthe density of ocean water more than salinity does. Therefore,the densest ocean water is found in the polar regions, wherethe ocean surface is coldest. Differences in density throughoutthe global ocean drive the circulation of ocean water, whichdistributes heat in the ocean.

What factors affect the circulation of ocean water?

6.3.c

6.3.c, 6.4.d

• The global ocean is dividedby the continents into fivemain oceans: Pacific Ocean,Atlantic Ocean, Indian Ocean,Southern Ocean, and ArcticOcean.

• Salts have collected in theocean for billions of years.Salinity is a measure of theamount of dissolved salts ina given mass of liquid.

• The three temperature zonesof ocean water are the sur-face zone, the thermocline,and the deep zone.

• Temperature and salinitydetermine the density ofocean water. The densityof ocean water drivesconvection currents.

1 In your own words, writedefinitions for salinity and thermocline.

2 Listing Name the majordivisions of the global ocean.

3 Analyzing Why does oceanwater taste salty?

4 Describing Describe the tem-perature layers in ocean water.

5 Comparing Which is moredense, ocean water or freshwater? Why?

6 Analyzing How is heat distrib-uted in the surface zone of theocean?

7 Identifying What causesconvection currents?

8 Predicting Consequences Ifall ocean water was the sametemperature all the way through,how would the movement ofocean water be affected?

INTERPRETING GRAPHICS Usethe graph below to answer the nextquestion.

Chlorine55.0%

Sodium30.6%

Magnesium7.7%

Sulfur3.7%

Calcium1.2%

Potassium1.1%

Other0.7%

9 Identifying RelationshipsWhat is the combined percent-age of magnesium and sulfurin the dissolved solids in oceanwater?

0 Identifying RelationshipsHow does density of ocean wateraffect how heat is distributed inthe ocean?

Topic: Exploring Earth’s OceanSciLinks code: HY70557