Why is the ocean salty?

Why is the Ocean Salty?

By Herbert Swenson
US Geological Survey Publication

All water, even rain water, contains dissolved chemicals which scientists call "salts." But not all water tastes salty. Water is fresh or salty according to individual judgment, and in making this decision man is more convinced by his sense of taste than by a laboratory test. It is one's taste buds that accept one water and reject another.

A simple experiment illustrates this. Fill three glasses with water from the kitchen faucet. Drink from one and it tastes fresh even though some dissolved salts are naturally present. Add a pinch of table salt to the second, and the water may taste fresh or slightly salty depending on a personal taste threshold and on the amount of salt held in a "pinch." But add a teaspoon of salt to the third and your taste buds vehemently protest that this water is too salty to drink; this glass of water has about the same salt content as a glass of sea water.

Obviously, the ocean, in contrast to the water we use daily, contains unacceptable amounts of dissolved chemicals; it is too salty for human consumption.

HOW SALTY IS THE OCEAN?...

How salty the ocean is, however, defies ordinary comprehension. Some scientists estimate that the oceans contain as much as 50 quadrillion tons (50 million billion tons) of dissolved solids.

If the salt in the sea could be removed and spread evenly over the Earth's land surface it would form a layer more than 500 feet thick, about the height of a 40-story office building. The saltiness of the ocean is more understandable when compared with the salt content of a fresh-water lake. For example, when 1 cubic foot of sea water evaporates it yields about 2.2 pounds of salt, but 1 cubic foot of fresh water from Lake Michigan contains only one one-hundredth (0.01) of a pound of salt, or about one sixth of an ounce. Thus, sea water is 220 times saltier than the fresh lake water. What arouses the scientist's curiosity is not so much why the ocean is salty, but why it isn't fresh like the rivers and streams that empty into it. Further, what is the origin of the sea and of its "salts"? And how does one explain ocean water's remarkably uniform chemical composition? To these and related questions, scientists seek answers with full awareness that little about the oceans is understood.

Illustration: Sources of salts in the ocean.

THE ORIGIN OF THE SEA...

In popular language, "ocean" and "sea" are used interchangeably. Today's seas are the North and South Pacific, North and South Atlantic, Indian and Arctic Oceans and the Antarctic waters or seas.

Scientists believe that the seas are as much as 500 million years old because animals that lived then occur as fossils in rocks which once were under ancient seas. There are several theories about the origin of the seas, but no single theory explains all aspects of this puzzle. Many earth scientists agree with the hypothesis that both the atmosphere and the oceans have accumulated gradually through geologic time from some process of "degassing" of the Earth's interior. According to this theory, the ocean had its origin from the prolonged escape of water vapor and other gases from the molten igneous rocks of the Earth to the clouds surrounding the cooling Earth. After the Earth's surface had cooled to a temperature below the boiling point of water, rain began to fall and continued to fall for centuries. As the water drained into the great hollows in the Earth's surface, the primeval ocean came into existence. The forces of gravity prevented the water from leaving the planet.

SOURCES OF THE SALTS...

Sea water has been defined as a weak solution of almost everything. Ocean water is indeed a complex solution of mineral salts and of decayed biologic matter that results from the teeming life in the seas. Most of the ocean's salts were derived from gradual processes such the breaking up of the cooled igneous rocks of the Earth's crust by weathering and erosion, the wearing down of mountains, and the dissolving action of rains and streams which transported their mineral washings to the sea. Some of the ocean's salts have been dissolved from rocks and sediments below its floor. Other sources of salts include the solid and gaseous materials that escaped from the Earth's crust through volcanic vents or that originated in the atmosphere.

IF FRESH WATER FLOWS OUT TO THE SEA, WHY IS THE SEA STILL SALTY?...

The Mississippi, Amazon, and Yukon Rivers empty respectively into the Gulf of Mexico, the Atlantic Ocean, and the Pacific Ocean, all of which are salty. Why aren't the oceans as fresh as the river waters that empty into them? Because the saltiness of the ocean is the result of several natural influences and processes, the salt load of the streams entering the ocean is just one of these factors.

In the beginning the primeval seas must have been only slightly salty. But ever since the first rains descended upon the young Earth hundreds of millions of years ago and ran over the land breaking up rocks and transporting their minerals to the seas, the ocean has become saltier. It is estimated that the rivers and streams flowing from the United States alone discharge 225 million tons of dissolved solids and 513 million tons of suspended sediment annually to the sea. Recent calculations show yields of dissolved solids from other land masses that range from about 6 tons per square mile for Australia to about 120 tons per square mile for Europe. Throughout the world, rivers carry an estimated 4 billion tons of dissolved salts to the ocean annually. About the same tonnage of salt from the ocean water probably is deposited as sediment on the ocean bottom, and thus, yearly gains may offset yearly losses. In other words, the oceans today probably have a balanced salt input and outgo.

Past accumulations of dissolved and suspended solids in the sea do not explain completely why the ocean is salty. Salts become concentrated in the sea because the Sun's heat distills or vaporizes almost pure water from the surface of the sea and leaves the salts behind. This process is part of the continual exchange of water between the Earth and the atmosphere that is called the hydrologic cycle. Water vapor rises from the ocean surface and is carried landward by the winds. When the vapor collides with a colder mass of air, it condenses (changes from a gas to a liquid) and falls to Earth as rain. The rain runs off into streams which in turn transport water to the ocean. Evaporation from both the land and the ocean again causes water to return to the atmosphere as vapor and the cycle starts anew. The ocean, then, is not fresh like river water because of the huge accumulation of salts by evaporation and the contribution of raw salts from the land. In fact, since the first rainfall, the seas have become saltier.

SEA WATER IS NOT SIMPLE...

Scientists have studied the ocean's water for more than a century, but they still do not have a complete understanding of its chemical composition. This is partly due to the lack of precise methods and procedures for measuring the constituents in sea water. Some of the problems confronting scientists stem from the enormous size of the oceans, which cover about 70 percent of the Earth's surface, and the complex chemical system inherent in a marine environment in which constituents of sea water have intermingled over vast periods of time. At least 72 chemical elements have been identified in sea water, most in extremely small amounts. Probably all the Earth's naturally occurring elements exist in the sea. Elements may combine in various ways and form insoluble products (or precipitates) that sink to the ocean floor. But even these precipitates are subject to chemical alteration because of the overlying sea water which continues to exert its environmental influence.

SALINITY AND ITS VARIABILITY...

Oceanographers report salinity (total salt content) and the concentrations of individual chemical constituents in sea water -- chloride, sodium, or magnesium for example -- in parts per thousand, for which the symbol o/oo is used. That is, a salinity of 35 o/oo means 35 pounds of salt per 1,000 pounds of sea water. Similarly, a sodium concentration of 10 o/oo means 10 pounds of sodium per 1,000 pounds of water.

The salinity of ocean water varies. It is affected by such factors as melting of ice, inflow of river water, evaporation, rain, snowfall, wind, wave motion, and ocean currents that cause horizontal and vertical mixing of the saltwater.

THE SALTIEST WATER...

The saltiest water (40 o/oo ) occurs in the Red Sea and the Persian Gulf, where rates of evaporation are very high. Of the major oceans, the North Atlantic is the saltiest; its salinity averages about 37.9 o/oo. Within the North Atlantic, the saltiest part is the Sargasso Sea, an area of about 2 million square miles, located about 2,000 miles west of the Canary Islands. The Sargasso Sea is set apart from the open ocean by floating brown seaweed "sargassum" from which the sea gets its name. The saltiness of this sea is due in part to the high water temperature (up to 83º F) causing a high rate of evaporation and in part to its remoteness from land; because it is so far from land, it receives no fresh-water inflow.

Low salinities occur in polar seas where the salt water is diluted by melting ice and continued precipitation. Partly landlocked seas or coastal inlets that receive substantial runoff from precipitation falling on the land also may have low salinities. The Baltic Sea ranges in salinity from about 5 to 15 o/oo. The salinity of the Black Sea is less than 20 o/oo. Water of the Puget Sound in the Tacoma, Wash., area ranges in salt content from 21 to about 27 o/oo. This area is drained by a number of fresh-water streams which discharge an average of about 4.1 billion gallons of water per day into Puget Sound. Salinity of sea water along the coastal areas of the conterminous United States varies with the month of the year as well as with geographic location. For example, the salinity of the ocean water off Miami Beach, Fla., varies from about 34.8 o/oo in October to 36.4 o/oo in May and June, while diagonally across the country, off the coast of Astoria, Oregon, the salinity of sea water varies from 0.3 o/oo in April and May to 2.6 o/oo in October. The water off the coast of Miami Beach has a high salt content because it is undiluted sea water. Off the coast of Astoria, however, the sea water is less saline because it is mixed with the fresh water of the mighty Columbia.

Sometimes river water travels far from shore before it mixes with sea water. This is shown by data gathered from a study of the Columbia River, which, in an average year, carries to the ocean enough water to cover an area of 1 million acres to a depth of 197 feet. Using a radio- active tracer, scientists at Oregon State University have followed the river's water from its mouth near Astoria to a point southwest of Coos Bay, 217 miles away.

The salt content of the open oceans, free from land influences, is rarely less than 33 o/oo and seldom more than 38 o/oo. Throughout the world, the salinity of sea water averages about 35 o/oo. This average salinity was obtained by William Dittmar in 1884 from chemical analyses of 77 sea water samples collected from many parts of the world during the scientific expedition of the British corvette, H.M.S. Challenger. The Challenger expedition, organized by the British Government at the suggestion of the Royal Society, set out to study the biology of the sea, examine the chemical and physical properties of the water, sample deposits on the ocean floor, and measure water temperatures. The voyage began in 1872 and ended almost 4 years later after covering 68,890 nautical miles. This expedition remains today the longest continuous scientific investigation of the ocean basins. Dittmar's 77 samples are still the only worldwide set of samples of sea water for which complete data (each principal constituent) on chemical composition are available. More recent data, reflecting improvements in analytical and sampling techniques, show slight deviations from Dittmar's results, but these changes do not affect the overall usefulness of his work. The average composition of the 77 samples is as shown on the following table.

The salinity of water in the open sea is not fixed at 35 o/oo even in areas distant from land; that figure is only an average. On a worldwide basis, a maximum salinity of 36 o/oo occurs at about latitudes 20º N. and 20º S. The average salinity of sea water, 35 o/oo , occurs at the Equator. A minimum salinity of 31 o/oo corresponds approximately with latitude 60º N., whereas lowest salinities of 33 o/oo in the Southern Hemisphere occur at latitude 60º S. At the Equator, where salinity is 35 o/oo, the dilution of sea water by rain is offset by the loss of water by evaporation. But in the latitudes bordering the Equator the opposite condition prevails -- evaporation exceeds rainfall because high temperatures plus increased winds accelerate evaporation losses.

HOW SEA LIFE AFFECTS SEA WATER'S COMPOSITION...

Inasmuch as the oceans receive most of their water from the rivers, the ratios (as distinguished from the total amounts) of different chemical constituents should be about the same in both regardless of total salt content. But this is not so. Comparisons of Dittmar's data on ocean water with the average salt concentrations in river waters of the world is as shown on the preceding table (above).

Sea water and river water obviously are very different from each other:

(1) Sodium and chloride (the components of common table salt) constitute a little more than 85 percent of the dissolved solids in ocean water and give to the water its characteristic salty taste, but they represent less than 16 percent of the salt content of river water.

(2) Rivers carry to the sea more calcium than chloride, but the oceans nevertheless contain about 46 times more chloride than calcium.

(3) Silica is a significant constituent of river water but not of sea water.

(4) Calcium and bicarbonate account for nearly 50 percent of the dissolved solids in river water yet constitute less than 2 percent of the dissolved solids in ocean water. These variations seem contrary to what one would expect.

Part of the explanation is the role played by marine life -- animals and plants -- in ocean water's composition. Sea water is not simply a solution of salts and dissolved gases unaffected by living organisms in the sea. Mollusks (oysters, clams, and mussels, for example) extract calcium from the sea to build their shells and skeletons. Foraminifers (very small one-celled sea animals) and crustaceans (such as crabs, shrimp, lobsters, and barnacles) likewise take out large amounts of calcium salts to build their bodies. Coral reefs, common in warm tropical seas, consist mostly of limestone (calcium carbonate) formed over millions of years from the skeletons of billions of small corals and other sea animals. Plankton (tiny floating animal and plant life) also exerts control on the composition of sea water. Diatoms, members of the plankton community, require silica to form their shells and they draw heavily on the ocean's silica for this purpose.

Some marine organisms concentrate or secrete chemical elements that are present in such minute amounts in sea water as to be almost undetectable: Lobsters concentrate copper and cobalt; snails secrete lead; the sea cucumber extracts vanadium; and sponges and certain seaweeds remove iodine from the sea.

Sea life has a strong influence on the composition of sea water. However, some elements in sea water are not affected to any apparent extent by plant or animal life. For example, no known biological process removes the element sodium from the sea.

In addition to biological influences, the factors of solubility and physical-chemical reaction rates also help to explain the composition of sea water. The solubility of a constituent may limit its concentration in sea water. Excess calcium (more calcium than the water can hold) may be precipitated out of the water and deposited on the sea floor as calcium carbonate. Presumably as a result of physical-chemical reactions not well understood, the metal manganese occurs as nodules in many places on the ocean floor. Similarly, phosphorite (phosphate rock) is found in large amounts on the sea bottom off southern California and in lesser amounts in several other places.

NEAR-CONSTANT RATIOS OF MAJOR CONSTITUENTS...

Although the composition of sea water differs from that of river water, the proportions of the major constituents of sea water are almost constant throughout the world. Dittmar's 77 samples showed no significant global differences in relative composition, and his average concentrations are used today to represent the ratios of major constituents in sea water. The analyses, which Dittmar made over a period of 9 years, further showed that chloride, sodium, magnesium, sulfate, calcium, and potassium make up 99 percent of the dissolved solids in sea water. Dittmar's findings may be expressed in another way: although the salinity or total salt content may vary from place to place, the ratio of any one major constituent of sea water (chloride as an example) to the total content is nearly constant. However, the ratios of the less abundant elements (aluminum, copper, tin, and bismuth, for example) to total salt content are not constant nor are those of dissolved gases such as oxygen, carbon dioxide, and nitrogen. But establishment of the near constancy of the ratios of major constituents of sea water is important because it enables scientists to measure one principal element and then, by projection of ratios and correction for temperature and pressure, to calculate the other components in the water, thereby determining its salinity.

SUMMARY...

The ocean is salty because of the gradual concentration of dissolved chemicals eroded from the Earth's crust and washed into the sea. Solid and gaseous ejections from volcanoes, suspended particles swept to the ocean from the land by onshore winds, and materials dissolved from sediments deposited on the ocean floor have also contributed. Salinity is increased by evaporation or by freezing of sea ice and it is decreased as a result of rainfall, runoff, or the melting of ice. The average salinity of sea water is 35 o/oo, but concentrations as high as 40 o/oo are observed in the Red Sea and the Persian Gulf. Salinities are much less than average in coastal waters, in the polar seas, and near the mouths of large rivers.

Sea water not only is much saltier than river water but it also differs in the proportion of the various salts. Sodium and chloride constitute 85 percent of the dissolved solids in sea water and account for the characteristic salty taste. Certain constituents in sea water, such as calcium, magnesium, bicarbonate, and silica, are partly taken out of solution by biological organisms, chemical precipitation, or physical-chemical reactions. In open water the chemical composition of sea water is nearly constant. Because of the stable ratios of the principal constituents to total salt content, the determination of one major constituent can be used to calculate sea water salinity. For minor constituents and dissolved gases the composition is variable and therefore ratios cannot be used to calculate salt Circulation and mixing, density and ocean currents, wind action, water temperature, solubility, and biochemical reactions are some of the factors that explain why the composition of water in the open sea is almost constant from place to place.

This information is from a general interest publication ("Why is the Ocean Salty?" By Herbert Swenson) prepared by the U.S. Geological Survey to provide information about the earth sciences, natural resources, and the environment.

To obtain a catalog of additional titles in the series "General Interest Publications of the U.S. Geological Survey," write:

U.S. Geological Survey

Branch of Distribution

P.O. Box 25286

Denver, CO 80225

For sale by the U.S. Government Printing Office Superintendent of Documents, Mail Stop: SSOP, Washington, DC 20402-9328

Illustrate sea-floor spreading and subduction by building a model

How to Build a Model Illustrating Sea-Floor Spreading and Subduction

by

John C. Lahr

Open-File Report 99-132, On-Line Edition

(MS Word97 Version for Printing)

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Denver Federal Center
Box 25946, Mail Stop 966
Denver, CO 80225
Phone: (303) 273-8596
Email: lahr@usgs.gov

Introduction

This report describes how to build a model of the outer 300 km (180 miles) of the Earth that can be used to develop a better understanding of the principal features of plate tectonics, including sea-floor spreading, the pattern of magnetic stripes frozen into the sea floor, transform faulting, thrust faulting, subduction, and volcanism.

In addition to a paper copy of this report, the materials required are a cardboard shoebox, glue, scissors, straight edge, and safety razor blade.

Structure of the Earth

The Earth consists of an iron-rich core with a radius of 3,500 km (2,100 miles), surrounded by a 2,800-km- (1,680-mile-) thick mantle of mostly silicon, magnesium, and oxygen, and finally an 80-km- (50-mile-) thick lithosphere. While 96% of the volume of the core is liquid, there is a solid inner core with a radius of 1,200 km (720 miles). Electric currents within the metallic-liquid outer core create the Earth's magnetic field. This magnetic field is oriented approximately parallel to the rotation pole of the Earth.

.

Plate Tectonics

The mantle, which is much more solid than the outer core, is slowly convecting due to the increase of temperature with depth within the Earth. This motion can be compared to the convective motion of water in a pan that is being heated on a stove; however, the movement of the mantle is much, much slower. The lithosphere, the outer hard shell of the Earth, is broken into a dozen or so major pieces, called plates, and these plates are moving with respect to one another. At one time North America, South America, Europe, and Africa were joined together in one giant continent that has since broken apart to form the Atlantic Ocean Basin. The following figure (Stacy, 1977) shows how these plates once fit together.

The process of sea-floor spreading created the lithosphere under the Atlantic Ocean. As North America and South America moved away from Europe and Africa, the resulting crack was filled by mantle material, which cooled and formed new lithosphere. The process continues today. Molten mantle materials continually rise to fill the cracks formed as the plates move slowly apart from each other. This process creates an underwater mountain chain, known as a mid-ocean ridge, along the zone of newly forming lithosphere.

Magnetic Stripes

Molten rock erupts along a mid-ocean ridge, then cools and freezes to become solid rock. The direction of the magnetic field of the Earth at the time the rock cools is "frozen" in place. This happens because magnetic minerals in the molten rock are free to rotate so that they are aligned with the Earth's magnetic field. After the molten rock cools to a solid, these minerals can no longer rotate freely. At irregular intervals, averaging about 200-thousand years, the Earth's magnetic field reverses. The end of a compass needle that today points to the north will instead point to the south after the next reversal. The oceanic plates act as a giant tape recorder, preserving in their magnetic minerals the orientation of the magnetic field present at the time of their creation. Geologists call the current orientation "normal" and the opposite orientation "reversed."

In the figure below, two plates are moving apart. A mid-ocean ridge marks the location where molten rocks are moving up, cooling, and forming new ocean floor. The zones of normal magnetization are indicated by ////// shading of the oceanic crust.

The figure below shows the observed magnetic pattern along the mid-Atlantic Ridge south of Iceland. This figure is from the excellent publication "A Teacher's Guide to the Geology of Hawaii Volcanoes National Park" (Mattox, 1992), which is available on the World Wide Web at http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part9.html. The Guide starts at http://volcano.und.nodak.edu/vwdocs/vwlessons/atg.html .

Based on the pattern and spacing of the oceanic magnetic stripes and the inferred motion of the plates, the age of the ocean floor can be determined. In the figure below by Müller and others (1997) ( http://gdcinfo.agg.emr.ca/app/jgr_paper.html ), the age of the ocean floor is depicted by colors (http://gdcinfo.agg.emr.ca/app/images/agemap.GIF ).

Convergent Margins

In some places, oceanic plates collide with continental plates. When this occurs, the heavier oceanic lithosphere sinks beneath the continental plate. This process, called subduction, creates a very deep trough near the line of contact between the oceanic and continental plates. This trough is called an oceanic trench. As an oceanic plate is subducted into the Earth it is subjected to increased pressure and temperature. These conditions cause some lightweight materials to melt and rise to the surface to form volcanoes. As a result, long chains of volcanoes, called volcanic arcs, are located above subducted plates, usually above the location where the plate has reached a depth of about 100 km.

Earthquakes

Geologic deformation is usually very slow, measured in centimeters per year, so the dynamic processes that continually reshape the Earth are, for the most part, unnoticed. Earthquakes are an occasional reminder that this deformation is indeed taking place, and infrequent, but potentially damaging, large earthquakes pose a hazard that is all too often ignored.

Earthquakes occur wherever two plates slip past one another, such as along the San Andreas Fault in California. This type of faulting is called "strike-slip." Earthquakes also occur where one plate slides under the other, as is happening in southern Alaska. "Thrust" earthquakes have been responsible for the two largest earthquakes ever recorded, the 1960 M 9.5 Chile earthquake and the 1964 M 9.2 Alaska earthquake. Pull-apart earthquakes occur where the lithosphere is being stretched, such as along mid-ocean ridges. These earthquakes generate "normal-faulting" events. When two ridges are connected by a fault, this is called a transform fault, and the fault is the source of strike-slip events.

When completed, your model will look something like the figure below. Follow the directions on the following pages to complete the model. Coloring with paint or magic markers can be

added to enhance the model. Note that the shaded portions of the sea floor have the same magnetization as today, while the unshaded portions have reversed magnetization.

Operation of the Model

Slide the sea floor into the three slits in the shoebox and pull it down from underneath so that the numbers "4" are just visible. Slide the sea floor into the trench as well. This is the position the sea floor would have had 4 million years ago. Now slowly pull down with one hand on the sea floor beneath the trench, while pulling with the other hand on opposite end of the sea floor. As the sea floor diverges at the ridges, new sea floor will continually appear until the dark gray bands indicating the current epoch of normal magnetic polarity have been revealed.

Now move the sea floor back to the 4 my before present position, and this time, while the sea floor is moving, watch the boundaries where slip is occurring. Where would the three styles of earthquake -- namely thrust, normal, and strike-slip -- occur?

The USGS National Earthquake Information Center has placed seismicity maps for many regions of the World on their World Wide Web site: http://gldss7.cr.usgs.gov/neis/general/seismicity/seismicity.html . Look at some of these maps and see if you can tell where the plate boundaries are located. Where are the deepest earthquakes located and why are they there?

From the ages of the sea floor and the scale of the model, which is the same in the vertical and horizontal directions, one can determine that the plate in this model is being subducted at a rate of about 50 km/my. This works out to 5 cm/y, which is within the 1 to 10 cm/y range that most plates are moving. As an interesting comparison, this is about the rate that fingernails grow!

Suggested Additional Reading

Kious, W. J., and Tilling, R I., 1992, This Dynamic Earth, The Story of Plate Tectonics, U. S. Geological Survey, Booklet 92-TDE, 77 p.

This publication can be found on the World Wide Web at http://pubs.usgs.gov/publications/text/dynamic.html .

Acknowledgements

The author would like to acknowledge the careful reviews by E. Cranswick and P. Detra. In particular, the latter's suggestion to reduce the size of the model to that of a shoebox was most helpful.

References

Mattox, S. R., 1992, A teacher's guide to the geology of Hawaii Volcanoes National Park, published by the Hawaii Natural History Association, P.O. Box 74, Hawaii National Park, HI 96718, (808) 967-7604.

Müller, R. D., Roest, W. R., Royer, J.-Y., Gahagan, L. M., and Sclater, J. G., 1997, Digital isochrons of the World's ocean floor, Journal of Geophysical, V. 102 (B2), p. 3211-3214.

Stacey, F. D., 1977, Physics of the Earth, John Wiley & Sons, New York, 414 pages.

Sea-floor Spreading Model Directions

1) Use a shoe box with approximate dimensions 6.5" x 13" by 5" deep.

2) Trim as indicated the pages labeled "Left portion of side of shoe box" and "Right ...". Glue these two pages together to form the cross section through the Earth.

3) Turn the shoe box over, so that the opening is down. Glue the cross section sheets that you have just glued together to the long side of the shoe box. The surface of the ocean should be even with the top of the shoe box while the volcano
wiill stick up a bit.

4) Trim as indicated the pattern for cutting slits in the shoe box. Orient the slit-pattern sheet as in the figure above, so that the mid-ocean ridge slit and the trench slit match the cross section on the side of the box. Cut the four slits through the shoe box. Discard the slit pattern and the four tabular pieces of cardboard from the slits.

5) Cut the four slits through the shoe box. Discard the slit pattern and the four tabular pieces of cardboard from the slits.

How does the depth of the ocean affect its temperature?

Chapter 4 - Ocean Chemistry

Scientists use a variety of instruments to measure salinity, temperature, and oxygen in the world's oceans. This information is then used to generate Hydrographic maps that depict the physical and chemical structure of the world's oceans.

The temperature of the ocean generally decreases with depth. Why?

The highest average temperature is about 25 ^oC at the surface. This temperature can still be found at 50 m depth. However cooler temperatures occur off the west coast of South America at this depth. Why? At about 200 m depth the pools of 25 ^oC water have almost disappeared. By 600 m there are just traces in the western North Atlantic. One can see that the Pacific is colder than the Atlantic. By 1100 m the Pacific is generally <>oC. Interestingly one can see a tongue of warm water in the eastern Atlantic, near the mouth of the Mediterranean. Why? Are there any other pools of warm water? By 1750 m depth the water is generally <4>oC, but one can still see the tongues of warm water. Why? How great is the overall change in temperature with depth in the ocean?

The pattern for salinity is somewhat different. Take a look at the maps for the surface, 50 m, 200 m, 600 m, and 1100 m. Instead of generally decreasing with depth, salinity increases. Why? How great is the change in salinity? Is it as great as the temperature change? Why is there a tongue of high salinity water in the eastern North Atlantic? Are there any other areas that show a particularly high salinity?

Sound

The physical properties of seawater affect the transmission of sound. The speed of sound decreases with decreasing temperature and increases with increasing pressure. The relationship of these two factors results in a zone of minimum sound velocity in the ocean referred to as the SOFAR channel. This channel is important for a variety of reasons. Sound is used by a variety of marine organisms to communicate and it appears that many marine organisms use the SOFAR channel for this purpose.

Acoustic Thermometry is the field of study that relies on sound to measure the temperature of the oceans. Scientists have proposed using sound to assess global warming. This field experiment has generated quite a bit of controversy. In order to move forward with the experiment additional studies were needed to determine the potential impact on marine mammals.

Chemical resources

• The ocean is a source of many natural chemical resources;
salt is one of them.

Hydrologic cycle

While 97% of the earth's surface water occurs in the oceans the remainder is found in several other reservoirs. In fact the water moves from reservoir to reservoir fueled by the energy from the sun. Water evaporates, is transported, and eventually returns to the earth's surface as precipitation. The movement between reservoirs is called the Hydrologic Cycle.