UNIT 5 THE BLUE PLANET: PHYSICAL AND CHEMICAL MAKEUP OF THE OCEANS

A. INTRODUCTION

Overview

Early explorers knew only the surface of the seas and their maps showed them as featureless blanks. Once the oceans and continents had been mapped, the observations of seafarers provided the first information on the currents and winds of the oceans. The laying and maintenance of the transatlantic cables provided impetus to obtaining knowledge of the deep ocean environment. Though the oceans are thin compared to their breadth, the greatest volume of the oceans remains inaccessible to the investigator using only SCUBA gear. Modern oceanography uses a wide array of tools to probe the oceanic depths, including depth sounders, physical and chemical measurements of seawater, and cores taken from the sea floor. Plate tectonic processes determine the geography of the ocean floor, while physical, chemical, and biological processes contribute the blanket of sediment that coats the sea floor. Explaining the chemistry of seawater requires interaction between the water and the mantle at the spreading ridges, leading to the conclusion that world geological and geochemical processes are more interrelated than we had previously thought.

B. AN OCEANIC PERSPECTIVE

1. The Ocean Basins

The ocean waters cover 71% of the surface of the Earth and are the most distinctive feature of our planet. Take out a map of the world (or look at Figure 2-3) and consider the dimensions of the oceans. The scale of most flat maps is accurate only at the equator, so measure the widths in kilometers of the Atlantic and Pacific Oceans along the equator in order to get an idea of their size.

How deep is the ocean bottom? From the earlier units you already know that the depth to the ocean floor varies considerably, being shallow on the continental shelves and extending to great depths in the oceanic trenches. You can turn to the hypsometric diagram in Figure 2-1 for an average depth to the abyssal plains -- approximately 4.5 km (15,000 ft). Now compare this depth to the width that you obtained for the Pacific Ocean. The proportion is such that if you were to construct a true scale model of the water in the Pacific Basin, its thickness would be very nearly that of the paper on which your map is printed. Even the deepest point in the oceans (the Mariana Trench) at 11,033 m (36,198 ft), would be represented on your model as an inconspicuous doubling of its thickness in the very small area occupied by the trench. The waters of the oceans, then, constitute a very thin layer covering the mud-covered ocean floor.

In spite of its relative thinness, however, a column of water 5 kilometers in thickness is still extremely heavy, exerting a pressure of some 520 kilograms per square centimeter. This is over 500 times atmospheric pressure at sea level, and is a formidable adversary to explorers who wish to venture to such depths. For most of our history our explorations of the oceans have been confined to the top few meters of the waters and the vast expanse of the oceanic surface.

2. Early Explorations

Before oceanography emerged as a science, knowledge of the oceans was being compiled by sailors. As early as 600 B.C., Phoenician explorers had circumnavigated Africa, and maps of the world began to expand outwards from the Mediterranean. From the first millennium B.C. on, Indian, Chinese, and Arabian sailors explored the Indian Ocean, and from the ninth to fifteenth centuries, Arabian and Persian pilots compiled navigational instructions that included information on winds and currents in addition to the mapping of coasts, islands, and ports. Knowledge of the geography of the world grew with oceanic exploration, and reliable world maps began to emerge. By 330 B.C. Aristotle had deduced the fact that the world was spherical by observing the shape of Earth's shadow on the moon during a lunar eclipse and in 250 B.C. Eratosthenes, a librarian in Alexandria, measured the radius of the Earth to remarkable precision. He also published a world map that included Europe and northern Africa and extended as far east as India. His map represented the land as being surrounded by an all-encompassing ocean.

In A.D. 140 Ptolemy published a map that included China and a bit more of Africa, but he represented the Atlantic and Indian Oceans as enclosed seas, like the Mediterranean. In addition, he used a value for the radius of the Earth that was substantially smaller than that of Eratosthenes. By the time that Columbus set out in search of a new route to the spice islands of the East Indies, more complete maps of the eastern hemisphere continents existed. Along with most other educated people of his age, Columbus knew full well that the world was spherical in shape. In constructing his own charts of the world, however, he made two extremely interesting errors that conspired to influence the course of history. The first was that Columbus chose to follow Ptolemy's lead in using far too small a radius for the Earth. On this shrunken world, he wrapped a Eurasian continent that was significantly too large. Since the measurement of longitude requires accurate timepieces, he could really only guess the distance eastward from Europe to Southeast Asia from travel times of boats and overland expeditions.

In any case, the combination of these two errors -- wrapping a too-large Eurasian continent around a too-small globe -- conspired to convince Columbus that he could set out to the west and, after traveling for less than 5,000 kilometers, arrive in Japan -- a vast saving in time and expense over the more traditional route that first rounded the Cape of Good Hope on the southern tip of Africa, and then crossed the Indian Ocean in an eastward direction.

The rest is ironic history, and explains why Native Americans came to be called "Indians." Columbus sighted land just about where he had expected to, and was convinced that he had, in fact, reached the fabled islands of the Indies. To his dying day, he never did realize that he had discovered a whole New World.

But Columbus did more than just sail in search of wealth (which he never found). Along the way, he made scientific observations that established the fact that the declination of the Earth's magnetic field -- the angle between true north and magnetic north -- has different values at different places on Earth. This practice of taking observations during the course of long cruises would prove to be the seed from which modern oceanography sprouted.

Almost all early knowledge of the oceans was derived from the experiences of seafarers. But a truly clever scientist could still make important discoveries without even setting out to sea. Such was the case with Benjamin Franklin's discovery of the Gulf Stream. Franklin was Postmaster General for the American Colonies from 1764 to 1775 and wondered why mail packets sailing from Falmouth, England to New York took weeks longer than heavier-laden merchant ships traveling from London to Rhode Island. In Maritime Observations, published by Franklin in 1786, he says:

• About the year 1769 or 70, there was an application made by the board of customs at Boston, to the lords of the treasury in London, complaining that the packets between Falmouth and New York, were generally a fortnight longer in their passages, than merchant ships from London to Rhode Island, and proposing that for the future they should be ordered to Rhode Island instead of New York.

  …it appearing strange to me that there should be such a difference between two places, scarce a day's run asunder, especially when the merchant ships are generally deeper laden, and more weakly manned than the packets, and had from London the whole length of the river and channel to run before they left the land of England, while the packets had only to go from Falmouth, I could not but think the face misunderstood or misrepresented. There happened then to be in London, a Nantucket sea-captain of my acquaintance, to whom I communicated the affair. He told me he believed the fact might be true; but the difference was owing to this, that the Rhode Island captains were acquainted with the gulf stream, which those of the English packets were not. We are well acquainted with that stream, says he, because in our pursuit of whales, which keep near the sides of it, but are not to be met with in it, we run down along the sides, and frequently cross it to change our side: and in crossing it have sometimes met and spoke with those packets, who were in the middle of it, and stemming a current, that was against them to the value of three miles an hour; and advised them to cross it and get out of it; but they were too wise to be counselled by simple American fisherman. When the winds are but light, he added, they are carried back by the current more than they are forwarded by the wind: and if the wind be good, the subtraction of 70 miles a day from their course is of some importance. I then observed that it was a pity no notice was taken of this current upon the charts, and requested him to mark it out for me, which he readily complied with, adding directions for avoiding it in sailing from Europe to North America. I procured it to be engraved by order from the general post office … and copies were sent down to Falmouth for the captains of the packets, who slighted it however …

  Figure 5-1 A Tracing of Benjamin Franklin's 1786 Map of the Gulf Stream

  

Observations of sea captains continued to be an important source of oceanographic information, but as time went on, expeditions whose goals were purely scientific became more numerous and influential. Significant impetus for these studies was provided by the desire to know more about the deep ocean environment on the part of the managers responsible for the maintenance of the transatlantic cables that were laid starting in 1858.

Perhaps the most famous and successful of the early scientific voyages was that made by H.M.S. Challenger .M.S.;which set out from England in 1872. The trip lasted 3-1/2 years, covered 68,890 nautical miles, and took physical, chemical, and biological observations at hundreds of places in the Atlantic, Pacific, and Indian Oceans. The scientific results took two decades to analyze and eventually filled 29,500 pages in 50 volumes. It ranks as one of the great scientific achievements of all time.

Throughout the nineteenth century, only the vaguest outlines began to emerge of the shape of the ocean floor. The only measurement method available was the sounding line, in which a light line with a weight on its end was lowered over the ship's side until the weight came to rest on the ocean floor. The person lowering the line might then notice the change in the apparent weight of the line and could mark the point at which this occurred. The depth to the ocean floor would then be just the length of the line that had been played out.

The difficulty with this method is that once four kilometers of line have been played out, the weight of the line is more than that of the weight on the bottom, and the heaving of the ship in waves and the fact that it is impossible to bring the ship to a total halt makes it very hard to judge just when the weight has struck bottom. It was not until the development of the acoustic echo sounder in the twentieth century that a detailed view of the ocean floors became possible. It works on the principle of sending out a pulse of sound from the ship and measuring the time needed for the pulse to travel down to the bottom and for its echo to return to the ship. Now, by assuming a constant velocity of sound in seawater, it was possible to make continuous soundings of the ocean floor while the ship was moving.

A notable oceanographic expedition was that of the ship Meteor, which set out in 1925 for 25 months and crisscrossed the South Atlantic Ocean, gathering more than 70,000 soundings of the ocean floor. For the first time it became possible to at least partially fill in the blank blue spaces on world globes. The exploration of the remaining two-thirds of the Earth had begun.

In this period oceanography grew as a discipline, with the establishment of major new oceanographic institutes such as Scripps Institution of Oceanography in La Jolla, California, and Woods Hole Oceanographic Institution in Woods Hole, Massachusetts in the United States.

Submarine warfare during World War II generated interest on the part of naval authorities for more detailed knowledge of the ocean floor, and with the end of the war, mapping of the deep sea continued with renewed vigor, eventually resulting in the highly detailed bathymetric (ocean floor) maps that are now available. In addition, scientific instruments which allowed precise measurement of magnetic fields were developed.

Within the span of a single decade, the heights and depths of the Earth were visited by humans. In 1953, Mt. Everest was climbed for the first time, and in 1960 the bathyscaph Trieste slowly settled into the depths of the Mariana Trench at its lowest point, named Challenger Deep after H.M.S. Challenger.

Perhaps the most ambitious deep ocean project since the war, and one of the most successful to date, was the Deep Sea Drilling Project. A large ship, aptly named the Glomar Challenger, was constructed with a hole in the middle and a drilling rig mounted astride it. Six engines and propellers could keep the ship nearly motionless even in heavy seas while drilling bits were lowered through kilometers of water to core the ocean bottom.

In this process, a hole is bored in the ocean floor using a bit that has a hole in its center. Into this hole, a cylinder of sediment and rock is extruded into a chamber and can be brought back up to the surface for detailed analysis. These ocean cores have been taken at over 500 sites around the world and constitute an invaluable record of the upper layers of the ocean floor. Dating of the sediments has provided direct confirmation of sea-floor spreading by showing how the ocean floor becomes older as you proceed away from the ridge crests, and in addition has provided evidence on topics as diverse as studies of magnetic field behavior, fluctuations in world climate, and a history of circulation patterns.

3. The Third Dimension

When the word oceanographer is spoken, most people probably envision someone in scuba gear, investigating coral reefs or sunken wrecks. If this were the complete picture, however, oceanography would be a two-dimensional science and oceanographers would be confined to a tiny fraction of the total volume of the oceans -- the water-air interface. Free-swimming divers are restricted to only the top hundred meters or so of the oceans, but modern oceanographers are determined to explore the third dimension of the oceans as well, taking their investigations right down to the ocean floor.

Until recently oceanographers had to make their measurements from ships on the two-dimensional water-air interface, using acoustic sounders and instrument packages lowered into the depths to extend their view into the third dimension. Now, three important developments are extending that view. One is the small submersible research vessel that can take the oceanographer down into the great depths of the ocean for first-hand observations. Another is the use of robotics for unmanned precise sampling and mapping. The third is the satellite or manned orbiting vehicle. In 1978, Seasat, the first satellite devoted entirely to oceanographic measurements, was launched, providing ocean scientists with powerful new tools for viewing their domain. In 1984 a space shuttle soared into orbit carrying the first oceanographer to make his observations from space. It seemed very fitting that the shuttle bore a name distinguished in the annals of the science: Challenger. After several successful flights, the shuttle Challenger met an untimely and tragic end, exploding shortly after launch and killing all astronauts and crew aboard, including the first schoolteacher to ride in space.

C. OBSERVATIONAL TECHNIQUES

Let us briefly review the kinds of measurements that oceanographers make and the equipment that they use. An oceanographic research vessel must not only provide living facilities for the scientists and crew, but must also have laboratory space, a winch for lowering instrument packages into the depths of the sea, and an on-board computer for analysis of the data while the scientists are still at sea.

A wide variety of instrumentation is used on a research vessel. We have already mentioned the acoustic depth sounder, which sends out sound pulses and measures the distance to the bottom by measuring the time needed for the sound to go down and back to the ship. By the way, the term sounding predates by far the use of sound to measure depth -- the weighted line previously used was called a "sounding line" -- and provides us with a highly appropriate coincidence. The acoustic sounder can sometimes provide us with a view of more than just the topography of the ocean floor. Because some sediments resting there are relatively transparent to sound waves, it is often possible, b y recording echoes from deeper layers, to get a picture of the shape of the sediment layers from the sounder as well.

Seismology allows us to examine the interior of the earth, and its use at sea provides a detailed look at the structure of the sea floor and the mantle beneath it. It is not practical, however, for a ship to sit at one station for a long enough period to wait for natural earthquakes to provide the necessary seismic waves, and so explosive charges or other artificial sources are used instead.

Different techniques have been employed for gathering seismic data at sea. One uses two ships, one moving and producing seismic waves at intervals, while the other is stationary and records them using special low-frequency microphones, called hydrophones, that are placed in water. Another arrangement uses a single ship that produces the seismic wave, usually with an air gun that fires about once per second. The seismic wave travels to the bottom, penetrates the sea floor, and is reflected from the various layers of rock beneath the sea floor to depths of a few kilometers. The echoes from these layers return to a string of hydrophones being towed by the ship. In this way, a continuous profile of sea-floor structure may be obtained as the ship moves along its course. These seismic techniques work similarly to those used to explore the continental crust for minerals and oil.

Measurements at depth are often carried out by instruments lowered at the end of a cable to whatever depth desired by the scientist. A "pinger" in the instrument package allows the determination of just how far above the ocean floor it is. A pinger works similarly to the acoustic depth sounder in that it emits a sound pulse that is received by a microphone on the surface. The time required for the sound wave to travel directly from the pinger is measured, as is the time required for the sound to bounce off the ocean floor and return to the ship. The greater the difference between the arrival times of the direct and the echoed sound waves, the farther the pinger is from the ocean floor. When the two waves arrive at exactly the same time, the pinger has arrived on the bottom.

Many different kinds of instruments may be lowered on cables. Among these are cameras, both film and television, and the lights necessary for them to record anything in the total darkness of the deep ocean. Light from the Sun fails to penetrate any farther down than about 100 m (330 ft). Water samples may be taken from various depths by bottles that are opened automatically when they arrive at the desired level. Electrode systems measure the electrical resistance of ocean water and determine its salinity, since dissolved salts make water more conductive to electricity.

• Figure 5-2 The Operation of a Piston Corer

  

An important device in the study of the soft sediments that blanket the ocean floor is the piston corer, illustrated in Figure 5-2. The device consists of a cylindrical coring tube with a movable piston inside. As it is lowered to the ocean floor, a smaller tube, the "trigger core," takes a sample of the topmost layers, which are the ones most disturbed in the main tube. In the process, the main core tube is released and falls, thrusting itself into the sediment. The piston is held at the level of the sediment surface while the core tube continues to penetrate the sediment. The piston acts much like the plunger in a hypodermic needle used to withdraw blood, sucking the sediment up into the core tube with a minimum of disturbance to the sediment layers. When the corer is pulled back up to the surface, the sediment in the core tube goes with it and can be removed on shipboard for study. Cores as long as thirty meters have been recovered using piston corers.

In addition to corers, dredges and grab samplers may be used to recover samples from the ocean floor. A grab sampler operates much like the clamshell bucket used in excavation machinery to bite off chunks of dirt in construction projects.

When measurements over an extended period of time are required, instrumented buoys may be used. One type uses a floating buoy that is anchored to the sea bottom with a cable. Instruments may ride the buoy or the cable at specified depths, and can record data concerning weather or ocean conditions for long periods of time. Free-floating buoys may be used to track oceanic currents for long distances, and more recently subsurface buoys, whose buoyancy has been carefully adjusted to float at a specified depth are being employed to chart the wanderings of the hidden layers of the ocean.

In addition, instrument-bearing tripods have been lowered to the ocean floor for the purpose of observing sea-bottom phenomena. Powered by batteries for periods of six months to a year, the package may contain sophisticated microcomputers that can sense the environment and turn on particular instruments when something of interest occurs. These undersea robots can be made to perform chemical, physical, and biological experiments on the spot, recording data in a digital form for later retrieval. When it is time to recover the instruments, an acoustic signal from a ship on the surface triggers a release mechanism that separates the instrument package from weights that hold it to the ocean floor and flotation chambers provide sufficient buoyancy to allow the package to float to the surface where it can be picked up and reused.

• Figure 5-3 Cutaway View of the Submersible Alvin

  Reprinted from Woods Hole Oceanographic Institution. Included with permission.

  

Submersible research vehicles have evolved to a point of considerable sophistication. Figure 5-3 shows a cutaway view of the submersible vessel Alvin, owned by the U. S. Office of Naval Research. In order to provide protection from the crushing pressures of the deep ocean environment, the passengers are confined to a thick-walled spherical chamber only two meters (six feet) in diameter. Nevertheless, they are able to observe through thick glass portholes, and a mechanical arm allows them to pick up objects and manipulate machinery on the ocean floor. Other countries that operate research submersibles are France and Japan.

D. GEOPHYSICS OF THE OCEAN FLOOR

The geography of the ocean floor is dominated by the ocean basin floor and the ocean ridge system. The ocean basin floors are those parts of the oceanic plates that are between the spreading ridges and trenches. Their elevation stands at an intermediate depth within the range of three to five kilometers. The spreading ridges owe their height to the buoyant force of the low-density hot or molten rock beneath them (see Figure III-8). This low density mass of rock below the ridge plays the role of an isostatic "root" and supports the weight of the ridge.

The ridge is also spreading, creating new plate on either side of it. As it does so, the lithospheric plates recede from the source of heat at the ridge and slowly cool and become more dense, sinking in the process. As long as the ocean floor remains in isostatic equilibrium, the extent to which the ocean floor has sunk is a function primarily of the age and temperature of that segment of ocean floor. On the other hand, the width of the ridge will be a strong function of how fast the ridge is spreading. Figure 5-4 shows this effect. Both ridges are the same height, but the fast-spreading ridge is much wider than the slow-spreading one because new, hot, and therefore high-elevation ridge is being carried away rapidly from the crest in the former case. The result is that the fast-spreading ridge has a much larger volume than the slow-spreading ridge.

The more volume there is in the oceanic ridges, the more water is displaced by them, with the result that worldwide sea levels will stand higher on the continents during times of faster average sea-floor spreading rates than during times of slower spreading rates. For this reason, it has been proposed that those periods of geological time during which the sea covered substantial portions of the continents, such as during the Cretaceous period, were times of rapid sea-floor spreading.

The transform faults that offset oceanic spreading ridges also contribute to the shape or topography of the ocean floor (see Subsection III-E-3). These are often the sites of very rugged terrain, caused by the sliding of the two plates past one another for millions of years, the results of countless earthquakes. These fracture zones, as they are called, often extend for thousands of kilometers along the ocean floor.

• Figure 5-4 Volume of Spreading Ridge is Affected by its Spreading Rate

  Slow spreading rates are typical of the Atlantic Ridges while fast spreading is found on the East Pacific Rise.

  

The trenches, of course, are the result of the subduction of the ocean floor. Here one plate is being forced down and under the other, and in the process, both are buckled downwards (see Figure 3-7). Into these great depths move the ocean floor and its load of sediments. It was initially thought that all the sediment would accumulate in the trench or be added to the plate that was not being subducted, but recent seismic studies show that substantial amounts of oceanic sediment are being subducted along with the oceanic crust. For the most part, these are very wet sediments, and so in this way a part of the ocean waters is recycled back into the mantle. Some of it at least will be erupted through the mouths of the andesitic volcanoes associated with the subduction zone and returned to the atmospheric/oceanic environment. As we shall see, this is not the only case in which ocean waters are brought into close interaction with mantle rocks.

1. Sediments and Sedimentation

Most of the ocean floor is blanketed in soft sediments of one kind or another. Only young surfaces, found on the spreading ridges or on active volcanic undersea mountains, have large areas of bare rock exposed. There is a continuous rain of sediment down through the ocean waters, adding layer after layer, year after year. The result is that the sediment blanket becomes thicker with increasing age of the ocean floor. Near the ocean ridges, the layer is thin or absent, but it increases steadily as we go farther from the ridge (Figure 5-5).

• Figure 5-5 Thickness of Sedimentation Increases with Distance from Ridges

  The thickness of sediments is shown greatly exaggerated in relation to the height of the oceanic ridge.

  

Oceanic sediments fall into two main categories -- terrigenous and biogenic.

Terrigenous sediments originate on the continents, as their name implies. The sediment load of rivers is a major contributor, but wind can carry terrigenous sediments large distances offshore. Most of these sediments enter the ocean at the shoreline, but soon disperse in a variety of ways throughout the continental margin, which consists of the continental shelf, slope, and rise (see Figure II-1).

The continental shelves are relatively flat and end fairly abruptly at the continental slope. Here the sea bottom slopes downward gradually at an average slope of about 4 degrees. The continental rise is a still more gradual slope (generally less than half a degree) extending out to the deep sea floor. The greatest accumulations of sediment are found on the continental rise, having migrated there under the influence of gravity. In fact, in the North Atlantic Ocean, sediments have accumulated on the continental rise since the opening of the Atlantic in the Jurassic, sometimes reaching a thickness of 10 km (6 mi). This is somewhat thicker than the average depth of the oceans.

Coarse terrigenous sediments are found in the deep ocean at high latitudes, where melting icebergs can dump large quantities of terrigenous sediment far from land.

Biogenic sediments owe their origin to life in the oceans. Most of it comes from the skeletal remains of microscopic plants and animals that live in the biota-rich environment of the uppermost water layer. In the shallow-water environment of the continental shelves, reefs also contribute their mass to biogenic sediments. Ocean-bottom sediments rich in material of biological origin are referred to as ooze.

Inorganic clays form much of the deep-ocean sediment layer. In many places, calcium carbonate-rich skeletal remains dissolve in their long trip from the surface waters to the cold depths of the ocean floor. Carbon dioxide from the atmosphere is dissolved in seawater, producing carbonic acid. This is the same mechanism by which carbon dioxide gas is put into carbonated soft drinks to give them their fizz. The result is an acidity that attacks the calcium carbonate of the skeletal remains and dissolves them. Soda pop holds more carbonation when it is cold than when it is warm, and the same is true for seawater. Cold bottom water tends to be more acidic as a result, and if the supply of biogenic sediment is not so high, all the skeletal material may dissolve in the acidic water before it reaches the bottom. In warmer waters, or in areas of higher biotic production, calcium-rich oozes can be the dominant sediment.

Some sediments are chemically precipitated directly from seawater. Most notable among these are phosphorites and manganese nodules. Phosphorites are phosphorus-rich deposits that tend to form on the sea floor near the continents and manganese nodules are rounded concretions that contain concentrations of iron, manganese, copper, and other metals. The nodules, ranging in size from 1 - 20 cm across, are mostly found in the deep ocean where sedimentation rates are low. They form by precipitation from seawater, coating some object such as a shark's tooth, growing at extremely slow rates on the order of one millimeter per million years. As you will see in the Mineral Resources Unit (Unit 11), manganese nodules may prove to have economic value in the future as a source of metals.

Volcanic dust may be distributed very far over the oceans by major eruptions. The dust settles out of the atmosphere and slowly rains down onto the ocean floor to join other sediments.

The rate of sediment accumulation varies greatly depending on the type and place. In general, sedimentation is rapid for terrigenous sediments near the continents, with accumulation rates of around 20 cm per thousand years being common. On the other hand, deep ocean sediments accumulate much more slowly at rates that average only 2 mm per thousand years. It is this slow but steady rate of accumulation that makes sediment cores from the deep ocean floor so valuable as repositories of scientific information spanning vast stretches of time.

The great difference between accumulation rates for the continental margins and the deep ocean floor means that sediment thickness near the land can become very great. The sheer weight of this load of sediment has an isostatic effect, depressing the continental shelves. As a result, the edge of the continent often sinks somewhat, and this appears as a rising of sea level in that area. The east coast of the United States is a case in point, with the drowned river valleys of the Maine coast and the Chesapeake Bay as evidence of the effect. In these places, the sea reaches up the valleys of former rivers, creating long brackish bays and tidewaters.

Still another effect of the sediment buildup near the continents is the occurrence of turbidity currents. The continental margin is in many places cut by undersea canyons, often leading from the mouths of rivers out to the deep ocean floor. Sediments accumulate in the heads of these canyons until the pile becomes unstable, at which point the sediments can begin to slide down the canyon, gathering speed and picking up more sediment along the way. Traveling at substantial speed, the turbidity current can travel for long distances -- sometimes right down the continental slope and out onto the abyssal plains, which is the flat surface of a turbidite sediment accumulation.

The existence of turbidity currents was originally deduced from indirect evidence -- the breaking of submarine cables when strong ones pass, and the presence of layers of coarse-grained terrigenous sediments containing shells of shallow-water organisms sandwiched between layers of normal deep ocean sediment, indicating that the terrigenous material had been transported rapidly over long distances.

2. Benthic Storms

Another sediment-moving phenomenon, discovered only recently, is the abyssal or benthic storm. Unlike a turbidity current, which is an underwater avalanche triggered by great forces such as an earthquake, the benthic storm behaves like a blizzard in the deep ocean. In a benthic storm, a rapidly-moving current sweeps over large areas of the deep seafloor, picking up sediment and causing dramatic changes in its path. In some areas, the storm scours the bottom; in other places, it deposits enormous loads of clay and silt. This "stormy" current may last for as long as two weeks at a time and lift sediment 300 feet off the bottom. In the western North Atlantic, scientists have recorded benthic storms moving as fast as 75 cm per second (30 in. per second).

The recent discovery of benthic storms surprised most oceanographers because, with the exception of the infrequent turbidity current, it was thought that all deep sea currents moved very slowly. Scientists are not yet able to predict benthic storms; nor do they know what causes them. In addition to turbidity currents and the more frequent benthic storms, the deep seascape is affected by a steady, continuous circulation of frigid bottom water. In the polar regions, the surface waters sink because they are colder and denser than the water in the lower latitudes. As the cold waters at each pole sink, they begin to flow toward the equator, gradually mingle, and eventually rise to the surface, beginning the entire process anew. The paths these cold waters take is determined by various factors such as the topography of the ocean bottom. While this global current system moves slowly -- reaching speeds of only 5-15 cm per second (2 - 6 in per second) -- it constitutes a continuous, relentless movement of massive amounts of water that play a major role in sweeping bottom sediment into drifts in a peculiar pattern throughout the world ocean that has remained unchanged for millions of years.

Scientists theorize that the great quantities of sediment stirred up by benthic storms may be picked up and carried downstream by the less energetic but persistent cold water currents. Some of the largest sediment accumulations, which are in the North and South Atlantic, are 1,000 km long, 200 km wide and 2 km thick (600 mi long, 120 mi wide, and 6,500 ft thick).

3. The Mapping of the Sea Floor

As we saw earlier in this unit, the shape of the ocean floor did not become known until fairly recent times. Even currently used maps are compilations of many thousands of echo sounding records along the tracks of ships. Some new techniques, however, promise to change this. The first is an adaptation of the usual echo sounder, called side-scanning sonar. Instead of pointing the sound pulse straight down below the ship, the sonar pulse is sent out to the side so it reaches the ground at a grazing angle. Those parts of the ocean floor that slope toward the sensor reflect sound more effectively than those that are horizontal or slope away. Shipboard electronics are able to convert the returned signal into a picture of the sea floor that may be viewed while the ship is still underway. Using this method, a swath several kilometers wide may be surveyed along the path of the ship that carries the sonar gear. Two different versions of this scheme are now operational: one, called GLORIA, is towed behind the research vessel and can obtain images of a swath 20 km (12 mi) or more in width along the path of the vessel, while the other, called SeaMARC I, is towed near the ocean floor and can obtain higher resolution images, though of a narrower swath. The images that these devices obtain of the sea floor look somewhat like landscapes lit by a low-angle sun.

Sea Beam is an elaboration of the usual echo sounder. Instead of a single acoustic beam aimed straight down below the ship, Sea Beam sends out 16 beams in a fan shape, each designed to measure the distance to a particular point on the sea floor along a line that is perpendicular to the ship's travel. With this system, a contour map showing the topography of the sea floor can be generated. With all of these methods, shipboard computers process the data received from the sonar devices and present them in forms that are easy to interpret. Scientists on board can examine the results and modify their cruise plans if necessary to maximize the time they spend in the most interesting areas. Cruise time is extremely expensive, and this ability to analyze results almost immediately is an important cost-saving measure.

Another technique uses measurements from an orbiting satellite and overcomes a significant shortcoming of all the other methods: the availability of data only along the tracks of ships. The satellite method promises to provide, for the first time, a broad view of ocean-floor topography featuring a uniform standard of resolution and accuracy throughout all the oceans.

The technique involves measuring, to extremely high precision, the elevation of the sea surface. This was accomplished by a satellite named Seasat, that used radar waves to make the measurements, accurate to centimeters. It may at first seem surprising that the ocean's surface mirrors (on a much reduced scale) the topography of the sea floor, but Figure 5-6 shows how this works.

Gravity is a function of all objects that have mass, acting to attract a mass to all others around it. A mountain or plateau on the sea floor is a massive object, and this excess of mass tends to attract the surrounding seawater to it. In effect, the mountain's mass distorts the local gravity field, as shown in the diagram, and this produces a mounding of seawater above the extra mass. The satellite sees this as a bulge on the ocean surface that is a much reduced version of the sea-floor topography.

• Figure 5-6 The Gravity Field Helps to Map the Ocean Floor

  

There is, however, a significant difference in how these two methods (echo sounding and satellite ranging) view the ocean floor. The satellite measurements rely on the distortion of the gravity field by excess mass on or below the sea floor, and so it provides a view not only of the topography, but also of density differences that may exist in the oceanic crust. In combination, the two methods promise to give us a very detailed view of the shape and structure of the ocean floor, comparable to maps that have been available for the continents for the past hundred years. The exploration of the other two-thirds of the Earth's surface has begun in earnest.

E. CHEMISTRY AND PHYSICS OF SEAWATER

1. The Composition of Seawater

Water is one of the most remarkable and unusual substances in the universe. Because it is so abundant on Earth and therefore so familiar to us, it has actually skewed our view of what constitutes normal behavior. For example, we all know that ice floats on water, and so we might guess that this is typical behavior for the solid and liquid phases of substances. In fact, the converse is usually true for most materials -- the solid is usually more dense than the liquid, and so, for instance, solid basalt sinks in a pool of its own lava rather than floats on top.

Indeed, the fact that water is liquid at all on Earth is purely a function of its unusual properties. Most other substances made of similarly light atoms (methane, for example) are gaseous at room temperature and only liquefy at much colder temperatures. Another notable property of water is its ability to dissolve other substances. No other common liquid is able to dissolve so many materials.

All of these properties are a result of the atomic makeup of the water molecule. Composed of two hydrogen and one oxygen atoms (H₂O), the water molecule has a polarized structure because the hydrogen atoms have a positive electric charge while the oxygen atom has a negative charge (Figure 5-7). This configuration, in which one "end" of the molecule is positively charged and the other "end" is negatively charged, accounts for many of water's unusual properties.

Unlike electrical charges attract, while like charges repel one another. For this reason, whenever a bunch of water molecules get together, they tend to line up in such a way as to mutually attract one another. This cohesion of the molecules explains why the melting and boiling points of water are so high -- in a gas, the molecules fly about with little regard for one another, but in liquids, and especially in solids, they interact with one another in a more cohesive manner. When water freezes, however, the molecules arrange themselves in a very regular way that has an open hexagonal structure. Because the structure is so open, ice is less dense than water; because it is hexagonal, snowflakes assume a magnificently delicate six-fold symmetry.

• Figure 5-7 The Water Molecule

  

But it is water's role as a solvent that most concerns us here. Seawater carries dissolved within it a large number of substances in the form of ions -- portions of molecules that have been torn apart to form charged atoms or groups of atoms. For instance, when common table salt (sodium chloride, symbolized as NaCl) is dissolved in water, the molecule is torn apart into a positively charged sodium ion (Na+) and a negatively charged chlorine ion (Cl-). These are each attracted to the negatively or positively charged parts of the water molecule. Because they are now separate from one another, the Na+ and Cl- ions no longer form a single salt molecule, but have been incorporated into the liquid water as individual ions. This is an important point because, as we shall see, it is not necessary for the sodium and chlorine to have come into the ocean in the form of salt -- indeed, each element may have come from a different source. But when seawater is evaporated, the ions are left behind and, without the polarized water molecules to keep them apart, they will combine to form salt.

Seawater contains nearly all of the elements, though many are found in extremely small concentrations. The four most important ions in terms of concentration are the chlorine (Cl^-), sodium (Na⁺), sulfate (SO₄^2-), and magnesium (Mg²⁺) ions, in decreasing order of importance. Next most important are the dissolved gases: carbon dioxide (CO₂), nitrogen (N₂), and oxygen (O₂). These are not torn apart, but are incorporated into the water as whole molecules. They are not held nearly as tightly by the water molecules as the ions, and so can escape fairly readily. A glass of water that has been allowed to stand in the open for a long time comes to taste "flat" -- what has happened is that the dissolved gases in it have escaped. Many faucets have an aerator attached that mixes air with the tap water to increase the concentration of dissolved gases, improving the water's taste.

You may have noticed that carbon dioxide was mentioned before nitrogen and oxygen, while in the atmosphere, carbon dioxide is present in much smaller amounts than the other two gases. In fact, carbon dioxide dissolves readily in water (as any soft drink or beer drinker knows) with the result that something like 60 times more carbon dioxide is dissolved in seawater than is present in the entire atmosphere. The oceans are thus an important regulator of carbon dioxide in the atmosphere -- an effect that we shall see in the unit The Atmosphere is of extreme importance to Earth's climate.

Next in order of concentration are a group of ions that are vital to biological processes: the nutrients. Chief among these are the nitrate (NO₃^-), nitrite (NO₂^-), and phosphate (PO₄^-) ions. These are present in concentrations typically of only a few parts per million, but they are essential to the presence of life in the oceans.

Finally, there are a large number of trace elements, present in concentrations of only parts per billion. Some of these, such as iodine, iron, lead, and mercury, have important effects on life in the ocean and on other organisms, such as ourselves, that consume seafood. Iodine is essential to human health, and before it was artificially added to table salt, seafood was a principal source. Mercury, on the other hand, is extremely toxic to humans, and increasing levels of this substance in some waters, due to industrial pollution, have posed serious health hazards.

An important result obtained from the voyage of the H.M.S. Challenger. M. S. ;in the 1870s was the discovery that the relative proportions of the dissolved ions in seawater were the same regardless of where in the world the sample was taken. There may be more or less water in which the salts are dissolved, but that is the principal variation. It would seem from this, that ocean water has been pretty well mixed.

2. Why is the Ocean Salty?

You may recall from Unit 1, A Sense of Time, that Sir Edmund Halley in 1715 tried to determine the age of the Earth from the assumption that salts carried by the world's rivers were the source of the saltiness of the oceans. While his assumption that the oceans were continuing to become more salty was wrong, until recent years his other assumption was generally accepted. Even so, there has been a long-standing problem with this explanation in that there is a serious mismatch between the ions prevalent in the oceans and those found in river water.

The ions most commonly found in river water are the carbonate (CO₃^2-), calcium (Ca²⁺), and sulfate (SO₄^2-) ions, while those most prevalent in ocean water are the chloride and sodium ions. Some of these discrepancies are easily explained in terms of chemical reactions that take place in the oceans, such as the removal of the carbonate ion by biological processes. Others, such as an excess of chlorine and bromine in the ocean, cannot be explained in this manner.

In an attempt to find a solution to this problem, scientists turned to another source for the excess ions -- the mantle. It had been found that ultramafic rocks, formed deep within the Earth, contained gas with a composition that contained chlorine, bromine, and other elements that matched oceanic composition much more closely than did river water. An alternative explanation was proposed: the excess volatile ions, like chlorine, must have come from the mantle, while the rivers supplied some of the other ions like sodium and magnesium. But how did these ions find their way from the mantle into the oceans without using the rivers as a conduit? The only logical mechanism seemed to involve the spreading ridges, where material from the mantle was involved in the process of creating new ocean floor.

The most recent step in the development of this idea was taken in 1977 when the submersible Alvin visited the crest of a spreading ridge in the vicinity of the Galapagos Islands off the coast of Peru. Here torrents of hot water were found issuing from vents in the rock, showing that ocean water circulates freely through the fresh rock of the ridge, creating hot springs. The circulation of water through hot rock is known to geologists from such places as Yellowstone Park in Wyoming and is termed hydrothermal circulation. This was just the kind of mechanism that was needed to introduce the missing ions from mantle sources.

Soon, hydrothermal vents were found in other places along the ridge crests, and estimates based on the likely total activity of vents made it clear that the entire volume of water in the oceans could circulate through them in only five to ten million years. Because of the high temperatures involved, chemical reactions between mantle rock and ocean water can proceed rapidly and it is now felt that the oceanic vents may well be the dominant influence on the chemical composition of seawater.

The development of our ideas about the chemistry of seawater provides us with a fairly typical example of how scientific explanation evolves. As stated throughout this course, the best theory is that which best explains all the available observations, and Halley's use of river salts was quite valid for his time, since all that he knew was that both river water and seawater contained salts. Detailed observations of their compositions did not yet exist. Once these observations were made, however, it was realized that the river water theory was flawed and the search was on for a better explanation. Other possible sources for the missing ions were sought and a mantle source was hypothesized on circumstantial evidence -- mantle rocks contained the needed ions in approximately the right proportions. This new idea, however, remained in the relatively weak category of hypothesis until it could be confirmed by direct observation of the vent mechanism on the ocean floor.

Detectives and scientists have much in common in their approach, only for the scientist it is nature and not mankind that sets the mysteries.

3. Salinity, Temperature, and Density of Seawater

Because the relative proportions of dissolved ions are essentially constant, we may use the measure of salinity as one descriptor of seawater. Salinity is just the total amount of dissolved material that is present in seawater, expressed in parts per thousand. Salinity can vary from low values of about 10 parts per thousand found in the Baltic Sea to values of 40 parts per thousand in the Red Sea. The salinity of normal ocean water is in the range of 34 to 36 parts per thousand.

There are a number of factors that can affect salinity. For instance, a large input of fresh water near the mouth of a major river may produce a local zone of low salinity. The Baltic Sea in northern Europe is fed by many rivers and it has only very narrow connections to the open ocean of the North Sea. Many bays that are fed by large rivers, such as the Chesapeake in the United States, contain brackish water -- water of low salinity.

On the other hand, areas in which evaporation is very high, such as the Red Sea and the Mediterranean, often are characterized by water of high salinity. The water is evaporated but the salts are left behind. The freezing of sea ice also has the effect of increasing salinity of seawater. When ice is frozen from salt water, the ice selectively excludes salts. While seawater is too salty to drink, sea ice frozen from seawater is in fact potable, containing only a small amount of salt. In response to this withdrawal of salt, the seawater remaining below the ice becomes more salty than before.

The density of seawater is determined by the salinity, temperature, and pressure. Of these, the salinity and temperature are most important. Cold water is more dense than warm water, and saline water is more dense than fresh water.

The vertical structure of the ocean is determined by density differences, with denser waters occurring in the deep ocean and less dense waters being found near the surface. Vertical mixing is slow, with the result that we find a layering of ocean water based on the density at any given location, but the form of the layering is not consistent everywhere. Each layer contains water that reflects conditions at its point of origin.

As a rough generalization, we may divide the ocean into three layers: surface waters, the thermocline, and deep waters. Surface waters are well-mixed, of uniform salinity and extend only to depths of 50 - 100 meters. They vary substantially in temperature and salinity from one locality to another, according to the influences of evaporation, fresh water, and the formation of sea ice, discussed previously. The thermocline is a thin region of rapid change in temperature dividing the surface waters from the deep waters. This often begins a region of rapid salinity change as well, that may extend to greater depths.

The deep waters are characterized by relatively uniform salinity and temperature. They are quite cold, with the temperature generally within the range of 3 - 4°C (37 - 39°F). This is the temperature at which the density of water is the greatest. Deep bottom waters for the most part are formed in the Antarctic region, where surface waters are cooled by the frigid climate and made more saline by the production of sea ice. The combination of cold temperature and high salinity produces the densest seawater found in the world. Dense bottom water is formed in the Arctic Ocean as well, but because that ocean basin is ringed by continents, little of the bottom water produced there escapes.

The kind of complexity that can result is shown in Figure 5-8. Cold water forming near the surface at high latitudes tends to sink and move toward the equator at depth, while water warmed in the tropics moves toward the poles. In this way, heat is transferred from the equator to the poles, and the oceans exert a very powerful moderating influence on world climate, distributing heat from the Sun in a far more equitable manner than if the oceans were not present.

An important attribute of deep bottom water is that it is rich in oxygen and nutrients. In high latitudes, the cold water at the surface is able to dissolve more gases from the atmosphere, and everywhere nutrients such as phosphates are carried down from the surface layer in particles of organic matter. The organic debris decays on the ocean floor, releasing the nutrients into the bottom layer. We shall see the importance of this point in the next unit, when we discuss the El Niño phenomenon.

RECOMMENDED READING

Brown (1960) introduces the history of map making in a splendid manner. Oceanography is presented in detail by Ingramanson and Wallace (1979), Ross (1982), Grant (1987) and Stowe (1983). The oceanographic environment is exhaustively detailed in Kennish (1994). Specific detail to seamanship (Van Dorn, 1993) and naval matters (Williams et al., 1968) make especially interesting reading.

• Lloyd A. Brown, Map making, the art that became a science, Little, Brown and Company, 1960.
• M. Gross Grant, Oceanography, Prentice-Hall, 1987.
• D. A. Ingramanson and W. J. Wallace, Oceanology: an Introduction, Second edition, Wadsworth Publishing Company, 1979.
• Michael J. Kennish, Practical handbook of marine science, Second Edition, CRC Press, 1994.
• David A. Ross, Introduction to Oceanography, Third Edition, Prentice-Hall, Inc., 1982.
• Keith S. Stowe, Ocean Science, Second Edition, John Wiley and Sons, 1983.
• William G. Van Dorn, Oceanography and seamanship, Second edition, Cornell Maritime Press, 1993.
• Jerome Williams, John Higginson, and John Rohrbough, Sea and Air, the naval environment, United States Naval Institute, 1968.