Ocean and Oceanography - geography.

« gravity, so the level of the ocean will be lower over valleys.

Using this method, a complete survey of the ocean floor was accomplished in less than two years.

Maps madefrom data on the level of the ocean surface have been compared with maps made with direct depth measurements and the two types have corresponded well. Using sonar, depth measurements are made by measuring the time for a sound wave to travel from the surface of the ocean to the ocean floor, and to return ( see Sounding).

Often several returns are recorded, indicating that some sound waves are bouncing off of several layers of sediment below the ocean floor.

More extensivestudies of the sediment below the ocean floor are carried out by teams of two ships: one ship fires an explosive in the water and the other uses sensitive instruments torecord the sound waves as they reach the second ship.

Some waves travel directly to the second ship; others travel to the ocean floor, are refracted (bent) within the layersof sediment, and then travel to the second ship.

By measuring the different times sound from the first boat takes to reach the second, the strength of the explosive, and thedistance between the boats, the thickness of the sediments can be determined. III COMPOSITION OF SEDIMENT The ocean floor is covered by an average of 0.5 km (0.3 mi) of sediment, but the thickness varies up to about 7 km (4.3 mi) in the Argentine Basin in the South Atlantic.Some regions, particularly the central parts of the midocean ridges where new crust is formed, have little, if any, sediment on them.

The sediments are studied by dredgingand by deep-sea exploration projects such as the Ocean Drilling Program, which obtains core samples of seafloor sediment from all the world’s oceans ( see Deep-Sea Exploration). The sediments are found to consist of rock particles and organic remains; the compositions depend on depth, distance from continents, and local variants such as submarinevolcanoes or high biological productivity.

Clay minerals, which are formed by the weathering of continental rocks and carried out to sea by rivers and wind, are usuallyabundant in the deep sea.

Thick deposits of such detrital material are often found near mouths of rivers and on continental shelves; fine particles of clay are spread throughthe ocean and accumulate slowly on the deep-ocean floor.

These sediments are stirred up and periodically redistributed by fierce current-generated disturbances that arecalled benthic storms because they occur in the sparsely populated deep-sea habitat known as the benthic zone ( see Marine Life).

Also accumulating as sediment in the benthic zone are the calcium carbonate shells of small organisms such as foraminifera and the siliceous shells of marine protozoans ( see Diatom; Protozoa). IV DATING TECHNIQUES Vast quantities of microscopic plants and animals live near the ocean surface.

When they die, their remains drift down to the ocean floor and accumulate in thick layers ofsediment.

When studied in sedimentary core samples, which can represent many millions of years of deposits, they provide a detailed and continuous history of the earth’senvironmental changes.

The record is particularly informative for the most recent 2 million to 5 million years, during which major fluctuations in global climate haveoccurred.

Successive ice ages can be traced by the relative scarcity or abundance of the shells of warm-water and cold-water diatoms in various layers of a sedimentarycore, as the organisms migrated to more hospitable habitats.

Geochemical records of these same fluctuations are revealed by determining the ratios of two isotopes ofoxygen, oxygen-16 and oxygen-18, in the shells of foraminifera.

The ratio of the two isotopes is proportional to the temperature of the water in which the organism grew;hence, a temperature record is preserved when each organism dies and its shell drifts to the ocean floor.

The records of climatic fluctuation found in ocean-floor sedimentsare much more continuous than similar records on land; they also lend themselves to worldwide correlation.

The absolute ages of climatic changes can be determined bycorrelating the evidence of temperature changes with radioactive-dating techniques ( see Chronology; Dating Methods; Radioactivity).

Thorium-230 dating is applicable to samples younger than 300,000 years, potassium-argon dating to samples in the range of 75,000 years, and carbon-14 dating to samples younger than 40,000 years.Several other radioactive dating techniques are available for samples of very recent age.

A geophysical dating method is also commonly used; it determines the magneticorientation of sediment particles, since it is now known that the earth’s magnetic field has reversed its orientation several times in the past few million years ( see Earth: The Core and Earth's Magnetism ).

Such dating techniques indicate that the ocean basins are no older than 200 million years. V COMPOSITION OF SEAWATER Seawater is a dilute solution of several salts derived from weathering and erosion of continental rocks.

The salinity of seawater is expressed in terms of total dissolved saltsin parts per thousand parts of water.

Salinity varies from nearly zero in continental waters to about 41 parts per 1,000 in the Red Sea, a region of high evaporation, andmore than 150 parts per 1,000 in the Great Salt Lake.

In the main ocean, salinity averages about 35 parts per 1,000, varying between 34 and 36.

The major cations, orpositive ions, present, and their approximate abundance per 1,000 parts of water are as follows: sodium, 10.5; magnesium, 1.3; calcium, 0.4; and potassium, 0.4 parts.The major anions, or negative ions, are chloride, 19 parts per 1,000, and sulfate, 2.6 parts.

These ions constitute a significant portion of the dissolved salts in seawater,with bromide ions, bicarbonate, silica, a variety of trace elements, and inorganic and organic nutrients making up the remainder.

The ratios of the major ions vary littlethroughout the ocean, and only their total concentration changes.

The major nutrients, although not abundant in comparison with the major ions, are extremely importantin the biological productivity of the sea.

Trace metals are of specific importance for certain organisms, but carbon, nitrogen, phosphorus, and oxygen are almost universallyimportant to marine life.

Carbon is found mainly as bicarbonate, HCO 3-; nitrogen as nitrate, NO 3-; and phosphorus as phosphate, PO 43-. VI TEMPERATURE The temperature of surface ocean water ranges from 26°C (79°F) in tropical waters to -1.4°C (29.5°F), the freezing point of seawater, in polar regions.

Surfacetemperatures generally decrease with increasing latitude, with seasonal variations far less extreme than on land.

In the upper 100 m (330 ft) of the sea, the water is almostas warm as at the surface.

From 100 m to approximately 1,000 m (3,300 ft), the temperature drops rapidly to about 5°C (41°F), and below this it drops gradually aboutanother 4° to barely above freezing.

The region of rapid change is known as the thermocline. Scientists are increasingly concerned about warming ocean temperatures due to increased amounts of greenhouse gases, such as carbon dioxide and methane, in theatmosphere.

The phenomenon known as global warming could have a negative effect on many forms of marine life that are sensitive to ocean temperature, such as coraland plankton, and also on ocean currents due to the release of fresh water from melting polar ice caps and glaciers. VII OCEAN CURRENTS. »