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OCEANOGRAPHY

Branches of Oceanography

1. Physical Oceanography - study of the motions of seawater, particularly

waves currents and tidal motion.2. Chemical Oceanography - chemistry of seawater and reactions between

the atmosphere and hydrosphere. More recently looks at how changes inseawater temperature (El Nino) and salinity affect global climate.

3. Biological Oceanography - study of life in the oceans, includes marinebiology and ecology.

4. Geological Oceanography - study of the shape and geologic features ofthe ocean floor.

Geology of the Ocean Floor

The ocean basins are characterized by a series or recognizablegeologic/topographic features (Figure). While the size of each feature varieswithin the various ocean basins they are always present:

1. Continental shelf - the gently (

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accumulate on the ocean floor as "fining upward" sequences ofsedimentary rocks.

4. Abyssal plain - the ocean floor (covers about 30% of the earth'ssurface). The average water depth is around 5000 meters. Consistsof a layer of unconsolidated sediment underlain by sedimentary

rock and pillow basalt (Figure).

Sea Floor Sediments

Sediments found on the floor of the ocean (abyssal plain) fall into three distinctcategories. The percentages of each vary from place to place within the oceanbasin and appear to be a function of deep ocean currents, prevailing windpatterns and local volcanism.

Lithogenous sediment - derived from the weathering of continental rocksand volcanic eruptions.

Biogenous sediment - comprised of the remains of organisms. When thesediment contains 30% or more organic material it is termed ooze. Oozesare further subdivided into calcareous oozes, which are only found inwater depths less than 3000 meters, and siliceous ooze that occurthroughout the deeper portions of the ocean basin.

Hydrogenous sediment - precipitated directly from seawater. Mostcommon type of hydrogenous sediment is a manganese nodule. How andwhy they form remains something of a mystery, but probably requires a

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contribution from hydrothermal waters generated by heat from subseavolcanoes.

Composition of Seawater

Ion wt %

Chlorine 1.92

Sodium 1.07

Sulfate 0.25

Magnesium 0.13

Calcium 0.04

Potassium 0.04

Others 0.02

TOTAL 3.47

The average salinity of seawater is 3.47% but oceanographers choose to reportsalinities in parts per thousand (ppt). Since wt% is the same as parts perhundred, all we need to do is multiply by 10. This gives a value of 34.7 ppt for

average salinity. Ocean water is very homogeneous but locally the salinity canvary from 33 ppt near the Poles to 41 ppt in arid, enclosed basins such as theRed Sea.

Scientists recognize the ocean is actually comprised of a series of layers(Figure). These layers represent differences in water temperature and salinity.The layering is a function of geographic latitude and water depth. The threelayers are:

Surface Zone - warmest water. Does not extend beyond 50 north orsouth of the equator. Is only about 2% of the ocean's volume. In this zone

the water is thoroughly mixed due to thermal energy from the Sun. Thiszone does not extend below the depth to which appreciable sunlightpenetrates seawater (a few hundred meters).

Transition Zone (pycnocline) - density changes rapidly with depth. Sincedensity of seawater is a measure of salinity and temperature we arelooking at the zone in which the effect both are changing. This zone is alsoabsent near the poles. About 18% of all seawater. Extends to a depth of1800 meters.

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Deep Zone - 80% of all seawater. Temperature and salinity are veryuniform and show little or no local variation.

Tides

Tides are caused by the gravitational attraction of the moon and the sun(Figure). Although the mass of the moon is much less than that of the sun it isalso much closer and hence its tidal pull is about twice that of the sun. A tidal daylasts 24 hours and 53 minutes, the time for the moon to make one completerevolution about the earth. However, the complete tidal cycle takes 19 yearsbecause of two complications:

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the moon and earth have elliptical orbits so their distance from oneanother and the sun varies. Since tidal forces are due to the pull of gravityand the force of gravity is inversely proportional to distance the height oftides will be a function of distance to the sun and moon. Further, tidalforces can be additive when the sun and moon are aligned relative to

earth producing very high (Spring) tides (Figure).

the tilt of earth's axis also effects the position of the sun and moon withrespect to the equator. This causes local variation in tidal height as afunction of latitude and season (Figure).

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Types of Tides (Figure)

1. Semidiurnal - two high and two low tides of the same height in 24 hours.2. Diurnal - one high and one low tide in a 24 hour period. Common along

Gulf Coast of U.S.3. Mixed - two high tides and two low tides of differing heights during a 24

hour period (California)

Not well understood what causes the different types of tides, but probably afunction of the geometry of the coastline. Open coastlines such as West Coastexperience mixed tides, while partially enclosed basins like the Caribbeanexperience diurnal tides with highs and lows varying by less than a meter

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