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 in seawater temperature (El Nino) and salinity affect global climate.

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

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

Geology of the Ocean Floor

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

    1. Continental shelf - the gently (<1) sloping platform at the edge of the continent. The shelf is generally thought to be an extension of the continent and not really a part of the ocean basin. The average water depth on the shelf is about 75 meters, varying from zero at the shoreline to about 150 meters near its edge. A typical continental shelf is 60 kilometers wide, but it exceeds 100 kilometers off the Florida coast and is less than a few kilometers wide in places along the West Coast of South America. The rock underlying the thin veneer of sediments is granite similar to the basement rock elsewhere beneath the continents.

    2. Continental slope - the continental slope marks the transition between the shelf and deep ocean floor. It has an average slope of 3-6. This way not sound like much, but over a distance of 100 kilometers water depth increases from 75 meters to 4000 meters. Typically, continental slopes are crisscrossed by a series of deep submarine canyons the origin of which is controversial. Some represent drowned stream valleys, but others were clearly never above sea level and can not have been cut downward by stream erosion.

    3. Continental rise - represents the accumulation of sediment at the base of the continental slope. Result is a gentler slope and the buildup of "turbidite" deposits. Uplifted turbidite deposits are common along the coastline of southern California, particularly at Blacks and Torrey Pines beaches north of San Diego. Southern California turbidites are thought to form during major earthquakes which cause sediments to slide off the edge of the shelf and accumulate on the ocean floor as "fining upward" sequences of sedimentary rocks.

    4. Abyssal plain - the ocean floor (covers about 30% of the earth's surface). The average water depth is around 5000 meters. Consists of 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 distinct categories. The percentages of each vary from place to place within the ocean basin and appear to be a function of deep ocean currents, prevailing wind patterns and local volcanism.

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 report salinities in parts per thousand (ppt). Since wt% is the same as parts per hundred, 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 can vary from 33 ppt near the Poles to 41 ppt in arid, enclosed basins such as the Red 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 three layers are:


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 is also much closer and hence its tidal pull is about twice that of the sun. A tidal day lasts 24 hours and 53 minutes, the time for the moon to make one complete revolution about the earth. However, the complete tidal cycle takes 19 years because of two complications:



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 a function of the geometry of the coastline. Open coastlines such as West Coast experience mixed tides, while partially enclosed basins like the Caribbean experience diurnal tides with highs and lows varying by less than a meter