OCEANS AND THEIR MARGINS

adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University

Cover about 71% of the surface of the Earth.
The greatest ocean depth of 11,035 m occurs in the Mariana Trench
Have an average depth of 3,800 m.
Have a present volume of about 1.35 billion cubic kilometers, but the volume fluctuates with the growth and melting of glacial ice.
Salinity, a measure of amount of dissolved ions in the oceans, ranges between 33 and 37 parts per thousand.
The dissolved ions have been concentrated in seawater as a result of chemical weathering (Na, Ca, Mg, S, K, Br, and HCO3 ) and degassing of the mantle by volcanic activity (Cl
Seawater would contain higher concentrations of dissolved ions if some were not removed by chemical precipitation, plants and animals, and absorption onto clay minerals.
Salinity varies in the oceans (see figure 13.3 in your text) because:
Surface waters evaporate, rain and stream water is added, and ice forms or thaws.
Salinity is higher in mid-latitude oceans
Salinity is higher in restricted areas of the oceans like the Mediterranean and Red Seas (up to 41 parts per thousand).
Salinity is lower near the equator because precipitation is higher.
Salinity is low near the mouths of major rivers because of input of fresh water.

Circulation is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

In each hemisphere cooler waters from higher latitudes circulate toward the equator where they are warmed and circulate back toward the poles.

Wave Erosion- Rigorous erosion of sea floor takes place in surf zone, i.e. between shoreline and breakers. Waves break at depths between 1 and 1.5 times wave height. Thus for 6m tall waves, rigorous erosion of sea floor can take place in up to 9 m of water. Waves can also erode by abrasion and flinging rock particles against one another or against rocks along the coastline.
Wave refraction- Waves generally do not approach shoreline parallel to shore. Instead some parts of waves feel the bottom before other parts, resulting in wave refraction or bending.

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Deltas -- Deltas form where sediment supply is greater than ability of waves to remove sediment. An example is the Mississippi River Delta, which is composed of several lobes that were deposited within the last several thousand years. Erosion of the older delta lobes has taken place due to subsidence, sea level rise, and lack of new sediment being supplied to the delta because of the human-made levee system.
Spits - elongated deposits of sand or gravel that projects from the land into open water. Spits usually form at the mouth of a bay due to long shore current and beach drift. Generally they curve inward towards the bay due to refraction of the waves around the mouth of the bay.
Bay Barriers - if a spit extends across a bay, it is called a bay barrier. Exchange of water between the bay and the ocean is accomplished through the groundwater system.
Tombolos - a spit that connects the mainland to an offshore island is called a tombolo.

Barrier Islands - A barrier island is a long narrow ridge of sand just offshore running parallel to the coast. Separating the island and coast is a narrow channel of water called a lagoon. Most barrier islands were built during after the last glaciation as a result of sea level rise. Barrier islands are constantly changing. They grow parallel to the coast by beach drift and longshore drift, and they are eroded by storm surges that often cut them into smaller islands.

Reefs and Atolls - Reefs consist of colonies of organisms, like corals, which secrete calcium carbonate. Since these organisms can only live in warm waters and need sunlight to survive, reefs only form in shallow tropical seas. In the deeper oceans reefs can build up on the margins of volcanic islands, but only do so after the volcanoes have become extinct. After the volcanism ceases, the volcanic island begins to erode and also begins to subside, due to the weight of newly added material. As the island subsides, the reefs continue to grow upward. Eventually, the original volcanic island subsides and is eroded below sea level. But, the reefs trap sediment and a circular or annular island, called an atoll.

Storms - great storms such as hurricanes or other winter storms can cause erosion of the coastline at much higher rate than normal. During such storms beaches can erode rapidly and heavy wave action can cause rapid undercutting and mass-wasting events of cliffs along the coast.
Tsunamis - a tsunami is a giant sea wave generated by an earthquake. Such waves travel at speeds up to 950 km/hr, have wavelengths up to 200 m, and upon approaching a shallow coastline, can have wave heights up to 30 m. Such waves have great potential to wipe out cities located along coastlines.
Landslides - on coasts with cliffs, the main erosive force of the waves is concentrated at the base of the cliffs. As the waves undercut the cliffs, they may become unstable and mass-wasting processes like landslides will result. Massive landslides can also generate tsunamis.