World Geography

OCEANS

Division of ocean depths

• The World Ocean is a global, interconnected continuous body of saline water. Approximately 71% of the earth’s surface is covered by the ocean
• For human convenience, the ocean has been divided into several smaller divisions known as oceans and seas
• There are five major divisions of the world ocean: Pacific Ocean, Atlantic Ocean, Indian Ocean, Arctic Ocean and Southern Ocean
• Evaporation of water from the oceans is the source of most rainfall, and ocean temperatures determine climate and wind patterns on land
• Life within the ocean evolved about 3 billion years prior to life on land. More than 230,000 marine life forms are currently known, but the actual number may be 10 times as much
  

Physical properties

• The total area of world ocean is 361 x 10⁶ sq km, and volume is approx 1.3 billion cu km
• The average depth of the ocean is 3790 m and maximum depth is 10,923 m
• The average density of sea water is 1.025 g/ml and has a freezing point of -2 C
• Sea water contains more dissolved ions than all types of freshwater, especially sodium and chloride. On average, sea water has a salinity of 3.5 %.
• The causes of high salinity of sea water include
  • River runoff causing concentration of sodium in the ocean
  • Sodium leaching out of the ocean floor when the ocean was formed
  • Chloride abundance due to volcanic activity and hydrothermal vents on the ocean floor
• Ocean salinity has been stable for billions of years

Sea level

• Mean Sea Level (MSL) is a measure of the average height of the ocean’s surface
• Mean Sea level is usually taken to be the half way point between mean high tide and mean low tide
• Sea level change can be measured in two ways
  • Local change: local mean sea level can be affected by vertical movement of land, and changes in atmospheric pressure, ocean currents and local ocean temperature
    
  • Eustatic change: is the alteration of global sea levels, such as changes in volume of water in the world oceans and changes in volume of ocean basins
    
• Short term changes in sea level can arise from tides, atmospheric pressure, storm surges, El Nino etc
  
• Medium term changes in sea level arise mainly from two factors: atmospheric temperature and the mass of water locked up as fresh water in rivers, lakes, glaciers, ice caps etc
  
• Geological changes in sea levels mainly arise from changes in the configuration of continents and sea floors due to plate tectonics and seafloor spreading
  
• On a geological time scale, long term sea level has always been higher than today (except at the Permian-Triassic boundary 250 million years ago). As a result, sea level is more likely to rise than fall today, even due to small changes in climate
• Over the past 100 years, sea level has been rising at an average of 1.8 mm per year. The majority of this rise is attributed to thermal expansion of ocean water due to increase in ocean temperatures

Ocean currents

• Ocean current is a continuous directed movement of ocean water generated by wind, Coriolis force, temperature and salinity gradients, and tides
  

  Major ocean currents of the world
• Ocean currents greatly affect earth’s climate by transferring heat from the tropics to polar regions, and transferring precipitation to coastal regions
• The most famous example of ocean currents is the Gulf Stream, which makes northwest Europe much more temperate than any other region at that latitude
• Surface ocean currents are generally driven by wind, and circulate in clockwise direction in the northern hemisphere and anticlockwise in the southern hemisphere (due to prevailing winds). Surface currents make up about 10% of all ocean currents
• Deep ocean currents, called thermohaline circulation, are driven by water density and temperature gradients. Also known as the world’s conveyor belts, these deep ocean currents supply heat to polar regions and thereby regulate sea ice formation. Deep ocean currents make up about 90% of all ocean currents
• Ocean currents are measured in Sverdrup (Sv), with 1 Sv being equivalent to a flow rate of 1 million cu. m per second

MARINE GEOGRAPHY

Oceanic basins

• Oceanic basins are large geologic basins (large scale rock strata) that are below sea level
• In a sense, oceanic basins are the complement to continents
• Ocean basins serve as sedimentary basins that collect sediments eroded from continents
• Ocean basins can be actively changing or inactive depending on plate tectonics. The Atlantic and Antarctic Ocean basins are actively growing while the Mediterranean is shrinking. Inactive ocean basins include the Gulf of Mexico, the Sea of Japan and the Bering Sea

Schematic of a continental shelf

• The continental shelf is the extended perimeter of a continent which is currently under sea
• Continental shelves were part of the continents during glacial periods (when sea levels were low) but are under sea during interglacial periods (like today)
• The continental shelf usually ends at a point of decreasing slope, called the shelf break. The sea floor below the shelf break is called the continental slope. Below the slope is the continental rise, which merges into the deep ocean floor (called abyssal plain)
• Due to the availability of sunlight in shallow waters, continental shelves teem with life, compared to the biotic deserts in the deep ocean abyssal plains
• Continental shelves consist of thick sediments from the continents
• Continental shelves extend on average about 80 km from the coast. The largest shelf, the Siberian Shelf in the Arctic Ocean, stretches to about 1500 km, while certain areas have no shelves at all such as the coast of Chile and the west coast of Sumatra (Indonesia)
• The United Nations Convention on the Law of the Sea (UNCLOS) defines the extent and regulates usage of continental shelves by sovereign nations
  • The continental shelf was defined as the natural prolongation of land to the continental margin’s outer edge, or 200 nautical miles from the coast, whichever is greater. 
    
  • However, the shelf is to not exceed 350 nautical miles, and it is to not exceed 100 nautical miles beyond the 2500m isobath
  • The coastal nations have the exclusive right to harvest mineral and non-living material in the subsoil of the continental shelf
  • Coastal states also have exclusive rights to living resources “attached” to the shelf, but not to creatures living there freely

Mid Ocean Ridges

• A mid ocean ridge is an underwater mountain range formed by plate tectonics
• Mid ocean ridges are caused by seafloor spreading i.e. magma rising through the crust and emerging as lava which then cools to form new oceanic crust
• A mid ocean ridge demarcates the boundary between two tectonic plates, and is called a divergent plate boundary
• The various mid-ocean ridges of the world are connected and form a single global mid ocean ridge system which covers every ocean. Thus, the mid ocean ridge system is the longest mountain range in the world (over 65,000 km)
• Mid ocean ridges are geologically active, with new magma constantly emerging onto the ocean floor

Ocean Trenches

• Ocean trenches are large scale long but narrow depressions on the sea floor. They are the deepest parts of the ocean floor
• Trenches are found at convergent plate boundaries, where one plate subducts (descends) beneath another. On average, oceanic crust moves into trenches at a rate of about 0.1 sq m per second
• They are usually located parallel to volcanic arcs at a distance of about 200 km
• Ocean trenches typically extend about 3-4 km below the level of the surrounding sea floor
• The deepest ocean depth known is the Challenger Deep point of the Mariana Trench in the Pacific Ocean (10,911 m)

Trench	Location	Depth	Notes
Mariana Trench	Western Pacific Ocean (near Philippines and Japan)	10,911 m	Deepest known part of the ocean Lowest elevation on the surface of the earth’s crust Maximum depth is recorded at Challenger Deep, a small valley at its southern end Formed by the subduction of Pacific plate under Mariana plate
Tonga Trench	Southern Pacific (near New Zealand)	10,882 m	Formed by subduction of Pacific plate under Tonga plate and Indo-Australian plate Fastest plate velocity recorded on earth (24 cm per year)
Kuril-Kamchatka Trench	Northern Pacific	10,542 m
Philippine Trench	Philippines (Pacific Ocean)	10,540 m
Kermadec Trench	New Zealand (Pacific Ocean)	10,047 m

Extraterrestrial oceans

• The earth is the only known planet to have liquid water on its surface
• However, liquid water is known to be present under the surface on Jupiter’s moons Europa, and possibly on Ganymede and Callisto
• It is believed that Venus once had liquid water and oceans on its surface, but they have now vanished
• Saturn’s moon Titan is thought to have subterranean water ocean under its crust (which consists of ice and hydrocarbons)

OCEANIC HABITATS

Corals are nocturnal feeders. Here, in the dark, the coral polyps extend their tentacles to feed on zooplankton

• Coral reefs are aragonite structures formed by living animal colonies.Aragonite is a carbonate mineral, one of the two major naturally occurring crystalline forms of calcium carbonate (the other being calcite)
• Reefs consist mostly of stony corals. These corals are built from polyps that secrete an exoskeleton of calcium carbonate
• Coral reefs grow best in shallow, clear, sunny waters. They are usually found in shallow depths in the tropics, but deep cold water reefs also exist although on a much smaller scale
• Coral reefs are some of the richest and most diverse ecosystems in the world. They occupy less than 1% of world ocean surface, but provide habitat to about 25% of all marine species
• Reefs are found in ocean waters containing few nutrients. High nutrient levels, such as found in agricultural runoff, can harm reefs by encouraging excess algae growth
• Coral reefs are under threat from climate change, ocean acidification, overfishing and overuse of reef resources

Coral reef	Location	Notes
Great Barrier Reef	Queensland, Australia (northeast Australia)	Largest coral reef system in the world World’s biggest structure made by living organisms Area of approx 344,000 sq km
Belize Barrier Reef	Belize	Second largest coral reef in the world Part of the Mesoamerican Barrier Reef that stretches along eastern coast of Central America from Mexico to Honduras
New Caledonia Barrier Reef	New Caledonia (southwest Pacific)	French territory in southwest Pacific Home to endangered dugong, and nesting site for green sea turtle
Andros (Bahamas) Barrier Reef	Bahamas (Caribbean)
Red Sea Coral Reef	Red Sea
Pulley Ridge	Florida, USA (southeast USA)	Deepest photosynthetic coral reef in the world (about 60-80 m)
Maldives	Indian ocean	Consists of about 1200 coral islands
Raja Ampat Islands	Indonesia	Contains the highest marine life diversity in the world

Deep sea and trenches

• As the ocean depth increases, sunlight decreases and water pressure increases.
• In general, sunlight is not able to penetrate the ocean water beyond a depth of 200 m. This depth is considered to be the beginning of aphotic zone (deep sea). Unusual and unique creatures inhabit these depths including giant squid, gluper eel, angler fish and vampire squid
• In the trenches, water pressure is extreme and sunlight is non existent. However, small flounder fish (family Soleidae) and shrimp have been observed even at these depths
• Seamounts (extinct undersea volcanoes that rise to shallow depths) provide natural habitats for fish and other species to spawn and feed
• Hydrothermal vents on the ocean floor support unique life forms, deriving essential nutrients from the chemicals released by volcanic activity

Open Ocean

• The open ocean is relatively unproductive due to lack of nutrients. However, simply due to its vastness, it possesses the largest number of life forms in total
• In the aphotic zone, energy for life forms is mainly supplied in the form of detritus, which is non living organic material consisting of dead organisms and fecal material
• The open ocean consists mainly of jelly fish and its predators like the mola mola

Intertidal and shores

• Intertidal zones are those areas close to the shore, which are constantly being covered and exposed by the tides
• These areas can be underwater anywhere from daily to very infrequently
• A huge array of life forms is found in this zone. This includes crabs, snails etc