The oceans that constitute nearly three-fourth of the surface of the earth are critical to the earth’s ecosystem and play a dominant role in moderating climate. They provide food and more significantly a major part of the oxygen that we breathe. The origin of life itself can be traced to the oceans. In fact, this maybe the reason why the chemistry of the human blood is similar to sea water (Theberge, 2007). The oceans provide easy and commercially viable navigational routes that connect distant lands from north to south and east to west corners of the world. While their vast depths of water exhibit great biodiversity, the ocean floor is endowed with huge mineral resources including oil and gas. The recognition of the value of oceans in the development of economy of coastal nations has given rise to the concept of the ‘blue economy’ in recent times, which is related to maritime trade and activities such as energy, trade, tourism, fisheries, marine mineral resources, shipping and more.
Five oceans of the earth
The Arctic, Atlantic, Indian, Pacific and Southern Oceans together constitute the world oceans and form the most significant component of the hydrosphere (Fig. 1). The Pacific Ocean, largest of all oceans, occupies 32 per cent of the earth’s surface and connects with Arctic Ocean in the north and Southern Ocean in the south. The areas bounding the Pacific Ocean are geologically very active. The famous Mariana Trench located in this ocean represent a deep subduction zone that has the deepest point on earth (about 11 km). The Atlantic Ocean, like the Pacific connects Arctic Ocean in the north and the Southern Ocean in the south. The floor of the Atlantic Ocean is marked by Mid Atlantic Ridge which is one of the longest submarine mountain range, running from Iceland in the north to 58oS latitude.
The Indian Ocean is the third largest ocean. Though it does not extend up to the Arctic, it connects with the Southern Ocean. The Ocean floor of Indian Ocean is marked by a complex mid oceanic ridge that bifurcates to Southwest and Southeast Indian Ridge near Rodrigues Island. The Arabian Sea and the Bay of Bengal on two sides of the Indian peninsula are known for huge piles of sediments, the Indus and the Bengal fans, on the ocean floor.
The Arctic and the Southern Ocean located at the northern and southern polar regions are considered the lungs of the earth, as they cool the warm ocean currents. These oceans differ in the sense that while Arctic Ocean is surrounded by Europe, Asia and North America; the Southern or the Antarctic Ocean encompasses the southern portion of the Pacific, Atlantic and Indian Oceans that lie beyond 60oS latitude. Major parts of these two oceans are frozen during the winter season. While the Southern Ocean retains multiyear ice, the sea ice in the Arctic Ocean is fast depleting due to rising sea temperatures. The Arctic Ocean is the smallest of the five oceans and its exchange with other oceans occur mainly through the Atlantic Ocean (between Greenland and Norway) while the fresh water input is received from Canada, north of Norway and Siberia.
There are differences among geographers regarding the northern boundary of the Southern Ocean. While some believe this boundary is defined by the body of water which lies south of the northern limit of the southern circulation, others regard the Antarctic Convergence as the natural boundary (Pyne, 1986). However, as per the International Hydrographic Organisation (IHO), the waters south of the 60th parallel are to be considered as that of Southern Ocean. The Southern Ocean circulation is defined by the west to east flow of the Antarctic Circumpolar Current (ACC).
The ocean floor
The earth’s crust differs in composition between the continents and the oceans. The rocks constituting the ocean floor are younger, denser and basaltic in nature as compared to the continental crustal rocks that are older, granitic and lighter in density. The oldest ocean rocks have been dated around 170 million years while some of the continental rocks from Canadian Shield, Africa and Indian peninsular region are more than 3.5 billion years old (Valdiya, 2016). The crustal evolution is a dynamic process and is governed by plate tectonics. The theory of plate tectonics divides the earth’s lithosphere into 14 main plates which move relative to each other due to convection currents in the upper mantle.
These plates collide, subduct or are pulled apart from each other, making their boundaries the loci of volcanic eruptions and /or earthquakes. The out pouring of lava from the interior of earth gives rise to new crust that spreads away from the mid oceanic ridges (Fig. 2) at the ocean floor. The junction points of these ridges (triple junctions) are often sites of rich hydrothermal vents. The ocean floor has a rough surface with mounds and trenches. At several places such as in Pacific and the Indian Ocean, nodules rich in manganese, nickel and iron are found. Such deposits—polymetallic nodules (PMN) are being explored by various geophysical methods.
Even though sunlight does not reach the deeper parts of the seas, life exists even at abyssal depths. Such deep sea organisms survive without photosynthesis in hypoxic environment by the chemosynthesis processes where chemical energy is changed into food energy. For example, tubeworms and mussels are often found feeding at the sites of hydrothermal vents. The bedrocks of the oceans, closer to continental margins are covered by layers of sediments brought down by the rivers or deep ocean currents.
Most of these sediments and those driven by deep ocean currents form a ‘rise’ at the junction of abyssal depth and the continental slope that covers the boundary between continental and oceanic crust. (Fig. 3). The ocean floor may also be covered by marine debris brought from a great distance, mounds, trenches and volcanic craters.
Are world oceans homogeneous?
Morphologically, oceans have been divided into geomorphic zones depending upon depth and gradient into shelf, continental slope, continental rise and abyssal depth or ocean floor (Fig. 3). These zones also correspond to benthic or pelagic zones depending upon the marine communities present there. The maritime zones of the oceans, from coast towards sea are—territorial sea; contiguous zone; exclusive economic zone (EEZ); extended continental shelf; and, deep sea bed or high seas. While the limit of territorial sea is at 12 nautical miles (NM) measured from the base line of a nation, the region of 24 NM, measured from the base line has been defined as the contiguous zone of that nation. EEZ of a coastal nation extends up to 200 NM from the base line, which can further be extended to a maximum of 350 NM from the base line (extended continental shelf) provided the conditions put forth in Article 76 of the United Nations Convention on the Law of the Sea (UNCLOS III) are met.
Sea water display diverse physical characteristics, hydrochemistry and varied marine life both laterally and vertically. Oceans are spread from the northern high latitudes
to tropical zones around equator to high southern latitudes.
Different water masses are identifiable in oceans. Water mass or a water body is an identifiable body of water with a common formation history and with physical properties (temperature, salinity, chemical isotopic ratios), distinct from surrounding water (Emery and Meincke, 1986). These water masses are known by the names of the localities and/or vertical depth at which these are located. Some of the prominent water masses are—Antarctic bottom water, Antarctic intermediate water, Subantarctic mode water, North Atlantic deep water, Arctic intermediate water, North Pacific intermediate water, the central waters of various oceanic basins, etc. These water bodies are in a constant state of motion (Fig. 4) as the complex interaction of the sun, the moon and the earth drives both, the oceanic as well as
the atmospheric circulation (Lumpkin and Spear, 2007).
The water movement or the ocean circulation is propagated by various regional currents. While the surface currents are a result of wind patterns the large scale ocean circulation or the thermohaline circulation is driven by the
Surface currents like the Gulf Stream that runs polewards from the equatorial Atlantic Ocean lose heat gradually as they reach at higher latitudes. This together with influx of melt water makes water denser, which consequently sinks forming North Atlantic Deep Water (Fig. 4). The upwelling of this water in the Southern Ocean results in bringing up of nutrient rich water to the surface, creating favourable conditions for mixing. Among the important ocean currents are the:
Antarctic Circumpolar Current
Deep Ocean Current
Western Boundary Currents (Gulf Stream, Kuroshio, Labrador, Oyashio, Agulhas, Brazil and East Australia Current)
Eastern Boundary Currents (California, Canary, Peru and Benguela Current).
Oceans are important constituent of our earth system and a major player in the hydrological cycle. It moderates the climate and is the primary sink for CO2. Ocean floors demonstrate a complex bathymetry with mountain ranges stretching from pole to pole and deep marine trenches. Oceans are source of livelihood of millions of people, directly or indirectly. The living and non living resources in the form of fisheries and minerals, including fossil fuel provide humans with food and raw materials for energy and other mineral based industries. Our knowledge about the ocean and their dynamic processes is increasing rapidly, owing to marine expeditions that deploy various geophysical techniques and oceanographic tools to unearth the mysteries of deep oceans.
Emery W. J. and and J., Meppincke, 1986. Global water masses-summary and review. Oceanologica Acta, 9 (4): 383–391.
Lumpkin R., and K. Spear, 2007. Global ocean meridional overturning, Journal of Physical Oceanography, 37(10): 2550-2562.
Pyne, S. J. 1998; The Ice: A Journey to Antarctica. (A study of Antarctica’s exploration, earth-sciences, icescape, esthetics, literature, and geopolitics). University of Washington Press.
Theberge, A. E., 2007. Hidden Depths. The Atlas of the Oceans.:NOAA, Harper Collins.
Valdiya, K.S., 2016 . The Making of India. Geodynamic Evolution, Switzerland: Springer.