oceans

oceans. The waters of the oceans provide the most voluminous habitat on the planet; 71% of the earth's surface is covered by sea to an average depth of 3,800 metres (12,470 ft). The volume of water in the oceans is estimated to be 1.368 × 10⁹ cub. km (3.283 × 10⁸ cub. mls.), which is 97% of all the water on the planet.

About 5% of the earth's surface is covered by the shallow waters of the continental shelves that extend down to depths of up to 200 metres (650 ft). At the outer edge of the shelf seas is the shelf-break, where the slope of the seabed suddenly steepens. Across the continental slope ocean depths increase to 3,000 metres (10,000 ft) and the lower margin of the slope where the gradient slackens is known as the continental rise. Continental slopes and rises account for 13% of the earth's area. The slopes are underlain by continental rocks whereas beneath the rise is a talus (scree) slope of rock debris that has slipped from the slope. Beyond the rise stretch abyssal depths of 3,000–6,000 metres (10,000–20,000 ft), which cover about 51% of the earth's surface, and are underlain by oceanic crust (see geological oceanography) that is constantly being formed along the mid-ocean ridges. Offshore oil and gas reserves are found in sedimentary rocks beneath the seabed of the continental shelves and slopes. They occur in regions which in the geological past have been highly productive, such as in the Persian Gulf, the Gulf of Mexico, the North Sea, and off Angola, Indonesia, and Brazil.

Along the margins of the Pacific Ocean and some island arcs the ocean crust is being subducted—buckled down beneath the continents. Here occur the deepest depths in trenches of over 6,000 metres (20,000 ft). The continents are asymmetrically distributed across the surface of the earth, and so the intervening oceans are very disparate. For example 60.7% of the northern hemisphere is covered with ocean compared with 80.9% in the southern hemisphere. Consequently sea-surface temperatures range more widely at temperate latitudes in the northern hemisphere than in the southern hemisphere. The boundaries of four of the major oceans are largely determined by the distribution of the continental land masses.

Pacific Ocean.

This is the largest and oldest ocean. Its total area is around 165.38 × 10⁶ sq. km (63.84 × 10⁶ sq. mls.), with an average depth of 4,200 metres (13,750 ft), and its maximum depth of 11,015 metres (36,130 ft) in the Marianas Trench is the deepest anywhere. Its total volume is about 695 × 10⁶ cub. km (167 × 10⁶ cub. mls.) In the south it is fully open to the Southern Ocean, but to the north it has only a shallow connection with the Arctic Ocean via the Bering Strait. It connects with the Atlantic only though the Drake Passage to the south of the tip of South America, but as recently as 5 million years ago, there was a shallow water connection across the Panama Isthmus. The South Pacific is connected to the Indian Ocean near the equator via the shallow sills between the islands of the Indonesian archipelago and to the south of Australia at full ocean depths.

Atlantic Ocean.

This is the second largest ocean, and has an area only half that of the Pacific, 82.22 × 10⁶ sq. km (31.74 × 10⁶ sq. mls.). It is a ‘young’ ocean; the North Atlantic began to open during the Jurassic era about 200 million years ago. Previous to that there was only one supercontinent, Pangea. The South Atlantic began to open up about 100 million years ago. The shapes and geology of Africa and South America are complementary and can be fitted together like pieces of a jig-saw. Because of its relative youth, the average depth of the Atlantic is only 3,600 metres (11,800 ft), and its volume is about 296 × 10⁶ cub. km (71.04 × 10⁶ cub. mls.). Its maximum depth of 9,560 metres (31,350 ft) is in the Puerto Rico Trench. It is the only ocean with a deep-water connection with the Arctic Ocean.

Warm Atlantic water flows north eastwards via the Norwegian Sea and feeds the Spitsbergen Current. This flow not only keeps the climate of north-west Europe much milder than that of north-west America, but it has a major influence on the ecology of the Arctic and keeps the seas to the north of Norway free of ice. These warm surface flows into the Arctic are balanced by cold outflows that spill over the ridges that stretch between Greenland, Iceland, the Faeroes, and Scotland, and also the flow of the East Greenland Current southwards along Greenland and Canada.

In the south, the Atlantic connects with the Indian Ocean around the southern tip of South Africa and large eddies of water pass around the Cape from the Indian Ocean. Further south around Cape Horn there is a connection with the Pacific. To the south, the Antarctic Convergence marks its boundary with the Southern Ocean.

Indian Ocean.

This has an area of 73.48 × 10⁶ sq. km (28.36 × 10⁶ sq. mls.), and a volume of 282.9 × 10⁶ cub. km (68.90 × 10⁶ cub. mls.). It is connected to the Southern Ocean to the south, but to the north it is bounded by continental Asia. Its climate is modulated by the interaction between the atmosphere and the Asian land mass, resulting in seasonally reversing monsoons. When the winds reverse, so do the surface currents particularly to the north of the equator. So in the Arabian Sea and to a lesser extent in the Bay of Bengal, the fertility of the ocean oscillates between being very rich during the south-west monsoon, and very unproductive during the north-east monsoon.

Arctic Ocean.

This has an area of 14.1 × 10⁶ sq. km (5.44 × 10⁶ sq. mls.). It is a truly polar ocean, as it lies mostly within the Arctic Circle and is almost entirely encircled by land. Nearly half its area of consists of shallow seas, particularly north of Russia. Its greatest depth of 4,400 metres (14,400 ft) lies under the pack ice close to the North Pole. Its connection with the North Pacific via the Bering Strait is only a few kilometres wide and is very shallow, whereas its connection with the North Atlantic is broad and deep. The inflow of warm water from the Atlantic pushes the southern boundary of the winter pack ice far to the north. Even so, much of the Arctic Ocean remains covered throughout the year with pack ice that can be up to five years old, and 1.5–4 metres (5–13 ft) thick. Currently the Arctic pack ice is thinning so quickly that much of the Arctic may become ice free by the end of this century. While this may finally open up the North-West Passage for commercial fisheries, it will have a devastating effect on charismatic Arctic species like the polar bear and marine mammals like the walrus, narwhal, and belugas. In the summer the area of the Arctic covered by pack ice normally shrinks by about 10%. The vast outflows of the great Siberian rivers keep the surface salinity of the Arctic quite low, so that the sea freezes at slightly warmer temperatures.

Southern Ocean.

Unlike the Arctic Ocean, which lies mostly in the Arctic Circle, only a relatively small part of this ocean lies south of the Antarctic Circle. This is why its former, popular name of Antarctic Ocean has been dropped. It is bounded in the south by the continent of Antarctica, but to the north it is open to the other major oceans, and is not separated from them by any geographical features. Its northern boundary is the Antarctic Convergence, whose position varies both seasonally and interannually. The major feature of its circulation is the Antarctic circumpolar current, which is driven by prevailing westerly winds of the Roaring Forties (and the ‘Filthy Fifties’). This current began to flow about 35 million years ago when the continent of Australia separated from the continent of Antarctica sufficiently to create an unimpeded circumpolar deep-water connection.

The Antarctic ice cap started to develop about 14 million years ago, becoming a major influence on the climate of the Southern Ocean and the evolution of characteristic Southern Ocean species such as the penguin. The extreme conditions, with water temperatures that can fall as low as -2 °C (28.4 °F), have led to the evolution of some special adaptations. Some fish have antifreeze in their blood and tissues, possibly one of the marine pharmaceuticals of the future, as the blood of most species will freeze if water temperatures fall below -1.5 °C (29.3 °F).

The extent of pack ice in the Southern Ocean fluctuates enormously seasonally from about 20 million sq. km (7.7 × 10⁶ sq. mls.) at the end of winter to 5 million sq. km (1.93 × 10⁶ sq. mls.) in late summer. Most is renewed each year; only in the Weddell Sea, as in the Arctic, is there much multi-year ice. Immense tabular icebergs are spawned from the broad ice-shelves of the Ross and Weddell Seas where the depth of the continental shelf has been depressed to 400–500 metres (1,300–1,640 ft), by the massive weight of the ice sheet.

Along the margins of the pack ice the seas are highly productive, and attract large concentrations of marine mammals and seabirds. During the 19th and 20th centuries sealing and whaling exploited the fur seals, penguins, and whales. See also seas.

Bibliography

Cramer, D. , Great Waters: An Atlantic Passage (2001).
Hardy, Sir Alister , Great Waters (1967).

M. V. Angel