seas

seas. In the ancient world traditionally there were seven seas—the Red Sea, the Mediterranean, the Persian Gulf, the Black Sea, the Adriatic Sea, the Caspian Sea, and the Indian Ocean. Seven is a mystical number. The world was created in seven days, there are seven deadly sins, seven graces, seven divisions to the Lord's Prayer, and seven ages in the life of man. The Romans named the seven salt-water lagoons off Venice ‘Septem Mare’. Rudyard Kipling further popularized the use of the term through the title of one of his books of poems.

Nowadays it tends to embrace all the oceans (two of the five oceans, the Pacific and the Atlantic, are divided into North and South). A sea usually describes an area of water that is geographically distinct, so the North Sea, the Baltic Sea, the Caribbean Sea, the Bering Sea, and the Sea of Okhotsk are bodies of water that are semi-enclosed by land or island arcs. However, others, like the Sargasso and the Arabian Seas, are areas of the open ocean with special characteristics that set them apart. The Baltic and the North Seas are quite shallow, whereas the Mediterranean, Black, and Red Seas have areas of water that are over 2,000 metres (6,500 ft) deep and offer exceptional deep-sea environments.

The straits that give entry into the Mediterranean and Red Seas, Gibraltar and Bab-el-Mandeb respectively, are narrow and shallow and inhibit deep-water interchange with the neighbouring oceans. Both are situated at arid latitudes, so the volumes of freshwater inputs by rainfall and by rivers are very much smaller than the volumes of water lost by evaporation. In the eastern Mediterranean this imbalance has been accentuated by the reductions in outflows from the Nile following the building of the High Aswan Dam. The imbalance both lowers sea level and increases the salinity at the far ends of the two seas. As a result strong surface currents constantly flow in through the two straits. This was of great help to Nelson when he was blockading the French fleet during the Revolutionary and Napoleonic Wars (1793–1815). It also results in the deep water in these seas being unusually warm and salty.

In the western Mediterranean the water below 300 metres (985 ft) is 12.6 °C (54.6 °F) all the way to the bottom at over 2,000 metres (6,500 ft), whereas outside in the Atlantic the temperature at 2,000 metres is about 3–4 °C (37.4–39.2 °F). Deep-water temperatures in the Red Sea are even more extreme at 21.6 °C (71 °F). In contrast, the deep water of the Japan Sea is sub-zero year-round.

The Black Sea is even more extraordinary, being a Mediterranean-type sea within the Mediterranean. It opens into the main sea via two straits, the Dardanelles and the Bosporus, through which there is very limited exchange. It is quite deep, over 2,000 metres (6,500 ft), but for its size it receives substantial freshwater inputs from major European rivers, the Danube, the Dniester, and the Dnieper. These rivers drain the fertile steppes and introduce high concentrations of nutrients. So below a depth of 150 metres (490 ft) the water of the Black Sea is anoxic, that is, totally devoid of oxygen, and rich in hydrogen sulphide. Indeed, it gained its name from the way an iron implement, if lowered deep into the sea, comes up blackened with sulphide. The Arabian Sea is the region of the north-west Indian Ocean along the coast of Oman, and is one of the most variable seas as a result of the monsoons. See also high seas; narrow seas; oceans; united nations conference on the law of the sea.

M. V. Angel