Oxygen deficiency (hypoxia) in polluted bodies of water has become a common public concern in practically all of the industrially developed countries. How does this new threat affect the World Ocean, if at all?

Today we have no clear evidence of any growing oxygen deficiency in the World Ocean and of its poisoning with hydrogen sulfide. Why? Because appropriate observations are few and rare. As for the Black Sea, however, scientists have been monitoring the distribution of hydrogen sulfide in its water for many decades now ^* According to their findings, it is only the surface layer of water from 150 to 200 meters deep that contains enough oxygen to sustain all of the Black Sea flora and fauna. But the bulk of water in that sea has levels of hydrogen sulfide incompatible with any common life forms. This being so, a number of projects have been proposed for improving the situation. One suggests building a dam across the Bosphorus which could reduce the size of the hydrogen sulfide zone and cause its ultimate disappearance-just as was the case in the distant geological past when the dropping level of the ocean in the Ice Age practically closed the strait.

Other projects for the Black Sea involve its hydrological regime. According to calculations, by obstructing the Low Bosphorus current it would become possible to intensify the vertical intermixing of sea water thus increasing the rate of exchange between the upper (oxygenated) and lower (hydrogen sulfide) zones. This should lead, sooner or later, to the extinction of the latter. But attractive as it may appear, this project is not an ideal solution because according to some experts the main mechanism of water circulation in the Black Sea is connected not with the water exchange through the Bosphorus, and a gradual displacement of water upwards, but with the atmosphere.

There are several more "hydrogen sulfide" zones in the World Ocean, such as the Walfish Bay off the south-west coast of Africa in the Atlantic. Experts identified there what they call an anaerobic (oxygen deficient-hydrogen sulfide) and almost lifeless zone more than 50 km across which stretches for more than 300 km along the coast.

And there are also some similar areas in the Indian Ocean and the Pacific. Off the coast of Peru, for example, hydrogen sulfide zones crop up at intervals of about 7 years due to the inflows of the warm current El-Nino. ^** A second "Black Sea" of a comparable size was identified back in the 1950s by researchers of the Woodohol Institute of Oceanography (USA). This is the Carioco Depression on the continental shelf in the Caribbean north of Venezuela. Registered there was a thick hydrogen sulfide "lens" with a period of semicirculation of water of about 24,000 years.

Thus, starting from the Sea of Arabia and the Gulf of Oman and stretching further northwest is a belt of hydrogen sulfide zones, including the Caspian and the Black Seas, the Gotland Depression in the Baltic and the fiords of Norway, the coast of Kamchatka and the region of the Philippines and ending up with the Carioco Depression. Possibly some new regions with hydrogen sulfide zones will be detected in the future, say, off Alaska or the Aleutian islands.

According to some experts, present-day hydrogen sulfide zones are confined to geologically active regions characterized by orogeny (mountain formation) ,volcanism, earthquakes and formation of depressions in the World Ocean. But, be that as it may, these regions cannot be written off, for they affect, first, the ecology of our planet, secondly, the fish catches and, thirdly, the distribution of oil deposits.

^* See: V. Avilov, S. Avilova, "The Mystery of Black Sea Sediments", Science in Russia, No. 2, 1995 . - Ed.

^** See: A. Polonsky, "Mystery of El Nino", Science in Russia, No. 6, 1993.- Ed.

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