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Anoxic sea water
 
 
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Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
. This condition is generally found in areas that have restricted water exchange.

In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier (sill) as well as by a pronounced density stratification. Anoxic conditions will occur if the rate of oxidation
Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
 of organic matter by bacteria
Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
 is greater than the supply of dissolved oxygen.

Anoxic waters are a natural phenomenon, and have occurred during the geological history of the Baltic Sea
Baltic Sea

The Baltic Sea is a brackish inland sea located in Northern Europe, from 53?N to 66?N latitude and from 20?E to 26?E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Denmark islands....
.






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Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
. This condition is generally found in areas that have restricted water exchange.

In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier (sill) as well as by a pronounced density stratification. Anoxic conditions will occur if the rate of oxidation
Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
 of organic matter by bacteria
Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
 is greater than the supply of dissolved oxygen.

Anoxic waters are a natural phenomenon, and have occurred during the geological history of the Baltic Sea
Baltic Sea

The Baltic Sea is a brackish inland sea located in Northern Europe, from 53?N to 66?N latitude and from 20?E to 26?E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Denmark islands....
. Recently, there have been some indications that eutrophication
Eutrophication

Eutrophication is an increase in chemical nutrients — compounds containing nitrogen or phosphorus — in an ecosystem, and may occur on land or in water....
 has increased the extent of the anoxic areas in areas including the Baltic Sea, and the Gulf of Mexico.

Anoxic conditions result from several factors; for example, stagnation
Water stagnation

Water stagnation occurs when water stops flowing. Stagnant water can be a major environmental hazard....
 conditions, density stratification, inputs of organic material, and strong thermocline
Thermocline

The thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below....
s. The bacterial production of sulfide
Sulfide

The term sulfide refers to several types of chemical compounds containing sulfur in its lowest oxidation number of −2.Formally, "sulfide" is the dianion, S2−, which exists in strongly alkaline aqueous solutions formed from H2S or alkali metal salts such as Li2S, Na2S, and K2...
 starts in the sediments, where the bacteria find suitable substrates, and then expands into the water column.

When oxygen is depleted in a basin, bacteria first turn to the second-best electron acceptor, which in sea water is nitrate. Denitrification
Denitrification

Denitrification is a microbially facilitated process of dissimilatory nitrate reduction that may ultimately produce molecular nitrogen through a series of intermediate gaseous nitrogen oxide products....
 occurs, and the nitrate will be consumed rather rapidly. After reducing some other minor elements, the bacteria will turn to reducing
Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
 sulfate
Sulfate

In inorganic chemistry, a sulfate is a salt of sulfuric acid....
. If anoxic sea water becomes reoxygenized, sulfides will be oxidized to sulfate according to:

HS- + 2 O2 ? HSO4-

Anoxic basins

  • Bannock Basin, eastern Mediterranean Sea
    Mediterranean Sea

    The Mediterranean Sea is a sea or Ocean off the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Europe, on the south by Africa, and on the east by Asia....
    ;
  • Black Sea
    Black Sea

    The Black Sea is an inland sea sea bounded by southeastern Europe, the Caucasus and the Anatolia and is ultimately connected to the Atlantic Ocean via the Mediterranean Sea and Aegean Seas and various straits....
     Basin, off eastern Europe, below 50 metres (150 feet);
  • Caspian Sea
    Caspian Sea

    The Caspian Sea is the largest enclosed body of water on Earth by area, variously classed as the List of lakes by area or a full-fledged sea. It has a surface area of 371,000 square kilometers and a volume of 78,200 cubic kilometers ....
     Basin, below 100 metres (300 feet);
  • Cariaco Basin
    Cariaco Basin

    The Cariaco Basin is an east-west trending pull-apart basin located on the continental shelf off the eastern coast of Venezuela. It is a deep depression composed of two sub-basins, the eastern basin and the western basin, each of ~1400m depth, separated by a saddle of ~900m....
    , off north central Venezuela
    Venezuela

    Venezuela , officially the Bolivarian Republic of Venezuela , is a country on the northern coast of South America.The country comprises a continental mainland and numerous islands located off the Venezuelan coastline in the Caribbean Sea....
    ;
  • Gotland Deep
    Gotland Basin

    The Gotland Basin is the large central basin in the Baltic Sea between Sweden and the Baltic countries. It is subdivided into the Gdansk Deep , the Western Gotland Basin and the Eastern Gotland Basin....
    , in the Baltic off Sweden
    Sweden

    Sweden , officially the Kingdom of Sweden , is a Nordic countries on the Scandinavian Peninsula in Northern Europe. Sweden has land borders with Norway to the west and Finland to the northeast, and it is connected to Denmark by the ?resund Bridge in the south....
    ;
  • Mariager Fjord
    Mariager Fjord

    With a length of approximately 35 km , Mariager Fjord is the longest fjord in Denmark . Mariager Fjord cuts into the Jutland peninsula from the Kattegat sea and ends at the town of Hobro; other important towns along the fjord are Hadsund and Mariager from which the fjord takes its name....
    , off Denmark
    Denmark

    Denmark is a Scandinavian country in northern Europe and the senior member of the Kingdom of Denmark. It is the southernmost of the Nordic countries....
    ;
  • Orca Basin
    Orca Basin

    The Orca Basin is a mid-slope, silled, mini-basin in the northern Gulf of Mexico. It is unique amongst the mini-basins in this area, in containing a large hypersaline lake of Hypoxia brine....
    , northeast Gulf of Mexico
    Gulf of Mexico

    The Gulf of Mexico is the ninth largest body of water in the world. Considered a smaller part of the Atlantic Ocean, it is an oceanic basin largely surrounded by the North American continent and the island of Cuba....
    ;
  • Saanich Inlet
    Saanich Inlet

    Saanich Inlet is a body of salt water that lies between the Saanich Peninsula and the Malahat highlands of Vancouver Island, British Columbia, Canada....
    , off Vancouver Island
    Vancouver Island

    Vancouver Island is a large island in British Columbia, Canada, one of several North American regions named after George Vancouver, the British Royal Navy officer who explored the Pacific Ocean coast of North America between 1791 and 1794....
    , Canada
    Canada

    Canada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean....
    ;


See also

  • Anoxic event
    Anoxic event

    Oceanic anoxic events or anoxic events occur when the Earth's oceans become completely depleted of oxygen below the surface levels. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past....
  • Dead zone (ecology)
    Dead zone (ecology)

    Dead zones are hypoxia areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s....
  • Meromictic
    Meromictic

    A meromictic lake has layers of water which do not intermix. In ordinary, "holomictic" lakes, at least once each year there is a physical mixing of the surface and the deep waters....
  • The Revenge of Gaia
    The Revenge of Gaia

    The Revenge of Gaia: Why the Earth is Fighting Back - and How we Can Still Save Humanity is a book by James Lovelock....