Anoxic sea water
Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen. 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 (silt) as well as by a pronounced density stratification, in which, for instance, heavier hypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate of oxidation
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 of organic matter by bacteria
Bacteria are a large domain of prokaryotic 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 throughout geological history. Anoxic basins exist at present, for example, in the Baltic Sea
Baltic Sea
The Baltic Sea is a brackish mediterranean 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 Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...

, and elsewhere (see below). Recently, there have been some indications that eutrophication
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...

 has increased the extent of the anoxic areas in areas including the Baltic Sea, and the Gulf of Mexico.

Causes and effects

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.-Dangers:Malaria and dengue are among the main dangers of stagnant water, which can become a breeding ground for the mosquitoes that transmit these diseases.Stagnant or Stailment water can be...

 conditions, density stratification, inputs of organic material, and strong thermocline
A 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
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

 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
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...

. Denitrification
Denitrification is a microbially facilitated process of 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 reactions describe all chemical reactions in which atoms have their oxidation state changed....

In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...

. If anoxic sea water becomes reoxygenized, sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

s will be oxidized to sulfate according to the chemical equation:
HS + 2 O2 → HSO4

In the Baltic Sea the slowed rate of decomposition under anoxic conditions has left remarkably preserved fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s retaining impressions of soft body parts, in Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....


Anoxic basins

  • Bannock Basin, eastern Mediterranean Sea
    Mediterranean Sea
    The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...

  • Black Sea
    Black Sea
    The Black Sea is bounded by Europe, Anatolia and the Caucasus and is ultimately connected to the Atlantic Ocean via the Mediterranean and the Aegean seas and various straits. The Bosphorus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects that sea to the Aegean...

     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 world's largest lake or a full-fledged sea. The sea has a surface area of and a volume of...

     Basin, below 100 metres (300 feet);
  • Cariaco Basin
    Cariaco Basin
    350px|thumb|right|Cariaco Basin, VenezuelaThe Cariaco Basin lies off the north central coast of Venezuela and forms the Gulf of Cariaco. It is bounded on the east by Margarita Island, Cubagua Island, and the Araya Pennisula; on the north by Tortuga Island and the Tortuga Banks; on the west by Cape...

    , off north central Venezuela
    Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...

  • 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. Within the Eastern Gotland Basin is the Gotland Deep which is an anoxic basin...

    , in the Baltic off Sweden
    Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

  • L'Atalante basin
    L'Atalante basin
    L'Atalante basin is a hypersaline brine lake at the bottom of the Mediterranean Sea about west of the island of Crete. It is named for one of the oceanographic research vessels involved in its discovery in 1993...

    , eastern Mediterranean Sea
  • 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 is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...

  • Orca Basin
    Orca Basin
    The Orca Basin is a mid-slope, silled, mini-basin in the northern Gulf of Mexico some 300 km southwest of the Mississippi River mouth on the Louisiana continental slope. It is unique amongst the mini-basins in this area, in containing a large brine pool of anoxic salt brine...

    , northeast Gulf of Mexico
    Gulf of Mexico
    The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

  • 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. Located just northwest of Victoria, the inlet is 24 km long , has a surface area of 65 km2 , and its maximum depth is 225 m . Great...

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

    , Canada
    Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends 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. Anoxic events may have caused...

  • Dead zone (ecology)
    Dead zone (ecology)
    Dead zones are hypoxic areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. These occur near inhabited coastlines, where aquatic life is most concentrated...

  • Hypoxia (environmental)
    Hypoxia (environmental)
    Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...

  • Meromictic
    A meromictic lake has layers of water that 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 source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.