Black Sea

:For the XTC album, see Black Sea (album).

Geology

The Black Sea is the largest anoxic, or oxygen-free, marine system. This is a result of the great depth of the sea and the relatively high salinity (and therefore density) of the water at depth; freshwater and seawater mixing is limited to the uppermost 100 to 150 m, with the water below this interface (called the pycnocline) being exchanged only once every thousand years. There is therefore no significant gas exchange with the surface, and as a result decaying organic matter in the sediment consumes any available oxygen. In these anoxic conditions some extremophile microorganisms are able to use sulfate (SO42−) for oxidation of organic material, producing hydrogen sulfide (H2S) and carbon dioxide. This mix is extremely toxic (a lungful would be fatal to a human), resulting in a sea that has almost all of its ecology living in that top layer down to a depth of approximately 180 m (600 ft). The relative lack of micro-organisms and oxygen has allowed deep-sea expeditions to recover ancient (on the order of thousands of years) human artifacts, such as boat hulls and the remains of settlements.

Related Topics:
Oxygen - Pycnocline - Decay - Organic - Sediment - Extremophile - Sulfate - Hydrogen sulfide - Carbon dioxide - Micro-organisms

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Large amounts of organic material reach the bottom of the sea and accumulate in the sediments in concentrations of up to 20%. These kinds of sediments are called sapropel.

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While it is agreed that the Black Sea has been a freshwater lake (at least in upper layers) with a considerably lower level during the last glaciation, its postglacial development into a marine sea is still a subject of intensive study and debate. There are catastrophic scenarios such as put forward by William Ryan and Walter Pitman as well as models emphasizing a more gradual transition to saline conditions and transgression in the Black Sea. They are based on different theories about the level the freshwater lake had reached by the time the Mediterranean Sea was high enough to flow over the Dardanelles and the Bosphorus. On the other hand, a study of the sea floor on the Aegean side shows that in the 8th millennium BCE there was a large flow of fresh water out of the Black Sea (New Scientist, 4 May 2002, p. 13).

Related Topics:
Aegean - 8th millennium BCE - 4 May - 2002

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