Sapropel

Sapropel (a contraction of ancient Greek words sapros and pelos, meaning putrefaction

Putrefaction

Putrefaction is the decomposition of animal proteins, especially by anaerobic microorganisms, described as putrefying bacteria. Decomposition is a more general process. Putrefaction usually results in amines such as putrescine and cadaverine, which have a putrid odor...

 and mud, respectively) is a term used in marine geology

Marine geology

Marine geology involves geophysical, geochemical, sedimentological and paleontological investigations of the ocean floor and coastal margins. Marine geology has strong ties to physical oceanography and plate tectonics....

 to describe dark-coloured sediments that are rich in organic matter. Organic carbon concentrations in sapropels commonly exceed 2% in weight.

Sapropels are thought to develop during episodes of reduced oxygen availability in bottom waters, such as an Oceanic 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...

 (OAE).

Sapropel (a contraction of ancient Greek words sapros and pelos, meaning putrefaction

Putrefaction

Putrefaction is the decomposition of animal proteins, especially by anaerobic microorganisms, described as putrefying bacteria. Decomposition is a more general process. Putrefaction usually results in amines such as putrescine and cadaverine, which have a putrid odor...

 and mud, respectively) is a term used in marine geology

Marine geology

Marine geology involves geophysical, geochemical, sedimentological and paleontological investigations of the ocean floor and coastal margins. Marine geology has strong ties to physical oceanography and plate tectonics....

 to describe dark-coloured sediments that are rich in organic matter. Organic carbon concentrations in sapropels commonly exceed 2% in weight.

Formation

Sapropels are thought to develop during episodes of reduced oxygen availability in bottom waters, such as an Oceanic 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...

 (OAE). Most studies of sapropel formation mechanisms infer some degree of reduced deep-water circulation. Oxygen can only reach the deep-sea by new deep-water formation and consequent "ventilation" of deep basins. Therefore, a reduction in deep-water circulation will eventually lead to a serious decrease in deep-water oxygen concentrations due to biological oxygen demand associated with the decay of organic matter that sinks into the deep-sea as a result of export production from surface waters. Oxygen depletion in bottom waters then favours the enhanced preservation of the sinking organic matter during burial in the sediments. However, organic-rich sediments may also form in well-ventilated settings with highly productive surface waters, where the high biological oxygen demand simply exceeds the re-supply of oxygen to bottom waters by deep circulation.

Significance

Sapropelic deposits from global Ocean Anoxic Events form important oil source rock

Source rock

In petroleum geology, source rock refers to rocks from which hydrocarbons have been generated or are capable of being generated. They form one of the necessary elements of a working hydrocarbon system. They are organic rich sediments that may have been deposited in a variety of environments...

s. Detailed process studies of sapropel formation have concentrated on the fairly recent eastern Mediterranean sapropels,
the last of which was deposited between 9.5 and 5.5 thousand years ago.

The Mediterranean sapropels of the Pleistocene reflect increased density stratification in the isolated Mediterranean basin. They record a higher organic carbon concentration than non-sapropel times; an increase in the δ¹⁵N and corresponding decrease in δ¹³C tells of rising productivity as a result of nitrogen fixation. This effect is more pronounced further east in the basin, suggesting that increased precipitation was most pronounced at that end of the sea.