Organic fertilizer Sapropel.
Silty rotted deposition of plant and animal origin at the bottom of closed water basins

Sapropel, unconsolidated sedimentary deposit rich in bituminous substances. It is distinguished from peat in being rich in fatty and waxy substances and poor in cellulosic material. 

Sapropels are thought to develop during episodes of reduced oxygen availability in bottom waters, such as an oceanic anoxic event. 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. There are two main causes of : A reduction in deep-water circulation or a raised upper level oxygen demand.

A reduction in deep-water circulation will eventually lead to a serious decrease in deep-water oxygen concentrations due to biochemical 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. Organic-rich sediments may also form in well-ventilated settings that have highly productive surface waters; here the high surface demand simply extracts the oxygen before it can enter the deep circulation currents so depriving the bottom waters of oxygen.

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