text: Sapropels have been recorded in the Mediterranean sediments since the closure of the Eastern Tethys Ocean 13.5 million years ago. The formation of sapropel events in the Mediterranean Sea occurs approximately every 21,000 years and last between 3,000 and 5,000 years. The first identification of these events occurred in the mid-20th century. Since then, their formulative conditions of have been investigated.
  The occurrence of sapropels has been related to the Earth's orbital parameters (Milankovitch cycles). The precession cycles influence the African monsoon, which influences the Mediterranean circulation through increases in freshwater inputs.
  Sapropels develop during episodes of reduced oxygen availability in bottom waters, such as an oceanic anoxic event (OAE). Most studies of formational 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 OAE: reduction in deep-water circulation or raised oxygen demand from upper level.
  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. This sinks into the deep sea as a result of export production from surface waters. Oxygen depletion in bottom waters then favors the enhanced preservation of the  organic matter during burial by 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 current thus depriving the bottom waters of oxygen.