Sapropel

Sapropel is a term used in marine geology to describe dark-colored sediments that are rich in organic matter. Sapropels events occurs periodically (around each 21000yr) and are specific to the Mediterranean sea. Organic carbon concentration in sapropels commonly exceeds 2% in weight.

Their occurrence has been found to be related of the Earth’s orbital parameter. The Earth’s pression cycles influences the African monsoon, which influences the Mediterranean circulation via an increase in freshwater inputs. Sapropels develop during episodes of reduced oxygen availability in bottom waters, such as an oceanic anoxic events (OAE).

• Oxygen can only reach the deep sea by new deep water formation and consequent “ventilation” of deep water. There are two main causes of OAE –
  • A reduction in Deep-water circulation
  • Raised upper level oxygen demand

Reduction in deep water circulation will eventually lead to a serious decrease in deep water oxygen concentration 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 water.

The oxygen depletion in bottom waters than favors 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.

• Sapropels deposits from global ocean anoxic events form important oil source rocks.