(a contraction of ancient Greek words sapros
, meaning putrefaction
and mud, respectively) is a term used in marine geology
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
(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.
Sapropelic deposits from global Ocean Anoxic Events form important oil source rocks
. 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 and corresponding decrease in 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.