Diagenesis and preservation of organic matter

Roughly 90% of the organic matter that exits the euphotic zone of the ocean is degraded in the water column. Of the c.10% of the organic carbon flux that reaches the sea floor, only about one tenth escapes oxidation and is buried. Degradation of the organic matter that reaches the ocean sediments drives many of the reactions that control sediment diagenesis and benthic flux. 

Standard free energies of formation from Stumm and Morgan (1981). CH2O represents organic matter (AG/= —129kJ mol ). 

Aerobic respiration (CH20),(NH3),(H3P04),+(x+2y)02 XCO2 + (x + /)H20 + /HNO3 +2H3PO4 

Sulfate reduction 2(CH20),(NH3)/H3P04),4-xS0 - - XH2S + 2xHCOj + 2/NH3 + 22H3PO4 

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The quantity and quality of organic matter preserved during diagenesis of sediment determines the generative potential of the source rock and whether it will be prone to expel oil or gas. Quantity is determined by amount of organic input, the degree to which it is preserved (either as primary or... 

Cowie G. L. and Hedges J. 1. (1993) A comparison of organic matter sources, diagenesis and preservation in oxic and anoxic coastal sites. Chem. Geol. 107, 447 -451. 

A comparison of organic matter (OM) components in cores from Dabob Bay (oxic bottom waters) and Saanich Inlet (anoxic bottom waters) indicates that 02 availability ultimately has little or no independent effect on OM preservation in these environments (Cowie Hedges, 1992). Comparative analyses of organic compounds in sediment traps and bottom sediments from Saanich Inlet indicates that the anoxic benthic interface is an important site of diagenesis, and that selective removal takes place at both compound-class and molecular levels (Cowie etal., 1992). Preferential loss of marine organic material is indicated by the calculated delta-C-13 value and biochemical composition of the substrate. 

Macko SA, Engel MH, and Parker PL (1993) Early diagenesis of organic matter in sediments Assessment of mechanisms and preservation by the use of isotopic molecular approaches. In Engel MH and Macko SA (eds.) Organic Geochemistry, pp. 211-224. New York Plenum. 

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