Preservation of reactive organic matter in marine sediments

It is generally assumed that the reactivity of organic matter and the amount preserved in sedimentary deposits necessarily increases with total sedimentation rate. In some environments, such as deltas, where supply of unreactive terrigenous debris may vary independently of reactive organic matter input, the amount of reactive organic material preserved can in fact theoretically correlate either directly or inversely with sedimentation rate. The amount preserved can be shown quantitatively by transport-reaction models to depend on (1) the relative importance of electron acceptor concentration in the overlying water, (2) advection during sedimentation, (3) dilution by sedimentation, (4) solute diffusion, (5) initial flux of organic matter, and (6) the magnitudes of reaction rate constants. Both single reaction rate constant and multiple reaction rate constant models suggest that, at steady state, maximum preservation with respect to a given oxidant occurs when D_sk = w², where D_s = whole sediment diffusion coefficient of the electron acceptor, k = first-order rate constant of the dominant organic fraction, and w = sedimentation rate. This is the likely basis for the reported correlation between average reactivity, k, of carbon in a deposit, percent carbon preservation, and w^1.5–w². Because of the variety of factors which determine these relationships, such correlations are probably valid only within specific classes of depositional environments.