A model for the biological degradation of recent sedimentary organic matter

Abstract

Biological degradation in a given environment is considered to be the sum of numerous elementary reactions. If degradation is to be followed in a sedimentary core, the steady state hypothesis is required, and the time scale in the core is t= h/ V h being the depth in the core, and V the sedimentation rate. If, for the sake of simplicity, one single global reaction is assumed, then its control parameters depend not only on V, but also on the depths of the measurements. Now first order kinetics is assumed for both the elementary reactions and the single global reaction. Control parameters, apart from q 0 (concentration at water/sediment interface) are the rate constants, r for the global reaction, r ij for the individual reactions, and α ij fraction of total organic matter involved in the ijth reaction. Using data from this work and from the literature, it is shown that consistent trends result when r is plotted against V/ H ( H being the depth of the deepest sample), but not against V alone. Comparisons of actual trends vs numerical simulations with several sets of r ij , α ij , show that sulfate reduction is dominated by rapid reactions, but total organic matter decay is dominated by slow reactions, compatible with geologic times. Thus bacteria can be geologically significant. Both sulfate reduction and microbiological degradation of total organic matter appear as processes of moderate efficiency. The high efficiency of biological degradation in oxic sediments relies on benthic activity at the boundary layer and in the bioturbated zone.