Organic matter reactivity and sedimentation rates in the ocean

Abstract

Distributions of sulfate, ammonia, and phosphorus in pore waters of some pelagic and near-shore oceanic sediments were modeled in terms of inorganic and organic matter-pore water reactions and diflusional and advective transport. The chemical kinetic parameters of the models were evaluated from concentration profiles reported in the literature, using a nonlinear least-squares fit technique. The com- puted first-order reaction rate constants for reduction of SO4^2- and production of NH4+ and P (Ki) are proportional to the second power of the sedimentation rate (U): Ki α U^2. The computed fluxes of the three species in pore waters at the sediment-water interface (Fz=0) are proportional to the first power of the sedimentation rate: Fz=0 α U. The two proportionality relationships are based on the dependence of ki on (he reactive solid surface area and the mathematical formulation of the flux Fz=0. For the world ocean, the estimated fluxes of SO4^2— (into sediment), NH4+, and P (out of sediment) across the sediment-water interface correspond to the renewal times of these species in the ocean of between 2x10^7 and 2x10^8 yr. Nitrogen and phosphorous are removed faster than carbon from the reactive fraction of organic matter in the course of diagenesis, as suggested by the higher values of C/N (and C/P) ratios at sites of lower sedimentation rates. This observation agrees in general with the fact that the computed activation energies for cleavage of C-C and C-H bonds are several kcal/mol higher than the activation energies characteristic of the C-N, C-P, and C-O bond cleavages.