Organic Carbon Reaction Kinetics in Bioturbated Sediments

Most organic carbon delivered to the seafloor is degraded within the bioturbated layer. Theory and empirical evidence have shown that organic carbon reactivity relates to the age of a particle. However, due to particle mixing, the age-depth linear relation induced by sediment accretion is obfuscated. Here we combine a Lagrangian particle tracking model that resolves the age distribution of particles in the bioturbated zone and couple it to age-dependent organic carbon degradation. Depth profiles for organic carbon concentration, reactivity and degradation rate are presented for sediments receiving low and high reactivity organic carbon in coastal, continental slope and deep-sea environments. Our results show that a simple first-order kinetics model suffices for well-mixed sediments and systems receiving pre-processed materials. A reactive continuum approach is needed for poorly mixed sediments receiving highly reactive organic carbon and for sediments below the bioturbated layer. Animals inhabiting marine sediments mix particles and impact organic carbon degradation kinetics and degradation rates. Experiments and theory have shown that organic carbon reactivity declines as degradation progresses with time. However, most studies employ a constant reactivity. Here we couple an individual particle mixing model with age-dependent reactivity and show that a constant reactivity model is adequate for well-mixed sediments and for sediments receiving organic carbon that has already been degraded elsewhere. An age-dependent reactivity model is needed for poorly mixed sediments receiving highly reactive organic carbon. Age-dependent organic carbon kinetics is coupled to a Lagrangian particle tracking model Organic carbon concentrations, reactivities and degradation rates in the bioturbated zone are presented A simple first-order (one-G) model suffices for well-mixed sediments and sediments receiving pre-processed organic carbon