Abstract

Most organic matter delivered to the seafloor is degraded within the bioturbated layer. Theory and empirical evidence have shown that organic matter 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 matter degradation. Depth profiles for organic carbon concentration, reactivity and degradation rate are presented for sediments receiving low-quality and high-quality organic matter 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 high-quality organic matter and for sediments below the bioturbated layer.