Age distribution and organic matter degradation in bioturbated sediment

Bioturbation is an important process in the early diagenesis of soft marine sediment. Benthic infaunal activity, such as feeding, burrowing and ploughing redistributes particles within the topmost layers of the sediment. Recently deposited particles are mixed into deeper sediment depth layers and old material remains longer near the surface. A sediment layer thus contains an assemblage of particles from young to very old ages. Under certain assumptions, bioturbational mixing can be modelled as a diffusive process with the macroscopic mixing coefficient D_B. Here we model the age distribution of the bioturbated sedimentary record with a depth dependent mixing coefficient D_B(z). The potential age bias introduced by mixing is typically higher than multiples of the mean mixed layer residence time, which scales linearly with the ratio of mixed layer depth and sediment accumulation rate. Scaling the mixing intensity has only a minor effect, as most marine environments are mixing dominated.

The rate of organic matter degradation can been modelled empirically as an age dependent process, with recently deposited, fresh organic matter having higher reactivities than older and more refractory material. With insights into the age distribution, this allows to couple the degradation of organic matter with bioturbation and estimate the burial of carbon.