A mechanistic analysis of bioturbation

The intensity of biogenic sediment mixing is often expressed as a "biodiffusion coefficient" (Db), quantified by fitting a diffusive model of bioturbation to vertical tracer profiles. Frequently when using steady-state particle-bound radioisotope tracers, the biodiffusion coefficient exhibits a dependence on tracer half-life: short-lived radioisotopes (e.g., 234Th) tend to yield significantly larger values than longer-lived radioisotopes (e.g., 210Pb). Similarly, when employing introduced particles as transient tracers the biodiffusion coefficient is inclined to be greatest in the short term and decrease thereafter. Transient tracer experiments further suggest that the nature of bioturbation changes as time progresses with initial advective or nonlocal transport giving way to diffusive mixing. It has been hypothesized that these trends are the results of differential mixing by benthic fauna, whereby recently deposited particles are mixed preferentially over other particles.;Alternative models of bioturbation must specify how particles are redistributed by benthic fauna, if they are not mixed diffusively. To this end, LABS is used to derive quantitative descriptions of particle transport for a variety of mixing modes. Such a decomposition of bioturbation into its constituent processes provides much needed insight into the different mixing mechanisms involved in biogenic reworking of sediments.;To investigate how the biodiffusion model interprets bioturbation on a variety of time scales, a mechanistic model is advanced named the Lattice-Automaton Bioturbation Simulator (LABS) within which tracer experiments like those undertaken in nature can be performed. Results from LABS demonstrate that the observed trends are generated in the absence of differential mixing when the biodiffusion model is applied on sufficiently short time scales such that its assumptions are no longer valid. Failure of the model, however, is not usually apparent from tracer profiles meaning that the model can be easily misapplied. Furthermore, an initial advective or nonlocal phase is shown to be expected when using introduced particles as a result of boundary effects.