Transient tracer dynamics in a lattice-automaton model of bioturbation

Mixing of sediments by benthic fauna represents a dominant transport process of solids in the majority of surficial marine deposits. Short-term mixing rates are often determined by fitting a transient, diffusive model of bioturbation to vertical profiles of introduced particles (e.g., luminophores). Previous field studies adopting this approach have noted that mixing intensity decreases with time and the nature of mixing progresses from advective or nonlocal transport to diffusive mixing. These observations have been attributed to age-dependent mixing. The present study employs a lattice-automaton model to investigate the short-term behavior of conservative transient tracers. Simulations demonstrate that despite constant, indiscriminate mixing of particles, mixing intensity, as quantified by the diffusion model, appears as a function of time due to the model being invalid on short time scales. Failure of the model, however, is not apparent from tracer profiles. Furthermore, the errors incurred by misapplying the biodiffusion model can be considerable with estimates of mixing intensity reaching up to 3000% of the actual value. The transition from advective to diffusive mixing is also demonstrated and found to be the product of a boundary effect. Results presented here suggest that, on average, at least nine mixing events are required before the biodiffusion model becomes an appropriate description of bioturbation. Thus, providing the time scale of the tracer is sufficiently large relative to the time scale of mixing, the model provides a reasonable description of bioturbation