Conveyor belt deposit-feeding organisms prevalent in both marine and freshwater systems have a profound effect on sediment properties and transport processes. These organisms ingest sediments over a range of depths while depositing gut contents from tails protruding above the sediment surface and imposes an accelerated rate of sediment and pore water burial within the feeding zone. Most previous efforts to combine sediment diagenesis with the effects of biogenic reworking characterize mixing as exclusively diffusive and ignore such major advective effects. Here a model is developed, based on fundamental diagenetic equations for transport and reaction, for the distribution of tracers in accumulating sediments subject to compaction and diffusive as well as advective redistribution by benthic organisms. Conveyor belt (CB) feeding is characterized as a first-order process with a depth-dependent rate constant which is either localized (Gaussian) or distributed (integrated Gaussian).

Biogenic diffusivity of bulk sediments is allowed these alternative depth dependences as well. The model assumes simple linear adsorption of the tracer between solid and solution phases and uses a time-dependent flux at (x=0) which is a combination of that originating externally and the depth-integrated contributions from feeding. Particle selectivity is introduced by applying mass conservation separately to transport of the tracer and bulk sediments. Properties of the model are illustrated for tracers in nondispersive systems strongly bound to sediment solids. CB recycling gives rise to transient reflections on passage of a tracer pulse through the zone of bioturbation. Reflections readily disappear in the presence of various integrative processes. The system time resolution is defined in terms of the downward propagation of dual tracer pulses and shown to be systematically degraded by intensified feeding and selective transport. A simple expression is developed for estimating the magnitude of particle-selective changes in steady state surface concentrations in terms of the particle selectivity factor and the efficiency of sediment reworking.

Unrecognized particle selectivity can lead to false inferences about changes in the rate of delivery of tracers to sediments. Selective CB recycling can cause a tracer such as a 210Pb to be retained largely within the bioturbationzone, so that relatively little activity reaches the region of sediment useful for geochronological interpretation. Increased selectivity accentuates the sensitivity of the profile to details of the reworking process. The model, applied to previous studies of CB deposit feeders in tracer-labeled freshwater microcosms, shows the need to include particle diffusion as well as advection in the treatment of CB transport.