A simple model of flow—Sediment—Organism interaction

Abstract

Previous models of organism-sediment interaction have been limited primarily to eddy diffusion-type mixing. While such models are useful in interpreting some stratigraphic records, they cannot be expected to provide mechanistic insights into the diversity of interactions among sedimentary processes, organism activities, and boundary layer flows. To provide a more general formulation capable of dealing with all three types of processes in concert, a simple, discrete-time Markov model is generated. This version of the model deals explicitly only with particulate and not liquid phases. The initial formulation is a two-compartment, ergodic model of deposit feeding. Particle selection by deposit feeders results in incorporation of the selected particles into fecal pellets, and fecal pellet disaggregation completes the cycle. Particles which are more strongly selected or which are incorporated into more robust pellets thus spend more time and reach higher relative concentration in the fecal-pellet compartment. The next level of complexity is achieved by adding burial from the “free” sediment (non-fecal-pellet) compartment. This model predicts that particles which are more strongly selected or incorporated into more robust fecal pellets will have greater residence times in surficial sediments (both pellets and “free” sediments) before they become part of the stratigraphic record, and will therefore achieve higher concentrations in surficial sediments than will less preferred particles. The full flow-sediment-organism system is reached by adding the potential for lateral advection of particles. The probability of such advection is allowed to differ between pellets and “free” sediments. Increasing selection and robustness of fecal pellets then can either increase or decrease relative concentration and residence time of a given particle type in surficial sediments. The model thus provides a means of explaining differences in stratigraphic patterns among materials deposited simultaneously. It also suggests that deposit-feeder selection may increase the average age (i.e., time since initial deposition) of surficial sediments by maintaining preferred particle types near the surface. Selection may likewise control the texture of surficial sediments-in a manner which is easily confused with purely physical sorting. Parameters which require careful measurement in testing these simple predictions are selectivity by deposit feeders (probability of selection), rate of fecal-pellet breakdown under realistically simulated conditions (probability of pelletal disaggregation), differential erodibilities of pellets versus “free” sediments (relative probabilities of lateral advection), and rates of burial of “free” sediments versus pelletized sediments (probabilities of burial).