HR: 1330h
AN: B33D-06    [Abstracts]
TI: Tracer Dynamics in a Lattice-Automaton Model of Bioturbation
AU: * Reed, D C
EM: Daniel.Reed@dal.ca
AF: Departmentof Oceanography, Dalhousie University, 1355 Oxford Street, Halifax, NS B3H 4J1 Canada
AU: Huang, K
EM: khuang@dal.ca
AF: Departmentof Oceanography, Dalhousie University, 1355 Oxford Street, Halifax, NS B3H 4J1 Canada
AU: Boudreau, B P
EM: bernie.boudreau@dal.ca
AF: Departmentof Oceanography, Dalhousie University, 1355 Oxford Street, Halifax, NS B3H 4J1 Canada
AB: Biogenic sediment mixing is commonly described as a diffusive process, quantified by modelling the vertical distribution of particle-bound radioisotopes. The resulting diffusion coefficient (D_b) that characterises the intensity of the mixing regime often exhibits a dependence on tracer half-life; short-lived radioisotopes (e.g.²³⁴Th) tend to yield notably larger D_b values than longer-lived radioisotopes (e.g.²¹⁰Pb). While it has previously been hypothesized that this dependence is due to differential mixing of tracers by particle selective benthos, modelling work presented here demonstrates that this trend can result from a more fundamental mechanism: violation of the assumptions required for bioturbation to be considered diffusive. The model employed in this study, the Lattice-Automaton Bioturbation Simulator (LABS), is a computational model comprising a two-dimensional sediment-water lattice inhabited by automatous entities. Stochastic and deterministic rules define the behaviour of these "automatons" to mimic real fauna, passing through the sediment-water matrix displacing particles by burrowing, feeding, etc., and thus mixing the sediment. Every particle in the matrix is tagged with the same array of radioisotopes so that all tracers experience exactly the same degree of mixing; tracer profiles are achieved by averaging lateral "slices" of the sediment-water lattice. Fitting a biodiffusion model to such profiles allows mixing coefficients to be determined from the various tracers and compared to a theoretically calculated D_b value. The point at which tracer determined D_b values deviate from the theoretical mixing coefficient coincides with the violation of the biodiffusion model's spatial criterion, while violation of the frequency criterion manifests itself as an increased temporal variability in D_b. The biodiffusion model breakdown is rarely apparent from tracer profiles, emphasizing the need to evaluate the model criteria from biological parameters rather than relying on obvious indications of model breakdown, e.g. subsurface maxima.
DE: 4804 Benthic processes/benthos
DE: 4842 Modeling
DE: 4860 Radioactivity and radioisotopes
SC: Biogeosciences [B]
MN: 2005 Joint Assembly