Steadystate tracer dynamics in a latticeautomaton model of bioturbation
Abstract
The intensity of biogenic sediment mixing is often expressed as a "biodiffusion coefficient" ( D_{b}), quantified by fitting a diffusive model of bioturbation to vertical profiles of particlebound radioisotopes. The biodiffusion coefficient often exhibits a dependence on tracer halflife: shortlived radioisotopes (e.g. ^{234}Th) tend to yield notably larger D_{b} values than longerlived radioisotopes (e.g. ^{210}Pb). It has been hypothesized that this is a result of differential mixing of tracers by particleselective benthos. This study employs a latticeautomaton model of bioturbation to explore how steadystate tracers with different halflives are mixed in typical marine settings. Every particle in the model is tagged with the same array of radioisotopes, so that all tracers experienced exactly the same degree of mixing. Two different estimates of the mixing intensity are calculated: a tracerderived D_{b}, obtained in the standard way by fitting the biodiffusion model to resulting tracer profiles, and a particletracking D_{b}, derived from the statistics of particle movements. The latter provides a tracerindependent measure of mixing for use as a reference. Our simulations demonstrate that an apparent D_{b} tracerdependence results from violating the underlying assumptions of the biodiffusion model. Breakdown of the model is rarely apparent from tracer profiles, emphasizing the need to evaluate the model's criteria from biological and ecological parameters, rather than relying on obvious indications of model breakdown, e.g., subsurface maxima. Simulations of various marine environments (coastal, slope, abyssal) suggest that the time scales of shortlived radioisotopes, such as ^{234}Th and ^{7}Be, are insufficient for the tracers to be used with the biodiffusion model. ^{210}Pb appears an appropriate tracer for abyssal sediments, while ^{210}Pb and ^{228}Th are suitable for slope and coastal sediments.
 Publication:

Geochimica et Cosmochimica Acta
 Pub Date:
 December 2006
 DOI:
 10.1016/j.gca.2006.03.026
 Bibcode:
 2006GeCoA..70.5855R