Transition‐matrix model of bioturbation and radionuclide diagenesis

Abstract

Bioturbation rates in muddy sediments are thought to be due primarily to the reworking activities of benthic deposit feeders. However, current mathematical models of bioturbation do not explicitly link rates of particle mixing with realistic biological reworking mechanisms. To address this problem, I present a transition-matrix model of bioturbation that quantitatively links the reworking activities of individual organisms and community-level particlemixing rates. Solutions to the model are presented for two kinds of tracers; particle-reactive radionuclides with a constant input flux and conservative tracers added to the sediment as a pulse. The model was used to predict the vertical profiles of excess 234 Th and 210 Pb in the field. The model parameters were determined from benthic community-structure data. Model predictions were then compared to measured profiles of these tracers. On the basis of this comparison, I inferred that maldanid polychaetes at the study site were collecting sediment at the sedimentwater interface and depositing it at depth. This transport mechanism had a large effect on the predicted tracer profiles. A sensitivity analysis of the model indicated that deposit feeding by the two most abundant species, Mediomastus ambiseta and Nucula annulata, was the most important process determining the burial rate of the tracers. The model results also indicated that the combined effects of deposit feeding and sedimentation were sufficient to determine the vertical distributions of excess 234 Th and 210 Pb at the study site.

Quantification of sediment reworking rates in bioturbation research: a review

“…The underlying principle is that tracers initially deposited at the sediment -water interface, or placed at some horizon within the sediment column (see Gilbert et al 2007), are displaced due to the action of benthic fauna. Sediment reworking coefficients are then computed using mathematical models that are fitted to the vertical tracer profile (Boudreau 1986a,b, Wheatcroft et al 1990, François et al 1997, 2001 (Fig. 4).…”

In situ quantification of bioturbation using time-lapse fluorescent sediment profile imaging (f-SPI), luminophore tracers and model simulation

“…Consideration of behaviourally driven tracer mixing over short timescales is essential to the understanding of the fate of organic matter and other important constituents of the sediment profile. As the burial of particles is unpredictable, the adoption of a diffusive analogy averaged over extended time periods is likely to lead to an inaccurate classification of benthic activity (for exceptions, see Shull 2001, Shull & Yasuda 2001. One alternative method has been the formulation of complex models tailored towards particular species or functional types (e.g.…”

Particle age distributions and O2 exposure times: Timescales in bioturbated sediments

“…Hence it becomes questionable when the mixing length scale is larger than the distance over which the concentration of a tracer of interest is changing significantly (e.g., in the presence of Zoophycos [Leuschner et al, 2002;Loewemark and Werner, 2001]), or when the mixing events are relatively rare [Boudreau, 1986;Meysman et al, 2003]. For certain types of bioturbation, such as food caching [Jumars et al, 1990] and conveyor belt feeding , a non-local transport description may be more appropriate [Shull, 2001]. Nonetheless, despite its potential limitations, the simple biodiffusion model has been remarkably successful at reproducing the depth profiles of inert tracers, radioisotopes and reactive particulate constituents in sediments.…”