KeywordsOrganic Carbon Content Molecular Diffusion Time Slice Biogeochemical Cycle Radioactive Tracer
Bioturbation is the mixing and displacement of particles in marine and freshwater sediments caused by benthic fauna mainly during foraging and the construction of burrows. As a result particulate proxies and microfossils such as tests of foraminifera deposited at the seafloor may not be found in the time slice corresponding to its deposition, and the interpretation of the geological record can be hampered. Other effects of animal activity, such as biodeposition and bioresuspension, but also fluid transport will not be considered in this section although they are essential for biogeochemical cycles and are used as subprocesses of bioturbation in recent biological literature (cf. Kristensen et al. 2012).
Measurement and Modeling
It describes particle mixing in analogy to molecular diffusion. This assumption holds true in the case of many animals moving particles in a stochastic way in small steps (local mixing). Statistically this creates a transport of tracers along the concentration gradient, as does the Brownian movement on the molecular level, and can be calculated via Fick’s law of diffusion. The mixing coefficient D b provides a quantitative measure.
Some animals transport food particle directly in one step from the surface to depth, or particles may drop into open burrows. In these cases the diffusion analogy is inadequate, and this advective transport (nonlocal mixing) has to be considered separately, i.e., the step length and frequency of such events have to be determined and to be included into the model, as, for example, in the gallery-diffusion model by Francois et al. (2002).
Order of Magnitude
From published data, Boudreau (1994) calculated a worldwide mean mixing depth of 9.8 ± 4.5 cm. It is, however, well documented that animal burrows may reach as deep as 2 m (Thalassinidea). Mixing coefficients D b may range from 0.0002 to 370 cm2 years−1. The latter results are highly dependent on the habitat and the tracers used. The shorter the half-life of the tracer, the higher is DB. This result is mainly explained by the fact that some animals are highly selective and prefer fresh organic matter.
So far attempts to find general relationships between sediment mixing and biomass or organic carbon content failed, indicating the strong species-specific effect of bioturbation.
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