Profiles are not always as clean as in the ^210 Pb example from Rockall Trough (Fig.
14.17c). A brief, larger mixing event can occur and disrupt the orderly exponential
progression of small mixing steps and radioactive decay. Repeated traverses of the
sediment by large echinoderms (Plate 13.1) can introduce breaks and shoulders into
the downward curve. Wheatcroft et al. (1990) make an extended argument that steady
diffusion is not how bioturbation really works, even if diffusion mathematics
produces reasonably good fits to the cleaner profiles of isotopes and other tracers.
Biological activity in sediment will occur in stronger and weaker pulses. Much of
sediment movement by deposit feeders will be horizontal with very slight vertical
displacements, such that most sediment mixing is not really represented by one-
dimensional (vertical) diffusivity estimates. Moreover, they point out that vertical
transfer of sediment often is not continuous across the mixing layer, but that animals,
particularly worms, will usually mine at one level and deposit tailings at the other end
of the body. An echiurid, for example, will sweep particles from the sediment surface
with its proboscis, ingest them, and then deposit the somewhat changed material as
feces at the bottom of its burrow some centimeters down. Sediment in the layer
between is not affected. Thus the right model might be advective rather than diffusive.
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