They give a diagram (Fig. 14.19) of different burrowing modes that produce sediment
transfers of different kinds.
Fig. 14.19 Cartoons of the principal modes of sediment mixing by benthic animals, all
largely self-explanatory, but see Wheatcroft et al. (1990) for full explanations.
(^) (After Wheatcroft et al. 1990.)
A tracer with a very short half-life in sediment is oxygen, which diffuses into
interstitial water from the water column above. The majority of oxygen profiles
determined by inserting microelectrodes slowly down from the sediment surface (Fig.
14.20) show a simple, curving form tending to zero a short distance down, a form
explicable from a shifting balance between molecular diffusion from above through
pore water (see below) and respiration. Half-life is not the correct representation for
oxygen, since its utilization by benthic metabolism is approximately zero order (that
is, not dependent upon oxygen concentration). Moreover, oxygen is not a tracer in the
same sense as an isotope in the solid phase, since it can diffuse through pore-water
spaces independently from bioturbation. It is continuously supplied from above as it is
used in the sediment. However, in most places, particularly nearshore, if the supply
from above were suddenly capped, the complete utilization times would be of the
order of 2 hours. That is why the diffusive penetration is often shallow. Most, but not
all, oxygen profiles are unaffected by bioturbation. Intermittent stirring events that