added to sediment in a basin off southern California. There is a slight tendency for
smaller particles to be moved faster and to have larger dispersion coefficients. The
largest particles were moved but could not be fitted with a dispersion model.
(After Wheatcroft 1992.)
(^) Boudreau (1998) has shown that L is independent of water column depth (Fig.
14.22a). For a large number of isotope profiles, mostly ^210 Pb, it averages 9.8 ± 4.5 cm
(±SD) (but see Teal et al. 2008). It also is not affected by sedimentation rate, which is
usually correlated with the supply of organic matter and inversely correlated with
water column depth. Wheatcroft et al. (1990) have speculated that L is set by
downward compaction of the sediment (Fig. 14.22b), that it gets progressively harder
to move through sediment as its water content decreases downward. This hypothesis
of rising cost of deeper burrowing is difficult to test. Boudreau (1998, 2004), in
contrast, proposes that bioturbation stops rather close to the sediment surface because
most of the non-refractory organic matter is used up in sediments deeper than a few
centimeters. There simply is no return to burrowers from seeking food deeper. It
should also be remembered that, especially on continental margins where organic
input is highest, sediments below a few centimeters are usually anoxic, have negative
redox potential, and contain toxic reduced substances including sulfide. Thus,
burrowers would encounter difficulty in sustaining activity below a few centimeters.