homogenize the oxygen profile below the diffusion limit are quickly eradicated by the
rapid utilization of oxygen by sediment microorganisms. The low frequency of deeper
oxygen penetration, although it does occur, is an indication that most of the sediment
is resting between bioturbation events most of the time.
Fig. 14.20 Microelectrode profile of oxygen concentration in sediment at 15 m depth
in Aarhus Bay, Denmark. Data are dots. Solid horizontal line is sediment surface;
horizontal dashed line is at the top of the diffusive boundary layer (DBL). Solid fitted
line is a zero-order model of consumption (no dependence on concentration) at a rate
of 0.108 μmol cm−3 h−1. Dashed curve is a best-fit first-order model.
(After Rasmussen & Jørgensen 1992.)
(^) Particle size may make a difference in the rate of downward mixing into the
sediment column. Wheatcroft (1992) used a submersible equipped with a large spice
shaker to spread smooth, spherical glass beads ranging from 8 to 420 μm diameter
over a marked square meter of sediment at 1240 m in the Santa Catalina Basin off
California. Cores collected on another visit 997 days later showed (Fig. 14.21) that the
beads had spread downward to about 7 cm depth. Because the tracer was inserted as a
“plane source” at the sediment surface, the form of the expected distribution is not a
simple exponential, but rather a curve as fitted to the data in the figure. Best-fit DB
values decrease with increasing particle size, but inspection of the figure (and others
in the paper) suggests the differences are small. All but the largest sizes, which are
almost pebbles as seen by small, deep-sea macrofauna, were moved to the same L,
and the general form of the curves is the same.
Fig. 14.21 Dispersion curves fitted to progressively larger sizes of tracer particles