mixed downward into the sediment by the burrowing of worms; by the churning and
ingestion of passing sea cucumbers; and by the tail swooshes of passing macrourid
fish. It eventually reaches a depth in the sediment called the bioturbation limit, L,
which varies beneath oxic waters from about ∼1.5 to ∼20 cm, averaging between 5
(Teal et al. 2008) and 10 cm (Boudreau 1998) across all depths. As sediment is
progressively added to the pile, the layer being mixed moves up, remaining at the
surface. If 1 cm is added at the surface, then 1 cm just below depth L will be protected
from further mixing. Throughout its residence in the mixing layer the ^14 C is reduced
by beta decay with a half-life of 5730 years. The amount of reduction can be
interpreted in terms of an apparent age, which the mixing continually averages
between the most recently arrived carbon and that added at all different ages back to
the time of deposition of the layer just at L. However, this physical averaging does not
produce an accurate mean time since deposition for the layer as a whole. That is
because inorganic carbon in the water column ages and loses ^14 C after sequestration
below the sea surface, and carbon isotopes do not fully equilibrate to atmospheric
proportions in the surface layers where primary production occurs. Thus, settling
particulate organic matter already has an apparent ^14 C age of hundreds to thousands of
years (except that, since bomb testing, it can appear to be younger than zero age).
Sediments in the bioturbated layer do increase in apparent age from coastal sites at
about 400 years to deep-sea abyssal plains at 12,000 years (Emerson et al. 1997;
Hedges et al. 1999), a function of relative rates of organic matter input and most
importantly of overall sedimentation rate.
(^) Below L, there is no further mixing, so (^14) C decreases exponentially, as expected
from radioactive decay, and apparent age increases with depth. Thomson et al. (2000)
provide an example (Fig. 14.17) from just west of Rockall Bank northwest of the
British Isles, which shows L = 17.4 cm. The slight increase in ^14 C in the upper 3 cm is
generally attributed to bomb-test contamination. “Conventional years” are ^14 C ages
compared to a tree-ring standard. Sedimentation rate at the Rockall site, estimated
from these and other data, is 4.4 cm (1000 yr)−1.
Fig. 14.17 Profiles of bioturbation tracers from a sediment core taken from 1100 m on
Rockall Plateau, northeast Atlantic. (a) ^14 C, (b) ^14 C age, and (c) ^210 Pb excess.
(^) (a & b from data kindly provided by J. Thomson; c after Thomson et al. 2000.)