bioturbation using a diffusion model. Waiting times between mixing events are taken
in this model to be small relative to the total time available, and a continuous mixing
rate DB is determined as a bulk diffusion coefficient by analogy to Fick’s Law:
(^) in which dC/dz is the concentration gradient of isotope downward in the sediment and
λ is the radioactive decay rate. DB, a bioturbative diffusion coefficient, can be
estimated from the isotope profile, and for the site near Rockall Bank the result is
0.088 cm^2 yr−1. Like L, DB depends upon tracer half-life, but in the opposite direction
of course. Smith et al. (1997) provide a comparison (Fig. 14.18) for sites along a
Pacific transect near the equator at 140°W, showing that DB for excess thorium-234 (a
uranium-238 daughter with half-life of 24 days) is markedly greater than for ^210 Pb.
The L values for ^234 Th were only 2–3 cm (Pope et al. 1996). Estimates of DB for both
isotopes were correlated with organic carbon input as estimated from sediment traps.
Apparently, greater food supply leads to more biological activity, which leads to
greater rates of bioturbation, a point carefully reviewed by Boudreau (2004).
Fig. 14.18 Mean (±SD) sediment mixing coefficients (Db) vs. POC flux into traps 700
m above bottom at stations along 140°W at the latitudes near the equator as indicated.
(a) Excess ^234 Th. (b) Excess ^210 Pb. Coefficients are higher for tracers with longer
half-life. Bioturbation is faster where more organic matter is supplied to feed
sediment-mixing animals.
(After Smith et al. 1997.)