Biological Oceanography

(^) Values of L estimated from isotope information are not constant, but depend upon
the half-life of the isotope used. A very commonly used tracer of sediment mixing is
“excess” lead-210 (^210 Pb). This is an intermediate product in the serial decay of
radium; its half-life is 22 years. In sediment, the excess ^210 Pb is that which has settled
from the water column attached to particles and can be differentiated from that
sustained by radium in the sediment. This excess is measured by counting its decay
products, as well as determining the amount of radium to permit subtraction of
“radium supported” ^210 Pb. Down-core data (Fig. 14.17c) from the same core do not
have the same form as the ^14 C profile; excess ^210 Pb falls off exponentially from near
the sediment surface, reaching zero at 8–10 cm, which is used as its estimate of L. The
half-life is short enough that below L there is none. The difference between the two
tracers is caused by the difference in their half-life and also by the intermittence of
bioturbation. Most of the time the sediment must just be sitting there, its stacking not
much disturbed. Once in a while, a worm moves some aside to make a burrow, and
surface sediment falls down the hole. Then it sits there again. The mixing events will
happen often enough to homogenize a more durable tracer, but not a more ephemeral
one, and more durable tracers will eventually be mixed to greater depths by the more
occasional visits of larger animals.
(^) The pattern of (^210) Pb and several other tracers can be used to estimate a rate of