THE DEEP PACIFIC OCEAN FLOOR
195
reduction, accounts for a substantial proportion (18–
54%) of the organic carbon respired by Californiaslope sediment communities (Fig. 6.6). This reflects
the relatively high flux rates of organic carbon, by
deep-sea standards, occurring on the California-slope
floor, and is direct evidence that microbes (in particular
bacteria) mineralize a major fraction of the organic
matter reaching slope sediments. Despite a down-slope
decline in sediment-community respiration, rates of
respiration at the bottom of the California slope are
still 3-fold to 10-fold greater than in the oceanic
abyssal Pacific (Table 6.1). In addition, at a given
water depth, rates of respiration on the California slope
substantially exceed those on the northwest Atlantic
margin (Jahnke and Jackson, 1987; Jahnke, 1996). This
likely results both from high primary productivity along
the California margin caused by upwelling (Jahnke and
Jackson, 1987) and from the narrowness of the slope,
facilitating downslope transport of coastal production.
Bioturbation, or the movement of sediment particles
by animals, is a key ecosystem process in lowenergy, depositional environments, such as much of
the deep sea. Bioturbation results from the sum of
deposit-feeding, locomotion and home-building activities of benthos; rates of bioturbation thus provide
an integrative measure of the physical activity of
sediment assemblages. Biogenic sediment mixing also
has an impact on the rates of chemical reactions
in sediments, including the recycling and burial of
organic carbon and particle-bound pollutants (Officer
and Lynch, 1989; C.R. Smith, 1992). Because the
rates of bioturbation are generally very high compared
to rates of sediment accumulation, sediment mixing
also substantially smears the paleontological record
preserved in deep ocean sediments.
Rates of bioturbation are typically evaluated using
naturally occurring radionuclides, such as
234 Th (halflife = 24 days) and
210 Pb (half-life = 22 years), that are
adsorbed in the water column by sinking particles.
These adsorbed radionuclides provide an “excess” signal that disappears from particles, through radioactive
decay, after they have been deposited on the seafloor.
Occasionally, exotic tracer particles have also been
introduced to the deep-sea floor to evaluate mixing
rates. Bioturbation is typically parameterized as an
eddy-diffusion, or “bioturbation,” coefficient (units of
cm
2 y
−1 ) within a surface-sediment mixed layer ranging
from 3 to 20 cm in thickness (C.R. Smith, 1992;
Boudreau, 1998; Smith and Rabouille, 2002).
Rates of bioturbation have been evaluated at a
number of sites along the California margin, as
well as on the nearby Washington slope. The Santa
Catalina Basin in particular has served as a test site
for mechanistic studies of deep-sea sediment mixing.
Several major points have emerged from these margin
studies.
(1) Measured rates of sediment mixing vary with the
particle type and radiotracer. For example, Wheatcroft
(1992) experimentally documented 10-fold faster mixing rates for 10-mm diameter beads than for 100-mm
beads at 1240 m depth in the Santa Catalina Basin.
This difference was ascribed to size-dependent ingestion and mixing of particles by deposit feeders,
whose feeding and defecating activities are thought
to contribute substantially to deep-sea bioturbation
(C.R. Smith, 1992; Wheatcroft, 1992). In addition to
size-dependent bioturbation, tracer-dependent mixing
has been demonstrated in the Santa Catalina Basin
(C.R. Smith et al., 1993), where mean bioturbation
coefficients for
234 Th (60 cm
2 y
−1 ) were a hundredfold higher than for
210 Pb (0.43 cm
2 y
−1 ) in precisely
the same sediments. Such tracer-dependent bioturbation, in which tracers with shorter characteristic
time scales (e.g.,
234 Th) are mixed faster than those
with longer time scales (e.g.,
210 Pb), appears to be
widespread in the deep sea, and has been thought
to result from age-dependent mixing (Smith et al.,
1993, 1997). According to the age-dependent mixing
hypothesis, recently deposited particles relatively rich
in excess
234 Th, and labile organic matter (e.g.,
phytodetritus), are preferentially ingested by deposit
feeders; the preferential ingestion and defecation of
such “young” particles causes the short-lived tracer
234 Th to be, on average, mixed faster than its longerlived counterparts, such as
210 Pb. Recent studies on
the California slope indicate that deposit feeders do
indeed preferentially ingest young particles rich in
234 Th (Lauerman et al., 1997; Miller et al., 2000), and
that fresh phytoplankton cells often are initially mixed
faster into sediments than are food-poor sediments of
similar grain size (Smith et al., 2002; Fornes et al.,
2002); both results are predicted by the age-dependent
mixing hypothesis.
(2) A second generalization to emerge from bioturbation studies on the northeast Pacific slope is
that, for a given tracer type, mixing coefficients
within and between sites are highly variable. For
example, between depths of 500 and 1933 m on the
Washington slope, Carpenter et al. (1982) found mixing
coefficients for
210 Pb spanning more than an order of
195
reduction, accounts for a substantial proportion (18–
54%) of the organic carbon respired by Californiaslope sediment communities (Fig. 6.6). This reflects
the relatively high flux rates of organic carbon, by
deep-sea standards, occurring on the California-slope
floor, and is direct evidence that microbes (in particular
bacteria) mineralize a major fraction of the organic
matter reaching slope sediments. Despite a down-slope
decline in sediment-community respiration, rates of
respiration at the bottom of the California slope are
still 3-fold to 10-fold greater than in the oceanic
abyssal Pacific (Table 6.1). In addition, at a given
water depth, rates of respiration on the California slope
substantially exceed those on the northwest Atlantic
margin (Jahnke and Jackson, 1987; Jahnke, 1996). This
likely results both from high primary productivity along
the California margin caused by upwelling (Jahnke and
Jackson, 1987) and from the narrowness of the slope,
facilitating downslope transport of coastal production.
Bioturbation, or the movement of sediment particles
by animals, is a key ecosystem process in lowenergy, depositional environments, such as much of
the deep sea. Bioturbation results from the sum of
deposit-feeding, locomotion and home-building activities of benthos; rates of bioturbation thus provide
an integrative measure of the physical activity of
sediment assemblages. Biogenic sediment mixing also
has an impact on the rates of chemical reactions
in sediments, including the recycling and burial of
organic carbon and particle-bound pollutants (Officer
and Lynch, 1989; C.R. Smith, 1992). Because the
rates of bioturbation are generally very high compared
to rates of sediment accumulation, sediment mixing
also substantially smears the paleontological record
preserved in deep ocean sediments.
Rates of bioturbation are typically evaluated using
naturally occurring radionuclides, such as
234 Th (halflife = 24 days) and
210 Pb (half-life = 22 years), that are
adsorbed in the water column by sinking particles.
These adsorbed radionuclides provide an “excess” signal that disappears from particles, through radioactive
decay, after they have been deposited on the seafloor.
Occasionally, exotic tracer particles have also been
introduced to the deep-sea floor to evaluate mixing
rates. Bioturbation is typically parameterized as an
eddy-diffusion, or “bioturbation,” coefficient (units of
cm
2 y
−1 ) within a surface-sediment mixed layer ranging
from 3 to 20 cm in thickness (C.R. Smith, 1992;
Boudreau, 1998; Smith and Rabouille, 2002).
Rates of bioturbation have been evaluated at a
number of sites along the California margin, as
well as on the nearby Washington slope. The Santa
Catalina Basin in particular has served as a test site
for mechanistic studies of deep-sea sediment mixing.
Several major points have emerged from these margin
studies.
(1) Measured rates of sediment mixing vary with the
particle type and radiotracer. For example, Wheatcroft
(1992) experimentally documented 10-fold faster mixing rates for 10-mm diameter beads than for 100-mm
beads at 1240 m depth in the Santa Catalina Basin.
This difference was ascribed to size-dependent ingestion and mixing of particles by deposit feeders,
whose feeding and defecating activities are thought
to contribute substantially to deep-sea bioturbation
(C.R. Smith, 1992; Wheatcroft, 1992). In addition to
size-dependent bioturbation, tracer-dependent mixing
has been demonstrated in the Santa Catalina Basin
(C.R. Smith et al., 1993), where mean bioturbation
coefficients for
234 Th (60 cm
2 y
−1 ) were a hundredfold higher than for
210 Pb (0.43 cm
2 y
−1 ) in precisely
the same sediments. Such tracer-dependent bioturbation, in which tracers with shorter characteristic
time scales (e.g.,
234 Th) are mixed faster than those
with longer time scales (e.g.,
210 Pb), appears to be
widespread in the deep sea, and has been thought
to result from age-dependent mixing (Smith et al.,
1993, 1997). According to the age-dependent mixing
hypothesis, recently deposited particles relatively rich
in excess
234 Th, and labile organic matter (e.g.,
phytodetritus), are preferentially ingested by deposit
feeders; the preferential ingestion and defecation of
such “young” particles causes the short-lived tracer
234 Th to be, on average, mixed faster than its longerlived counterparts, such as
210 Pb. Recent studies on
the California slope indicate that deposit feeders do
indeed preferentially ingest young particles rich in
234 Th (Lauerman et al., 1997; Miller et al., 2000), and
that fresh phytoplankton cells often are initially mixed
faster into sediments than are food-poor sediments of
similar grain size (Smith et al., 2002; Fornes et al.,
2002); both results are predicted by the age-dependent
mixing hypothesis.
(2) A second generalization to emerge from bioturbation studies on the northeast Pacific slope is
that, for a given tracer type, mixing coefficients
within and between sites are highly variable. For
example, between depths of 500 and 1933 m on the
Washington slope, Carpenter et al. (1982) found mixing
coefficients for
210 Pb spanning more than an order of