THE DEEP PACIFIC OCEAN FLOOR
205
during November and December 1992, falling from
roughly 2.3 g C m
−2 y
−1 between 2ºS and 2ºN to roughly
0.3 g C m
−2 y
−1 at 12ºS and 9ºN (Fig. 6.11). At all
these stations, at least 70–90% of the organic carbon degradation occurs in the oxygenated, top 5 cm
of sediment, indicating that oxygen is the primary
electron acceptor during organic-matter mineralization
(Hammond et al., 1996). This contrasts sharply with
California slope habitats where anaerobic metabolism
(e.g., denitrification and sulfate reduction) may control
up to 54% of organic matter degradation (Fig. 6.9).
Between 2ºS and 2ºN, seafloor respiration rates showed
substantial variability on time scales of months,
apparently in response to changes in the flux of
particulate organic carbon induced by El Ni˜ no events
(Hammond et al., 1996; Berelson et al., 1997). This
variability in seafloor respiration rates (i.e., in organiccarbon mineralization rates) is highly consistent with
the results from modeling of organic-matter reactions,
suggesting that most (70 to 90%) of the degrading
organic carbon in eutrophic equatorial sediments is
very labile, with a degradation half-life of ~20 days
(Hammond et al., 1996).
Bioturbation has been well studied along the EqPac
transect and, at any point, appears to result from
the summation of three processes: (1) eddy-diffusive
mixing of the top 2 to 8 cm of sediment by small
macrofauna and meiofauna; (2) pulsed homogenization
of the top 2 to 3 cm of sediment by plowing urchins
(Fig. 6.6); and (3) episodic transport of surface
sediments to depths between 3 and 27 cm by echiurans
and other animals that feed on surface sediments
from a central burrow and then defecate within their
burrows (Smith et al., 1997). Eddy diffusive mixing
along the EpPac transect exhibited both a rough
correlation with the flux of particulate organic carbon
and substantial dependence on the tracer used and
the time scale. At eutrophic stations (2ºS to 5ºN),
eddy-diffusion coefficients (D b ) for both
234 Th and
210 Pb were at least 10-fold higher than for the same
isotopes at the mesotrophic site (9ºN), and correlation
coefficients between D b and the flux of particulate
organic carbon along the whole transect were >0.88
for each isotope ( p < 0.05). The depth to which
210 Pb
was mixed also decreased from ~8 cm at eutrophic
stations to ~2 cm at 9ºN (Smith and Rabouille, 2002).
In addition, D b values for the short-lived isotope
234 Th
(half life = 24 d) were 5 to 70 times greater than those
for
210 Pb (half life = 22 yr) in the same cores and over
the same depth intervals (Smith et al., 1997). This
tracer-dependent mixing provides strong support for
the “age-dependent mixing” hypothesis, which predicts
that recently deposited, relatively organic-rich particles
are ingested and mixed at higher rates than are foodpoor particles (see detailed discussion of age-dependent
mixing above ( p. 195). Phytodetritus, which is rich in
234 Th and labile organic compounds (Smith et al., 1996;
Stephens et al., 1997), is likely the target of this agedependent ingestion and mixing.
The second major form of mixing along the eutrophic portions of the EqPac transect results from
urchins plowing through near-surface sediments (see
Fig. 6.6). X-radiographs of box-core sediments indicate
that urchin plowing homogenizes a swathe roughly
10 cm wide and 2–3 cm deep (Smith et al., 1997). This
mixing produces a vertical “shoulder” in the profiles of
excess
210 Pb, which then disappears over time owing
to the diffusive mixing of smaller macrofauna and
meiofauna described above. By modeling the disappearance of urchin shoulders in
210 Pb profiles, Hoover
(1995) estimated that urchins rework approximately
10 to 15% of the seafloor per year, and a random
spot on the seafloor is stirred by a passing urchin
every 5 to 7 years. Thus, urchin mixing may have
profound effects on sediment processes with recovery
times longer than a few years, such as degradation
of moderately labile particulate organic carbon and,
perhaps, macrofaunal succession. Urchin burrowing
has been shown to affect the diversity and community
structure of shallow-water communities (Thayer, 1983;
Austen et al., 1998) suggesting that urchin disturbance
in the abyssal equatorial Pacific also influences the
structure of infaunal assemblages.
The final form of mixing in equatorial Pacific
sediments is the transport of superficial sediments to
depths of 3 to 27 cm within the sediment column
by echiuran worms and other burrow dwellers (Smith
et al., 1997). At the eutrophic EqPac stations, roughly
15 to 30% of the excess inventory of
234 Th was found
at depths of 2 to 4 cm, indicating that many particles are
subducted centimeters into the sediment column within
100 days of arrival on the seafloor (Pope et al., 1996).
Some of this subduction apparently results from the
caching of food-rich phytodetritus in the burrows of
infaunal megabenthos, such as echiuran worms (Smith
et al., 1996, 1997).
Recolonization rates following anthropogenic disturbance of sediments have been evaluated on the
abyssal equatorial seafloor as part of the DISCOL
experiment (Fig. 6.1). In order to explore the potential
205
during November and December 1992, falling from
roughly 2.3 g C m
−2 y
−1 between 2ºS and 2ºN to roughly
0.3 g C m
−2 y
−1 at 12ºS and 9ºN (Fig. 6.11). At all
these stations, at least 70–90% of the organic carbon degradation occurs in the oxygenated, top 5 cm
of sediment, indicating that oxygen is the primary
electron acceptor during organic-matter mineralization
(Hammond et al., 1996). This contrasts sharply with
California slope habitats where anaerobic metabolism
(e.g., denitrification and sulfate reduction) may control
up to 54% of organic matter degradation (Fig. 6.9).
Between 2ºS and 2ºN, seafloor respiration rates showed
substantial variability on time scales of months,
apparently in response to changes in the flux of
particulate organic carbon induced by El Ni˜ no events
(Hammond et al., 1996; Berelson et al., 1997). This
variability in seafloor respiration rates (i.e., in organiccarbon mineralization rates) is highly consistent with
the results from modeling of organic-matter reactions,
suggesting that most (70 to 90%) of the degrading
organic carbon in eutrophic equatorial sediments is
very labile, with a degradation half-life of ~20 days
(Hammond et al., 1996).
Bioturbation has been well studied along the EqPac
transect and, at any point, appears to result from
the summation of three processes: (1) eddy-diffusive
mixing of the top 2 to 8 cm of sediment by small
macrofauna and meiofauna; (2) pulsed homogenization
of the top 2 to 3 cm of sediment by plowing urchins
(Fig. 6.6); and (3) episodic transport of surface
sediments to depths between 3 and 27 cm by echiurans
and other animals that feed on surface sediments
from a central burrow and then defecate within their
burrows (Smith et al., 1997). Eddy diffusive mixing
along the EpPac transect exhibited both a rough
correlation with the flux of particulate organic carbon
and substantial dependence on the tracer used and
the time scale. At eutrophic stations (2ºS to 5ºN),
eddy-diffusion coefficients (D b ) for both
234 Th and
210 Pb were at least 10-fold higher than for the same
isotopes at the mesotrophic site (9ºN), and correlation
coefficients between D b and the flux of particulate
organic carbon along the whole transect were >0.88
for each isotope ( p < 0.05). The depth to which
210 Pb
was mixed also decreased from ~8 cm at eutrophic
stations to ~2 cm at 9ºN (Smith and Rabouille, 2002).
In addition, D b values for the short-lived isotope
234 Th
(half life = 24 d) were 5 to 70 times greater than those
for
210 Pb (half life = 22 yr) in the same cores and over
the same depth intervals (Smith et al., 1997). This
tracer-dependent mixing provides strong support for
the “age-dependent mixing” hypothesis, which predicts
that recently deposited, relatively organic-rich particles
are ingested and mixed at higher rates than are foodpoor particles (see detailed discussion of age-dependent
mixing above ( p. 195). Phytodetritus, which is rich in
234 Th and labile organic compounds (Smith et al., 1996;
Stephens et al., 1997), is likely the target of this agedependent ingestion and mixing.
The second major form of mixing along the eutrophic portions of the EqPac transect results from
urchins plowing through near-surface sediments (see
Fig. 6.6). X-radiographs of box-core sediments indicate
that urchin plowing homogenizes a swathe roughly
10 cm wide and 2–3 cm deep (Smith et al., 1997). This
mixing produces a vertical “shoulder” in the profiles of
excess
210 Pb, which then disappears over time owing
to the diffusive mixing of smaller macrofauna and
meiofauna described above. By modeling the disappearance of urchin shoulders in
210 Pb profiles, Hoover
(1995) estimated that urchins rework approximately
10 to 15% of the seafloor per year, and a random
spot on the seafloor is stirred by a passing urchin
every 5 to 7 years. Thus, urchin mixing may have
profound effects on sediment processes with recovery
times longer than a few years, such as degradation
of moderately labile particulate organic carbon and,
perhaps, macrofaunal succession. Urchin burrowing
has been shown to affect the diversity and community
structure of shallow-water communities (Thayer, 1983;
Austen et al., 1998) suggesting that urchin disturbance
in the abyssal equatorial Pacific also influences the
structure of infaunal assemblages.
The final form of mixing in equatorial Pacific
sediments is the transport of superficial sediments to
depths of 3 to 27 cm within the sediment column
by echiuran worms and other burrow dwellers (Smith
et al., 1997). At the eutrophic EqPac stations, roughly
15 to 30% of the excess inventory of
234 Th was found
at depths of 2 to 4 cm, indicating that many particles are
subducted centimeters into the sediment column within
100 days of arrival on the seafloor (Pope et al., 1996).
Some of this subduction apparently results from the
caching of food-rich phytodetritus in the burrows of
infaunal megabenthos, such as echiuran worms (Smith
et al., 1996, 1997).
Recolonization rates following anthropogenic disturbance of sediments have been evaluated on the
abyssal equatorial seafloor as part of the DISCOL
experiment (Fig. 6.1). In order to explore the potential