160
Lisa A. LEVIN and Andrew J. GOODAY
rates might result from proximity to land, both to the
west and east, and associated channeling into the sites
of organic matter from terrestrial and shallow marine
sources. Respiration requirements exceeded measured
fluxes of particulate organic carbon in the Straits of
Florida and the TOTO site by a factor of 5, leading
various investigators to suggest that macrophytic algae
and seagrass may have supplied some of the missing
material (Wiebe et al., 1976). A time series of nine
SCOC measurements made by benthic lander at an
abyssal station 83 km southeast of Bermuda (BATS),
during a period of three years, indicated remarkable
constancy (0.030±0.0035 mmol cm
−2 d
−1 ) with no seasonal variation.
Eastern Atlantic
Measurements made in the Gulf of Guinea off
Northwest Africa indicate very high SCOC (3.93 ml O 2
m
−2 h
−1 ) at 278 m and much reduced values at
4000 m (0.65 ml O 2 m
−2 h
−1 ). Both are probably elevated by upwelling, and the shallow site may be
influenced by terrigenous input from nearby rivers
(Hinga et al., 1979). Patching et al. (1986) measured SCOC at depths of 2880 m in the Rockall
Trough and 4980 m on the Porcupine Abyssal Plain.
Mean values were 2.2 times greater in Rockall
Trough (99–203 mmol O 2 m
−2 h
−1 ) than on the Porcupine Abyssal Plain (58–77 mmol O 2 m
−2 h
−1 ). The high
rates observed might be related to the timing of
the measurements, which were taken just after the
spring phytoplankton bloom in surface waters, or to
methodology bias (Patching et al., 1986). Seasonal
measurements made in situ in the Porcupine Seabight
(2000 m) by Lampitt et al. (1995) yielded SCOC
values between those in the Rockall Trough and on
the Porcupine Abyssal Plain (74–125 mmol O 2 m
−2 h
−1 ),
but indicated no evidence of seasonal variation. Cores
with phytodetritus visible on the surface did not yield
SCOC values different from those lacking this material,
indicating either degradation so rapid that it was missed
or very long community response times (Lampitt
et al., 1995). Measurements in June 1985 of around
30 mmol O 2 m
−2 h
−1 , made on the Porcupine Abyssal
Plain by Lampitt et al. (1995), were half those made
earlier in the year (May 1980, April 1982) by Patching
et al. (1986) in the same region. Pfannkuche (1992,
1993) obtained comparable SCOC values slightly
further south at the BIOTRANS site (4590 m), where
values ranged from 15 to 38 mmol O 2 m
−2 h
−1 during
spring and summer. He reported higher SCOC values
in July and August than in spring.
Overview
In general, bathyal and abyssal SCOC data from
the eastern Atlantic seem to be higher than in situ
values at equivalent depths from the northwestern
Atlantic, although SCOC measurements made on the
Hatteras Abyssal Plain were comparable to those on
the Porcupine Abyssal Plain and at the BIOTRANS
Site (Table 5.4). The Northwest Atlantic margin off
Cape Hatteras experiences extremely high carbon flux
(Blair et al., 1994), and this influence may extend to the
adjacent abyssal areas. The few data available suggest
that the organic-carbon input to the Northeast Atlantic
seabed may be subject to more seasonal variation
than in the Northwest Atlantic. Both the increased
SCOC rates and greater variability may be related
to heavy, pulsed input of phytodetritus to the seabed
during spring blooms in the northeastern section of the
Atlantic. Seasonality of SCOC has been documented
in both the Atlantic (Pfannkuche, 1992) and the
Pacific (Smith et al., 1994). However, the Atlantic
and Pacific differ in the extent to which measured
fluxes of particulate organic carbon appear to meet
the respiratory demands of the benthic community.
Evidence to date suggests that this flux in the North
Atlantic is more than sufficient to account for the
SCOC (Smith and Hinga, 1983), whereas this is not
the case in the Pacific (Smith, 1987).
Bioturbation
Most quantitative estimates of bioturbation in deepsea sediments are based on a diffusive mixing coefficient (D b ) derived from profiles of radiotracers
scavenged on particles in the seabed. D b is calculated
from the following steady-state, unidirectional equation
as derived by Nozaki et al. (1977):
D b
ð
2 N
ðx 2 − S
ðN
ðx
+ P − lN = 0,
where N , radiotracer concentration in bulk sediment
(atoms g
−1 ); ð, bulk sediment density (g cm
−3 ); S, sedimentation rate (cm sec
−1 ); D b , mixing coefficient (cm
2
sec
−1 ); P, production rate of radiotracer (atoms g
−1
sec
−1 ); l, decay constant of tracer (sec
−1 ).
Most estimates of D b for sediments of the continental
rise and abyss in the Atlantic have been based on
210 Pb profiles with a half life of 22 years (Boudreau,
1994). Values determined for marine sediments at
depths between 1410 and 5160 m consistently fall
between 0.02 and 1.0 cm
2 yr
−1 (Table 5.5, Fig. 5.8A).
These values are several orders of magnitude lower
Lisa A. LEVIN and Andrew J. GOODAY
rates might result from proximity to land, both to the
west and east, and associated channeling into the sites
of organic matter from terrestrial and shallow marine
sources. Respiration requirements exceeded measured
fluxes of particulate organic carbon in the Straits of
Florida and the TOTO site by a factor of 5, leading
various investigators to suggest that macrophytic algae
and seagrass may have supplied some of the missing
material (Wiebe et al., 1976). A time series of nine
SCOC measurements made by benthic lander at an
abyssal station 83 km southeast of Bermuda (BATS),
during a period of three years, indicated remarkable
constancy (0.030±0.0035 mmol cm
−2 d
−1 ) with no seasonal variation.
Eastern Atlantic
Measurements made in the Gulf of Guinea off
Northwest Africa indicate very high SCOC (3.93 ml O 2
m
−2 h
−1 ) at 278 m and much reduced values at
4000 m (0.65 ml O 2 m
−2 h
−1 ). Both are probably elevated by upwelling, and the shallow site may be
influenced by terrigenous input from nearby rivers
(Hinga et al., 1979). Patching et al. (1986) measured SCOC at depths of 2880 m in the Rockall
Trough and 4980 m on the Porcupine Abyssal Plain.
Mean values were 2.2 times greater in Rockall
Trough (99–203 mmol O 2 m
−2 h
−1 ) than on the Porcupine Abyssal Plain (58–77 mmol O 2 m
−2 h
−1 ). The high
rates observed might be related to the timing of
the measurements, which were taken just after the
spring phytoplankton bloom in surface waters, or to
methodology bias (Patching et al., 1986). Seasonal
measurements made in situ in the Porcupine Seabight
(2000 m) by Lampitt et al. (1995) yielded SCOC
values between those in the Rockall Trough and on
the Porcupine Abyssal Plain (74–125 mmol O 2 m
−2 h
−1 ),
but indicated no evidence of seasonal variation. Cores
with phytodetritus visible on the surface did not yield
SCOC values different from those lacking this material,
indicating either degradation so rapid that it was missed
or very long community response times (Lampitt
et al., 1995). Measurements in June 1985 of around
30 mmol O 2 m
−2 h
−1 , made on the Porcupine Abyssal
Plain by Lampitt et al. (1995), were half those made
earlier in the year (May 1980, April 1982) by Patching
et al. (1986) in the same region. Pfannkuche (1992,
1993) obtained comparable SCOC values slightly
further south at the BIOTRANS site (4590 m), where
values ranged from 15 to 38 mmol O 2 m
−2 h
−1 during
spring and summer. He reported higher SCOC values
in July and August than in spring.
Overview
In general, bathyal and abyssal SCOC data from
the eastern Atlantic seem to be higher than in situ
values at equivalent depths from the northwestern
Atlantic, although SCOC measurements made on the
Hatteras Abyssal Plain were comparable to those on
the Porcupine Abyssal Plain and at the BIOTRANS
Site (Table 5.4). The Northwest Atlantic margin off
Cape Hatteras experiences extremely high carbon flux
(Blair et al., 1994), and this influence may extend to the
adjacent abyssal areas. The few data available suggest
that the organic-carbon input to the Northeast Atlantic
seabed may be subject to more seasonal variation
than in the Northwest Atlantic. Both the increased
SCOC rates and greater variability may be related
to heavy, pulsed input of phytodetritus to the seabed
during spring blooms in the northeastern section of the
Atlantic. Seasonality of SCOC has been documented
in both the Atlantic (Pfannkuche, 1992) and the
Pacific (Smith et al., 1994). However, the Atlantic
and Pacific differ in the extent to which measured
fluxes of particulate organic carbon appear to meet
the respiratory demands of the benthic community.
Evidence to date suggests that this flux in the North
Atlantic is more than sufficient to account for the
SCOC (Smith and Hinga, 1983), whereas this is not
the case in the Pacific (Smith, 1987).
Bioturbation
Most quantitative estimates of bioturbation in deepsea sediments are based on a diffusive mixing coefficient (D b ) derived from profiles of radiotracers
scavenged on particles in the seabed. D b is calculated
from the following steady-state, unidirectional equation
as derived by Nozaki et al. (1977):
D b
ð
2 N
ðx 2 − S
ðN
ðx
+ P − lN = 0,
where N , radiotracer concentration in bulk sediment
(atoms g
−1 ); ð, bulk sediment density (g cm
−3 ); S, sedimentation rate (cm sec
−1 ); D b , mixing coefficient (cm
2
sec
−1 ); P, production rate of radiotracer (atoms g
−1
sec
−1 ); l, decay constant of tracer (sec
−1 ).
Most estimates of D b for sediments of the continental
rise and abyss in the Atlantic have been based on
210 Pb profiles with a half life of 22 years (Boudreau,
1994). Values determined for marine sediments at
depths between 1410 and 5160 m consistently fall
between 0.02 and 1.0 cm
2 yr
−1 (Table 5.5, Fig. 5.8A).
These values are several orders of magnitude lower