364
John D. GAGE
However, a more recently published study (K.L. Smith
and Kaufmann, 1999) based on a much longer time
series (7 years) has shown a widening, long-term deficit
at this station. This may be related to increasing surface
water temperature with reduced plankton biomass,
resulting in a declining particulate export flux to the
deep-ocean bottom. Whether this trend will prove to
be persistent is as yet unknown.
Carbon dynamics at the deep-sea continental
margin
The deep-sea continental margin beyond the shelf edge
constitutes the continental slope and rise, the latter
a wedge of sediment at the base of the slope where
the gradient lessens into the flat area of the abyssal
plain. Studies on the continental margin have provided
some of the best, and most tantalizing, data. It is here
that comparisons of supply and demand by the bottom
community show the largest discrepancies (K.L. Smith,
1987; Jahnke et al., 1990).
This budgetary problem was uncovered during the
1980s on the upper continental slope along the Atlantic
margin of the United States, where it was suggested
that the large material flux to the continental slope
is derived from material produced on the shelf and
transported over the shelf edge (Walsh, 1991). The
organic flux measured in sediment traps on the
middle continental slope at about 1000 m depth was
insufficient to account for the measured sediment community oxygen consumption (Rowe et al., 1994). When
expressed as carbon equivalents (mg C org m
−2 day
−1 ),
values of sediment community oxygen consumption on
the continental slope are generally lower than nearbottom particle flux, except at mid slope depths where
it was slightly higher; the general discrepancy may
possibly be ascribed to along-slope and down-slope
advection, anaerobic metabolism (not measured by
oxygen demand), and burial. Net carbon flux to the bed
might be thought best measured close to the bottom
in order to collect near-bed advected material, as well
as vertical flux. But there remains uncertainty arising
from local resuspension and deposition – the so-called
‘rebound’ flux. However, if the measurements are made
at greater depths more serious budgetary discrepancies
have emerged. The differential in sediment community
oxygen consumption on the continental margin is
greater on the west coast of America than on the
east, and increases with depth. This has been linked to
resuspension by benthic storms along the continental
rise in the western Atlantic, which results in relatively
less bacterial decomposition of particles while they are
on the bottom (Rowe et al., 1994). Furthermore, this
turbulent regime on the continental rise may extend
the effects of the continental margin far out onto the
adjacent abyssal plain (Walsh et al., 1991).
Lampitt et al. (1995) found that particle flux
collected by sediment traps set at mid-slope would
be sufficient to fuel only 20% of measured sediment
community oxygen consumption. Possible overestimation of oxygen demand might have resulted from the
technique used, but it seems likely that very substantial
quantities of detrital flux on the seabed are transported
downslope in the bathyal Northeast Atlantic in order to
provide the organic carbon consumed.
The issue of whether there is significant downslope transport of particulate detrital material can
be addressed by comparison of downward flux from
surface production with estimates of benthic oxygen
demand. Rowe et al. (1994) found seabed oxygen
uptake, in terms of carbon equivalents (mg C m
−2 d
−1 )
to be less than particle flux measured in sediment traps
except at the mid-slope ‘depocentre’, where there is
an accumulation of organic carbon laterally advected
down the continental slope. Lampitt et al. (1995) also
found that vertical flux measured from deep sediment
traps at depths of 2000 m on the continental slope in
the Northeast Atlantic was sufficient to fuel only 20%
of demand. They concluded that substantial quantities
of particulate material, not intercepted or properly
collected by sediment traps (see Baker et al., 1988;
Gust et al., 1994), must be transported down-slope
in order to balance the discrepancy. The opposite
imbalance found by Rowe et al. (1994) at other stations
was partly attributed to lateral and near-bed down-slope
transport.
Studies on the slope off Southwest Ireland studied in
the OMEX program of the European Union has shown
that both hydrodynamic and biologically mediated
processes associated with lateral advection are of great
importance (Heip et al., 2001). Even in low-flow
conditions (<5 cm s
−1 ), lateral particle flux is orders
of magnitude greater than the vertical flux. Largesized particles in the benthic boundary layer at the
shelf edge are not deposited there, but transported to
deeper, quieter areas. The processes involved of particle
aggregation, biologically mediated modifications of
near-bed fluxes, and a resuspension loop within the
benthic boundary layer have been shown to be sufficient
to fuel observed benthic demand on the slope and
John D. GAGE
However, a more recently published study (K.L. Smith
and Kaufmann, 1999) based on a much longer time
series (7 years) has shown a widening, long-term deficit
at this station. This may be related to increasing surface
water temperature with reduced plankton biomass,
resulting in a declining particulate export flux to the
deep-ocean bottom. Whether this trend will prove to
be persistent is as yet unknown.
Carbon dynamics at the deep-sea continental
margin
The deep-sea continental margin beyond the shelf edge
constitutes the continental slope and rise, the latter
a wedge of sediment at the base of the slope where
the gradient lessens into the flat area of the abyssal
plain. Studies on the continental margin have provided
some of the best, and most tantalizing, data. It is here
that comparisons of supply and demand by the bottom
community show the largest discrepancies (K.L. Smith,
1987; Jahnke et al., 1990).
This budgetary problem was uncovered during the
1980s on the upper continental slope along the Atlantic
margin of the United States, where it was suggested
that the large material flux to the continental slope
is derived from material produced on the shelf and
transported over the shelf edge (Walsh, 1991). The
organic flux measured in sediment traps on the
middle continental slope at about 1000 m depth was
insufficient to account for the measured sediment community oxygen consumption (Rowe et al., 1994). When
expressed as carbon equivalents (mg C org m
−2 day
−1 ),
values of sediment community oxygen consumption on
the continental slope are generally lower than nearbottom particle flux, except at mid slope depths where
it was slightly higher; the general discrepancy may
possibly be ascribed to along-slope and down-slope
advection, anaerobic metabolism (not measured by
oxygen demand), and burial. Net carbon flux to the bed
might be thought best measured close to the bottom
in order to collect near-bed advected material, as well
as vertical flux. But there remains uncertainty arising
from local resuspension and deposition – the so-called
‘rebound’ flux. However, if the measurements are made
at greater depths more serious budgetary discrepancies
have emerged. The differential in sediment community
oxygen consumption on the continental margin is
greater on the west coast of America than on the
east, and increases with depth. This has been linked to
resuspension by benthic storms along the continental
rise in the western Atlantic, which results in relatively
less bacterial decomposition of particles while they are
on the bottom (Rowe et al., 1994). Furthermore, this
turbulent regime on the continental rise may extend
the effects of the continental margin far out onto the
adjacent abyssal plain (Walsh et al., 1991).
Lampitt et al. (1995) found that particle flux
collected by sediment traps set at mid-slope would
be sufficient to fuel only 20% of measured sediment
community oxygen consumption. Possible overestimation of oxygen demand might have resulted from the
technique used, but it seems likely that very substantial
quantities of detrital flux on the seabed are transported
downslope in the bathyal Northeast Atlantic in order to
provide the organic carbon consumed.
The issue of whether there is significant downslope transport of particulate detrital material can
be addressed by comparison of downward flux from
surface production with estimates of benthic oxygen
demand. Rowe et al. (1994) found seabed oxygen
uptake, in terms of carbon equivalents (mg C m
−2 d
−1 )
to be less than particle flux measured in sediment traps
except at the mid-slope ‘depocentre’, where there is
an accumulation of organic carbon laterally advected
down the continental slope. Lampitt et al. (1995) also
found that vertical flux measured from deep sediment
traps at depths of 2000 m on the continental slope in
the Northeast Atlantic was sufficient to fuel only 20%
of demand. They concluded that substantial quantities
of particulate material, not intercepted or properly
collected by sediment traps (see Baker et al., 1988;
Gust et al., 1994), must be transported down-slope
in order to balance the discrepancy. The opposite
imbalance found by Rowe et al. (1994) at other stations
was partly attributed to lateral and near-bed down-slope
transport.
Studies on the slope off Southwest Ireland studied in
the OMEX program of the European Union has shown
that both hydrodynamic and biologically mediated
processes associated with lateral advection are of great
importance (Heip et al., 2001). Even in low-flow
conditions (<5 cm s
−1 ), lateral particle flux is orders
of magnitude greater than the vertical flux. Largesized particles in the benthic boundary layer at the
shelf edge are not deposited there, but transported to
deeper, quieter areas. The processes involved of particle
aggregation, biologically mediated modifications of
near-bed fluxes, and a resuspension loop within the
benthic boundary layer have been shown to be sufficient
to fuel observed benthic demand on the slope and