326
John D. GAGE
1994). Other possible causes are variability in the
intensity of monsoon-driven upwelling (Haake et al.,
1993; Rixen et al., 1996), or the El Ni˜ no Southern
Ocean Oscillation (Karl et al., 1996), and in upwelling
(Baldwin et al., 1998).
Such variability means that the export flux to the
deep-sea bed will need to be measured over many
years in order to obtain an accurate integrated estimate
which can be related to the life spans of larger seabed
biota. Furthermore, studies during the North Atlantic
JGOFS experiments in 1989 uncovered unexpected
mesoscale spatial variability. A massive sedimentation
of particulate material was observed in autumn at one
site, but was not registered in a trap just 100 km away
(Honjo and Manganini, 1993; Newton et al., 1994).
This difference is probably related to the eddy field
at the time affecting the distribution of algal blooms
in the surface waters (Newton et al., 1994). However,
significant variability in particle flux at 100 metres
above the bottom was also measured in sediment traps
spaced from less than one metre to several kilometres
apart in the abyssal Pacific off California (Crassous and
Khripounoff, 1994).
PARTICLE DYNAMICS AT THE DEEP-SEA BENTHIC
BOUNDARY
I shall here consider physical processes of redistribution of particles at and near the bottom within the
benthic boundary layer. Other processes, mediated by
larger burrowing animals, also occur, which contribute
towards the biogenic mixing of sediments and, usually
vertical, particle transport known as bioturbation. But
this phenomenon, although important in its own right,
will not be considered in this chapter, except where
relevant later (p. 343) to the utilization of organic
particles by the benthic biota.
Dynamics of fine particles
Realization of the redistribution of sediment by currents on the deep-sea bed dates from large-scale
profiling of the sediment column by means of cores
and acoustic profiling. In many areas large-scale
sedimentary features, such as sediment drifts and
ridges, have been mapped, which are thought to result
from persistent thermohaline-driven current activity,
particularly when constrained by major bottom topography such as the continental slope and rise. At
smaller spatial scales the signature of bottom flow
may be recognized in side-scan acoustic imagery and
bottom photographs as bedforms ranging from dynamic
ripples to huge, slowly migrating mud waves. One
needs to examine such phenomena briefly in order to
assess the importance of such transport in providing
and redistributing food to the benthic boundary layer
community.
To understand the dynamics of such large-scale
particle redistribution, sedimentologists measure the
cloudiness (transparency) of the water with a nephelometer. This measures transmission (attenuation) and
the scattering of light by suspended particles. The result
depends on both the concentration and size of particles,
and on dissolved material in the water that reduces its
transparency. The finest suspended particles are made
up principally of clay-sized fine mineral particles forming fine sediment clouds (Thorpe and White, 1988).
Such sediment clouds are thought to be be derived
from resuspension by abyssal benthic storms that send
bottom material high up into the water column to form
nepheloid layers. More usually these sediment clouds
remain close to the bed, forming a benthic nepheloid
layer, or BNL (McCave, 1986). As a consequence
such fine particles show up as increasing, rather than
decreasing, in density nearer the bottom (see below).
Nepheloids may form persistent layers hundreds of metres thick. Intermediate nepheloid layers may also occur
through plume injection into the adjacent water column
driven by breaking internal tidal waves on the continental slope (e.g., Dickson and McCave, 1986; Gilbert
and Garrett, 1989; Ivey and Nokes, 1989; Thorpe
et al., 1990). Even in deeper water on the continental
rise nephelometry records have shown transient resuspension events (Beaulieu and Baldwin, 1998). These
may be associated with energetic internal waves (Vangriesheim and Khripounoff, 1990) or driven by vorticity in flow in the upper water column which is propagated to the bottom to cause ‘benthic storms’ (Gardner
et al., 1985). At the High Energy Benthic Boundary
Layer Experiment (HEBBLE) site on the Nova Scotia
Rise the topmost layer of bottom sediment is periodically resuspended and redeposited by benthic storms
that recur over a time-scale of weeks (see Chapter 2).
Intermediate and benthic nepheloids made up of
these fine particles are likened to atmospheric clouds
and mist. They appear to be conservative features
in the deep ocean that can be traced far out over
the abyssal plain, facilitated by isopycnal (lateral)
eddy diffusion which is much greater than vertical
John D. GAGE
1994). Other possible causes are variability in the
intensity of monsoon-driven upwelling (Haake et al.,
1993; Rixen et al., 1996), or the El Ni˜ no Southern
Ocean Oscillation (Karl et al., 1996), and in upwelling
(Baldwin et al., 1998).
Such variability means that the export flux to the
deep-sea bed will need to be measured over many
years in order to obtain an accurate integrated estimate
which can be related to the life spans of larger seabed
biota. Furthermore, studies during the North Atlantic
JGOFS experiments in 1989 uncovered unexpected
mesoscale spatial variability. A massive sedimentation
of particulate material was observed in autumn at one
site, but was not registered in a trap just 100 km away
(Honjo and Manganini, 1993; Newton et al., 1994).
This difference is probably related to the eddy field
at the time affecting the distribution of algal blooms
in the surface waters (Newton et al., 1994). However,
significant variability in particle flux at 100 metres
above the bottom was also measured in sediment traps
spaced from less than one metre to several kilometres
apart in the abyssal Pacific off California (Crassous and
Khripounoff, 1994).
PARTICLE DYNAMICS AT THE DEEP-SEA BENTHIC
BOUNDARY
I shall here consider physical processes of redistribution of particles at and near the bottom within the
benthic boundary layer. Other processes, mediated by
larger burrowing animals, also occur, which contribute
towards the biogenic mixing of sediments and, usually
vertical, particle transport known as bioturbation. But
this phenomenon, although important in its own right,
will not be considered in this chapter, except where
relevant later (p. 343) to the utilization of organic
particles by the benthic biota.
Dynamics of fine particles
Realization of the redistribution of sediment by currents on the deep-sea bed dates from large-scale
profiling of the sediment column by means of cores
and acoustic profiling. In many areas large-scale
sedimentary features, such as sediment drifts and
ridges, have been mapped, which are thought to result
from persistent thermohaline-driven current activity,
particularly when constrained by major bottom topography such as the continental slope and rise. At
smaller spatial scales the signature of bottom flow
may be recognized in side-scan acoustic imagery and
bottom photographs as bedforms ranging from dynamic
ripples to huge, slowly migrating mud waves. One
needs to examine such phenomena briefly in order to
assess the importance of such transport in providing
and redistributing food to the benthic boundary layer
community.
To understand the dynamics of such large-scale
particle redistribution, sedimentologists measure the
cloudiness (transparency) of the water with a nephelometer. This measures transmission (attenuation) and
the scattering of light by suspended particles. The result
depends on both the concentration and size of particles,
and on dissolved material in the water that reduces its
transparency. The finest suspended particles are made
up principally of clay-sized fine mineral particles forming fine sediment clouds (Thorpe and White, 1988).
Such sediment clouds are thought to be be derived
from resuspension by abyssal benthic storms that send
bottom material high up into the water column to form
nepheloid layers. More usually these sediment clouds
remain close to the bed, forming a benthic nepheloid
layer, or BNL (McCave, 1986). As a consequence
such fine particles show up as increasing, rather than
decreasing, in density nearer the bottom (see below).
Nepheloids may form persistent layers hundreds of metres thick. Intermediate nepheloid layers may also occur
through plume injection into the adjacent water column
driven by breaking internal tidal waves on the continental slope (e.g., Dickson and McCave, 1986; Gilbert
and Garrett, 1989; Ivey and Nokes, 1989; Thorpe
et al., 1990). Even in deeper water on the continental
rise nephelometry records have shown transient resuspension events (Beaulieu and Baldwin, 1998). These
may be associated with energetic internal waves (Vangriesheim and Khripounoff, 1990) or driven by vorticity in flow in the upper water column which is propagated to the bottom to cause ‘benthic storms’ (Gardner
et al., 1985). At the High Energy Benthic Boundary
Layer Experiment (HEBBLE) site on the Nova Scotia
Rise the topmost layer of bottom sediment is periodically resuspended and redeposited by benthic storms
that recur over a time-scale of weeks (see Chapter 2).
Intermediate and benthic nepheloids made up of
these fine particles are likened to atmospheric clouds
and mist. They appear to be conservative features
in the deep ocean that can be traced far out over
the abyssal plain, facilitated by isopycnal (lateral)
eddy diffusion which is much greater than vertical