184
Craig R. SMITH and Amanda W.J. DEMOPOULOS
(<0.25% organic carbon), consisting primarily of clay
particles transported by wind from continents and
volcanic eruptions (Berger, 1974).
Hard substrata in the deep Pacific Ocean are of
three major types. (1) Basalt rocks predominate within
1–2 kilometers of the central valley of the East
Pacific Rise, where oceanic crust is too new to have
accumulated sediments. (2) Rock faces with slopes
>22º typically are bare because they are too steep
to allow sediment accumulation. Such faces are most
common along continental margins (e.g., in submarine
canyons) and on the steep slopes of the islands and
seamounts that dot the Pacific. (3) The surfaces of
ferromanganese concretions, or manganese “nodules”
resting on the sediment surface also typically are
sediment-free. Such nodules are found predominantly
in red-clay regions of the Pacific, range in size from
0.5 to 20 cm in diameter, and may cover more than 60%
the plan area of the seafloor.
Near-bottom currents
Near-bottom currents fundamentally influence the nature of benthic habitats (Nowell and Jumars, 1984;
see also Thistle, Chapter 2). Under conditions of
very low flow, the horizontal flux of particles near
the seafloor may be inadequate to sustain suspension
feeders (Jumars and Gallagher, 1982) and chemical
exchange between bottom water and the seabed may
be limited by molecular diffusion (Archer et al., 1989).
At high current velocities, sediments may be eroded
and transported, flooding suspension feeders with nonnutritive mineral grains and burying sessile organisms
(Aller, 1989; Nowell et al., 1989). At intermediate
flow velocities, less dense particles, such as recently
settled phytoplankton, may be mobilized by currents
and deposited in pits and behind flow obstructions,
yielding food-rich patches (Lampitt, 1985; Yager et al.,
1993; Smith et al., 1996). In short, near-bottom flow
rates and bed shear stress may influence a broad
range of ecologically significant physical, chemical and
biological processes (see Nowell and Jumars, 1984 for
a review).
Currents in the relatively flat areas of the deep Pacific
seafloor, such as the vast regions of abyssal hills, are
generally sluggish, imposing shear stresses inadequate
to transport most sediment types. However, currents
of erosive magnitudes may occur in certain deep-sea
environments as a consequence of boundary currents,
high eddy energy or topographic intensification. The
Kuroshio and East Australian western boundary current
systems (Fig. 6.2) likely cause intermittent erosion
of sediments to water depths of 1500 m along the
western margins of the Pacific, although the sites and
frequencies of such erosive events are very difficult to
predict. In addition, the region of Kuroshio separation
from the Japan slope is characterized by high eddy
energy (Hollister and McCave, 1984; Hollister et al.,
1984); intermittent “storms” at intervals of days to
months, which erode and redeposit several centimeters
of abyssal sediments, are thus to be expected in this
area. Relatively high-velocity currents also occur, at
least occasionally, in submarine canyons as a result of
storms or tides (Shepard and Dill, 1966; Vetter and
Dayton, 1998), and through channels (e.g., transform
faults) and around peaks (e.g., seamounts) owing to
acceleration of tidal flows (Genin et al., 1986). The
frequency and intensity of such high-energy flows
are typically very site-specific, and depend on the
interactions of local tides, bottom topography and
low-frequency flow events (e.g., upwelling and Taylor
circulation: Genin et al., 1986; Gage and Tyler, 1991).
Bottom-water oxygen
All deep-sea animals require oxygen as an electron
acceptor for oxidative metabolism. When bottom-water
oxygen concentrations fall below 0.5 ml °
−1 in the deep
sea, oxygen availability becomes an important factor
and benthic community structure varies with oxygen
concentration (Diaz and Rosenberg, 1995; Levin and
Gage, 1998). Above this threshold, other factors control
the nature and abundance of seafloor life. On most
of the deep Pacific seafloor, bottom-water oxygen
concentrations exceed the threshold of 0.5 ml °
−1 .
However, beneath relatively productive waters, such as
the eastern tropical Pacific and in coastal upwelling
zones, an oxygen-minimum zone may develop in the
water column, with oxygen concentrations approaching
zero at depths between 100 and 1000 m (Wishner
et al., 1990). This zone results from the oxidation of
organic particles sinking through the water column
from the highly productive euphotic zone; in the
North Pacific the oxygen-minimum zone is often
particularly well developed owing to the old “age”
(i.e., time since surface ventilation) and consequent
low oxygen concentrations of Intermediate and Deep
Water masses. Where this oxygen minimum intersects
the seafloor, bottom-water oxygen concentrations may
drop to zero (Wishner et al., 1990). Oxygen-stressed
Craig R. SMITH and Amanda W.J. DEMOPOULOS
(<0.25% organic carbon), consisting primarily of clay
particles transported by wind from continents and
volcanic eruptions (Berger, 1974).
Hard substrata in the deep Pacific Ocean are of
three major types. (1) Basalt rocks predominate within
1–2 kilometers of the central valley of the East
Pacific Rise, where oceanic crust is too new to have
accumulated sediments. (2) Rock faces with slopes
>22º typically are bare because they are too steep
to allow sediment accumulation. Such faces are most
common along continental margins (e.g., in submarine
canyons) and on the steep slopes of the islands and
seamounts that dot the Pacific. (3) The surfaces of
ferromanganese concretions, or manganese “nodules”
resting on the sediment surface also typically are
sediment-free. Such nodules are found predominantly
in red-clay regions of the Pacific, range in size from
0.5 to 20 cm in diameter, and may cover more than 60%
the plan area of the seafloor.
Near-bottom currents
Near-bottom currents fundamentally influence the nature of benthic habitats (Nowell and Jumars, 1984;
see also Thistle, Chapter 2). Under conditions of
very low flow, the horizontal flux of particles near
the seafloor may be inadequate to sustain suspension
feeders (Jumars and Gallagher, 1982) and chemical
exchange between bottom water and the seabed may
be limited by molecular diffusion (Archer et al., 1989).
At high current velocities, sediments may be eroded
and transported, flooding suspension feeders with nonnutritive mineral grains and burying sessile organisms
(Aller, 1989; Nowell et al., 1989). At intermediate
flow velocities, less dense particles, such as recently
settled phytoplankton, may be mobilized by currents
and deposited in pits and behind flow obstructions,
yielding food-rich patches (Lampitt, 1985; Yager et al.,
1993; Smith et al., 1996). In short, near-bottom flow
rates and bed shear stress may influence a broad
range of ecologically significant physical, chemical and
biological processes (see Nowell and Jumars, 1984 for
a review).
Currents in the relatively flat areas of the deep Pacific
seafloor, such as the vast regions of abyssal hills, are
generally sluggish, imposing shear stresses inadequate
to transport most sediment types. However, currents
of erosive magnitudes may occur in certain deep-sea
environments as a consequence of boundary currents,
high eddy energy or topographic intensification. The
Kuroshio and East Australian western boundary current
systems (Fig. 6.2) likely cause intermittent erosion
of sediments to water depths of 1500 m along the
western margins of the Pacific, although the sites and
frequencies of such erosive events are very difficult to
predict. In addition, the region of Kuroshio separation
from the Japan slope is characterized by high eddy
energy (Hollister and McCave, 1984; Hollister et al.,
1984); intermittent “storms” at intervals of days to
months, which erode and redeposit several centimeters
of abyssal sediments, are thus to be expected in this
area. Relatively high-velocity currents also occur, at
least occasionally, in submarine canyons as a result of
storms or tides (Shepard and Dill, 1966; Vetter and
Dayton, 1998), and through channels (e.g., transform
faults) and around peaks (e.g., seamounts) owing to
acceleration of tidal flows (Genin et al., 1986). The
frequency and intensity of such high-energy flows
are typically very site-specific, and depend on the
interactions of local tides, bottom topography and
low-frequency flow events (e.g., upwelling and Taylor
circulation: Genin et al., 1986; Gage and Tyler, 1991).
Bottom-water oxygen
All deep-sea animals require oxygen as an electron
acceptor for oxidative metabolism. When bottom-water
oxygen concentrations fall below 0.5 ml °
−1 in the deep
sea, oxygen availability becomes an important factor
and benthic community structure varies with oxygen
concentration (Diaz and Rosenberg, 1995; Levin and
Gage, 1998). Above this threshold, other factors control
the nature and abundance of seafloor life. On most
of the deep Pacific seafloor, bottom-water oxygen
concentrations exceed the threshold of 0.5 ml °
−1 .
However, beneath relatively productive waters, such as
the eastern tropical Pacific and in coastal upwelling
zones, an oxygen-minimum zone may develop in the
water column, with oxygen concentrations approaching
zero at depths between 100 and 1000 m (Wishner
et al., 1990). This zone results from the oxidation of
organic particles sinking through the water column
from the highly productive euphotic zone; in the
North Pacific the oxygen-minimum zone is often
particularly well developed owing to the old “age”
(i.e., time since surface ventilation) and consequent
low oxygen concentrations of Intermediate and Deep
Water masses. Where this oxygen minimum intersects
the seafloor, bottom-water oxygen concentrations may
drop to zero (Wishner et al., 1990). Oxygen-stressed