118
Lisa A. LEVIN and Andrew J. GOODAY
Sediments and mass movement
The deep Atlantic Ocean floor is covered by sediments
deposited by near-shore and volcanic processes, by
turbidity currents and related gravity-driven processes,
by bottom currents and by pelagic sedimentation
(Emery and Uchupi, 1984). Sokolova (1997) considered the extensive Russian literature on sedimentation
processes in the North and South Atlantic. Sands and
gravels of terrestrial or biogenic origin are generally
restricted to the shelf and upper slope. Sediments
become progressively finer with increasing depth and
distance from land (Emery and Uchupi, 1984; Lampitt
et al., 1986), although in some areas submarine
canyons channel coarser sediments onto the continental
slope and rise (Mart et al., 1979; Auffret, 1985;
Weston, 1985). Over much (67%) of the Atlantic
Ocean, however, the surface sediments are carbonate
oozes (CaCO 3 content 30–50%) with a mean particle
size of <100 mm (Apostolescu et al., 1978; Emery
and Uchupi, 1984; Auffret, 1985; Lampitt et al.,
1986; Udintsev, 1990), a sand-sized fraction consisting
predominantly of planktonic foraminiferal tests, and
an organic-carbon content generally <0.5% (Emery
and Uchupi, 1984, fig. 345B therein). In the central
South Atlantic, the calcareous oozes are dominated
by pteropod shells. Siliceous (diatomaceous) oozes
cover about 7% of the Atlantic floor, mainly in the
region to the north of the Antarctic continent (Sverdrup
et al., 1970), where their accumulation reflects levels
of benthic and benthopelagic dissolution rather than
an increased flux of opal (biogenic silicate) to the
sea floor (Lampitt and Antia, 1997). The principal
clay minerals in deep Atlantic sediments are kaolinite
and chlorite, with greater proportions of kaolinite
beneath the tropics. Off the Amazon and western
Sahara Desert, the kaolinite/chlorite ratio is >10,
indicating large kaolinite inputs from these sources
(Pinet, 1998).
In general, Atlantic sediments have a much higher
calcium carbonate content than Pacific sediments.
Differences in ocean chemistry result in a calcium
carbonate (calcite) compensation depth (CCD) which
is much deeper (>5000 m) in the Atlantic, particularly
in eastern basins which are less influenced by corrosive
AABW, than in the Pacific (Sverdrup et al., 1970;
Berger, 1975; Biscaye et al., 1976). As a result, areas
where red clay (from which the carbonate has been
removed by dissolution) accumulates are restricted
to deep, relatively small basins, for example west
and northwest of the Cape Verde Islands, under the
Sargasso Sea in the central subtropical Atlantic and in
the western South Atlantic (Emery and Uchupi, 1984,
fig. 341C therein). The aragonite (essentially pteropodderived) compensation depth is also relatively deep in
the Atlantic, normally being located between 2000 and
3000 m (Berger, 1978) but rising to 400 m under the
Northwest African upwelling area (Ganssen and Lutze,
1982).
Sedimentation rates for Central Atlantic foraminiferal
oozes are generally in the range 1–5 cm ky
−1 compared
with >5 cm ky
−1 for the largely terrigenous sediments
which accumulate around the margins of the North Atlantic (Brown et al., 1989; Pinet, 1998). The sediment
drifts (e.g., the Feni Drift) which are developed in parts
of the northeastern Atlantic (Johnson and Schneider,
1969) are also characterized by sedimentation rates of
>5 cm ky
−1 (Thomson et al., 1993). These enhanced
rates reflect the advection of fine material by bottom
currents.
During the recent geological past, large areas
around the Atlantic margin have been disturbed by
gravity-driven mass movements, including slumps,
slides, debris flows and turbidity currents, which have
modified the seafloor to a significant extent (Stoker
et al., 1998). These categories may be related; for
example, some debris flows seem to be associated
with turbidity currents (Masson et al., 1996). Mass
movements are well documented off Northwest Europe,
Northwest Africa, Southern Africa, the United States
between New York and Cape Hatteras, and Brazil
(Emery and Uchupi, 1984, fig. 43 therein). Massive
sediment transport has been intensively studied on the
continental rise and abyssal plain off Northwest Africa,
an important area for the development of modern ideas
about continental-margin sedimentation (Jacobi and
Hayes, 1982; Simms et al., 1991; Masson et al., 1996).
Here, debris flows, notably the unusually large Canary
and Saharan flows (Embley, 1976; Masson et al., 1994,
1996), have occurred on the upper continental rise,
transporting 600 km
3 of sediment from the shelf over
a very short period, perhaps days or even hours – as
much as the southern California basins have received
from turbidity currents in a million years. Elsewhere
in the Northeast Atlantic, the mid-Norwegian margin
(the ‘Storegga’ area, 62ºN) is notable for a series of
enormous slides, the most recent of which occurred
about 7000 years ago (Bugge et al., 1988). These slides
and associated debris flows and turbidity currents have
Lisa A. LEVIN and Andrew J. GOODAY
Sediments and mass movement
The deep Atlantic Ocean floor is covered by sediments
deposited by near-shore and volcanic processes, by
turbidity currents and related gravity-driven processes,
by bottom currents and by pelagic sedimentation
(Emery and Uchupi, 1984). Sokolova (1997) considered the extensive Russian literature on sedimentation
processes in the North and South Atlantic. Sands and
gravels of terrestrial or biogenic origin are generally
restricted to the shelf and upper slope. Sediments
become progressively finer with increasing depth and
distance from land (Emery and Uchupi, 1984; Lampitt
et al., 1986), although in some areas submarine
canyons channel coarser sediments onto the continental
slope and rise (Mart et al., 1979; Auffret, 1985;
Weston, 1985). Over much (67%) of the Atlantic
Ocean, however, the surface sediments are carbonate
oozes (CaCO 3 content 30–50%) with a mean particle
size of <100 mm (Apostolescu et al., 1978; Emery
and Uchupi, 1984; Auffret, 1985; Lampitt et al.,
1986; Udintsev, 1990), a sand-sized fraction consisting
predominantly of planktonic foraminiferal tests, and
an organic-carbon content generally <0.5% (Emery
and Uchupi, 1984, fig. 345B therein). In the central
South Atlantic, the calcareous oozes are dominated
by pteropod shells. Siliceous (diatomaceous) oozes
cover about 7% of the Atlantic floor, mainly in the
region to the north of the Antarctic continent (Sverdrup
et al., 1970), where their accumulation reflects levels
of benthic and benthopelagic dissolution rather than
an increased flux of opal (biogenic silicate) to the
sea floor (Lampitt and Antia, 1997). The principal
clay minerals in deep Atlantic sediments are kaolinite
and chlorite, with greater proportions of kaolinite
beneath the tropics. Off the Amazon and western
Sahara Desert, the kaolinite/chlorite ratio is >10,
indicating large kaolinite inputs from these sources
(Pinet, 1998).
In general, Atlantic sediments have a much higher
calcium carbonate content than Pacific sediments.
Differences in ocean chemistry result in a calcium
carbonate (calcite) compensation depth (CCD) which
is much deeper (>5000 m) in the Atlantic, particularly
in eastern basins which are less influenced by corrosive
AABW, than in the Pacific (Sverdrup et al., 1970;
Berger, 1975; Biscaye et al., 1976). As a result, areas
where red clay (from which the carbonate has been
removed by dissolution) accumulates are restricted
to deep, relatively small basins, for example west
and northwest of the Cape Verde Islands, under the
Sargasso Sea in the central subtropical Atlantic and in
the western South Atlantic (Emery and Uchupi, 1984,
fig. 341C therein). The aragonite (essentially pteropodderived) compensation depth is also relatively deep in
the Atlantic, normally being located between 2000 and
3000 m (Berger, 1978) but rising to 400 m under the
Northwest African upwelling area (Ganssen and Lutze,
1982).
Sedimentation rates for Central Atlantic foraminiferal
oozes are generally in the range 1–5 cm ky
−1 compared
with >5 cm ky
−1 for the largely terrigenous sediments
which accumulate around the margins of the North Atlantic (Brown et al., 1989; Pinet, 1998). The sediment
drifts (e.g., the Feni Drift) which are developed in parts
of the northeastern Atlantic (Johnson and Schneider,
1969) are also characterized by sedimentation rates of
>5 cm ky
−1 (Thomson et al., 1993). These enhanced
rates reflect the advection of fine material by bottom
currents.
During the recent geological past, large areas
around the Atlantic margin have been disturbed by
gravity-driven mass movements, including slumps,
slides, debris flows and turbidity currents, which have
modified the seafloor to a significant extent (Stoker
et al., 1998). These categories may be related; for
example, some debris flows seem to be associated
with turbidity currents (Masson et al., 1996). Mass
movements are well documented off Northwest Europe,
Northwest Africa, Southern Africa, the United States
between New York and Cape Hatteras, and Brazil
(Emery and Uchupi, 1984, fig. 43 therein). Massive
sediment transport has been intensively studied on the
continental rise and abyssal plain off Northwest Africa,
an important area for the development of modern ideas
about continental-margin sedimentation (Jacobi and
Hayes, 1982; Simms et al., 1991; Masson et al., 1996).
Here, debris flows, notably the unusually large Canary
and Saharan flows (Embley, 1976; Masson et al., 1994,
1996), have occurred on the upper continental rise,
transporting 600 km
3 of sediment from the shelf over
a very short period, perhaps days or even hours – as
much as the southern California basins have received
from turbidity currents in a million years. Elsewhere
in the Northeast Atlantic, the mid-Norwegian margin
(the ‘Storegga’ area, 62ºN) is notable for a series of
enormous slides, the most recent of which occurred
about 7000 years ago (Bugge et al., 1988). These slides
and associated debris flows and turbidity currents have