THE DEEP ATLANTIC OCEAN
119
transported 6000 km
3 of sediment from the shelf to
depths of 3500 m over horizontal distances of 800 km.
Not surprisingly, turbidite deposition is a very important feature of Atlantic sedimentation and turbiditeformed abyssal plains are present off most of the
continents, except the southern part of South America
(Brown et al., 1989). The best-documented example is
the Madeira Abyssal Plain, which is composed of a
sequence of large turbidite deposits separated by thin
pelagic layers. The turbidites were derived from several
sources on the Northwest African margin, and their emplacement appears to be related to changes in climate
and sea-level over the past 700 000 years (Weaver et al.,
1992). Despite the prevalence of turbidite deposits,
the actual currents have rarely been observed. The
best-known turbidity flow occurred in 1929, when an
earthquake on Grand Banks, Newfoundland, triggered
a sediment slump which developed into a turbidity
current extending 800 km from its source across the
abyssal plain (Heezen and Ewing, 1952). Thunell et al.
(1999) observed a striking increase in near-bottom
concentrations of suspended sediment and mass flux
through the water column in the Venezuela Basin
following a 1997 earthquake. These mass-movement
events must have had a devastating impact on the
benthic fauna at the time of their emplacement, and
may have a continuing effect through their influence on
the granulometry (Huggett, 1987) and organic-matter
content (Thunell et al., 1999) of the sediment.
While most of the material on the Atlantic seabed
is pelagic or terrigenous in origin, authigenic
4 deposits
occur in certain areas. Manganese nodules (composed
of a mixture of iron and manganese oxides) were
first recovered from the Atlantic near the Canary
Islands during the Challenger Expedition. They are
most extensively developed in the Argentine, Brazil and
Cape Basins in the South Atlantic and in the Sargasso
Sea in the North Atlantic. These Atlantic nodules have
a somewhat lower manganese content (16%) and higher
iron content (21%) than those in the Pacific and Indian
Oceans (Brown et al., 1989). Phosphorites (authigenic
calcium phosphate) occur on the shelf off south-west
Africa (Price and Calvert, 1978), and unconsolidated
phosphatic deposits are found in various areas around
the Atlantic margin, including the shelf and upper slope
off North Carolina and Northwest Africa (e.g., Riggs
et al., 1985; Summerhayes et al., 1972).
ORGANIC-MATTER FLUX AND BENTHOPELAGIC
COUPLING
General patterns of primary production
Apart from spatially limited areas of hydrothermal
vents and seeps (see Chapter 4), almost all food
available to the deep-sea benthos is derived from
primary production in the euphotic zone. The ‘Dahlem
map’ (Berger, 1989, fig. 11) provides an overview of
estimated primary production for the Atlantic Ocean
based on previous maps and remote-sensed satellite
data. For the North Atlantic, more precise data on
new primary production (i.e., production depending
on nutrients imported into the euphotic zone) can
be derived from surface chlorophyll concentrations
determined from satellite imagery (Campbell and
Aarup, 1992). Campbell and Aarup identified three
areas characterized by different patterns of seasonal
production and increasing levels of overall production:
(i) a subtropical zone in which production reaches
a maximum in the winter and minimum in the late
summer (new production = 18 g C m
−2 y
−1 ), (ii) a midlatitude zone characterized by a spring bloom followed
by oligotrophic conditions throughout the summer (new
production = 24 g C m
−2 y
−1 ), and (iii) a subpolar zone
with minimum production in the winter and maximum
in the late summer (new production = 43 g C m
−2 y
−1 ).
These three zones represent 20% of the area of the
Atlantic and 43% of the North Atlantic, and exclude
the entire tropical region where primary production is
strongly influenced by upwelling and river inputs.
Longhurst (1995, 1998) and Longhurst et al. (1995)
have presented a comprehensive scheme to classify
seasonal cycles of primary production and consumption
in the World Ocean. Sathyendranath et al. (1995)
focused in detail on the Atlantic Ocean. Longhurst
and his colleagues recognized four primary ecological
domains (three oceanic and one coastal) which have
characteristic seasonal cycles of water-column stability,
nutrient supply and solar illumination. These are
divided into 57 biogeochemical provinces defined on
the basis of local features such as currents, fronts and
topography. The important Atlantic provinces include,
from north to south: North Atlantic Drift (NADR), Subtropical Gyre [STGW (West) and STGE (East)], North
Atlantic Tropical Gyre (NATR), Western Tropical Atlantic (WTRA), Eastern Tropical Atlantic (ETRA), and
South Atlantic Tropical Gyre (SATG) (Sathyendranath
4 Authigenic: generated locally (in situ), usually by a geochemical reaction (e.g., precipitation of Mn or Fe oxides to form nodules).
119
transported 6000 km
3 of sediment from the shelf to
depths of 3500 m over horizontal distances of 800 km.
Not surprisingly, turbidite deposition is a very important feature of Atlantic sedimentation and turbiditeformed abyssal plains are present off most of the
continents, except the southern part of South America
(Brown et al., 1989). The best-documented example is
the Madeira Abyssal Plain, which is composed of a
sequence of large turbidite deposits separated by thin
pelagic layers. The turbidites were derived from several
sources on the Northwest African margin, and their emplacement appears to be related to changes in climate
and sea-level over the past 700 000 years (Weaver et al.,
1992). Despite the prevalence of turbidite deposits,
the actual currents have rarely been observed. The
best-known turbidity flow occurred in 1929, when an
earthquake on Grand Banks, Newfoundland, triggered
a sediment slump which developed into a turbidity
current extending 800 km from its source across the
abyssal plain (Heezen and Ewing, 1952). Thunell et al.
(1999) observed a striking increase in near-bottom
concentrations of suspended sediment and mass flux
through the water column in the Venezuela Basin
following a 1997 earthquake. These mass-movement
events must have had a devastating impact on the
benthic fauna at the time of their emplacement, and
may have a continuing effect through their influence on
the granulometry (Huggett, 1987) and organic-matter
content (Thunell et al., 1999) of the sediment.
While most of the material on the Atlantic seabed
is pelagic or terrigenous in origin, authigenic
4 deposits
occur in certain areas. Manganese nodules (composed
of a mixture of iron and manganese oxides) were
first recovered from the Atlantic near the Canary
Islands during the Challenger Expedition. They are
most extensively developed in the Argentine, Brazil and
Cape Basins in the South Atlantic and in the Sargasso
Sea in the North Atlantic. These Atlantic nodules have
a somewhat lower manganese content (16%) and higher
iron content (21%) than those in the Pacific and Indian
Oceans (Brown et al., 1989). Phosphorites (authigenic
calcium phosphate) occur on the shelf off south-west
Africa (Price and Calvert, 1978), and unconsolidated
phosphatic deposits are found in various areas around
the Atlantic margin, including the shelf and upper slope
off North Carolina and Northwest Africa (e.g., Riggs
et al., 1985; Summerhayes et al., 1972).
ORGANIC-MATTER FLUX AND BENTHOPELAGIC
COUPLING
General patterns of primary production
Apart from spatially limited areas of hydrothermal
vents and seeps (see Chapter 4), almost all food
available to the deep-sea benthos is derived from
primary production in the euphotic zone. The ‘Dahlem
map’ (Berger, 1989, fig. 11) provides an overview of
estimated primary production for the Atlantic Ocean
based on previous maps and remote-sensed satellite
data. For the North Atlantic, more precise data on
new primary production (i.e., production depending
on nutrients imported into the euphotic zone) can
be derived from surface chlorophyll concentrations
determined from satellite imagery (Campbell and
Aarup, 1992). Campbell and Aarup identified three
areas characterized by different patterns of seasonal
production and increasing levels of overall production:
(i) a subtropical zone in which production reaches
a maximum in the winter and minimum in the late
summer (new production = 18 g C m
−2 y
−1 ), (ii) a midlatitude zone characterized by a spring bloom followed
by oligotrophic conditions throughout the summer (new
production = 24 g C m
−2 y
−1 ), and (iii) a subpolar zone
with minimum production in the winter and maximum
in the late summer (new production = 43 g C m
−2 y
−1 ).
These three zones represent 20% of the area of the
Atlantic and 43% of the North Atlantic, and exclude
the entire tropical region where primary production is
strongly influenced by upwelling and river inputs.
Longhurst (1995, 1998) and Longhurst et al. (1995)
have presented a comprehensive scheme to classify
seasonal cycles of primary production and consumption
in the World Ocean. Sathyendranath et al. (1995)
focused in detail on the Atlantic Ocean. Longhurst
and his colleagues recognized four primary ecological
domains (three oceanic and one coastal) which have
characteristic seasonal cycles of water-column stability,
nutrient supply and solar illumination. These are
divided into 57 biogeochemical provinces defined on
the basis of local features such as currents, fronts and
topography. The important Atlantic provinces include,
from north to south: North Atlantic Drift (NADR), Subtropical Gyre [STGW (West) and STGE (East)], North
Atlantic Tropical Gyre (NATR), Western Tropical Atlantic (WTRA), Eastern Tropical Atlantic (ETRA), and
South Atlantic Tropical Gyre (SATG) (Sathyendranath
4 Authigenic: generated locally (in situ), usually by a geochemical reaction (e.g., precipitation of Mn or Fe oxides to form nodules).