THE DEEP ATLANTIC OCEAN
113
Danish Ingolf expeditions of 1895 and 1896 around
Greenland, Iceland, and the Færøerne were notable
for the use of fine-meshed screens (silk gauze) to sift
mud recovered by the trawl and dredge. These residues
yielded a diversity of small animals, for example,
70 tanaid species (49 of them new) and 121 isopod
species (61 of them new) (Hansen, 1913, 1916).
There were parallel developments on the North
American margin. An early pioneer was Pourt´ ales who
dredged to a maximum depth of 494 m in the Florida
Straits using the U.S. Coast Guard steamer Corwin
in the summer of 1867. During the following two
years Pourt´ ales and Louis Agassiz obtained abundant
animals down to a maximum depth of 1555 m on the
U.S. continental margin between Florida and the Grand
Bahama Bank from the steamer Bibb (Mills, 1983).
A decade later (1877–1880), under the direction of
Alexander Agassiz, another U.S. Coast Guard steamer,
the Blake, undertook a series of biological cruises on
the continental margin of the eastern United States,
during which animals were dredged from depths down
to almost 3000 m (Agassiz, 1888). These studies raised
issues that continue to concern deep-sea biologists –
not least, the question of food supply to the deep-sea
benthos.
After fifty years or so of relative quiescence, deepsea biology has undergone a renaissance since the
1960s, a period during which national and, more
recently, international programs have become more
focused on specific areas and issues (Table 5.1;
Fig. 5.1). Work conducted in the North Atlantic by
American and European scientists has contributed
substantially to the tremendous development of deepsea ecology during this period. Perhaps the most
significant advance resulted from the recognition by
H.L. Sanders, R.R. Hessler and their colleagues, that
smaller animals (macroinfauna – animals residing
within the sediment column which are retained on a
0.3 or 0.5 mm mesh) are abundant and highly diverse
in the deep sea, a discovery made during the mid1960s on the continental margin of the northeastern United States using novel sampling techniques
(Sanders et al., 1965; Hessler and Sanders, 1967).
Although to some extent foreshadowed by earlier
observations (for instance, those of Hansen referred
to above), this discovery overturned the paradigm of
low deep-sea species diversity which had been derived
from studying large epibenthic megafaunal animals
caught in coarse-meshed bottom nets. Many of the
ideas which have developed subsequently regarding
the magnitude and geographic patterns of deep-sea
species diversity have resulted from research carried
out on this intensively studied margin (e.g., Rex,
1983; Grassle and Morse-Porteous, 1987; Grassle and
Maciolek, 1992) and elsewhere in the Atlantic Ocean
(Rex et al., 1993, 1997; Allen and Sanders, 1996). Two
other major developments in deep-sea biology during
the last 20 years have been the discovery that the deepsea floor is subject to seasonal or unpredictable episodic
inputs of organic matter which play an important role
in structuring benthic communities, and the discovery
that certain areas are physically disturbed by periodic
strong currents (benthic storms: see Chapter 2). Again,
these developments arose largely from work carried out
in the North Atlantic Ocean.
The important contribution made by Russian scientists to knowledge of the biology of both the South
and North Atlantic Ocean should not be overlooked.
Much of the extensive Russian literature on the faunas
of the Atlantic slopes and abyssal plains, large-scale
faunal distribution patterns, and primary phytoplankton
production, is summarized in the English-language
volume edited by Gebruk et al. (1997).
GENERAL PHYSICAL AND CHEMICAL
CHARACTERISTICS
Seafloor topography and physiography
An excellent and detailed account of the physiography
and geology of the Atlantic Ocean has been given by
Emery and Uchupi (1984). According to the hypsometric curve of Emery and Uchupi (1984, fig. 9 therein),
the mean depth of the Atlantic between 60ºN and 60ºS
is 3730 m, very close to the average for the entire
World Ocean, with the modal depth lying between
4000 m and 5000 m. The most important first-order
feature is the Mid-Atlantic Ridge which divides the
ocean longitudinally into two halves, each with a series
of major basins delimited by secondary, more or less
transverse ridges (Fig. 5.2). The Ridge extends above
the 2000 m contour along most of its length and has a
major influence on the circulation of near-bottom water
masses (Tomczak and Godfrey, 1994). It is interrupted
by a series of transform faults including the Romanche
Fracture Zone near the equator and the Gibbs Fracture
Zone at 53ºN. Both these features provide routes for
bottom water to pass from one side of the ocean to the
other. Second-order physiographic features include the
113
Danish Ingolf expeditions of 1895 and 1896 around
Greenland, Iceland, and the Færøerne were notable
for the use of fine-meshed screens (silk gauze) to sift
mud recovered by the trawl and dredge. These residues
yielded a diversity of small animals, for example,
70 tanaid species (49 of them new) and 121 isopod
species (61 of them new) (Hansen, 1913, 1916).
There were parallel developments on the North
American margin. An early pioneer was Pourt´ ales who
dredged to a maximum depth of 494 m in the Florida
Straits using the U.S. Coast Guard steamer Corwin
in the summer of 1867. During the following two
years Pourt´ ales and Louis Agassiz obtained abundant
animals down to a maximum depth of 1555 m on the
U.S. continental margin between Florida and the Grand
Bahama Bank from the steamer Bibb (Mills, 1983).
A decade later (1877–1880), under the direction of
Alexander Agassiz, another U.S. Coast Guard steamer,
the Blake, undertook a series of biological cruises on
the continental margin of the eastern United States,
during which animals were dredged from depths down
to almost 3000 m (Agassiz, 1888). These studies raised
issues that continue to concern deep-sea biologists –
not least, the question of food supply to the deep-sea
benthos.
After fifty years or so of relative quiescence, deepsea biology has undergone a renaissance since the
1960s, a period during which national and, more
recently, international programs have become more
focused on specific areas and issues (Table 5.1;
Fig. 5.1). Work conducted in the North Atlantic by
American and European scientists has contributed
substantially to the tremendous development of deepsea ecology during this period. Perhaps the most
significant advance resulted from the recognition by
H.L. Sanders, R.R. Hessler and their colleagues, that
smaller animals (macroinfauna – animals residing
within the sediment column which are retained on a
0.3 or 0.5 mm mesh) are abundant and highly diverse
in the deep sea, a discovery made during the mid1960s on the continental margin of the northeastern United States using novel sampling techniques
(Sanders et al., 1965; Hessler and Sanders, 1967).
Although to some extent foreshadowed by earlier
observations (for instance, those of Hansen referred
to above), this discovery overturned the paradigm of
low deep-sea species diversity which had been derived
from studying large epibenthic megafaunal animals
caught in coarse-meshed bottom nets. Many of the
ideas which have developed subsequently regarding
the magnitude and geographic patterns of deep-sea
species diversity have resulted from research carried
out on this intensively studied margin (e.g., Rex,
1983; Grassle and Morse-Porteous, 1987; Grassle and
Maciolek, 1992) and elsewhere in the Atlantic Ocean
(Rex et al., 1993, 1997; Allen and Sanders, 1996). Two
other major developments in deep-sea biology during
the last 20 years have been the discovery that the deepsea floor is subject to seasonal or unpredictable episodic
inputs of organic matter which play an important role
in structuring benthic communities, and the discovery
that certain areas are physically disturbed by periodic
strong currents (benthic storms: see Chapter 2). Again,
these developments arose largely from work carried out
in the North Atlantic Ocean.
The important contribution made by Russian scientists to knowledge of the biology of both the South
and North Atlantic Ocean should not be overlooked.
Much of the extensive Russian literature on the faunas
of the Atlantic slopes and abyssal plains, large-scale
faunal distribution patterns, and primary phytoplankton
production, is summarized in the English-language
volume edited by Gebruk et al. (1997).
GENERAL PHYSICAL AND CHEMICAL
CHARACTERISTICS
Seafloor topography and physiography
An excellent and detailed account of the physiography
and geology of the Atlantic Ocean has been given by
Emery and Uchupi (1984). According to the hypsometric curve of Emery and Uchupi (1984, fig. 9 therein),
the mean depth of the Atlantic between 60ºN and 60ºS
is 3730 m, very close to the average for the entire
World Ocean, with the modal depth lying between
4000 m and 5000 m. The most important first-order
feature is the Mid-Atlantic Ridge which divides the
ocean longitudinally into two halves, each with a series
of major basins delimited by secondary, more or less
transverse ridges (Fig. 5.2). The Ridge extends above
the 2000 m contour along most of its length and has a
major influence on the circulation of near-bottom water
masses (Tomczak and Godfrey, 1994). It is interrupted
by a series of transform faults including the Romanche
Fracture Zone near the equator and the Gibbs Fracture
Zone at 53ºN. Both these features provide routes for
bottom water to pass from one side of the ocean to the
other. Second-order physiographic features include the