THE DEEP INDIAN OCEAN FLOOR
225
and are composed mostly of terrestrial plant material,
phytodetritus, and mineral grains transported by rivers
(Kennett, 1982; Levin and Gage, 1998; Levin et al.,
2000). In the Bay of Bengal, terrigenous sedimentation
from the Ganges is particularly extensive, reaching
depths of 5000 m. In the Arabian Sea, there is an
organic carbon maximum (4.9%) at 400 m, owing apparently to preferential preservation and accumulation
of organic matter under low-oxygen conditions in the
bottom water (Levin et al., 2000). The particle flux to
the deep Bay of Bengal is enhanced by the freshwater
input from the major rivers entering at the north of the
Bay (Ittekkot et al., 1991).
Sediments are at most very thin on the crests
of mid-ocean ridges, and essentially absent on the
ridge axes (Fig. 7.3). Thick siliceous oozes, composed
primarily of radiolarian and diatom tests, occur at
depths of ~5000 m south of the polar front and along
a few mid-ocean ridges, where there is high biological
productivity (Berger, 1974; Kennett, 1982). However,
because of the oligotrophic nature of the equatorial
Indian Ocean, siliceous sediments are rare in low
latitudes of the Indian Ocean compared to the Pacific
Ocean (Kennett, 1982). Red clay is present mostly in
the eastern and southern Indian Ocean, near the equator
and high latitudes. It is composed of fine-grained,
organic-poor sediments resulting from volcanic activity
at ridges (Berger, 1974; Pilipchuk et al., 1977; Kennett,
1982).
Hard substrata in the Indian Ocean consist of basalt
rocks, rock faces, and the surfaces of ferromanganese concretions. The morphology of the central
Indian Ocean Basin is composed of abyssal hills and
seamounts, as well as valleys and abyssal plains.
The topographic highs, which are in the proximity
of three major fracture zones, are composed of hard,
massive basalts occurring at the crests, along the
slopes and on the foothills as talus deposits (Sharma
et al., 1997). Owing to strong geostrophic currents and
consequent scouring of the sediments, the Wharton
Basin, the southern Mascarene Basin, and parts of the
Southwest Indian and Australian–Antarctic Basins have
little or no sediment (Kennett, 1982). Sediment in these
areas, when present, is mostly brown clay. Along the
Southwest Indian Ridge, German et al. (1998) have
identified hydrothermal activity. The associated fauna
inhabiting these hydrothermal regions is dominated by
shrimps and anemones (T. Shank, pers. comm.). In
the southeast and southwest Indian Ocean, and in the
Mozambique Basin, there are extensive pavements of
manganese nodules at depths of about 4000 m (Kolla
et al., 1980). It has been suggested that their presence
at the sediment–water interface is a result of benthic
biological activity and strong bottom currents. Benthic
organisms may nudge the nodules upward, maintaining
them near the sediment–water interface, while strong
currents may limit the deposition of sediment, allowing
the nodules to grow (Berger, 1974; Paul, 1976; Kennett,
1982).
Near-bottom currents
Currents in the deep sea influence sediment deposition
and the organisms that inhabit the seafloor (Nowell and
Jumars, 1984). In regions where there are strong currents, sediment deposition is minimal. These currents
can smother sessile and suspension-feeding organisms
with sediment grains. Where currents are weak, sediment chemistry and biology may be controlled by
diffusive processes. In these environments, suspension
feeders may suffer owing to the inadequate supply
of advected particles (e.g., Jumars and Gallagher,
1982). Thus, near-bottom currents not only may control
sediment deposition, but can also manipulate sediment
chemistry and the structure of the benthic communities
(Nowell and Jumars, 1984).
Both bottom and deep waters of the Indian Ocean
are derived from the Atlantic, and spread throughout
the deep sea by active flow (Wyrtki, 1973). Antarctic
Bottom Water warms as it spreads north, and fills the
deep basins of the central Indian Ocean. Therefore, the
abyssal Indian Ocean is an area of active deep-sea circulation. Antarctic Bottom Water supplies fine-grained
sediments from the Africa–Madagascar source to the
marginal areas of the Mozambique Basin and generates
wavy bedforms in this region. In addition, eddies
appear to penetrate to great depths in the Arabian Sea,
and the circulation influences the bottom topography,
causing depressions and rises (Das et al., 1980). There
are strong bottom currents, with speeds approaching
10–20 cm s
−1 , in the Wharton Basin and the southern
Mascarene Basin, and in parts of the Southwest Indian
Basin and Australian–Antarctic Basins, resulting in
minimal sediment deposition (Kennett, 1982; Gage and
Tyler, 1991). The active circulation ensures that the
abyssal bottom waters of the Indian Ocean remain
oxygenated.
There is no evidence for benthic storms (see
Chapter 2) in the deep Indian Ocean although they are
225
and are composed mostly of terrestrial plant material,
phytodetritus, and mineral grains transported by rivers
(Kennett, 1982; Levin and Gage, 1998; Levin et al.,
2000). In the Bay of Bengal, terrigenous sedimentation
from the Ganges is particularly extensive, reaching
depths of 5000 m. In the Arabian Sea, there is an
organic carbon maximum (4.9%) at 400 m, owing apparently to preferential preservation and accumulation
of organic matter under low-oxygen conditions in the
bottom water (Levin et al., 2000). The particle flux to
the deep Bay of Bengal is enhanced by the freshwater
input from the major rivers entering at the north of the
Bay (Ittekkot et al., 1991).
Sediments are at most very thin on the crests
of mid-ocean ridges, and essentially absent on the
ridge axes (Fig. 7.3). Thick siliceous oozes, composed
primarily of radiolarian and diatom tests, occur at
depths of ~5000 m south of the polar front and along
a few mid-ocean ridges, where there is high biological
productivity (Berger, 1974; Kennett, 1982). However,
because of the oligotrophic nature of the equatorial
Indian Ocean, siliceous sediments are rare in low
latitudes of the Indian Ocean compared to the Pacific
Ocean (Kennett, 1982). Red clay is present mostly in
the eastern and southern Indian Ocean, near the equator
and high latitudes. It is composed of fine-grained,
organic-poor sediments resulting from volcanic activity
at ridges (Berger, 1974; Pilipchuk et al., 1977; Kennett,
1982).
Hard substrata in the Indian Ocean consist of basalt
rocks, rock faces, and the surfaces of ferromanganese concretions. The morphology of the central
Indian Ocean Basin is composed of abyssal hills and
seamounts, as well as valleys and abyssal plains.
The topographic highs, which are in the proximity
of three major fracture zones, are composed of hard,
massive basalts occurring at the crests, along the
slopes and on the foothills as talus deposits (Sharma
et al., 1997). Owing to strong geostrophic currents and
consequent scouring of the sediments, the Wharton
Basin, the southern Mascarene Basin, and parts of the
Southwest Indian and Australian–Antarctic Basins have
little or no sediment (Kennett, 1982). Sediment in these
areas, when present, is mostly brown clay. Along the
Southwest Indian Ridge, German et al. (1998) have
identified hydrothermal activity. The associated fauna
inhabiting these hydrothermal regions is dominated by
shrimps and anemones (T. Shank, pers. comm.). In
the southeast and southwest Indian Ocean, and in the
Mozambique Basin, there are extensive pavements of
manganese nodules at depths of about 4000 m (Kolla
et al., 1980). It has been suggested that their presence
at the sediment–water interface is a result of benthic
biological activity and strong bottom currents. Benthic
organisms may nudge the nodules upward, maintaining
them near the sediment–water interface, while strong
currents may limit the deposition of sediment, allowing
the nodules to grow (Berger, 1974; Paul, 1976; Kennett,
1982).
Near-bottom currents
Currents in the deep sea influence sediment deposition
and the organisms that inhabit the seafloor (Nowell and
Jumars, 1984). In regions where there are strong currents, sediment deposition is minimal. These currents
can smother sessile and suspension-feeding organisms
with sediment grains. Where currents are weak, sediment chemistry and biology may be controlled by
diffusive processes. In these environments, suspension
feeders may suffer owing to the inadequate supply
of advected particles (e.g., Jumars and Gallagher,
1982). Thus, near-bottom currents not only may control
sediment deposition, but can also manipulate sediment
chemistry and the structure of the benthic communities
(Nowell and Jumars, 1984).
Both bottom and deep waters of the Indian Ocean
are derived from the Atlantic, and spread throughout
the deep sea by active flow (Wyrtki, 1973). Antarctic
Bottom Water warms as it spreads north, and fills the
deep basins of the central Indian Ocean. Therefore, the
abyssal Indian Ocean is an area of active deep-sea circulation. Antarctic Bottom Water supplies fine-grained
sediments from the Africa–Madagascar source to the
marginal areas of the Mozambique Basin and generates
wavy bedforms in this region. In addition, eddies
appear to penetrate to great depths in the Arabian Sea,
and the circulation influences the bottom topography,
causing depressions and rises (Das et al., 1980). There
are strong bottom currents, with speeds approaching
10–20 cm s
−1 , in the Wharton Basin and the southern
Mascarene Basin, and in parts of the Southwest Indian
Basin and Australian–Antarctic Basins, resulting in
minimal sediment deposition (Kennett, 1982; Gage and
Tyler, 1991). The active circulation ensures that the
abyssal bottom waters of the Indian Ocean remain
oxygenated.
There is no evidence for benthic storms (see
Chapter 2) in the deep Indian Ocean although they are