268
Paul A. TYLER
water circulation shows southward flow at depths of
~250 m and >1000 m, with northward flow at ~500 m.
Deep water is formed in the northern Red Sea,
especially in the Gulf of Suez, although the Gulf of
Aqaba makes a contribution. This new deep water is
immediately injected under the pycnocline, at a rate
of 0.11±0.02 Sverdrup, forming a southward-flowing
current (Cember, 1988). Deep-water residence time is
about 36 y. Flow of the dense brine water in the deeps
of the Red Sea is driven by local topography with
the temperature decreasing away from the brine source
(Karbe, 1987).
Compared to the surface and intermediate waters
of the Red Sea, the brines of the deep waters are
enriched in manganese, iron, zinc, cadmium and
copper, but depleted in magnesium, iodine, sulphate
and nitrate. Concentrations of hydrogen sulphide and
carbon dioxide differ between the various deep basins.
As a broad rule, the high-density discontinuity between
the brines and the overlying waters has prevented
convective mixing. In the intervening years since their
discovery in the early 1960s there has been little
change in the salinity and temperature of most of
the deeps (23.5 to 44.6ºC and 144 to 270 salinity)
with the exception of the Valdivia Deep (21º20.5
N,
37º57
E), where the temperature has increased by
4.1ºC and the salinity by 10. This change is attributed
to variation in the brine discharge (Anschutz et al.,
1999). Only the Atlantis II Deep has been surveyed
on a regular basis. Recent observations (Blanc and
Anschutz, 1995) suggest that the two-layer stratification
recorded originally has been supplemented by two
additional upper convective layers, possibly as a result
of the rate of heat input remaining constant (Anschutz
and Blanc, 1996). From these data Blanc and Anschutz
(1995) estimate that the hydrothermal discharge is in
the order of 200 ° s
−1 .
Deep-water sediments
The majority of sediments of the Red Sea are
poorly-sorted biogenous carbonates in the form of
foraminiferal, pteropod and heteropod oozes (Berger,
1978). Coccolithophorids also contribute significantly
to the sediment. Siliceous oozes are less common. The
inorganic fraction consists of quartz, feldspar and mica
derived from wind-blown particles from the deserts of
Arabia and Africa. The dominant size range of the
particles is in the <63 mm fraction, and the organicmatter content is <1%. Sediments in the very deep
parts of the Red Sea have a hydrothermal origin and
arise from the deep-water brines (Karbe, 1987). Within
the deeper parts of the Red Sea the sediments are marls
with a strong hydrothermal imprint giving rise to a
multi-coloured layered structure. Oxidized sediments
contain limonite, haematite and manganite, whilst those
under reducing conditions contain pyrite and chalcopyrite, minerals found at hydrothermal vents forming
chimneys. Other deposits contain ferrous sulphide,
sphalerite, iron montmorillonite and manganosiderite.
The sediment sequences vary greatly both within and
between deeps (Karbe, 1987). Monin et al. (1981)
have presented visual observations of the sea bed in
the deeper parts of the Red Sea. These observations
reveal a mosaic of rock outcrops with numerous basalt
fragments dusted with sediment, step-like scarps of
~4 m amplitude, as well as decimetre-scale ripples
believed to represent the brine surface.
Surface production and vertical flux
Throughout the Red Sea surface primary production
is low, mainly owing to the low levels of inorganic
nutrients (see Table 9.2). Vertical flux of organic
matter is in the form of a ‘ladder of migrations’
(Weikert, 1982)
2 but the transport of organic matter
below 1100 m depth by sinking is very limited owing
to the rapid decay of organic matter in the high
water temperatures. For the Atlantis II Deep, Weikert
(1982) gave values for particulate organic matter of
<100 mg °
−1 below 750 m and even less at greater
depths; but he was unsure of the reliability of his data.
Caribbean Sea
Morphology
The Caribbean Sea, underlain by oceanic crust
(Matthews and Holcombe, 1985), consists of the
eastern and western basins, separated by the Jamaica
Rise at depths of 1500 m (Fig. 9.4). The eastern basin,
with a maximum depth of 5400 m, is divided into
the Venezuela Basin and the Colombia Basin by the
Beata Ridge, although the Aruba Gap connects the
Venezuela and Colombia Basins at a depth of 4078 m.
The Venezuela Basin is separated from the smaller
Grenada Basin to the east by the Aves Ridge and has the
Muertos Trough along its northern side (Matthews and
2 The ‘ladder of migrations’ is a term used for the overlapping depth distribution (including vertical migration) of different species of
zooplankton. This ‘ladder’ allows the rapid removal of organic particulates into deep water.
Paul A. TYLER
water circulation shows southward flow at depths of
~250 m and >1000 m, with northward flow at ~500 m.
Deep water is formed in the northern Red Sea,
especially in the Gulf of Suez, although the Gulf of
Aqaba makes a contribution. This new deep water is
immediately injected under the pycnocline, at a rate
of 0.11±0.02 Sverdrup, forming a southward-flowing
current (Cember, 1988). Deep-water residence time is
about 36 y. Flow of the dense brine water in the deeps
of the Red Sea is driven by local topography with
the temperature decreasing away from the brine source
(Karbe, 1987).
Compared to the surface and intermediate waters
of the Red Sea, the brines of the deep waters are
enriched in manganese, iron, zinc, cadmium and
copper, but depleted in magnesium, iodine, sulphate
and nitrate. Concentrations of hydrogen sulphide and
carbon dioxide differ between the various deep basins.
As a broad rule, the high-density discontinuity between
the brines and the overlying waters has prevented
convective mixing. In the intervening years since their
discovery in the early 1960s there has been little
change in the salinity and temperature of most of
the deeps (23.5 to 44.6ºC and 144 to 270 salinity)
with the exception of the Valdivia Deep (21º20.5
N,
37º57
E), where the temperature has increased by
4.1ºC and the salinity by 10. This change is attributed
to variation in the brine discharge (Anschutz et al.,
1999). Only the Atlantis II Deep has been surveyed
on a regular basis. Recent observations (Blanc and
Anschutz, 1995) suggest that the two-layer stratification
recorded originally has been supplemented by two
additional upper convective layers, possibly as a result
of the rate of heat input remaining constant (Anschutz
and Blanc, 1996). From these data Blanc and Anschutz
(1995) estimate that the hydrothermal discharge is in
the order of 200 ° s
−1 .
Deep-water sediments
The majority of sediments of the Red Sea are
poorly-sorted biogenous carbonates in the form of
foraminiferal, pteropod and heteropod oozes (Berger,
1978). Coccolithophorids also contribute significantly
to the sediment. Siliceous oozes are less common. The
inorganic fraction consists of quartz, feldspar and mica
derived from wind-blown particles from the deserts of
Arabia and Africa. The dominant size range of the
particles is in the <63 mm fraction, and the organicmatter content is <1%. Sediments in the very deep
parts of the Red Sea have a hydrothermal origin and
arise from the deep-water brines (Karbe, 1987). Within
the deeper parts of the Red Sea the sediments are marls
with a strong hydrothermal imprint giving rise to a
multi-coloured layered structure. Oxidized sediments
contain limonite, haematite and manganite, whilst those
under reducing conditions contain pyrite and chalcopyrite, minerals found at hydrothermal vents forming
chimneys. Other deposits contain ferrous sulphide,
sphalerite, iron montmorillonite and manganosiderite.
The sediment sequences vary greatly both within and
between deeps (Karbe, 1987). Monin et al. (1981)
have presented visual observations of the sea bed in
the deeper parts of the Red Sea. These observations
reveal a mosaic of rock outcrops with numerous basalt
fragments dusted with sediment, step-like scarps of
~4 m amplitude, as well as decimetre-scale ripples
believed to represent the brine surface.
Surface production and vertical flux
Throughout the Red Sea surface primary production
is low, mainly owing to the low levels of inorganic
nutrients (see Table 9.2). Vertical flux of organic
matter is in the form of a ‘ladder of migrations’
(Weikert, 1982)
2 but the transport of organic matter
below 1100 m depth by sinking is very limited owing
to the rapid decay of organic matter in the high
water temperatures. For the Atlantis II Deep, Weikert
(1982) gave values for particulate organic matter of
<100 mg °
−1 below 750 m and even less at greater
depths; but he was unsure of the reliability of his data.
Caribbean Sea
Morphology
The Caribbean Sea, underlain by oceanic crust
(Matthews and Holcombe, 1985), consists of the
eastern and western basins, separated by the Jamaica
Rise at depths of 1500 m (Fig. 9.4). The eastern basin,
with a maximum depth of 5400 m, is divided into
the Venezuela Basin and the Colombia Basin by the
Beata Ridge, although the Aruba Gap connects the
Venezuela and Colombia Basins at a depth of 4078 m.
The Venezuela Basin is separated from the smaller
Grenada Basin to the east by the Aves Ridge and has the
Muertos Trough along its northern side (Matthews and
2 The ‘ladder of migrations’ is a term used for the overlapping depth distribution (including vertical migration) of different species of
zooplankton. This ‘ladder’ allows the rapid removal of organic particulates into deep water.