THE PERIPHERAL DEEP SEAS
263
~2500 m, and where the waters below a depth of
250 m are anoxic (Fredj and Laubier, 1985). The
surface properties of the Mediterranean were reviewed
in Volume 26 of this series (Miller, 1983; Ben-Tuvia,
1983).
The Mediterranean Sea has a complicated geological
history, being trapped between Africa and Europe, and
its present deep-sea fauna reflects part of this geologic
history. The Mediterranean was derived from the
Tethys Ocean, which itself was formed when Pangea
split ~200 My ago (Maldonado, 1986). The shape
of the Mediterranean that one recognizes today was
created ~40 My ago, although there remained a deep
connection between the Mediterranean and the Atlantic
Ocean. In the eastern Mediterranean the Hellenic
Trench system southwest of the island of Kriti (Crete)
represents the last fragments of the Tethys Ocean
as it is subducted beneath the European Plate. The
African continental crust is now reaching the Hellenic
Trench (Maldonado, 1986). The youngest basin is
the Pliocene/Quaternary Aegean Sea. In the western
Mediterranean there has been extensional rifting and
new oceanic crust has developed, the oldest oceanic
crust being the Oligocene Balearic Sea.
In the late Miocene (Messinian), the Mediterranean
became separated from the Atlantic by the orogenic
closure of the Strait of Gibraltar, and this started the socalled ‘Messinian Salinity Crisis’. The Mediterranean
was formed from a number of large lakes which
ultimately dried out leaving thick evaporite deposits
(Cita and Ryan, 1973). Estimated drying times have
been as little as 1000 y, and there have been suggestions
this may have occurred up to 17 times (Maldonado,
1986). The latest inundation of the Mediterranean occurred at the beginning of the Pliocene, approximately
5 My ago.
Hydrography
The main surface inflow of Atlantic surface water to
the Mediterranean Sea is through the Strait of Gibraltar
(36 salinity), driven by water-level differences between
the two water bodies. This water flows via two cyclonic
gyres in the Alboran Sea along the North African
coast, with branches feeding cyclonic loops extending
northwards in the western basin (Fig. 9.2a). As the
surface water flows eastwards, excess evaporation over
precipitation increases the salinity to 39. Formation of
intermediate and deep water takes place during winter
along the northern borders of the Mediterranean, in
both basins, under the influence of cold katabatic winds
blowing off the continental landmass to the north.
When cold winds blow down the Rhˆ one valley the
water column in the northern Mediterranean becomes
cold, and up to one-third of the deep water can
be formed in a single event. Levantine Intermediate
Water (LIW) is formed off the island of Rodhos
(Rhodes) (Fig. 9.2a) when winter cooling increases
the density of high-salinity water, and this water sinks
to a depth of ~400 m. Levantine Intermediate Water
extends as a midwater water mass westwards from its
source and bifurcates with a branch flowing both north
and south of Crete (Fig. 9.2b) (Tomczak and Godfrey,
1994).
The northern branch of Levantine Intermediate
Water flows into the Adriatic Sea at intermediate depths
(Fig. 9.2b). Under the influence of winter cooling the
surface and intermediate waters of the Adriatic mix
and form a deep-water mass down to the seabed at
1000 m (Fig. 9.2a). This deep water overflows the
700 m-deep sill at the entrance to the Adriatic Sea
and flows southeastward into the deepest part of the
eastern basin of the Mediterranean Sea. More recently,
there has been evidence that the densest waters of the
eastern Mediterranean are formed in the Aegean Sea
(Klein et al., 1999; Lascaratos et al., 1999) with a
formation rate
1 of 1 Sv, which is three times the rate of
formation in the Adriatic Sea. A new dense water mass
has recently been identified (Fig. 9.2a). It is formed in
the Aegean [called Cretan Sea Overflow Water (Klein
et al., 1999), leading to Cretan Deep Water (Tsimplis
et al., 1999)], is warmer but more saline than that
formed in the Adriatic, and has replaced about 20% of
the deep water in the eastern Mediterranean over the
past decade. This subtle change in the origin of the deep
waters is attributed to the very cold winters of 1987 and
1992–1993, which created favourable conditions for
deep-water formation in the Aegean (Lascaratos et al.,
1999). This regime shift is also believed to be a longterm impact from the building of the High Aswan Dam,
which reduced freshwater inflows from the Nile into
the eastern Mediterranean, leading to increased salinity,
and thus density. Past and present circulation patterns
and deep-water formation in the eastern Mediterranean
have been reviewed in detail by Malanotte-Rizzoli and
Hecht (1988) and Lascaratos et al. (1999).
1 A special unit, the Sverdrup (abbreviation Sv), is used for formation and movement of water masses in the ocean. It is defined as
10 6 m 3 s −1 .
263
~2500 m, and where the waters below a depth of
250 m are anoxic (Fredj and Laubier, 1985). The
surface properties of the Mediterranean were reviewed
in Volume 26 of this series (Miller, 1983; Ben-Tuvia,
1983).
The Mediterranean Sea has a complicated geological
history, being trapped between Africa and Europe, and
its present deep-sea fauna reflects part of this geologic
history. The Mediterranean was derived from the
Tethys Ocean, which itself was formed when Pangea
split ~200 My ago (Maldonado, 1986). The shape
of the Mediterranean that one recognizes today was
created ~40 My ago, although there remained a deep
connection between the Mediterranean and the Atlantic
Ocean. In the eastern Mediterranean the Hellenic
Trench system southwest of the island of Kriti (Crete)
represents the last fragments of the Tethys Ocean
as it is subducted beneath the European Plate. The
African continental crust is now reaching the Hellenic
Trench (Maldonado, 1986). The youngest basin is
the Pliocene/Quaternary Aegean Sea. In the western
Mediterranean there has been extensional rifting and
new oceanic crust has developed, the oldest oceanic
crust being the Oligocene Balearic Sea.
In the late Miocene (Messinian), the Mediterranean
became separated from the Atlantic by the orogenic
closure of the Strait of Gibraltar, and this started the socalled ‘Messinian Salinity Crisis’. The Mediterranean
was formed from a number of large lakes which
ultimately dried out leaving thick evaporite deposits
(Cita and Ryan, 1973). Estimated drying times have
been as little as 1000 y, and there have been suggestions
this may have occurred up to 17 times (Maldonado,
1986). The latest inundation of the Mediterranean occurred at the beginning of the Pliocene, approximately
5 My ago.
Hydrography
The main surface inflow of Atlantic surface water to
the Mediterranean Sea is through the Strait of Gibraltar
(36 salinity), driven by water-level differences between
the two water bodies. This water flows via two cyclonic
gyres in the Alboran Sea along the North African
coast, with branches feeding cyclonic loops extending
northwards in the western basin (Fig. 9.2a). As the
surface water flows eastwards, excess evaporation over
precipitation increases the salinity to 39. Formation of
intermediate and deep water takes place during winter
along the northern borders of the Mediterranean, in
both basins, under the influence of cold katabatic winds
blowing off the continental landmass to the north.
When cold winds blow down the Rhˆ one valley the
water column in the northern Mediterranean becomes
cold, and up to one-third of the deep water can
be formed in a single event. Levantine Intermediate
Water (LIW) is formed off the island of Rodhos
(Rhodes) (Fig. 9.2a) when winter cooling increases
the density of high-salinity water, and this water sinks
to a depth of ~400 m. Levantine Intermediate Water
extends as a midwater water mass westwards from its
source and bifurcates with a branch flowing both north
and south of Crete (Fig. 9.2b) (Tomczak and Godfrey,
1994).
The northern branch of Levantine Intermediate
Water flows into the Adriatic Sea at intermediate depths
(Fig. 9.2b). Under the influence of winter cooling the
surface and intermediate waters of the Adriatic mix
and form a deep-water mass down to the seabed at
1000 m (Fig. 9.2a). This deep water overflows the
700 m-deep sill at the entrance to the Adriatic Sea
and flows southeastward into the deepest part of the
eastern basin of the Mediterranean Sea. More recently,
there has been evidence that the densest waters of the
eastern Mediterranean are formed in the Aegean Sea
(Klein et al., 1999; Lascaratos et al., 1999) with a
formation rate
1 of 1 Sv, which is three times the rate of
formation in the Adriatic Sea. A new dense water mass
has recently been identified (Fig. 9.2a). It is formed in
the Aegean [called Cretan Sea Overflow Water (Klein
et al., 1999), leading to Cretan Deep Water (Tsimplis
et al., 1999)], is warmer but more saline than that
formed in the Adriatic, and has replaced about 20% of
the deep water in the eastern Mediterranean over the
past decade. This subtle change in the origin of the deep
waters is attributed to the very cold winters of 1987 and
1992–1993, which created favourable conditions for
deep-water formation in the Aegean (Lascaratos et al.,
1999). This regime shift is also believed to be a longterm impact from the building of the High Aswan Dam,
which reduced freshwater inflows from the Nile into
the eastern Mediterranean, leading to increased salinity,
and thus density. Past and present circulation patterns
and deep-water formation in the eastern Mediterranean
have been reviewed in detail by Malanotte-Rizzoli and
Hecht (1988) and Lascaratos et al. (1999).
1 A special unit, the Sverdrup (abbreviation Sv), is used for formation and movement of water masses in the ocean. It is defined as
10 6 m 3 s −1 .