Box 10.1: Deep Hypersaline Anoxic Basins in Eastern
Mediterranean Sea
Danielle Marty
In the early 1980s, the first deep depressions filled
with NaCl-saturated anoxic brines, known under the
English acronym “DHABs” for “deep hypersaline
anoxic basins,” were discovered in the Eastern Mediterranean Sea. These anoxic basins were usually
named after the names of the oceanographic vessel
that first explored them.
“Tyro” was the first basin discovered in 1983, in
the Strabo Trench, and within the year after “Bannock” (1984), on the southwestern slopes of the Mediterranean Ridge. Ten years or so after, three more
basins were discovered on the eastern part of the
Mediterranean Ridge, “l’Atalante” (1993), “Urania”(1993), and “Discovery” (1993–1994). In 2008, a
novel basin was discovered on the western part of the
Mediterranean Ridge and named “Thetis.”
Some similar deeps were previously observed in
smaller oceanic basins: in the Red Sea, where, among
some 25 deeps found since the late 1960s; “Atlantis II
Deep” discovered in 1968 is the largest (52 km
2
); and in
the Northern Gulf of Mexico, where “Orca Basin,”
discovered in 1975, covers an area of 400 km
2
.
All these basins have a common feature, the presence of evaporitic deposits in the subsurface
sediments at shallow depths; these evaporites are
Jurassic in age (Mesozoic Era) in the Gulf of Mexico
and Messinian in age (Late Miocene in Cenozoic Era)
in the Red Sea and Mediterranean Sea (Cita 2006).
Mediterranean DHABs are extreme deep-sea
biotopes, characterized by extremely high salinity
and corresponding density, elevated hydrostatic
pressure, absence of light and oxygen, and a sharp
chemocline between seawater and brines, some
meters in thickness. These unique physicochemical
characteristics demonstrate that DHABs were physically isolated from other habitats of the planet during thousands of years.
The Detection of DHABs
All DHABs have a similar sonar signature, with a
well-defined seismic reflection at the interface
between normal seawater and high saline brines.
D. Marty
Institut Me ´diterrane ´en d’Oce ´anologie (MIO), UM 110,
CNRS 7294 IRD 235, Universite ´ de Toulon, Aix-Marseille
Universite ´, Campus de Luminy, 13288 Marseille Cedex 9,
France
(continued)
Box 10.1 (continued)
Once the presence of brine accumulation recognized
by sound reflector, it is necessary to sample and verify
the presence of brines, to approve the hypersaline
deep.
Hydrological and Chemical Characteristics of Various
DHABs (Box Table 10.1)
The Formation of Mediterranean DHABs
About 5.96–5.33 million years ago, during the
Messinian (Late Miocene), tectonic processes of convergence between the African and Eurasian plates
caused the occlusion of the ancient Tethys Sea and
interruption of water inflow from the Atlantic Ocean.
The nearly complete desiccation of the Mediterranean,
called “the Messinian salinity crisis,” came along with
a decrease in Mediterranean sea level of about 1.5 km
and massive evaporite deposits on the seafloor: these
salt deposits could be as thick as some hundred meters,
one thousand meters, and even several thousand
meters. These gigantic evaporite deposits underlying
all the Mediterranean Basin, with more than 1 million
km
3 salts covering more than 2 million km
2
, are widely
regarded as one of the biggest evaporitic episodes, if
not the biggest that our planet knew. About 5.3 million
years ago, at the Messinian/Pliocene boundary, the
opening of the Gibraltar Strait allowed drastic refilling
of the Mediterranean by Atlantic waters intrusion, dissolution of the Messinian evaporites, and formation of
brine lakes at the bottom of the deepest depressions.
Location and Description of the DHABs of the
Mediterranean Sea
Mediterranean DHABs represent unique, extreme,
and largely unexplored habitats, lying at more than
3,500 m below sea level, containing very stable brines
entrapped into basins, deeper than the surrounding
seafloor: the brine columns exhibit thickness ranging
from 80 to 500 m and are homogeneous or stratified
into different layers (Box Fig. 10.1). Each of the six
brine lakes, physically isolated from each other
(Box Fig. 10.2), presents environmental peculiarities,
suggesting that the brines are derived from the
dissolution of different levels of the Messinian evaporitic succession; evaporites are essentially halite for
Bannock, Tyro, Urania, and L’Atalante and bischofite
for Discovery.
Tyro. The smallest basin: diameter c.a. 4 km, funnel shaped, with a rounded rim; it lies east of Kretheus
and west of Poseidon; both basins are not more anoxic
and brine filled since about 3,000 years. Tyro shows
(continued)
10 The Extreme Conditions of Life on the Planet and Exobiology
369
Mediterranean Sea
Danielle Marty
In the early 1980s, the first deep depressions filled
with NaCl-saturated anoxic brines, known under the
English acronym “DHABs” for “deep hypersaline
anoxic basins,” were discovered in the Eastern Mediterranean Sea. These anoxic basins were usually
named after the names of the oceanographic vessel
that first explored them.
“Tyro” was the first basin discovered in 1983, in
the Strabo Trench, and within the year after “Bannock” (1984), on the southwestern slopes of the Mediterranean Ridge. Ten years or so after, three more
basins were discovered on the eastern part of the
Mediterranean Ridge, “l’Atalante” (1993), “Urania”(1993), and “Discovery” (1993–1994). In 2008, a
novel basin was discovered on the western part of the
Mediterranean Ridge and named “Thetis.”
Some similar deeps were previously observed in
smaller oceanic basins: in the Red Sea, where, among
some 25 deeps found since the late 1960s; “Atlantis II
Deep” discovered in 1968 is the largest (52 km
2
); and in
the Northern Gulf of Mexico, where “Orca Basin,”
discovered in 1975, covers an area of 400 km
2
.
All these basins have a common feature, the presence of evaporitic deposits in the subsurface
sediments at shallow depths; these evaporites are
Jurassic in age (Mesozoic Era) in the Gulf of Mexico
and Messinian in age (Late Miocene in Cenozoic Era)
in the Red Sea and Mediterranean Sea (Cita 2006).
Mediterranean DHABs are extreme deep-sea
biotopes, characterized by extremely high salinity
and corresponding density, elevated hydrostatic
pressure, absence of light and oxygen, and a sharp
chemocline between seawater and brines, some
meters in thickness. These unique physicochemical
characteristics demonstrate that DHABs were physically isolated from other habitats of the planet during thousands of years.
The Detection of DHABs
All DHABs have a similar sonar signature, with a
well-defined seismic reflection at the interface
between normal seawater and high saline brines.
D. Marty
Institut Me ´diterrane ´en d’Oce ´anologie (MIO), UM 110,
CNRS 7294 IRD 235, Universite ´ de Toulon, Aix-Marseille
Universite ´, Campus de Luminy, 13288 Marseille Cedex 9,
France
(continued)
Box 10.1 (continued)
Once the presence of brine accumulation recognized
by sound reflector, it is necessary to sample and verify
the presence of brines, to approve the hypersaline
deep.
Hydrological and Chemical Characteristics of Various
DHABs (Box Table 10.1)
The Formation of Mediterranean DHABs
About 5.96–5.33 million years ago, during the
Messinian (Late Miocene), tectonic processes of convergence between the African and Eurasian plates
caused the occlusion of the ancient Tethys Sea and
interruption of water inflow from the Atlantic Ocean.
The nearly complete desiccation of the Mediterranean,
called “the Messinian salinity crisis,” came along with
a decrease in Mediterranean sea level of about 1.5 km
and massive evaporite deposits on the seafloor: these
salt deposits could be as thick as some hundred meters,
one thousand meters, and even several thousand
meters. These gigantic evaporite deposits underlying
all the Mediterranean Basin, with more than 1 million
km
3 salts covering more than 2 million km
2
, are widely
regarded as one of the biggest evaporitic episodes, if
not the biggest that our planet knew. About 5.3 million
years ago, at the Messinian/Pliocene boundary, the
opening of the Gibraltar Strait allowed drastic refilling
of the Mediterranean by Atlantic waters intrusion, dissolution of the Messinian evaporites, and formation of
brine lakes at the bottom of the deepest depressions.
Location and Description of the DHABs of the
Mediterranean Sea
Mediterranean DHABs represent unique, extreme,
and largely unexplored habitats, lying at more than
3,500 m below sea level, containing very stable brines
entrapped into basins, deeper than the surrounding
seafloor: the brine columns exhibit thickness ranging
from 80 to 500 m and are homogeneous or stratified
into different layers (Box Fig. 10.1). Each of the six
brine lakes, physically isolated from each other
(Box Fig. 10.2), presents environmental peculiarities,
suggesting that the brines are derived from the
dissolution of different levels of the Messinian evaporitic succession; evaporites are essentially halite for
Bannock, Tyro, Urania, and L’Atalante and bischofite
for Discovery.
Tyro. The smallest basin: diameter c.a. 4 km, funnel shaped, with a rounded rim; it lies east of Kretheus
and west of Poseidon; both basins are not more anoxic
and brine filled since about 3,000 years. Tyro shows
(continued)
10 The Extreme Conditions of Life on the Planet and Exobiology
369