Long term data series on Mediterranean Sea temperature, nutrients and hydrology changes
305
3.3 Western Mediterranean
A scenario similar to that proposed for the eastern basin is not acceptable to explain
sapropel deposits in the Alboran Sea. First, the offshore formation of dense water in
the Gulf of Lion is more or less preserved from coastal inputs of freshwater.
Secondly, the uplift of deep water and the outflow current of dense water across the
strait of Gibraltar depends on the density difference between Mediterranean and
Atlantic waters at the sill depth (300m). At present this density difference is high,
about 1.6 density units (instead of about 0.2 at the strait of Sicily), allowing the
uplift of very deep waters up to the sill depth (Bethoux, 1980). As far as evaporation
in the continental Mediterranean is greater than in the Atlantic, a strong decrease
of density difference between Mediterranean and Atlantic waters may only provide
from a change in the Atlantic circulation. In the Alboran Sea, the last sapropel
event occurred between 14.5 and 10.3 Ka (De Kaenel et aI., 1999), at the time of
maximum freshwater input in the North Atlantic during the last deglacial time,
according to the sea level change (Fairbanks (1989). In a previous scenario (Bethoux
and Pierre, 1999), it was assumed the decrease or stoppage of dense water formation
in the Norwegian Sea, due to the freshwater input, and the increase of the volume
of waters from Mediterranean origin in the Atlantic (the behaviour of Mediterranean
water being to be used in dense water formation in the Norwegian Sea). A
consequence of this Mediterranean increasing layer is a strong decrease of the
density difference across the strait of Gibraltar and a possible stagnation of deep
waters inside the western Mediterranean. The interest of this scenario and of the
discovery of about 70 sapropel layers in the Alboran Sea is to evidence iterative
changes in the Atlantic circulation, and consequently in the global climate every
21,000 years, one of the periods of the Earth orbit around the sun. This constitutes
a chronology of global climate events over the past 1.5 million years.
References
Bethoux J-P (1980) Mean water fluxes across sections in the Mediterranean Sea, evaluated on the
basis of water and salt budget and of observed salinities. Oceano!. Acta, 3 :79-88
Bethoux JP, PierreC (1998) Mediterranean functioning and sapropel formation: respective influences
of climate and hydrological changes in the Atlantic and the Mediterranean. Marine Geology,
153:29-39
Bethoux JP, Gentili B, Tailliez D (1998) Warming and freshwater budget change in the
Mediterranean since the 1940s, their possible relation to the greenhouse effect. Geophysical
Research letters, 25: 1023-1026
Bethoux JP, Gentili B (1996) The Mediterranean Sea: coastal and deep-sea signatures of climatic and
environmental changes. 1. Marine Systems, 7:383-3944
Bethoux JP, Morin P, Chaumery C, Connan 0, Gentili B, Ruiz-Pino 0 (1998) Nutrients in the
Mediterranean Sea, mass balance and statistical analysis of concentrations with respect to
environmental change. Marine Chemistry, 63: 155-169
Bethoux JP, Gentili B (1999) Functioning of the Mediterranean Sea: past and present changes related
to freshwater input and climate changes. 1. Marine Systems, 20:33-47
Bethoux JP, Gentili B, Morin P, Nicolas E, Pierre C, Ruiz-Pino D (1999) The Mediterranean Sea: a
305
3.3 Western Mediterranean
A scenario similar to that proposed for the eastern basin is not acceptable to explain
sapropel deposits in the Alboran Sea. First, the offshore formation of dense water in
the Gulf of Lion is more or less preserved from coastal inputs of freshwater.
Secondly, the uplift of deep water and the outflow current of dense water across the
strait of Gibraltar depends on the density difference between Mediterranean and
Atlantic waters at the sill depth (300m). At present this density difference is high,
about 1.6 density units (instead of about 0.2 at the strait of Sicily), allowing the
uplift of very deep waters up to the sill depth (Bethoux, 1980). As far as evaporation
in the continental Mediterranean is greater than in the Atlantic, a strong decrease
of density difference between Mediterranean and Atlantic waters may only provide
from a change in the Atlantic circulation. In the Alboran Sea, the last sapropel
event occurred between 14.5 and 10.3 Ka (De Kaenel et aI., 1999), at the time of
maximum freshwater input in the North Atlantic during the last deglacial time,
according to the sea level change (Fairbanks (1989). In a previous scenario (Bethoux
and Pierre, 1999), it was assumed the decrease or stoppage of dense water formation
in the Norwegian Sea, due to the freshwater input, and the increase of the volume
of waters from Mediterranean origin in the Atlantic (the behaviour of Mediterranean
water being to be used in dense water formation in the Norwegian Sea). A
consequence of this Mediterranean increasing layer is a strong decrease of the
density difference across the strait of Gibraltar and a possible stagnation of deep
waters inside the western Mediterranean. The interest of this scenario and of the
discovery of about 70 sapropel layers in the Alboran Sea is to evidence iterative
changes in the Atlantic circulation, and consequently in the global climate every
21,000 years, one of the periods of the Earth orbit around the sun. This constitutes
a chronology of global climate events over the past 1.5 million years.
References
Bethoux J-P (1980) Mean water fluxes across sections in the Mediterranean Sea, evaluated on the
basis of water and salt budget and of observed salinities. Oceano!. Acta, 3 :79-88
Bethoux JP, PierreC (1998) Mediterranean functioning and sapropel formation: respective influences
of climate and hydrological changes in the Atlantic and the Mediterranean. Marine Geology,
153:29-39
Bethoux JP, Gentili B, Tailliez D (1998) Warming and freshwater budget change in the
Mediterranean since the 1940s, their possible relation to the greenhouse effect. Geophysical
Research letters, 25: 1023-1026
Bethoux JP, Gentili B (1996) The Mediterranean Sea: coastal and deep-sea signatures of climatic and
environmental changes. 1. Marine Systems, 7:383-3944
Bethoux JP, Morin P, Chaumery C, Connan 0, Gentili B, Ruiz-Pino 0 (1998) Nutrients in the
Mediterranean Sea, mass balance and statistical analysis of concentrations with respect to
environmental change. Marine Chemistry, 63: 155-169
Bethoux JP, Gentili B (1999) Functioning of the Mediterranean Sea: past and present changes related
to freshwater input and climate changes. 1. Marine Systems, 20:33-47
Bethoux JP, Gentili B, Morin P, Nicolas E, Pierre C, Ruiz-Pino D (1999) The Mediterranean Sea: a