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J.P. Bethoux
Danube rivers) or bay bordered by a great city, but the changes in the nutrient ratios
of the western basin are signature that the human impact concerns a whole deep sea.
3 Sediment Signatures of Past Changes in Climate and
Circulation in the Mediterranean and in the North Atlantic
3.1 Organic Rich Layers (Sapropels)
Multiple layers of organic rich sediments, the sapropels, were discovered, first in the
eastern Mediterranean, in 1947-48 (Kullenberg, 1952), then more recently in the
Alboran Sea in 1996 (De Kaene1 et al., 1999). Preservation of organic mater in the
sediment is explained by anoxia in deep water, acting over several thousand years,
with two possible causes: an increase of new production or a change in deep water
ventilation. In the present circulation scheme, overconsumption of the oxygen of
deep waters by mineralisation of organic mater would require an increase by 4 to
5 times of the present new production, and a similar increase of phosphate input
over a long time. We cannot imagine such an increase of a natural input from land
weathering. The stoppage of deep water ventilation appears as the probable reason
of anoxia in deep waters, with different causes in western and eastern
Mediterranean (Bethoux and Pierre, 1999).
3.2 Eastern Mediterranean
In the eastern Mediterranean there are different areas for dense water formation, two
of them in more or less coastal area: the Adriatic and Aegean Seas, generally
providing the densest deep waters, the last one in the high sea waters of the
Levantine basin forming the Levantine intermediate waters. The stoppage of dense
water formation in the Adriatic and Aegean Seas, due to an increase of freshwater
input (by precipitation and river runoff) may affect the ventilation of eastern deep
water, at depth greater than about 600m. More, the water dynamic across the strait
of Sicily may be slowed down by a change in the water deficit between the eastern
and western basin (e.g., an increase offreshwater input, greater in the eastern than
in the western basin) up to limit the deep outflow to the intermediate water situated
at a depth lower than the sill (at about 400m depth). Consequently, anoxia may
spread over the whole basin, at depth greater than 500 to 800m depth, the reason is
the local change in freshwater budget, more or less enhanced by the Nile river
spates, referring to changes in the tropical areas. Consequently, the different
sapropel layers are signatures of a wetter climate over the Mediterranean and
probably over the tropical area.
J.P. Bethoux
Danube rivers) or bay bordered by a great city, but the changes in the nutrient ratios
of the western basin are signature that the human impact concerns a whole deep sea.
3 Sediment Signatures of Past Changes in Climate and
Circulation in the Mediterranean and in the North Atlantic
3.1 Organic Rich Layers (Sapropels)
Multiple layers of organic rich sediments, the sapropels, were discovered, first in the
eastern Mediterranean, in 1947-48 (Kullenberg, 1952), then more recently in the
Alboran Sea in 1996 (De Kaene1 et al., 1999). Preservation of organic mater in the
sediment is explained by anoxia in deep water, acting over several thousand years,
with two possible causes: an increase of new production or a change in deep water
ventilation. In the present circulation scheme, overconsumption of the oxygen of
deep waters by mineralisation of organic mater would require an increase by 4 to
5 times of the present new production, and a similar increase of phosphate input
over a long time. We cannot imagine such an increase of a natural input from land
weathering. The stoppage of deep water ventilation appears as the probable reason
of anoxia in deep waters, with different causes in western and eastern
Mediterranean (Bethoux and Pierre, 1999).
3.2 Eastern Mediterranean
In the eastern Mediterranean there are different areas for dense water formation, two
of them in more or less coastal area: the Adriatic and Aegean Seas, generally
providing the densest deep waters, the last one in the high sea waters of the
Levantine basin forming the Levantine intermediate waters. The stoppage of dense
water formation in the Adriatic and Aegean Seas, due to an increase of freshwater
input (by precipitation and river runoff) may affect the ventilation of eastern deep
water, at depth greater than about 600m. More, the water dynamic across the strait
of Sicily may be slowed down by a change in the water deficit between the eastern
and western basin (e.g., an increase offreshwater input, greater in the eastern than
in the western basin) up to limit the deep outflow to the intermediate water situated
at a depth lower than the sill (at about 400m depth). Consequently, anoxia may
spread over the whole basin, at depth greater than 500 to 800m depth, the reason is
the local change in freshwater budget, more or less enhanced by the Nile river
spates, referring to changes in the tropical areas. Consequently, the different
sapropel layers are signatures of a wetter climate over the Mediterranean and
probably over the tropical area.