Long Term Data Series on Mediterranean Sea
Temperature, Nutrients and Hydrology Changes
J.P. Bethoux
1 Introduction
The Mediterranean is a deep semi enclosed sea, under a continental climate. Its
surface water balance is negative (evaporation losses being greater than
precipitation) and water dynamics are controlled by winter dense water formation
and by two shallows silJs at Gibraltar and Sicily Straits. Consequently, the AlgeroProven<;al Basin (Western Mediterranean) is filled from 500m depth to the bottom,
at more than 2800m, by homogeneous deep water. Its physical characteristics
(temperature, salinity) are linked to the heat and water exchanges with the
atmosphere and the Atlantic Ocean. Heat exchange with the Atlantic being smalJ,
the temperature depends mainly from the heat exchanges through its surface, i.e.,
from the local climate coupled with the northern hemisphere atmospheric
circulation. Salinity depends on exchanges with the Atlantic and on evaporation,
precipitation and river runoff. Similarly, the geochemical state of the Mediterranean
Sea depends on exchanges with the Atlantic and on atmospheric and terrestrial
supplies which are linked to natural and mostly anthropic inputs, issuing from more
than 200 millions' inhabitants who live on the watershed. Unlike the open ocean,
for which the deep water response time to perturbations is of the order of 1000 years,
the residence time of the western deep water is about 15 years, as a result of
intensive horizontal and vertical circulation. Consequently, the Mediterranean
response to climatic or environmental changing trends is perceptible in a few years.
Analysis of marine data acquired in the deep waters of the Algero-Proven<;al basin
shows increases of temperature, salinity, nitrate and phosphate since the early
sixties. Such evolutions are linked to recent changes in climatic and environmental
forcing, as resulting from man's acti vities. More, in the sediment of the eastern and
western basins, there are numerous layers of sapropels, i.e., layers of some
centimetre thick containing organic carbon and proving a sedimentation process
with anoxic deep layer. This anoxia is a proof of change in the deep layer
oxygenation and circulation linked either to change in climate over the eastern basin
(the cause of eastern sapropels) or change in the Atlantic circulation off the strait of
Gibraltar (the probable cause of western sapropels). These marine events are useful
to quantify changes occurred either over the Mediterranean watershed (climate and
environment) or in the Atlantic ocean (past iterative changes in the global
circulation and paleoclimate) over the last 2 million years.
Temperature, Nutrients and Hydrology Changes
J.P. Bethoux
1 Introduction
The Mediterranean is a deep semi enclosed sea, under a continental climate. Its
surface water balance is negative (evaporation losses being greater than
precipitation) and water dynamics are controlled by winter dense water formation
and by two shallows silJs at Gibraltar and Sicily Straits. Consequently, the AlgeroProven<;al Basin (Western Mediterranean) is filled from 500m depth to the bottom,
at more than 2800m, by homogeneous deep water. Its physical characteristics
(temperature, salinity) are linked to the heat and water exchanges with the
atmosphere and the Atlantic Ocean. Heat exchange with the Atlantic being smalJ,
the temperature depends mainly from the heat exchanges through its surface, i.e.,
from the local climate coupled with the northern hemisphere atmospheric
circulation. Salinity depends on exchanges with the Atlantic and on evaporation,
precipitation and river runoff. Similarly, the geochemical state of the Mediterranean
Sea depends on exchanges with the Atlantic and on atmospheric and terrestrial
supplies which are linked to natural and mostly anthropic inputs, issuing from more
than 200 millions' inhabitants who live on the watershed. Unlike the open ocean,
for which the deep water response time to perturbations is of the order of 1000 years,
the residence time of the western deep water is about 15 years, as a result of
intensive horizontal and vertical circulation. Consequently, the Mediterranean
response to climatic or environmental changing trends is perceptible in a few years.
Analysis of marine data acquired in the deep waters of the Algero-Proven<;al basin
shows increases of temperature, salinity, nitrate and phosphate since the early
sixties. Such evolutions are linked to recent changes in climatic and environmental
forcing, as resulting from man's acti vities. More, in the sediment of the eastern and
western basins, there are numerous layers of sapropels, i.e., layers of some
centimetre thick containing organic carbon and proving a sedimentation process
with anoxic deep layer. This anoxia is a proof of change in the deep layer
oxygenation and circulation linked either to change in climate over the eastern basin
(the cause of eastern sapropels) or change in the Atlantic circulation off the strait of
Gibraltar (the probable cause of western sapropels). These marine events are useful
to quantify changes occurred either over the Mediterranean watershed (climate and
environment) or in the Atlantic ocean (past iterative changes in the global
circulation and paleoclimate) over the last 2 million years.