Guide The Mediterranean Sea: Its history and present challenges

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  2. Future Trends of Mediterranean Biodiversity
  3. The Mediterranean Sea : Its history and present challenges (eBook, ) []

These cool and relatively low-salinity waters circulate westwards along the North African coasts. A part of these surface waters does not pass the Strait of Sicily, but deviates towards Corsica before exiting the Mediterranean. The surface waters entering the eastern Mediterranean basin circulate along the Libyan and Israelian coasts. Upon reaching the Levantine Sea , the surface waters having experienced warming and saltening from their initial Atlantic state, are now more dense and deepen to form the Levantine Intermediate Waters LIW. Most of the water found anywhere between 50 and m deep in the Mediterranean originates from the LIW.

LIW are the only waters passing the Sicily Strait eastwards. After the Strait of Sicily, the intermediate waters circulate along the Italian, French and Spanish coasts before exiting the Mediterranean through the depths of the Strait of Gibraltar. Deep water in the Mediterranean originates from three main areas: the Adriatic Sea , from which most of the deep water in the eastern Mediterranean originates, the Aegean Sea , and the Gulf of Lion.

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Deep water formation in the Mediterranean is triggered by strong winter convection fueled by intense cold winds like the Bora. When new deep water is formed, the older waters mix with the overlaying intermediate waters and eventually exit the Mediterranean. The residence time of water in the Mediterranean is approximately years, making the Mediterranean especially sensitive to climate change.

Being a semi-enclosed basin, the Mediterranean experiences transitory events that can affect the water circulation on short time scales. In the mid s, the Aegean Sea became the main area for deep water formation in the eastern Mediterranean after particularly cold winter conditions. This transitory switch in the origin of deep waters in the eastern Mediterranean was termed Eastern Mediterranean Transient EMT and had major consequences on water circulation of the Mediterranean. Another example of a transient event affecting the Mediterranean circulation is the periodic inversion of the North Ionian Gyre, which is an anticyclonic ocean gyre observed in the northern part of the Ionian Sea , off the Greek coast.

The transition from anticylonic to cyclonic rotation of this gyre changes the origin of the waters fueling it; when the circulation is anticyclonic most common , the waters of the gyre originate from the Adriatic Sea. When the circulation is cyclonic, the waters originate from the Levantine Sea. These waters have different physical and chemical characteristics, and the periodic inversion of the North Ionian Gyre called Bimodal Oscillating System or BiOS changes the Mediterranean circulation and biogeochemistry around the Adriatic and Levantine regions.

Because of the short residence time of waters, the Mediterranean Sea is considered a hot-spot for climate change effects. The decrease in precipitation over the region could lead to more evaporation ultimately increasing the Mediterranean Sea salinity. In spite of its great biodiversity , concentrations of chlorophyll and nutrients in the Mediterranean Sea are very low, making it one of the most oligotrophic ocean regions in the world.

The Mediterranean Sea fits the definition of a desert as it has low precipitation and its nutrient contents are low, making it difficult for plants and animals to develop. There are steep gradients in nutrient concentrations, chlorophyll concentrations and primary productivity in the Mediterranean. Nutrient concentrations in the western part of the basin are about double the concentrations in the eastern basin.

The Alboran Sea , close to the Strait of Gibraltar , has a daily primary productivity of about 0.

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The productive areas of the Mediterranean Sea are few and small. High i. The Gulf of Lion has a relatively high productivity because it is an area of high vertical mixing, bringing nutrients to the surface waters that can be used by phytoplankton to produce Chlorophyll a. Primary productivity in the Mediterranean is also marked by an intense seasonal variability. In winter, the strong winds and precipitation [ Contradicts statement above that precipitation is low ] over the basin generate vertical mixing , bringing nutrients from the deep waters to the surface, where phytoplankton can convert it into biomass.

Between March and April, spring offers the ideal trade-off between light intensity and nutrient concentrations in surface for a spring bloom to occur. In summer, high atmospheric temperatures lead to the warming of the surface waters. The resulting density difference virtually isolates the surface waters from the rest of the water column and nutrient exchanges are limited.

As a consequence, primary productivity is very low between June and October. Oceanographic expeditions uncovered a characteristic feature of the Mediterranean Sea biogeochemistry: most of the chlorophyll production does not occur on the surface, but in sub-surface waters between 80 and meters deep. Redfield demonstrated that most of the world's oceans have an average N:P ratio around However, the Mediterranean Sea has an average N:P between 24 and 29, which translates a widespread phosphorus limitation.

Because of its low productivity, plankton assemblages in the Mediterranean Sea are dominated by small organisms such as picophytoplankton and bacteria. The geologic history of the Mediterranean Sea is complex.

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Underlain by oceanic crust , the sea basin was once thought to be a tectonic remnant of the ancient Tethys Ocean ; it is now known to be a structurally younger basin, called the Neotethys , which was first formed by the convergence of the African and Eurasian plates during the Late Triassic and Early Jurassic. Because it is a near-landlocked body of water in a normally dry climate, the Mediterranean is subject to intensive evaporation and the precipitation of evaporites.

The Messinian salinity crisis started about six million years ago mya when the Mediterranean became landlocked, and then essentially dried up. There are salt deposits accumulated on the bottom of the basin of more than a million cubic kilometres—in some places more than three kilometres thick. Scientists estimate that the sea was last filled about 5. Water poured in from the Atlantic Ocean through a newly breached gateway now called the Strait of Gibraltar at an estimated rate of about three orders of magnitude one thousand times larger than the current flow of the Amazon River.

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Coastal areas have submarine karst springs or vrulja s, which discharge pressurised groundwater into the water from below the surface; the discharge water is usually fresh, and sometimes may be thermal. The Mediterranean basin and sea system was established by the ancient African-Arabian continent colliding with the Eurasian continent. As Africa-Arabia drifted northward, it closed over the ancient Tethys Ocean which had earlier separated the two supercontinents Laurasia and Gondwana. At about that time in the middle Jurassic period roughly million years ago [ dubious — discuss ] a much smaller sea basin, dubbed the Neotethys , was formed shortly before the Tethys Ocean closed at its western Arabian end.

The broad line of collisions pushed up a very long system of mountains from the Pyrenees in Spain to the Zagros Mountains in Iran in an episode of mountain-building tectonics known as the Alpine orogeny. The Neotethys grew larger during the episodes of collisions and associated foldings and subductions that occurred during the Oligocene and Miocene epochs 34 to 5. Accordingly, the Mediterranean basin consists of several stretched tectonic plates in subduction which are the foundation of the eastern part of the Mediterranean Sea. Various zones of subduction contain the highest oceanic ridges, east of the Ionian Sea and south of the Aegean.

During Mesozoic and Cenozoic times, as the northwest corner of Africa converged on Iberia, it lifted the Betic-Rif mountain belts across southern Iberia and northwest Africa. There the development of the intramontane Betic and Rif basins created two roughly-parallel marine gateways between the Atlantic Ocean and the Mediterranean Sea.

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Dubbed the Betic and Rifian corridors , they gradually closed during the middle and late Miocene: perhaps several times. The start of the MSC was recently estimated astronomically at 5. After the initial drawdown [ clarification needed ] and re-flooding, there followed more episodes—the total number is debated—of sea drawdowns and re-floodings for the duration of the MSC.

It ended when the Atlantic Ocean last re-flooded the basin—creating the Strait of Gibraltar and causing the Zanclean flood —at the end of the Miocene 5. Some research has suggested that a desiccation-flooding-desiccation cycle may have repeated several times, which could explain several events of large amounts of salt deposition.

As mentioned, there were two earlier gateways: the Betic Corridor across southern Spain and the Rifian Corridor across northern Morocco. The Betic closed about 6 mya, causing the Messinian salinity crisis MSC ; the Rifian or possibly both gateways closed during the earlier Tortonian times, causing a " Tortonian salinity crisis " from Both "crises" resulted in broad connections between the mainlands of Africa and Europe, which allowed migrations of flora and fauna—especially large mammals including primates—between the two continents.

The Vallesian crisis indicates a typical extinction and replacement of mammal species in Europe during Tortonian times following climatic upheaval and overland migrations of new species: [85] see Animation: Messinian salinity crisis and mammal migrations , at right. The almost complete enclosure of the Mediterranean basin has enabled the oceanic gateways to dominate seawater circulation and the environmental evolution of the sea and basin. Circulation patterns are also affected by several other factors—including climate, bathymetry, and water chemistry and temperature—which are interactive and can induce precipitation of evaporites.

Deposits of evaporites accumulated earlier in the nearby Carpathian foredeep during the Middle Miocene , and the adjacent Red Sea Basin during the Late Miocene , and in the whole Mediterranean basin during the MSC and the Messinian age. Many diatomites are found underneath the evaporite deposits, suggesting a connection between their [ clarification needed ] formations. Today, evaporation of surface seawater output is more than the supply input of fresh water by precipitation and coastal drainage systems, causing the salinity of the Mediterranean to be much higher than that of the Atlantic—so much so that the saltier Mediterranean waters sink below the waters incoming from the Atlantic, causing a two-layer flow across the Strait of Gibraltar: that is, an outflow submarine current of warm saline Mediterranean water, counterbalanced by an inflow surface current of less saline cold oceanic water from the Atlantic.

The underlying energy grid was also intended to support a political union between Europe and, at least, the Maghreb part of Africa compare Eurafrika for the later impact and Desertec for a later project with some parallels in the planned grid. The end of the Miocene also marked a change in the climate of the Mediterranean basin. Fossil evidence from that period reveals that the larger basin had a humid subtropical climate with rainfall in the summer supporting laurel forests. The shift to a "Mediterranean climate" occurred largely within the last three million years the late Pliocene epoch as summer rainfall decreased.

The subtropical laurel forests retreated; and even as they persisted on the islands of Macaronesia off the Atlantic coast of Iberia and North Africa, the present Mediterranean vegetation evolved, dominated by coniferous trees and sclerophyllous trees and shrubs with small, hard, waxy leaves that prevent moisture loss in the dry summers.

Much of these forests and shrublands have been altered beyond recognition by thousands of years of human habitation.

There are now very few relatively intact natural areas in what was once a heavily wooded region. Because of its latitude and its land-locked position, the Mediterranean is especially sensitive to astronomically induced climatic variations, which are well documented in its sedimentary record. Since the Mediterranean is subject to the deposition of eolian dust from the Sahara during dry periods, whereas riverine detrital input prevails during wet ones, the Mediterranean marine sapropel -bearing sequences provide high-resolution climatic information.

These data have been employed in reconstructing astronomically calibrated time scales for the last 9 Ma of the Earth's history, helping to constrain the time of past geomagnetic reversals. Unlike the vast multidirectional ocean currents in open oceans within their respective oceanic zones ; biodiversity in the Mediterranean Sea is that of a stable one due to the subtle but strong locked nature of currents which affects favorably, even the smallest macroscopic type of volcanic life form. The stable marine ecosystem of the Mediterranean Sea and sea temperature provides a nourishing environment for life in the deep sea to flourish while assuring a balanced aquatic ecosystem excluded from any external deep oceanic factors.

It is estimated that there are more than 17, marine species in the Mediterranean Sea with generally higher marine biodiversity in coastal areas , continental shelves , and decreases with depth [88].

The Mediterranean Sea : Its history and present challenges (eBook, ) []

As a result of the drying of the sea during the Messinian salinity crisis , [89] the marine biota of the Mediterranean are derived primarily from the Atlantic Ocean. The North Atlantic is considerably colder and more nutrient-rich than the Mediterranean, and the marine life of the Mediterranean has had to adapt to its differing conditions in the five million years since the basin was reflooded.

The Alboran Sea is a transition zone between the two seas, containing a mix of Mediterranean and Atlantic species. The Alboran Sea has the largest population of bottlenose dolphins in the Western Mediterranean, is home to the last population of harbour porpoises in the Mediterranean, and is the most important feeding grounds for loggerhead sea turtles in Europe. The Alboran Sea also hosts important commercial fisheries, including sardines and swordfish.

The Mediterranean monk seals live in the Aegean Sea in Greece. In , the World Wildlife Fund raised concerns about the widespread drift net fishing endangering populations of dolphins, turtles, and other marine animals such as the spiny squat lobster. There was a resident population of killer whale in the Mediterranean until the s, when they went extinct, probably due to long term PCB exposure. There are still annual sightings of killer whale vagrants. For 4, years, human activity has transformed most parts of Mediterranean Europe, and the "humanisation of the landscape" overlapped with the appearance of the present Mediterranean climate.

Based on a broad variety of methods, e. The historical evolution of climate, vegetation and landscape in southern Europe from prehistoric times to the present is much more complex and underwent various changes. For example, some of the deforestation had already taken place before the Roman age. While in the Roman age large enterprises such as the latifundia took effective care of forests and agriculture, the largest depopulation effects came with the end of the empire.

Some [ who? Also, the climate has usually been unstable and there is evidence of various ancient and modern " Little Ice Ages ", [92] and plant cover accommodated to various extremes and became resilient to various patterns of human activity. Human activity was therefore not the cause of climate change but followed it.