Messinian salinity crisis
The Messinian Salinity Crisis (Italian: Crisi di salinità del Messiniano ) is the name for a section of the Earth's history in which the Mediterranean was partially or completely dried up. Here encamped in the deepest ocean basin to three kilometers mighty evaporation rocks ( evaporites ) from.
This happened in the period before about six million years ago to about five million years ago at the end of Messin, the last stage of the Miocene.
- 2.1 The deposition cycles
- 2.2 Chronology
- 2.3 Effects
- 2.4 Future Development
Discovery history
As early as 1833 the British geologist Charles Lyell was noticed in different fossil sites in Italy a striking Faunenschnitt, where many creatures that had previously inhabited the Mediterranean, disappeared and were replaced by other organisms. From the latter should then largely be seen today's fauna. This major event Lyell established the border between the geological epochs of the Miocene and the Pliocene.
First indications
In the plane of Valence in Southern France was discovered in the late 19th century during the placement of a drinking water well hidden beneath Quaternary gravels gorge that was inexplicably cut deep into the crystalline basement. Later, it was possible to detect this canyon in the entire valley of the Rhone between Lyon and the Camargue, where she was filled with marine sediments of the Pliocene. Some French and Italian paleontologist even then subjected to a temporary drying up of the Mediterranean into consideration in order to explain this phenomenon. At the beginning of the 20th century such ideas were common, but were regarded as highly speculative. So used the science fiction author HG Wells, who had studied in his youth, among others, Vincent Illing Geology in London, the idea in his short story The Grisly Folk.
1958 disclosed seismic measurements of the North American oceanographers Brackett Hersey a previously unknown geological structure, which always was about 100 to 200 meters below the bottom of the Mediterranean. Since this surface, the so-called "M- reflector ", today's profile of the seabed followed closely, it was obvious that it was a hard rock layer that had been deposited at a certain time uniformly and coherently throughout the Mediterranean. In addition to the seismic profiles were encountered structures, reminiscent of salt domes that rise from the depths and penetrated the overlying sediments pierced. Many geologists suspected at the time that the salt from the time of the Permian or Triassic would have to come, because during this geological era rich salt deposits were more than 200 million years ago in many places of the world emerged, including also those of the Zechstein series in Central Europe. Since the known Permian and Triassic salt deposits, however, in a relatively flat Epikontinentalmeer, ie had formed on continental crust and not in a deep ocean basins, the newly discovered structures were considered as evidence that the Mediterranean basin had eventually sunk during 200 million years after the Triassic.
Few geologists have speculated whether these salt domes could not have come about but at the same time with the small scattered evaporite deposits, which are exposed for example in the city of Messina in Sicily ( and who have given the Messin - stage her name ). More salt - and gypsum- bearing formations of this age were found in Piedmont, Tuscany, Calabria and in Spain, Morocco, Algeria, Tunisia, Greece, Turkey, Cyprus and Israel.
The discovery
The first tangible evidence of the former desiccation of the Mediterranean occurred in the summer of 1970 as part of the Leg -13 Expedition of the Glomar Challenger Tiefseebohrschiffs. The geologists under the scientific direction of William Ryan and Kenneth Hsu promoted in the Balearic Basin cores to light that revealed the surprising nature of the "M- reflector ". Already in the holes 121 to 123 smaller remnants of dolomite were ( a rock similar to limestone, but with magnesium-rich carbonate ) and gypsum drilled pebbles, without, however, more far-reaching conclusions have drawn from it. The plaster was finally able to have been flushed from the nearby mainland yes. In the bore 124 but then found at a depth of about 2000 meters below sea level stromatolites and pending anhydrite. In Stromatolites now is fine change bearings of solidified mud and lime, which is deposited by algae mats in the intertidal zone of shallow tropical waters. The so-called " chicken wire " anhydrite, however, is a calcium sulfate - such as plaster, but without entrapped water of crystallization - which almost exclusively in " salt marsh " ( sabkhas ), very hot and dry coastal plains, is precipitated where even the ground water temperatures above 30 ° C achieved. At lower temperatures only gypsum forms. The fossils (in this case microscopic calcareous shells of foraminifera ) in the marine deposits below the sabkha sediments do not date from the Permian, but come from the much younger stage of the Messinian.
Attempts to explain
At first glance, these findings seem to be totally incompatible with each other. On the one hand indicated the drilled rocks and sedimentary structures clearly indicate deposition in very shallow water. On the other hand, revealed the seismic data show that the "M- reflector " the deep soils of the Mediterranean Sea covered, as if he would have been there, formed in place at great depth. In addition, the evaporites could have occurred suddenly at the same time with a large number of small isolated occurrences on the surrounding mainlands that had previously only thought for minor local events and hardly communicated with each other.
An attempt to explain who was also represented by a member of the Leg -13 expedition, the sedimentologists Vladimir Nesteroff was: When were deposited the Messinian evaporites, the Mediterranean would still have been a shallow marginal sea, one after the pinch from the Atlantic Ocean in wide salt pan would have converted. However, the depression of the ocean basin could then no longer have eventually taken place during the Mesozoic or Cenozoic, but would very quickly have occurred less than five million years ago.
Other researchers, such as the leader of the expedition Bill Ryan himself, however, doubted the possibility of a so rapid " Ozeanisierung " of continental crust. In classical Geosynklinaltheorie such ideas about " collapse basin " and " reduce troughs " had been still acceptable, but since the advent of the new geotectonic model of plate tectonics in the sixties, they were more and more fall into discredit. Therefore we concluded that the anhydrite must have been formed in some way in deep water. In fact, deep-sea sediments during subsequent drilling not only above the salts found, but even among them. For this reason, we considered models, such as heavy brines or brines themselves could also accumulate at the base of a large column of water that would be concentrated enough to precipitate readily soluble minerals.
Ultimately, however, prevailed an idea that could unite the contradictory findings at last. The evaporites had indeed deposited under shallow water ( finally had the algae mats, which had formed the stromatolites can never exist in the lightless deep-sea ), but they were several thousand meters below the sea world. During the Strait of Gibraltar was closed, preventing the ingress of water from the Atlantic Ocean, the evaporites must have formed at the bottom of very deep, arid basin.
An important indication has already provided the borehole 133 west of Sardinia. Here found under the "M- reflector " no evaporites, but change superpositions of well-rounded gravels with intense red and green colored siltstones. Apparently it was the deposits of desert rivers that were passed down the Sardinian continental shelf and had formed on his foot alluvial fans. In borehole 134 already found rock salt, which is part of the evaporative minerals that precipitate almost as a last resort.
As a consequence, also came more and more evidence to light that the long-known deep-sea canyons were formed by the action of underwater avalanches not only in the Pleistocene before the mouths of the Rhone and other rivers such as the canyons in the Atlantic and Pacific, but already at end of the Miocene were cut into the steep flanks of the largely dried-up Mediterranean basin deep down to the present-day abyssal plains. For example, the bed of the Nile at Aswan was then already 750 meters below the present sea level, as it was realized in the establishment of the Nasser Dam from 1959 to 1970, at the mouth at Cairo even 2400 meters deep.
Other findings
However, it was the enormous volume of Messinian evaporites obtained, which, as we learned later, reached maximum thicknesses of up to three kilometers, not to have been deposited during a single drying event. The total dissolved salt in the Mediterranean would never have been enough to do so.
The deposition cycles
After a thorough investigation of the borehole 124 Kenneth Hsu recognized two years after the end of the drilling campaign that the nature of the strata clearly pointed to several cycles in which the Mediterranean was dried and refilled. At this time it was already the existence of a large brackish water lake ( Paratethys ) in Eastern Europe known.
Chronology
20 million years ago formed the precursor of the Mediterranean ocean, the Tethys, still a wide waterway between the Indian Ocean and the Atlantic opening. However, the Tethys was increasingly concentrated in the course of the following years until the African Plate collided with Asia Minor in the middle Miocene about 15 million years ago. This led to the unfolding of mountains chain in the Middle East and ended the connection of the Mediterranean to the Indian Ocean incurred. From now on only about ties to the Atlantic in the form of the Betic road in the south of the Iberian Peninsula ( Iberian block, Iberian small plate or simply Iberia ), north of the Betic Cordillera and the Rif Street in northwest Africa, south of the Rif Mountains. Today's Straits of Gibraltar was the mountain arc which Betic Cordillera and Rif another Association ( Gribraltar Arch ), closed.
The exact sequence and the exact reasons for the Messinian salinity crisis are still controversial. However, one may assume that the Mediterranean would evaporate without any inflow in some ten thousand years. While in the past mostly emanated from a global sea-level waste or, as is discussed by a lateral constriction of the remaining straits by tectonic movements since 2003, a model that led to a closing of the sea passages between the Atlantic and the Mediterranean to the large-scale motions in the upper mantle.
In the model, the authors suggest that the subduction of oceanic lithosphere beneath the Alboran Sea ( westernmost Mediterranean) bands of subcontinental mantle lithosphere below the southern edge of Iberia and Northwest Africa peeled. The removal of material at the bottom of the lithosphere along with the Aufströmen of mantle material into the vacant space led at the end of the Miocene to a rapid increase in the overlying remaining lithosphere including the crust and the ocean passages on the southern edge of Iberia and Northwest Africa. These processes in the upper mantle were reconstructed from the temporal and spatial evolution of the geochemical composition of volcanic rocks in southern Spain, northern Morocco and the seabed between the intermediate part of the Mediterranean ( Alboran Sea). On the basis of geochemical analyzes and age dating has been shown that the composition of the volcanic rocks in the region from 6.3 to 4.8 million years ago, thus largely coincided with the drying of the Mediterranean, drastically changed ( from subduction - to intraplate -type). This change strongly suggests a causal relationship between processes in the mantle and the Messinian salinity crisis. The model of thermo-mechanical ( geophysical ) calculations, which show that the processes in the upper mantle to an increase in the straits around half a mile and thus may have caused over the sea level is supported. This led to a closing of the sea passages, isolation and ultimately desiccation of the Mediterranean.
According Clauzon et al. (1996 ) began the salinity crisis before 5.75 million years, according to Krijksman, et al. (1999) but before 5.96 Ma. Both authors propose a division of the salinity crisis. While Clauzon assumes that in the first phase ( 5.75 to 5.60 Ma) only a moderate decline in sea level took place, in which were deposited only in the peripheral areas of the Mediterranean evaporites, and that it a phase ( 5.60 to 5, 32 Ma) was followed the complete pinch-off and evaporation, in which the evaporites in the deep basins and huge canyons were formed, Krijksman suggests, however, provide that the latter already in the first phase ( 5.59 to 5.50 Ma) had happened, while in the second phase would have ( 5.50 to 5.33 Ma) formed the cyclic Evaporitablagerungen in a large Lago- Mare basin ( " lake - sea ").
Prior to about 5.33 million years ago, at the turn of Miocene to Pliocene was done according to the latest findings initially a slight lowering of the land bridge between Europe and Africa, so that for several millennia spilled only small amounts of water from the Atlantic Ocean in the dry Mediterranean basin. Gradually the water dug deeper and deeper into the land bridge until finally einströmten through a 200 kilometers long and up to 11 km wide channel about 100 million cubic meters per second, while the flow channel per at a speed of 144 kilometers per hour at 40 centimeters day depth. In total, 500 cubic kilometers of rock were washed away. This meant that every day rose to the peak of this process, the water level in the Mediterranean basin by more than 10 meters, to a maximum of two years after the Mediterranean was replenished. Since then, this strait is the only natural connection between the Atlantic and Central and Black Sea.
In the latter case, the new and final flooding of the basin a lot less would have been more spectacular event than previously thought by a presumably relatively narrow but deep channel in the Strait of Gibraltar. The grandiose picture of a thousand -foot waterfall, a thousand times as powerful as the Niagara Falls who breaks roaring into the deep desert basin, as it was particularly popularized by Kenneth Hsu, so would probably be revised a bit. Also, findings in Sicily speak at least in the final stages for a speedy, but not for a catastrophic flooding of the Mediterranean.
The isolated evaporite deposits on the hard countries around the Mediterranean is the way, mostly by sediments in smaller, but higher marginal basins that were raised during later orogenic phases above sea level, for example in Italy, Sicily and Crete. The basin in southern Spain and northwest Africa, however, were up to the opening of the Straits of Gibraltar, the only connection to the Atlantic. Even minor tectonic movements or eustatic sea-level changes in this region could connect to the Atlantic, the Mediterranean, but also with the individual sub-basins with each block or restore. Thus, the tectonic and sedimentary evolution of the Betic and the Rif road road is probably the key to the understanding of the Messinian Salinity Crisis final.
Effects
In addition to the erosion of submarine canyons desiccation of the Mediterranean was also blamed for the deep karstification in the north and east of the Adriatic, as well as for the rapid removal of the Alps.
In assessing the climatic consequences of the Messinian salinity crisis, it is often difficult to distinguish cause and effect. Has the increased formation of sea ice triggered a global lowering of the sea and thus causes the constriction of the Mediterranean? Or does the binding of enormous amounts of salt, the salinity of the ocean is reduced, so that the freezing point of sea water increased and promoted the formation of sea ice? In any case, can be controlled during the Miocene a drier, steppe climate in parts of Central Europe to demonstrate during the Pliocene, after the flooding of the Mediterranean, the climate was more humid and cooler, down to the last ice age.
During the bottom of the Mediterranean was mostly dry and desert -like, to coniferous forests, the continental slopes spread from the surrounding plateaus from going down. The present-day Mediterranean islands were high mountains peaks with alpine flora. After the flooding this socializations survived, for example, in Sardinia and Corsica, while elsewhere retreated back into the high mountains. Apparently allowed the dehydration and the migration of many species of animals from Africa to Europe, as wild horses and even hippos, which then sometimes, like the caprine further developed in the Balearic Islands ( Myotragus balearicus ) after flooding to dwarf forms. Hsü even speculated whether the desertification of large parts of Africa that one makes often responsible for the " descent from the trees " of the early hominids could not have been also caused by the salinity crisis.
Future Development
Even today, the Mediterranean Sea is because of its high evaporation rate and the small opening of the Strait of Gibraltar again significantly saltier than for example the North Atlantic. Similarly, the Strait of Gibraltar is already back shallower than in the Pliocene. It may be assumed that it is likely to close in two or three million years.