Messinian salinity crisis
The Messinian salinity crisis (English: Messinian salinity crisis , abbreviated MSC ) is a section of the earth's history in which the Mediterranean was partially or completely dried up. Here, up to three kilometers thick evaporation rocks ( evaporites ) were deposited in the deepest sea basins . This happened between about six million years ago and about five million years ago at the end of the Messin , the last stage of the Miocene .
Discovery story
As early as 1833, the British geologist Charles Lyell had noticed a striking faunal section in various fossil sites in Italy , where many living things that had previously populated the Mediterranean disappeared and were displaced by other organisms. Today's fauna should largely emerge from the latter . With this striking event, Lyell established the boundary between the geological epochs of the Miocene and the Pliocene .
First clues
At the end of the 19th century, when a drinking water well was being built on the Valence plain in southern France, a ravine hidden under Quaternary gravel was discovered that was inexplicably cut deep into the crystalline subsoil. Later it was possible to prove this gorge in the entire valley of the Rhone between Lyon and the Camargue , where it was filled with marine sediments of the Pliocene. Even then, some French and Italian paleontologists considered a temporary desiccation of the Mediterranean Sea to explain this phenomenon. Such ideas were still widespread at the beginning of the 20th century, but were considered highly speculative. The science fiction author HG Wells , who studied geology with Vincent Illing in London in his youth , used the idea in his short story The Grisly Folk .
In 1958, seismic measurements by the North American oceanographer Brackett Hersey revealed a previously unknown geological structure that was always about 100 to 200 meters below the bottom of the Mediterranean Sea. Since this surface, the so-called "M-reflector", closely followed the present-day profile of the sea floor, it was obvious that it was a hard rock layer that at a certain point in time had deposited uniformly and coherently throughout the Mediterranean. In addition, structures appeared in the seismic profiles that were reminiscent of salt domes that rose from the depths and pierced the overlying sediments . At that time, many geologists suspected that the salt must have come from the Permian or Triassic period, because during these geological ages, abundant salt deposits were formed in many parts of the world more than 200 million years ago . a. also those of the Zechstein series in Central Europe. Since the previously known Permian and Triassic salt deposits are located in a relatively shallow Epicontinental Sea , i. H. over the continental crust and not in a deep ocean basin , the newly discovered structures were considered evidence that the Mediterranean basin sank sometime during the 200 million years after the Triassic.
Only a few geologists speculated whether these salt domes might not have formed at the same time as the small, scattered evaporite deposits, such as those exposed in the city of Messina in Sicily (and which gave the Messin stage its name). Other salt and gypsum-bearing formations of this age were found in Piedmont , Tuscany , Calabria as well as in Spain , Morocco , Algeria , Tunisia , Greece , Turkey , Cyprus and Israel .
The discovery
The first tangible evidence of the former drying up of the Mediterranean came in the summer of 1970 as part of the Leg-13 expedition of the deep-sea drilling ship Glomar Challenger . The geologists, under the scientific direction of William Ryan and Kenneth Hsü , unearthed drill cores in the Balearic Basin that revealed the surprising nature of the "M-reflector". Smaller remains of dolomite (a rock similar to limestone , only with magnesium-rich carbonate ) and gypsum pebbles were already drilled in holes 121 to 123, but no further conclusions were drawn from them. The plaster of paris could have been washed in from the nearby mainland. In borehole 124, however, stromatolites and anhydrite were found at a depth of about 2000 meters below sea level . Stromatolites are now fine alternating layers of solidified sludge and lime deposited by algae mats in the intertidal zone of shallow tropical waters. The so-called “chicken wire” anhydrite, on the other hand, is a calcium sulphate - like gypsum, but without enclosed crystal water - which is precipitated almost exclusively in “salt marshes” ( Sabchas ), very hot and dry coastal plains, in which even the groundwater temperatures above 30 ° C reached. At lower temperatures, only gypsum is formed. The fossils (in this case microscopic calcareous shells of foraminifera ) in the marine deposits beneath the Sabcha sediments do not date from the Permian, but come from the much later stage of the Messin.
Attempts to explain
At first glance, these findings appeared to be completely incompatible. On the one hand, the drilled rocks and sedimentary structures clearly indicated a deposit under very shallow water. On the other hand, the seismic data indicated that the “M-reflector” covered the deep floors of the Mediterranean Sea, as if it had formed there, on the spot, at great depth. In addition, the evaporites should suddenly have formed simultaneously with a large number of small isolated deposits on the surrounding mainland, which had previously only been regarded as insignificant local events and hardly associated with one another.
An attempt to explain, which was also represented by a member of the Leg-13 expedition, the sedimentologist Vladimir Nesteroff , was: When the Messinian evaporites were deposited, the Mediterranean must have been a shallow adjoining sea that turned into one after being cut off by the Atlantic large salt pan. The sinking of the ocean basin could then no longer have taken place at some point in the course of the Mesozoic or Cenozoic , but should have happened very quickly, less than five million years ago.
However, other researchers, such as the leader of the expedition Bill Ryan himself, doubted the possibility of such a rapid “oceanization” of the continental crust. In classical geosyncline theory , such ideas about "collapse basins" and "subsidence troughs" were still defensible, but since the emergence of the new geotectonic model of plate tectonics in the 1960s, they had become more and more discredited. It was therefore concluded that the anhydrite must have formed in some way in deep water. In fact, later drilling found deep-sea sediments not only above the salts, but also below. For this reason, models were considered as to how heavy brines or brines could accumulate at the base of a large column of water , which would be concentrated enough to precipitate easily soluble minerals .
Ultimately, however, an idea prevailed that could finally unite the contradicting findings. The evaporites had deposited under shallow water (after all, the algae mats that had formed the Stromatholites could never have existed in the lightless deep sea), but they were still several thousand meters below world sea level. While the Strait of Gibraltar was closed and prevented the penetration of water from the Atlantic, the evaporites must have formed on the bottom of very deep, desert-like basins.
Borehole 133 west of Sardinia already provided an important indication. Here there were no evaporites under the “M reflector”, but alternating layers of well-rounded gravels with intensely red and green colored silt stones . Apparently it was the deposits of desert rivers that had flowed down the Sardinian continental shelf and formed rubble fans at its foot. Rock salt , one of the evaporation minerals that is almost the very last to precipitate , has already been found in borehole 134 .
As a result, more and more indications came to light that the long-known deep-sea canyons in front of the mouths of the Rhone and other rivers had not only formed in the Pleistocene through the action of underwater avalanches like the canyons in the Atlantic and Pacific, but already on At the end of the Miocene, the steep flanks of the largely dried-up Mediterranean basin were cut deep down to today's deep-sea plains . For example, the bed of the Nile at Aswan was already 750 meters below today's sea level, as was determined when the Nasser Dam was built from 1959 to 1970, and at the mouth near Cairo it was even 2,400 meters deep.
Further findings
However, the enormous volume of the Messinian evaporites obtained, which, as was later determined, reached a maximum thickness of up to three kilometers, could not have been deposited in the course of a single desiccation event. All the salt dissolved in the Mediterranean would never have been enough.
The deposition cycles
After a thorough examination of borehole 124, Kenneth Hsü realized two years after the end of the drilling campaign that the nature of the strata clearly indicated several cycles in which the Mediterranean had been dried out and filled again. At this time he was also aware of the existence of a large brackish lake ( Paratethys ) in Eastern Europe.
- The oldest sediment in each cycle came either from the deep sea or from a large brackish lake. Fine-grained sediments on soils with calm water or from great depths show perfectly uniform stripes. As the pool dried up and the water depth decreased, the banding became more and more irregular as the waves increased. And when the places where sediments were deposited were only under water from time to time, stromatolite formed. Finally, after further drying up, the previously flooded area was completely dry, and anhydride was now precipitated from the salty Sabcha groundwater. Suddenly, however, either sea water sloshed over the Strait of Gibraltar - or a large amount of brackish water broke in from the Eastern European brackish water lake. Now the Balearic Basin filled up again, and fine-grained masses of mud, which the water ingress carried with it, abruptly superimposed the “chicken wire anhydrite”. In the course of the millions of years that comprised the so-called Messina phase of the late Miocene, this cycle was repeated at least eight to ten times.
chronology
20 million years ago, the precursor ocean of the Mediterranean, the Tethys , still formed a broad waterway between the Indian Ocean and the opening Atlantic. However, the Tethys was narrowed more and more in the course of the following time, until the African plate collided with the Middle East around 15 million years ago in the middle Miocene . This led to the unfolding of chain mountains in the Middle East and ended the connection of the emerging Mediterranean Sea to the Indian Ocean. From now on there were only connections to the Atlantic in the form of the Betic Strait in the south of the Iberian Peninsula ( Iberian Block , Iberian Small Plate or simply Iberia ), north of the Betic Cordillera , and the Rif Strait in Northwest Africa, south of the Rif Mountains . Today's Strait of Gibraltar was closed by the mountain arch that connected the Betic Cordillera and the Rif ( Gibraltar Arch ).
The exact process and the exact reasons for the Messinian salinity crisis are still controversial. However, it can be assumed that the Mediterranean would evaporate in a few tens of thousands of years without any inflow. While in the past it was mostly assumed that the sea level would drop globally or that the remaining ocean roads were narrowing laterally due to tectonic movements, a model has been discussed since 2003 according to which large-scale movements in the upper mantle led to the sea passages between the Atlantic and the Mediterranean being closed.
In the model, the authors propose that subduction of oceanic lithosphere beneath the Alborán Sea (westernmost Mediterranean) peeled off bands of subcontinental mantle lithosphere beneath the southern rim of Iberia and northwest Africa. The removal of material in the lower part of the lithosphere together with the upflow of mantle material into the released space led at the end of the Miocene to a rapid uplift of the remaining lithosphere including the crust and the sea passages on the southern edge of Iberia and northwestern Africa. These processes in the upper mantle were reconstructed from the temporal and spatial development of the geochemical composition of volcanic rocks in southern Spain, northern Morocco and the seabed of the intermediate part of the Mediterranean (Alborán Lake). On the basis of geochemical analyzes and age dating, it was possible to show that the composition of the volcanic rocks in the region changed drastically between 6.3 and 4.8 million years, i.e. largely at the same time as the drying up of the Mediterranean Sea (from subduction to intraplate type). This change strongly points to a causal connection between processes in the earth's mantle and the Messinian salinity crisis. The model is supported by thermomechanical ( geophysical ) calculations, which show that the processes in the upper mantle could have caused the ocean roads to rise by almost a kilometer and thus above sea level. This led to the closure of the sea passages, isolation and ultimately drying up of the Mediterranean.
According to Clauzon et al. (1996) the salinity crisis began 5.75 million years ago, according to Krijksman, et al. (1999), however, already 5.96 Ma. Both authors propose dividing the salinity crisis in two. While Clauzon assumes that in the first phase (5.75-5.60 Ma) only a moderate decline in sea level took place, during which evaporites were only deposited in the marginal areas of the Mediterranean Sea, and that a phase (5.60-5.60 Ma) took place thereupon , 32 Ma) followed the complete constriction and evaporation, in which the evaporites would have formed in the deep basins and the huge canyons, Krijksman suggests that the latter should already be in the first phase (5.59-5.50 Ma) happened, while in the second phase (5.50-5.33 Ma) the cyclic evaporite deposits formed in a large Lago-Mare basin (“sea lake”).
About 5.33 million years ago, at the turn of the Miocene to the Pliocene , the latest findings indicate that the land bridge between Europe and Africa was initially slightly lowered, so that only small amounts of water spilled from the Atlantic into the dried-up Mediterranean basin for several millennia. Gradually, the water dug deeper and deeper into the land bridge until finally about 100 million cubic meters per second flowed in through a 200 kilometer long and up to 11 kilometer wide canal, with a speed of 144 kilometers per hour the flow channel by 40 centimeters per second Day deepened. A total of 500 cubic kilometers of rock was washed away. As a result, at the height of this process, the water level in the Mediterranean basin rose daily by more than 10 meters until the Mediterranean was refilled after a maximum of two years. Since then, this strait has been the only natural connection between the Atlantic and the Mediterranean and Black Seas.
In the latter case, the renewed and final flooding of the basin through a presumably relatively narrow but deep channel in the Strait of Gibraltar would have been a much less spectacular process than previously thought. The grandiose image of a thousand meter high waterfall, a thousand times as powerful as Niagara Falls , which crashes into the deep desert basin with a roar, as it was particularly popularized by Kenneth Hsü, would have to be revised a little. Findings in Sicily, at least in the final phase, only speak in favor of rapid, but not catastrophic, flooding of the Mediterranean.
The isolated evaporite deposits on the mainlands around the Mediterranean are mostly sediments in smaller, but also higher-lying marginal basins, which were raised above sea level during later mountain formation phases, for example in Italy, Sicily and Crete. The basins in southern Spain and northwest Africa, on the other hand, formed the only connection to the Atlantic until the Strait of Gibraltar opened. Even slight tectonic movements or eustatic sea level fluctuations in this region could block or restore the connection with the Atlantic, with the Mediterranean, but also with the individual sub-basins. The tectonic and sedimentary development of the Betic Strait and the Rif Strait probably forms the key to a final understanding of the Messinian salinity crisis.
Effects
In addition to the erosion of the submarine canyons, the drying up of the Mediterranean Sea is also held responsible for the profound karstification in the north and east of the Adriatic and for the rapid erosion of the Alps .
When assessing the climatic consequences of the Messinian salinity crisis, it is often difficult to distinguish cause and effect. Has the increased formation of glaciers triggered a global lowering of the sea level and thus caused the Mediterranean Sea to be pinched off? Or has the binding of enormous amounts of salt reduced the salinity of the world's oceans, thereby increasing the freezing point of seawater and promoting the formation of ice? In any case, during the Miocene there was evidence of a drier, steppe-like climate in parts of Central Europe, while in the Pliocene, after the flooding of the Mediterranean Sea, the climate became more and more humid and cooler, right up to the last Ice Age .
While the floor of the Mediterranean was largely dry and desert-like, coniferous forests spread from the surrounding plateaus down the continental slopes. Today's Mediterranean islands formed high mountain peaks with alpine flora . After the flooding, these associations survived, for example, in Sardinia and Corsica , while elsewhere they withdrew again into the high mountains. Apparently, the desiccation also allowed the migration of many animal species from Africa to Europe, such as wild horses and even hippos , which then sometimes, like the goat-like in the Balearic Islands ( Myotragus balearicus ), developed into dwarf forms after the flooding. Hsü even speculated whether the desertification of large parts of Africa, which is often blamed for the early hominids “climbing down from the trees”, could not also have been caused by the salinity crisis.
Future development
The Mediterranean Sea is already significantly salty again than the North Atlantic, for example, because of its high evaporation rate and the low opening of the Strait of Gibraltar. The Strait of Gibraltar is already shallower again than in the Pliocene. One can assume that it will probably close again in two or three million years.
Trivia
In his award-winning science fiction novel The Last Day of Creation from 1981, the author Wolfgang Jeschke lets people travel back to the time of the Messinian salinity crisis.
See also
literature
- Kenneth J. Hsü: The Mediterranean was a desert. On research trips with the Glomar Challenger. Harnack, Munich 1984. ISBN 3-88966-012-6
Individual evidence
- ↑ Kenneth J. Hsü: The Mediterranean was a desert. On research trips with the Glomar Challenger. P. 112, Harnack, Munich 1984.
- ↑ Svend Duggen, Kaj Hoernle, Paul van den Bogaard, Lars Rüpke, Jason Phipps Morgan: Deep roots off the Messinian salinity crisis . In: Nature , Vol. 422, 2003, pp. 602-606. doi: 10.1038 / nature01553
- ↑ S. Duggen, K. Hoernle, P. van den Bogaard, D. Garbe-Schönberg: Post-collisional transition from subduction to intraplate-type magmatism in the westernmost Mediterranean: Evidence for continental-edge delamination of subcontinental lithosphere . In: Journal of Petrology , Vol. 46, 2005, No. 6 pp. 1155-1201, doi: 10.1093 / petrology / egi013
- ↑ Garcia-Castellanos, D., A. Villaseñor: Messinian salinity crisis regulated by competing tectonics and erosion at the Gibraltar Arc. In: Nature , Vol. 480, 2011, pp. 359–363, doi: 10.1038 / nature10651 ( alternative PDF link ( memento from July 9, 2015 on WebCite ); 3.7 MB)
- ↑ Georges Clauzon, Jean-Pierre Suc, Francois Gautier, André Berger, Marie-France Loutre: Alternate interpretation of the Messinian salinity crisis: Controversy resolved? . In: Geology , Vol. 24, 1996, No. 4, pp. 363-366. doi: 10.1130 / 0091-7613
- ↑ W. Krijgsman, FJ Hilgent, I. Raffi, FJ Sierros, DS Wilson: Chronology, causes and progression of the Messinian salinity crisis . In: Nature Vol. 400, 1999, pp. 652-655. doi: 10.1038 / 23231
- ^ A b c D. Garcia-Castellanos et al .: Catastrophic flood of the Mediterranean after the Messinian salinity crisis. In: Nature , Vol. 462, 2009, pp. 778-781, doi: 10.1038 / nature08555
Web links
- Salt crisis - the mega flood. Article at Spektrum.de
- The great flood. Researchers unravel the prehistoric catastrophe on the Mediterranean. Article at scinexx.de
- Rob Butler: The Messinian Salinity Crisis. University of Leeds website
- Messinian Online - Living in an Evaporitic World. ( Memento of July 9, 2013 in the Internet Archive ) Website on the Messinian salinity crisis