Messinian salinity crisis
The scale of gypsum formation in the Sorbas basin (Yesares member). The upward-growing cones sug...
|The scale of gypsum formation in the Sorbas basin (Yesares member). The upward-growing cones |
suggest precipitation on the sea floor (not within sediments).
The Messinian Salinity Crisis, also referred to as the Messinian Event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partly or nearly complete desiccation throughout the latter part of the Messinian age of the Miocene epoch, from 5.96 to 5.33 Ma (million years ago). It ended with the so-called "Zanclean flood", when the sea water invaded the Basin.
Sediment samples from below the deep seafloor of the Mediterranean Sea, which include evaporite minerals, soils, and fossil plants, show that about 5.96 million years ago in the late Miocene period the precursor of the Strait of Gibraltar closed tight, and the Mediterranean Sea, for the first time and then repeatedly, partially desiccated. 5.6 Ma ago the strait closed one last, final time, and because of the generally dry climate conditions, within a millennium the Mediterranean basin nearly completely desiccated, evaporating into a deep dry basin bottoming at some places 3 to 5 km below the world ocean level, with a few hypersaline Dead-Sea-like pockets. Around 5.5 Ma less dry climatic conditions allowed the basin to resume receiving more fresh water from rivers, with pockets of Caspian-like brackish waters getting progressively less hyper-saline, until the final reopening of the Strait of Gibraltar 5.33 Ma with the Zanclean flooding.
Even now the Mediterranean is saltier than the North Atlantic because of its near isolation by the Strait of Gibraltar and its high rate of evaporation. If the Strait of Gibraltar closes again, which is likely to happen in the near geological future (though extremely distantly on a human time scale), the Mediterranean would mostly evaporate in about a thousand years. After that, continued northward movement of Africa may obliterate the Mediterranean: see Mediterranean Ridge.
Naming and discovery
The Messinian salt deposits that are outcropping (because they were uplifted by tectonic activity during later episodes) in places like Messina (Sicily), northeast Libya, Italy, and southern Spain have been described since the 19th century and it is then that the salinity crisis theory started to be developed.
Charles Mayer-Eymar (1826–1907) of Zurich studied fossils between gypsum-bearing, brackish, and freshwater sediment layers and identified them as having been deposited just before the end of the Miocene Epoch. In 1867 he named the period the Messinian, for the region of Messina, Sicily. Since then salt-bearing and gypsum-bearing evaporite layers in many Mediterranean countries have been dated to that period.
In 1961, seismic surveying of the Mediterranean basin revealed a geological feature some 100–200 metres below the seafloor. This feature, dubbed the M reflector, closely followed the contours of the present seafloor, suggesting that it was laid down evenly and consistently at some point in the past. The origin of this layer was largely interpreted as related to salt deposition. However, different interpretations were proposed for the age of salt and its deposition.
Earlier suggestions from Denizot (1952) and Ruggieri (1967) proposed that this layer was Late Miocene in Age, and the same Ruggeri coined the term "Messinian Salinity Crisis".
New and high quality seismic data on the M-reflector were acquired in the Mediterranean Basin in 1970, published by e.g. Auzende et al. (1971) At the same time, the salt was cored during Leg 13 of the Deep Sea Drilling Program conducted from the Glomar Challenger under the supervision of co-chief scientists William B.F. Ryan and Kenneth J. Hsu. All these deposits were indisputably dated and interpreted for the first time as deep-basin products of the Messinian Salinity Crisis.
|Cones of gypsum, which formed on the sea floor as a result of evaporation. |
Evaporation of one metre of seawater precipitates around 1mm of gypsum.
The first solid evidence for the ancient desiccation of the Mediterranean Sea came in the summer of 1970, when geologists aboard the Deep Sea Drilling Program drillship Glomar Challenger brought up drill cores containing arroyo gravels and red and green floodplain silts; and gypsum, anhydrite, rock salt, and various other evaporite minerals that often form from drying of brine or seawater, including in a few places potash, left where the last bitter, mineral-rich waters dried up. One drill core contained a wind-blown cross-bedded deposit of deep-sea foraminiferal ooze that had dried into dust and been blown about on the hot dry abyssal plain by sandstorms and ended up in a brine lake interbedded between two layers of halite. These layers alternated with layers containing marine fossils, indicating a succession of drying and flooding periods.
Other evidence of drying comes from the remains of many (now submerged) canyons that were cut into the sides of the dry Mediterranean basin by rivers flowing down to the abyssal plain. For example, the Nile cut its bed down to several hundred feet below sea level at Aswan (where Ivan S. Chumakov found marine Pliocene foraminifers in 1967), and 8,000 feet (2,500 m) below sea level just north of Cairo.
In many places in the Mediterranean, fossilized cracks have been found where muddy sediment had dried and cracked in the sunlight and drought. In the Western Mediterranean series, the presence of pelagic oozes interbedded within the evaporites suggests that the area was repeatedly flooded and desiccated over the course of 700,000 years.
After the discovery, the definition, and the scientific acceptance of this particular event in Geological History, since the early seventies of the 20th Century, numerous Conferences, Workshops, M.Sc and Ph.D. Researches, Reviews and Special Volumes have been dedicated to this argument.:
- Decima and Wezel (1971)
- Hsu et al. (1972)
- Cita (1972)
- Drooger (1973)
- Heimann (1977)
- Catalano et al (1978)
- Rouchy (1981)
- Hsu (1983)
- Hsu (1985)
- Muller (1986)
- Benson (1984, 1991)
- Benson & Rakic-El Bied (1991)
- Cita (1992)
- Van Dijk (1992), Van Dijk et al. (1998)
- Roveri et al. (2008)
Based on palaeomagnetic datings of Messinian deposits that have since been brought above sea level by tectonic activity, the salinity crisis started at the same time over all the Mediterranean basin, at 5.96 ± 0.02 million years ago. It must be underlined that this episode comprises the second part of what is called the "Messinian" Epoch. The Epoch was characterised by several stages of tectonic activity and sea level fluctuations, and erosional and depositional events, all more or less interrelated (van Dijk et al., 1998). The Mediterranean-Atlantic strait closed tight time and again, and the Mediterranean Sea, for the first time and then repeatedly, partially desiccated. The basin was finally isolated from the Atlantic Ocean for a longer period, between 5.59 and 5.33 million years ago, resulting in a large or smaller (depending on the scientific model applied) lowering of the Mediterranean sea level. During the initial, very dry stages (5.6 - 5.5 Ma), there was extensive erosion, creating several huge canyon systems (some similar in scale to the Grand Canyon) around the Mediterranean. Later stages (5.50 - 5.33 Ma) are marked by cyclic evaporite deposition into a large "lake-sea" basin ("Lago Mare" event). About 5.33 million years ago, at the start of the Zanclean age (at the start of the Pliocene epoch), the barrier at the Strait of Gibraltar broke one last time, re-flooding the Mediterranean basin in the Zanclean flood; the basin has not desiccated since.
The amount of Messinian salts is larger than 4·1018 kg (Ryan, 2008, Sedimentology), exceeding by a factor 50 the amount of salt normally contained in the Mediterranean waters. This suggests either a succession of desiccations or a long period of hypersalinity during which incoming water from the Atlantic Ocean was evaporated with the level of the Mediterranean brine being similar to that of the Atlantic The nature of the strata points strongly to several cycles of the Mediterranean Sea completely drying and being refilled, with drying periods correlating to periods of cooler global temperature - which were therefore drier in the Mediterranean region. Each refilling was presumably caused by a seawater inlet opening, either tectonically, or by a river flowing eastwards below sea level into the "Mediterranean Sink" cutting its valley head back west until it let the sea in, similarly to a river capture. The last refilling was at the Miocene/Pliocene boundary, when the Strait of Gibraltar broke wide open permanently. Upon closely examining the Hole 124 core, Kenneth J. Hsu found that:
The oldest sediment of each cycle was either deposited in a deep sea or in a great brackish lake. The fine sediments deposited on a quiet or deep bottom had perfectly even lamination. As the basin was drying up and the water depth decreased, lamination became more irregular on account of increasing wave agitation. Stromatolite was formed then, when the site of deposition fell within an intertidal zone. The intertidal flat was eventually exposed by the final desiccation, at which time anhydrite was precipitated by saline ground water underlying sabkhas. Suddenly seawater would spill over the Strait of Gibraltar, or there would be an unusual influx of brackish water from the eastern European lake. The Balearic abyssal plain would then again be under water. The chicken-wire anhydrite would thus be abruptly buried under the fine muds brought in by the next deluge.
Research since then has confirmed that the desiccation-flooding cycle repeated itself at least eight times during the last 630,000 years of the Miocene epoch.
The notion of a completely waterless Mediterranean Sea has some corollaries.
|A possible palaeogeographical reconstruction of the west end of the|
Miocene Mediterranean. North to the left.
S current coastline Sorbas basin, Spain
R Rifean corridor
B Betic corridor
G Strait of Gibraltar
M Mediterranean sea
- At the time, the Strait of Gibraltar was not open, but other seaways (the Betic corridor to the north where the Sierra Nevada or Baetic Cordillera is now, or to the south where the Rifean corridor or corridors where the Rif Mountains are now) linked the Mediterranean to the Atlantic. These must have closed, isolating the basin from the open ocean.
- The high level of salinity cannot be tolerated by many known organisms, most likely reducing the biodiversity of much of the basin.
- The basin's low altitude would have made it extremely hot during the summer through adiabatic heating, a conclusion supported by the presence of anhydrite, which is only deposited in water warmer than 35°C (95°F).
- Rivers emptying into the basin would have cut their beds much deeper (at least a further 2,400 m or 8,000 feet in the case of the Nile, as the buried canyon under Cairo shows).
The climate of the abyssal plain during the drought is unknown. There is no situation on Earth directly comparable to the dry Mediterranean, and thus it is not possible to know its climate. There is not even a consensus as to whether the Mediterranean Sea even dried out completely; it seems likeliest that at least three or four large brine lakes on the abyssal plains remained at all times. The extent of desiccation is very hard to judge due to the reflective seismic nature of the salt beds, and the difficulty in drilling cores, making it difficult to map their thickness.
Nonetheless, one can study the forces at play in the atmosphere to arrive at a good speculation of the climate. As winds blew across the "Mediterranean Sink", they would heat or cool adiabatically with altitude. As a result, the rule that "cold air falls and hot air rises" has to be adjusted by considering how hot each bit of air would be if brought adiabatically to sea-level pressure.
In the empty Mediterranean Basin the summertime temperatures would likely have been extremely high. Using the Clausius-Clapeyron relation, one can derive an equation showing that a theoretical temperature of an area 4 kilometers (= 2.485 miles) below sea level would be about 40°C (72°F) warmer than the temperature at sea level. Thus one could predict theoretical temperature maximums of around 80°C (176 °F) at the lowest depths of the dry abyssal plain permitting little known life to survive there. One can also calculate that 2 to 3 miles (3.2 - 4.8 km) below sea level would have resulted in 1.45 to 1.71 atm (1,102 to 1,300 mmHg) of air pressure at the bottom. Although it was likely quite dry in the Basin, there is no direct way to measure how much drier it would have been compared to its surroundings. Areas with less severe depths would likely have been very dry.
Today the evaporation from the Mediterranean Sea supplies moisture that falls in frontal storms, but without such moisture, the Mediterranean climate that we associate with Italy, Greece, and the Levant would be limited to the Iberian Peninsula and the western Maghreb. Climates throughout the central and eastern basin of the Mediterranean and surrounding regions to the north and east would have been drier even above modern sea level. The eastern Alps, the Balkans, and the Hungarian plain would also be much drier than they are today, even if the westerlies prevailed as they do now. However, the Paratethys ocean provided water to the area north of the Mediterranean basin. The Wallachian-Pontic and Hungarian basins were underwater during the Miocene, modifying the climate of what is now the Balkans and other areas north of the Mediterranean basin. The Pannonian Sea was a source of water north of the Mediterranean basin until the middle Pleistocene before becoming the Hungarian plain. Debate exists whether the waters of the Wallachian-Pontic basin (and the possibly connected Pannonian Sea) would have had access (thus bringing water) to at least the eastern Mediterranean basin at times during the Miocene.
The Messinian event provided also an opportunity to many African species, including antelopes, elephant and hippopotamus to migrate into the empty basin, obviously close to the descending great rivers, to reach interior wetter cooler highlands such as Malta: as the sea level was dropping, as such species would not have been able to cross the wide hot empty sink at maximum dryness. After the return of the sea water, they remained on the islands, where they underwent insular dwarfism during the Pleistocene as on Crete (H. creutzburgi), on Cyprus (H. minor), on Malta (H. melitensis) and Sicily (H. pentlandi). Of these, the Cyprus Dwarf Hippopotamus, survived until the end of the Pleistocene or early Holocene. But some of these species may have crossed the sea when it was flooded, washed out to sea on rafts of floating vegetation, or with some species (e.g. elephants) by swimming.
Synchronism vs Diachronism - Deep water vs shallow water evaporites
Some major questions remain concerning the beginning of the crisis in the central Mediterranean Basin. The geometric physical link between the evaporitic series identified in marginal basins accessible for field studies, such as the Tabernas basin and Sorbas basin, and the evaporitic series of the central basins has never been made.
Using the concept of deposition in both shallow and deep basins during the Messinian (i.e. assuming that both Basin types existed during this period), two major groupings are evident: one that favours a synchronous deposition (image c) of the first evaporites in all the basins before the major phase of erosion (Krijgsman et al., 1999); and the other that favours a diachronous deposition (image a) of the evaporites through more than one phases of desiccation which would first have affected the marginal basins and later the central basins (Clauzon et al., 1996).
Another school suggests that desiccation was synchronous, but occurred mainly in shallower basins. This model would suggest that the sea level of the whole Mediterranean basin fell at once, but only shallower basins dried out enough to deposit salt beds. See image b.
As highlighted in the work of van Dijk (1992) and van Dijk et al (1998) the history of dessication and erosion was complexely interacting with tectonic uplift and subsidence events, and erosional episodes. They also questioned wether the basins now observed as "deep" were actually also deep during the Messinian Episode and gave different names to the end-member scenario's described above.
Distinguishing between these hypotheses requires the calibration of gypsum deposits. Gypsum is the first salt (calcium sulphate) to be deposited from a desiccating basin. Magnetostratigraphy offers a broad constraint on timing, but no fine detail. Therefore, cyclostratigraphy is relied upon to compare the dates of sediments. The typical case study compares the gypsum evaporites in the main Mediterranean basin with those of the Sorbas basin, a smaller basin on the flanks of the Mediterranean Sea that is now exposed in southern Spain. The relationship between these two basins is assumed to represent the relationships of the wider region.
Recent work has relied on cyclostratigraphy to correlate the underlying marl beds, which appear to be have given way to gypsum at exactly the same time in both basins. The proponents of this hypothesis claim that cyclic variations in bed compositions are astronomically tuned, and the beds' magnitude can be calibrated to show they were contemporaneous - a strong argument. In order to refute it, it is necessary to propose an alternative mechanism for generating these cyclic bands, or for erosion to have coincidentally removed just the right amount of sediment everywhere before the gypsum was deposited. The proponents claim that the gypsum was deposited directly above the correlated marl layers, and slumped into them, giving the appearance of an unconformable contact. However, their opponents seize upon this apparent unconformity, and claim that the Sorbas basin was exposed - therefore eroding - while the Mediterranean sea was depositing evaporites. This would result in the Sorbas basin being filled with evaporites at 5.5 million years ago (Ma), compared to the main basin at 5.96 Ma.
Recent works have highlighted a pre-evaporite phase corresponding to a prominent erosional crisis (also named "Messinian Erosional crisis"; the termination of the "Mes-1" unconformity bound depositional sequence of van Dijk, 1992) responding to a major drawdown of the Mediterranean seawater. Assuming that this major drawdown corresponds to the major Messinian drawdown, they concluded that the Mediterranean bathymetry significantly decreased before the precipitation of central basins evaporites. Regarding these works, a deep water formation seems unlikely. The assumption that central basin evaporites partly deposited under a high bathymetry and before the major phase of erosion should imply the observation of a major detritic event above evaporites in the basin. Such a depositional geometry has not been observed on data. This theory corresponds to one of the end-member scenarios discussed by van Dijk et al. (1998) .
The water from the Mediterranean would have been redistributed in the world ocean, raising global sea level by as much as 10 meters (~33 feet).The Mediterranean basin also sequestered below its seabed a significant percentage of the salt from Earth's oceans; this decreased the average salinity of the world ocean and raised its freezing point.
Several possible causes of the series of Messinian crises have been considered. While there is disagreement on all fronts, the most general consensus seems to agree that climate had a role in forcing the periodic filling and emptying of the basins, and that tectonic factors must have played a part in controlling the height of the sills restricting flow between the Atlantic and Mediterranean. The magnitude and extent of these effects, however, is widely open to interpretation.
In any case, the causes of the closing and isolation of the Mediterranean Sea from the Atlantic Ocean must be found in the area where nowadays the Strait of Gibraltar is located. In that area, one of the tectonic boundaries between the African Plate and the European Plate and its southern fragments such as the Iberian Plate, is located. This boundary Zone is characterised by the presence of an Arc Shaped tectonic feature, the Alboran Arc, which unites southern Spain with northern Africa. In the Present Day area of the Mediterranean Sea, three of these Arc shaped belts are present: the Alboran Arc, the Calabrian Arc, and the Egean Arc. The kinematics and dynamics of this Plate Boundary and of the Alboran Arc during the late Miocene are strictly related to the causes of the Messinain Salinity Crisis: Tectonic reconfiguration may have closed and re-opened passages; the region where the connection with the Atlantic Ocean was situated is permeated by strike-slip faults and rotating blocks of continental crust. As faulting accommodated the regional compression caused by Africa's convergence with Eurasia, the geography of the region may have altered enough to open and close seaways. However, the precise tectonic activity behind the motion can be interpreted in a number of ways. An extensive discussion can be found in Weijermars (1988).
Any model must explain a variety of features of the area:
- Shortening and extension occur at the same time in close proximity; sediments constrain the rates of uplift and subsidence quite precisely
- Fault-bounded continental blocks can often be observed to rotate
- The depth and structure of the lithosphere is constrained by records of seismic activity, as well as tomography
- The composition of igneous rocks varies - this constrains the location and extent of any subduction.
- A moving subduction zone may have caused periodic regional uplift. Changes in volcanic rocks suggest that subduction zones at the rim of the Tethys Sea may have rolled back westwards, changing the chemistry and density in magma underlying the western Mediterranean. However, this does not account for the periodic emptying and refilling of the basin.
- The same features can be explained by regional delamination, the loss of a layer of the entire lithosphere.
- Deblobbing, the loss of a "blob" of lithospheric mantle, and the subsequent upward motion of the overlying crust (which has lost its dense mantle "anchor") may also have caused the observed phenomena, although the validity of the "deblobbing" hypothesis has been called into question.
This has led to some bizarre and interesting combinations of the models, in attempts to approach the true state of affairs.
Changes in climate must almost certainly be invoked to explain the periodic nature of the events. They occur during cool periods of Milankovic cycles, when less solar energy reached the Earth. This led to less evaporation of the North Atlantic, hence less rainfall over the Mediterranean. This would have starved the basin of water supply from rivers and allowed its desiccation.
Contrary to many people's instincts, there is now a scientific consensus that global sea level fluctuations cannot have been the major cause, although it may have played a role. The lack of ice caps at the time means there was no realistic mechanism to cause significant changes in sea level - there was nowhere for the water to go, and the morphology of ocean basins cannot change on such a short timescale.
When the Strait of Gibraltar was ultimately breached, the Atlantic Ocean would have poured a vast volume of water through what would have presumably been a relatively narrow channel. This refill has been envisaged as resulting in a large waterfall higher than today's Angel Falls (979 m), and far more powerful than either the Iguazu Falls or the Niagara Falls, but recent studies of the underground structures at the Gibraltar Strait show that the flooding channel descended in a rather gradual way to the dry Mediterranean.
In popular culture
Even before 1961, there had been speculations about a possible dehydration of the Mediterranean Sea in the distant past.
|1920s speculative map of 50,000 years ago|
- In the first century, Pliny the Elder recounted a popular story in his Natural History according to which the Mediterranean Sea was created when the Atlantic ocean gained admission through the Strait of Gibraltar:
- In 1920, H. G. Wells published a popular history book in which it was suspected that the Mediterranean basin had in the past been cut off from the Atlantic. One piece of physical evidence, a deep channel past Gibraltar, had been noticed. Wells estimated that the basin had refilled roughly between 30,000 and 10,000 B.C. The theory he printed was that:
Without the inflow from the Atlantic the Mediterranean would evaporate much more water than it receives, and would evaporate down to two large lakes, one on the Balearic Abyssal Plain, the other further east.
The east lake would receive most of the incoming river water, and may have overflowed into the west lake.
All or some of this seabed may have had a human population, where it was watered from the incoming rivers.
There is a long deep submerged valley running from the Mediterranean out into the Atlantic.
But the Strait of Gibraltar is 320 m deep, and global sea levels during the most recent Ice Age are believed to have lowered sea levels by only about 100 m, so the basin was not dry during the Ice Age.
The previous statement is a logical fallacy because it assumes that the Strait of Gibraltar's depth has not changed since the last Ice Age. If it is assumed, rather, that the Strait of Gibraltar was once less than 100 m below the present sea level, a global sea level drop of 100 m (that lasted for sufficient time) would indeed cut off the Mediterranean from the Atlantic. When the sea level rose at the end of the Ice Age to less than 100 m below the present sea level once more, it is conceivable that the mass of water as large as that fills the Mediterranean Sea today moving at a high speed could erode even 220 m. Thus it is possible, in principle, for such an event as H.G. Wells hypothesized, to have occurred.
That there had been an extreme drying event in the region earlier, was to be discovered forty years later.
|Atlantropa outline map (German)|
- Robert E. Howard's Conan the Barbarian stories (1932) picture a Hyborian Age where the Mediterranean Sea and Black Sea are both dry.
- Poul Anderson's Time Patrol story "Gibraltar Falls" (1975) takes place while the Atlantic begins to fill the Mediterranean Sea; here "falls" means "waterfall".
- Harry Turtledove's novella "Down in the Bottomlands," which takes place on an alternate Earth where the Mediterranean Sea stayed empty, and void of water, and part of it is a national park to the countries around it, none of which are nations that we are familiar with in the real world. In this continuum, other differences that might have developed are:
- Egypt as we know it would not have developed: little or no agriculture would be possible in the Nile's canyon, and the Nile's alluvial fan in the "Mediterranean Sink" would be too hot for human habitation. The Nile canyon would have been a major barrier to travel.
- Harsher climates and extensive zones of impassable desert between habitable areas rendered the economic and material basis of classical civilizations so different that any ancient civilizations in Phoenicia, Greece, Rome, and Carthage would have been very different or impossible.
- An early Dan Dare science fiction comic story says as a side remark that (in its time line) the Mediterranean stayed dry until Neolithic times and was flooded by alien spaceships exploding, breaking the Strait of Gibraltar open.
- Atlantropa, also referred to as Panropa, was a gigantic engineering and colonization project devised by the German architect Herman Sörgel in the 1920s and promulgated by him until his death in 1952. Its central feature was a hydroelectric dam to be built across the Strait of Gibraltar, and the lowering of the surface of the Mediterranean Sea by up to 200 metres. Similar projects have appeared in fiction.
- A draft version of the Space Odyssey series says that (in its time line) the Mediterranean bed stayed dry into human times and that the legend of Atlantis derived from the Mediterranean reflooding.
- The episode "The Vanished Sea" of the Animal Planet/ORF/ZDF-produced television series The Future Is Wild posits a world 5 million years in the future where the Mediterranean Basin has again dried up, and explores what kind of life could survive the new climate.
- Julian May's 1980's science fiction books The Many Coloured Land and The Golden Torc are set in Europe just before and during the rupture at Gibraltar. The rupture and the rapid filling of the Mediterranean form a Wagnerian climax to The Golden Torc, in which aliens and time-traveling humans are caught up in this cataclysm.
- The Gandalara Cycle by Randall Garrett and Vicki Ann Heydron chronicles the adventures of Ricardo, a modern earth man, sent into the past, where he discovers an entire civilization at the bottom of the dry Mediterranean.
- Wolfgang Jeschke's time-travel novel, The Last Day of Creation takes part during the MSC