Messinian salinity crisis

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The Messinian Salinity Crisis, also referred to as the Messinian Event, is a period when the Mediterranean Sea evaporated partly or completely dry during the Messinian period of the Miocene epoch, 5.96 million years ago.^[3]

[edit] Naming

The first observation of the result of the crisis may have been when the period's name was created. In the mid 1800s, Professor Charles Mayer-Eymar (1826-1907) of Zurich studied some fossils between gypsum layers and identified them as being just before the end of the Miocene Epoch. He named the period the Messinian, for the region of Messina, Sicily; salt-bearing and gypsum-bearing layers in many Mediterranean countries have been dated to that period.^[4]

[edit] Discovery

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. Drilling experiments, conducted a decade later from the Glomar Challenger under the supervision of co-chief scientists William B.F. Ryan and Kenneth J. Hsu during Leg 13 of the Deep Sea Drilling Program, revealed the nature of the M reflector, a layer of evaporites up to 3 kilometres thick.

[edit] Evidence

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.

Sediment samples from below the deep seafloor of the Mediterranean Sea, which include evaporite minerals, soils, and fossil plants, show that about 5.9 million years ago in the late Miocene period the precursor of the modern Strait of Gibraltar closed tight, and the Mediterranean Sea evaporated into a deep dry basin with a bottom at some places 2 to 3 miles (3.2 to 4.9 km) below the world ocean level.^[5] Even now the Mediterranean is saltier than the North Atlantic because of its near isolation by the Straits 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 distant on a human time scale), the Mediterranean would evaporate dry in about a thousand years.^[6]

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 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 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. 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 (2,400 m) below sea level under Cairo. Fossilized cracks were found where muddy sediment had dried and cracked in the sunlight and drought. The area was repeatedly flooded and desiccated over the course of 700,000 years. About 5.4 million years ago, at the start of the Pliocene period the barrier at the Strait of Gibraltar broke, permanently reflooding the basin.

Some of these Messinian deposits have since been pushed up onto land by later tectonic activity in Messina (Sicily), northeast Libya, Italy, and southern Spain.

[edit] Extent

There is not currently a consensus as to whether the Mediterranean Sea 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, especially as the reflective seismic nature of the salt beds, and the difficulty in drilling cores, make it difficult to map their thickness.

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[edit] Several cycles

The enormous volume of extant Messinian evaporites could not have been deposited during a single event.^[7] Furthermore, the nature of the strata points strongly to several cycles of the Mediterranean Sea completely drying up and being refilled, with "drying up" 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:

[edit] Chronology

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. The basin was isolated from the Atlantic Ocean between 5.59 and 5.33 million years ago, resulting in a huge decrease in the Mediterranean sea level. During the initial stages (5.59 - 5.50 million years ago) there was extreme erosion, creating several huge canyon systems (some similar in scale to the Grand Canyon) around the Mediterranean. Later stages (5.50 - 5.33 million years ago) are marked by cyclic evaporite deposition into a large "lake-sea" basin.

[edit] Dehydrated geography

A possible palaeogeographical reconstruction of the Miocene Mediterranian. North to the left. *Red = current coastline *S = Sorbas basin, Spain *R = Rifean corridor *B = Betic corridor *G = Strait of Gibraltar *M = Mediterranean sea

The notion of a completely waterless Mediterranean Sea has some corollaries.

1. At the time, the Strait of Gibraltar was not open, but other seaways (The Betic corridor to the north, the Rifean corridor or corridors to the south) linked the Mediterranean to the Atlantic. These must have closed to isolate the basin from the open ocean.
2. The high level of salinity cannot be tolerated by many organisms, 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, evidence supported by the presence of anhydrite, which is only deposited in water warmer than 35°C (95°F).^{[citation needed]}
3. 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).
4. 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; 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. 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.^{[citation needed]}

[edit] Multiple basins

Hypotheses of evaporite formation during the MSC.
a: Diachronous deposition: Evaporites (pink) were deposited in landward basins first, and closer to the Atlantic as the extent of the Mediterranean Sea (dark blue) diminished towards the gateway. The light blue shows the original sea level.
b: Synchronous deposition in marginal basins. Sea level drops slightly, but the whole basin is still connected to the Atlantic. Reduced inflow allows the accumulation of evaporites in shallow basins only.
c: Synchronous, basin-wide deposition. Closure or restriction of the Atlantic seaway by tectonic activity (dark grey) causes evaporite deposition simultaneously across the entire basin; the basin may not need to empty completely, as salts are concentrated by evaporation.

As the level of the Mediterranean fell, smaller basins on its flanks became isolated from the main sea. The timing and mode of the desiccation of these smaller basins, such as the Tabernas basin and Sorbas basin, is still debated.

One school of thought holds that there was still limited connection to the open ocean, so basins further from the seaway dried out first, with shallow lakes between the deepest parts of the Mediterranean and the gateway. In other words, deposition was diachronous, as in image a.

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.

The final suggestion is that desiccation was synchronous and basin wide, with sea level potentially falling to rock bottom. Salts would be deposited in all Mediterranean basins, as in envisioned in image c.

Distinguishing between these hypotheses requires the calibration of gypsum deposits. Gypsum is the first salt 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.^[9] 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.^[9] 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.^[10][11]

[edit] Global effects

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).[1] 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.^[12]

[edit] Causes

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!

Tectonic reconfiguration may have closed and re-opened passages; the region 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.

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 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.

There are three contenders that may fit the data.

• A moving subduction zone may have caused 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.^[13] However, this does not account for the periodic emptying and refilling of the basin.

• The same features can be explained by regional delamination,^[14] the loss of a layer of the entire lithosphere.^[15]

• "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,^[16] although the validity of the "deblobbing" hypothesis has been called into question.^[17]

Of these, only rollback can explain the rotations observed. However, it is difficult to fit it with the pressure and temperature histories of some metamorphic rocks.^[18] This has led to some bizarre and interesting combinations of the models, in attempts to approach the true state of affairs.^[19][20]

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, global sea level fluctuations cannot have played a role. The lack of ice caps at the time means there is no realistic mechanism to cause significant changes in sea level - there's nowhere for the water to go, and the morphology of ocean basins cannot change on such a short timescale.

[edit] Replenishment

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. The resulting waterfall could have been higher than Angel Falls is today (979 meters), and far more powerful than either Iguassu Falls, Niagara Falls, or Victoria Falls.

[edit] In popular culture

1920s speculative map of 50,000 years ago

Even before 1961, there had been speculations about a possible dehydration of the Mediterranean Sea in the distant past.

• 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.^[21] The theory he printed was that: ^[21]
  • In the last Ice Age, so much ocean water was taken into the icecaps that world ocean level dropped below the sill in the Strait of Gibraltar.
  • 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. That there had been an extreme drying event in the region earlier, was to be discovered forty years later.

• Robert E. Howard's Conan the Barbarian stories (1932) picture a Hyborian Age where the Mediterranean Sea and Black Sea are both dry.
• 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.
• 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 prior to and during the rupture at Gibraltar. The rupture and the rapid filling of the Mediterranean form a Wagnerian climax to the The Golden Torc, in which aliens and time-traveling humans are caught up in this cataclysm.

[edit] Footnotes and references

[edit] Bibliography

• Kenneth J. Hsu, The Mediterranean Was a Desert: A Voyage of the Glomar Challenger, Princeton University Press, ISBN 0-691-02406-5

[edit] External links

• Arizona University: Geology 212, Lecture 17: "When the Mediterranean Dried Up". (Accessed 7/16/06)

1. The Messinian Salinity Crisis by Ian West (Internet Archive copy)
2. The Messinian Salinity Crisis by Rob Butler
3. Messinian online