WikiPortallus: Messinian Salinity Crisis

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]

Naming

In the first observation of the result of the episode, Professor Charles Mayer-Eymar (1826-1907) of Zurich studied some fossils between gypsum-bearing, brackish and freshwater layers and identified them as having been deposited just before the end of the Miocene Epoch. In a publication of 1867 he named the period the Messinian, for the region of Messina, Sicily; since then salt-bearing and gypsum-bearing layers in many Mediterranean countries have been dated to that period.^[4]

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.

Evidence

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 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. 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.

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.

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:

Dehydrated geography

Climate

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:

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 below sea level would be about 40°C warmer than the temperature at sea level. Thus one could predict theoretical temperature maximums of around 80°C at the lowest depths of the valley permitting little known life to survive there. One can also calculate that 2 to 3 miles below sea level would have resulted in 1.45 to 1.71 atm (1,090 to 1,310 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.^{citation needed}

Multiple basins

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 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.^[10] 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.^[10] 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.^[11]^[12]

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.^[7]

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.

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.

Replenishment

In popular culture

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

Footnotes and references

^ Retallack, G.J. (1997). "Neogene Expansion of the North American Prairie". PALAIOS 12 (4): 380-390. Retrieved on 2008-02-11.
^ Zachos, J.C.; Kump, L.R. (2005). "Carbon cycle feedbacks and the initiation of Antarctic glaciation in the earliest Oligocene". Global and Planetary Change 47 (1): 51-66.
^ ^a ^b Krijgsman, W.; Garcés, M.; Langereis, C.G.; Daams, R.; Van Dam, J.; Van Der Meulen, A.J.; Agustí, J.; Cabrera, L. (1996). "A new chronology for the middle to late Miocene continental record in Spain". Earth and Planetary Science Letters 142 (3-4): 367-380. Retrieved on 2008-03-01.
^ Kenneth J. Hsu, The Mediterranean Was a Desert, Princeton University Press, Princeton, New Jersey 1983. A Voyage of the Glomar Challenger.
^ Clauzon, Georges, Suc, Jean-Pierre, Gautier, François, Berger, André, Loutre, Marie-France (1996). "Alternate interpretation of the Messinian salinity crisis: Controversy resolved?" (abstract). Geology 24 (4): 363–366. doi:10.1130/0091-7613(1996)024<0363:AIOTMS>2.3.CO;2 (inactive 2008-06-23).
^ "Only the inflow of Atlantic water maintains the present Mediterranean level. When that was shut off sometime between 6.5 to 6 MYBP, net evaporative loss set in at the rate of around 3,300 cubic kilometers yearly. At that rate, the 3.7 million cubic kilometres of water in the basin would dry up in scarcely more than a thousand years, leaving an extensive layer of salt some tens of meters thick and raising global sea level about 12 meters." Cloud, P. (1988). Oasis in space. Earth history from the beginning, New York: W.W. Norton & Co. Inc., 440. ISBN 0-393-01952-7
^ ^a ^b Lecture 17: Mediterranean
^ Hsu, K.J. (1983). "A Voyage of the Glomar Challenger", The Mediterranean Was a Desert", Princeton University Press, Princeton, New Jersey.
^ Part 10: The Mediterranean Was a Desert
^ ^a ^b Krijgsman, W.; Fortuin, A.R.; Hilgen, F.J.; Sierro, F.J. (2001). "Astrochronology for the Messinian Sorbas basin (SE Spain) and orbital (precessional) forcing for evaporite cyclicity". Sedimentary Geology 140 (1-2): 43–60. doi:10.1016/S0037-0738(00)00171-8.
^ Riding, R.; Braga, J.C.; Martín, J.M. (2000). "Late Miocene Mediterranean desiccation: topography and significance of the ‘Salinity Crisis’ erosion surface on-land in southeast Spain: Reply". Sedimentary Geology 133 (3-4): 175–184. doi:10.1016/S0037-0738(00)00039-7.
^ Braga, J.C.; Martín, J.M.; Riding, R.; Aguirre, J.; Sánchez-almazo, I.M.; Dinarès-turell, J. (2006). "Testing models for the Messinian salinity crisis: The Messinian record in Almería, SE Spain". Sedimentary Geology 188: 131–154. doi:10.1016/j.sedgeo.2006.03.002.
^ Lonergan, L.; White, N. (1997). "Origin of the Betic-Rif mountain belt". Tectonics 16 (3): 504–522. doi:10.1029/96TC03937. Retrieved on 2008-04-04.
^ Turner, S. (1999). "Magmatism Associated with Orogenic Collapse of the Betic-Alboran Domain, SE Spain". Journal of Petrology 40 (6): 1011–1036. doi:10.1093/petrology/40.6.1011. Retrieved on 2008-04-04.
^ Seber, D.; Barazangi, M.; Ibenbrahim, A.; Demnati, A. (1996). "Geophysical evidence for lithospheric delamination beneath the Alboran Sea and Rif--Betic mountains". Nature 379 (6568): 785–790. doi:10.1038/379785a0.
^ Platt, J.P.; Vissers, R.L.M. (1989). "Extensional collapse of thickened continental lithosphere; a working hypothesis for the Alboran Sea and Gibraltar Arc" (abstract). Geology 17 (6): 540–543. doi:10.1130/0091-7613(1989)017<0540:ECOTCL>2.3.CO;2. Retrieved on 2008-04-04.
^ Jackson, J.A.; Austrheim, H.; McKenzie, D.; Priestley, K. (2004). "Metastability, mechanical strength, and the support of mountain belts". Geology 32 (7): 625–628. doi:10.1130/G20397.1.
^ Platt, J.P.; Soto, J.I.; Whitehouse, M.J.; Hurford, A.J.; Kelley, S.P. (1998). "Thermal evolution, rate of exhumation, and tectonic significance of metamorphic rocks from the floor of the Alboran extensional basin, western Mediterranean" (abstract). Tectonics 17 (5): 671–689. doi:10.1029/98TC02204. Retrieved on 2008-04-04.
^ Jolivet, L.; Augier, R.; Robin, C.; Suc, J.P.; Rouchy, J.M. (2006). "Lithospheric-scale geodynamic context of the Messinian salinity crisis". Sedimentary Geology 188: 9–33. doi:10.1016/j.sedgeo.2006.02.004.
^ Svend Duggen, Kaj Hoernle, Paul van den Bogaard, Lars Rüpke and Jason Phipps Morgan (2003). "Deep roots of the Messinian salinity crisis". Nature 422 (6932): 602–6. doi:10.1038/nature01553.
^ ^a ^b Wells, H. G. (1920). The Outline of History. Garden City, New York: Garden City Publishing Co., Inc..

Messinian Salinity Crisis

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