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, approximately 6 million years ago.
[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 head scientist 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
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 precursors 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. 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), and the Suez Canal closes, the Mediterranean would evaporate dry in about a thousand years.
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 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 and 8000 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 underwent repeated flooding and desiccation over 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 during later orogenies in Messina (Sicily), northeast Libya, Italy, and southern Spain.
[edit] Several cycles
The enormous volume of extant Messinian evaporites could not have been deposited during a single event.[2] Furthermore, the nature of the strata points strongly to several cycles of the Mediterranean Sea completely drying up and being refilled. 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. The cycle repeated itself at least eight or ten times during the million years that constituted the late Miocene Messinian stage." (Kenneth J. Hsu, The Mediterranean Was a Desert, Princeton University Press, Princeton, New Jersey 1983. A Voyage of the Glomar Challenger.)
[edit] Chronology
Based on palaeomagnetic datings of Messinian deposits tectonically brought above sea level, the salinity crisis started at the same time over all the Mediterranean basin, at 5.96 ± 0.02 million years ago. It 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) 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
The notion of a completely waterless Mediterranean Sea has some corollaries.
- The Strait of Gibraltar must have somehow reconfigured to disconnect the Mediterranean Sea from the Atlantic Ocean.
- The high level of salinity would have precluded almost all plant life or, by extension, animal life, making much of the basin a wasteland.[citation needed] 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).
- Rivers emptying into the basin would have cut their beds much deeper (at least a further 2400 m or 8000 feet with the Nile, as the buried canyon under Cairo shows). This later caused some consternation for the construction of the Aswan Dam, with an original river bed filled with debris 750 meters = c.2460 feet below sea level, although 1200 km away from the coast.
- 2 to 3 miles below sea level at 35°C would have resulted in 1.43 to 1.71 atm (1090 to 1300 mmHg) of air pressure at the very bottom. A wind blowing into the dry Mediterranean hollow, assuming dry adiabatic warming, would be about 30°C to 50°C hotter at the bottom than it was at sea level.[citation needed]
- 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 even if the westerlies prevailed as they do now. Civilizations characteristic of classical times would have been impossible in Egypt, Phoenicia, and Greece until the Mediterranean Sea filled.
[edit] Global effects
The water from the Mediterranean would have been redistributed in the world ocean, raising global sea level by anything up to 10 meters (~33 feet).[3] The Mediterranean basin also sequestered below its seabed a significant percentage of the salt from Earth's oceans; this decreased the average salinity of seawater and raised its freezing point.[4]
[edit] In popular culture
Had the breach in the Strait of Gibraltar not occurred, as it did not with the land around the Suez Canal, the course of human history could have been very different.
- An example of the possible differences can be found in Harry Turtledove's novel 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.
- Other differences that would likely have developed are:-[citation needed]
- Egypt as we know it could 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 would have 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.
[edit] Cause
Although several possible causes have been considered, including tectonic uplift or sea level drop due to glaciation, evidence has been found of a crust-mantle interaction. Changes in volcanic rocks suggest subducted Tethys Sea crust may have moved westward and changed the chemistry and density in magma underlying the western Mediterranean. The less dense material under the area could have raised it sufficiently to close the Atlantic connection.
[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 Niagara Falls or Victoria Falls.
[edit] References
- ↑ Clauzon, Georges, Suc, Jean-Pierre, Gautier, François, Berger, André, Loutre, Marie-France (1996). "Alternate interpretation of the Messinian salinity crisis: Controversy resolved?". Geology 24 (4): 363–366. DOI:<0363:AIOTMS>2.3.CO;2 10.1130/0091-7613(1996)024<0363:AIOTMS>2.3.CO;2
- ↑ 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
- Geology 212, Lecture 17: "When the Mediterranean Dried Up". (Accessed 7/16/06)
- W. KRIJGSMAN et al., "Chronology, causes and progression of the Messinian salinity crisis" Nature 400, 652 - 655
[edit] Bibliography
- Kenneth J. Hsu, The Mediterranean Was a Desert: A Voyage of the Glomar Challenger, Princeton University Press, ISBN 0-691-02406-5
