Messinian salinity crisis

The Messinian Salinity Crisis, also referred to as the Messinian Event, is the name given to a period when the Mediterranean Sea evaporated partly or completely dry during the Messinian period of the Miocene epoch, approximately 6 million years ago.

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 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 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. 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 were 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 Italy and Sicily.

Several cycles

The enormous volume of extant Messinian evaporites could not have been deposited during a single event.^* Furthermore, the nature of the strata themselves point strongly to several cycles of the Mediterranean Sea completely drying up and being refilled. 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 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.)

Chronology

The onset of the salinity crisis is synchronous over the entire Mediterranean basin, dated at 5.96 ± 0.02 million years ago. Isolation from the Atlantic Ocean was established between 5.59 and 5.33 million years ago, resulting in a significant 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.

Dehydrated geography

The notion of a completely dehydrated Mediterranean Sea has some fascinating 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. Furthermore, the basin's low altitude would have made it extremely hot during the summer, evidence supported by the presence of anhydrite; it 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 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 below sea level, although 1200 km away from the coast.

Global effects

The waters from the Mediterranean would have been redistrubuted 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 seawater and raised its freezing point.[http://www.geo.arizona.edu/geo2xx/geo212.034/Lect17.html This might have allowed seawater to more readily froze into sea ice, decreasing the Earth's temperature. The weather patterns around the Mediterranean, eventually home to many nascent cultures, were likely to have been significantly altered during the dry periods, presenting dramatically different conditions to migrating life. 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.

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.

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.

References

External links

http://www.soton.ac.uk/~imw/messin.htm
http://earth.leeds.ac.uk/tectonics/messinian/
http://www.messinianonline.it/

Regional geology | Mediterranean

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