Messinian salinity crisis

The Messinian salinity crisis ended with the Strait of Gibraltar finally reopening 5.33 Ma, when the Atlantic rapidly filled up the Mediterranean basin in what is known as the Zanclean flood.

[8] Even today, the Mediterranean is considerably saltier than the North Atlantic, owing to its near isolation by the Strait of Gibraltar and its high rate of evaporation.

[9]In the 19th century, the Swiss geologist and paleontologist Karl Mayer-Eymar (1826–1907) studied fossils embedded between gypsum-bearing, brackish, and freshwater sediment layers, and identified them as having been deposited just before the end of the Miocene Epoch.

[14] At the same time, the salt was cored during Leg 13 of the Deep Sea Drilling Project conducted from the Glomar Challenger under the supervision of co-chief scientists William B. F. Ryan and Kenneth Hsu.

The first drilling of the Messinian salt at the deeper parts of the Mediterranean Sea came in the summer of 1970, when geologists aboard the 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.

In the Western Mediterranean series, the presence of pelagic oozes interbedded within the evaporites suggests that the area was repeatedly flooded and desiccated over 700,000 years.

[19] 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 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.

[22] 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 intertidal flat was eventually exposed by the final desiccation, at which time anhydrite was precipitated by saline ground water underlying sabkhas.

Recent work has relied on cyclostratigraphy to correlate the underlying marl beds, which appear to have given way to gypsum at exactly the same time in both basins.

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.

This theory corresponds to one of the end-member scenarios discussed by van Dijk et al.[20] 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.

This boundary zone is characterised by an arc-shaped tectonic feature, the Gibraltar Arc, which includes southern Spain and northern Africa.

The kinematics and dynamics of this plate boundary and of the Gibraltar Arc during the late Miocene are closely related to the causes of the Messinian salinity crisis.

Tectonic movements may have closed and re-opened passages, as the region where the connection with the Atlantic Ocean was situated is permeated by strike-slip faults and rotating blocks of continental crust.

[citation needed] Glacioeustatic sea level falls with an amplitude of around 10 metres (33 ft) that began approximately 6.14 Ma were likely responsible for modulating the connection between the Mediterranean and the Atlantic.

The extent of desiccation is very hard to judge, owing to the reflective seismic nature of the salt beds, and the difficulty in drilling cores, making it difficult to map their thickness.

Under this extreme assumption, maxima would be near 80 °C (176 °F) at the lowest points of the dry abyssal plain, permitting no permanent life but extremophiles.

Further, the altitude 3–5 km (2–3 mi) below sea level would result in 1.45 to 1.71 atm (1102 to 1300 mmHg) air pressure, further increasing heat stress.

In addition, the model results indicated global stationary wave response to the introduction of the topographic depression causes patterns of warming and cooling by up to 4 °C (7.2 °F) around the Northern Hemisphere.

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.

[63] 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.

[22] An enormous deposit of unsorted debris washed in by a massive catastrophic flood-wash has been found in the seabed southeast of the south corner of Sicily.