Messinian salinity crisis
The Messinian salinity crisis was an event in which the Mediterranean Sea went into a cycle of partial or nearly complete desiccation throughout the latter part of the Messinian age of the Miocene epoch, from 5.96 to 5.33 Ma. It ended with the Zanclean flood, when the Atlantic reclaimed the basin.
Sediment samples from below the deep seafloor of the Mediterranean Sea, which include evaporite minerals, soils, and fossil plants, show that the precursor of the Strait of Gibraltar closed about 5.96 million years ago, sealing the Mediterranean off from the Atlantic. This resulted in a period of partial desiccation of the Mediterranean Sea, the first of several such periods during the late Miocene. After the strait closed for the last time around 5.6 Ma, the region's generally dry climate at the time dried the Mediterranean basin out nearly completely within a thousand years. This massive desiccation left a deep dry basin, reaching deep below normal sea level, with a few hypersaline pockets similar to today's Dead Sea. Then, around 5.5 Ma, wetter climatic conditions resulted in the basin receiving more fresh water from rivers, progressively filling and diluting the hypersaline lakes into larger pockets of brackish water. 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.
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. If the Strait of Gibraltar closes again, the Mediterranean would mostly evaporate in about a thousand years, after which continued northward movement of Africa may obliterate the Mediterranean altogether.
Naming and first evidence
In the 19th century, the Swiss geologist and paleontologist Karl Mayer-Eymar 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. In 1867, he named the period the Messinian after the city of Messina in Sicily, Italy. Since then, several other salt-rich and gypsum-rich evaporite layers throughout the Mediterranean region have been dated to the same period.Further evidence and confirmation
Seismic surveying of the Mediterranean basin in 1961 revealed a geological feature some 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. The origin of this layer was largely interpreted as related to salt deposition. However, different interpretations were proposed for the age of salt and its deposition.Earlier suggestions from Denizot in 1952 and Ruggieri in 1967 proposed that this layer was of Late Miocene age, and the same Ruggieri coined the term Messinian salinity crisis.
New and high-quality seismic data on the M-reflector were acquired in the Mediterranean Basin in 1970. 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. These deposits were dated and interpreted for the first time as deep-basin products of the Messinian salinity crisis.
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. 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, mixed with quartz sand blown in from nearby continents, and ended up in a brine lake interbedded between two layers of halite. These layers alternated with layers containing marine fossils, indicating a succession of drying and flooding periods.
The massive presence of salt does not require a desiccation of the sea. The main evidence for the evaporative drawdown of the Mediterranean comes from the remains of many 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 200 metres below sea level at Aswan, and below sea level just north of Cairo.
In many places in the Mediterranean, fossilized cracks have been found where muddy sediment had dried and cracked in the sunlight and drought. 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.
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. This episode comprises the second part of what is called the "Messinian" age of the Miocene epoch. This age was characterised by several stages of tectonic activity and sea level fluctuations, as well as erosional and depositional events, all more or less interrelated.The Mediterranean-Atlantic strait closed tight time and time again, and the Mediterranean Sea, for the first time and then repeatedly, partially desiccated. 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 lowering of the Mediterranean sea level. During the initial, very dry stages, there was extensive erosion, creating several huge canyon systems around the Mediterranean. Later stages are marked by cyclic evaporite deposition into a large "lake-sea" basin.
About 5.33 million years ago, at the start of the Zanclean age, the barrier at the Strait of Gibraltar broke one last time, re-flooding the Mediterranean basin in the Zanclean flood, favouring slope destabilization. The basin has not desiccated since.
Several cycles
The amount of Messinian salts has been estimated as around and more than 1 million cubic kilometres, 50 times the amount of salt normally in the Mediterranean waters. This suggests either a succession of desiccations or a long period of hypersalinity during which incoming water from the Atlantic Ocean was evaporated with the level of the Mediterranean brine being similar to that of the Atlantic. The nature of the strata points strongly to several cycles of the Mediterranean Sea completely drying and being refilled, with drying periods correlating to periods of cooler global temperatures, 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:Research since then has suggested that the desiccation-flooding cycle may have repeated several times during the last 630,000 years of the Miocene epoch. This could explain the large amount of salt deposited. Recent studies, however, show that the repeated desiccation and flooding is unlikely from a geodynamic point of view.
Synchronism versus diachronism—deep water versus shallow water evaporites
Some major questions remain concerning the beginning of the crisis in the central Mediterranean Basin. The geometric physical link between the evaporitic series identified in marginal basins accessible for field studies, such as the Tabernas Desert and Sorbas Basin, and the evaporitic series of the central basins has never been made.Using the concept of deposition in both shallow and deep basins during the Messinian, two major groupings are evident: one that favours a synchronous deposition of the first evaporites in all the basins before the major phase of erosion; and the other that favours a diachronous deposition of the evaporites through more than one phases of desiccation which would first have affected the marginal basins and later the central basins.
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.
As highlighted in the work of van Dijk
and van Dijk et al. the history of desiccation and erosion was complexly interacting with tectonic uplift and subsidence events, and erosional episodes. They also questioned again like some previous authors had done, whether the basins now observed as "deep" were actually also deep during the Messinian Episode and gave different names to the end-member scenarios described above.
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 have given way to gypsum at exactly the same time in both basins.
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. However, their opponents seize upon this apparent inconformity, 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 .
Recent works have highlighted a pre-evaporite phase corresponding to a prominent erosional crisis responding to a major drawdown of the Mediterranean seawater.
Assuming that this major drawdown corresponds to the major Messinian drawdown, they concluded that the Mediterranean bathymetry significantly decreased before the precipitation of central basins evaporites. Regarding these works, a deep water formation seems unlikely. The assumption that central basin evaporites partly deposited under a high bathymetry and before the major phase of erosion should imply the observation of a major detritic event above evaporites in the basin. Such a depositional geometry has not been observed on data. This theory corresponds to one of the end-member scenarios discussed by van Dijk et al.