Messinian Salinity Crisis - Causes

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 (see, e.g., van Dijk et al. (1998)).

In any case, the causes of the closing and isolation of the Mediterranean Sea from the Atlantic Ocean must be found in the area where nowadays the Strait of Gibraltar is located. In that area, one of the tectonic boundaries between the African Plate and the European Plate and its southern fragments such as the Iberian Plate, is located. This boundary Zone is characterised by the presence of an Arc Shaped tectonic feature, the Gibraltar Arc, which includes southern Spain and northern Africa. In the Present Day area of the Mediterranean Sea, three of these Arc shaped belts are present: the Gibraltar Arc, the Calabrian Arc, and the Aegean Arc. The kinematics and dynamics of this Plate Boundary and of the Gibraltar Arc during the late Miocene are strictly related to the causes of the Messinian Salinity Crisis: Tectonic reconfiguration may have closed and re-opened passages; the region where the connection with the Atlantic Ocean was situated 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. An extensive discussion can be found in Weijermars (1988).

Any model must explain a variety of features of the area:

• Shortening and extension occur at the same time in close proximity; sediments constrain the rates of uplift and subsidence quite precisely
• Fault-bounded continental blocks can often be observed to rotate
• The depth and structure of the lithosphere is constrained by records of seismic activity, as well as tomography
• The composition of igneous rocks varies — this constrains the location and extent of any subduction.

There are three contending geodynamic models that may fit the data, models which have been discussed in an equal way for the other Arc shaped features in the Mediterranean (see for a systematic review van Dijk & Okkes, 1990):

• A moving subduction zone may have caused periodic regional uplift. Changes in volcanic rocks suggest that subduction zones at the rim of the Tethys Sea may have rolled back westwards, changing the chemistry and density in magma underlying the western Mediterranean. However, this does not account for the periodic emptying and refilling of the basin.

• The same features can be explained by regional delamination, the loss of a layer of the entire lithosphere.

• Deblobbing, the loss of a "blob" of lithospheric mantle, and the subsequent upward motion of the overlying crust (which has lost its dense mantle "anchor") may also have caused the observed phenomena, although the validity of the "deblobbing" hypothesis has been called into question.

Of these, only the first model, invoking rollback, can explain the rotations observed. However, it is difficult to fit it with the pressure and temperature histories of some metamorphic rocks.

This has led to some bizarre and interesting combinations of the models, in attempts to approach the true state of affairs.

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, there is now a scientific consensus that global sea level fluctuations cannot have been the major cause, although it may have played a role. The lack of ice caps at the time means there was no realistic mechanism to cause significant changes in sea level — there was nowhere for the water to go, and the morphology of ocean basins cannot change on such a short timescale.