When the Mediterranean dried up

At the end of the Miocene (from 6 to 5.3 Ma) the connection between the Atlantic Ocean and the Mediterranean Sea was blocked somehow.  Over 700 thousand years evaporation deposited a thick layer of salt that now lies beneath much of the Mediterranean basin.  This is known as the Messinian salt crisis.  Equally dramatic, the straits reopened suddenly to allow seawater to flood back in the early Pliocene, in a hydrological catastrophe.  How the Mediterranean basin became cut off has been ascribed to a 60 m sea-level fall, crustal shortening associated with nappe formation in the Betic Cordillera of Spain and the Atlas mountains, or by some kind of tectonic uplift.  Timing of the Messinian crisis rules out the first two options, but sedimentation in the former “gateway”, a shallow seaway through what is now southern Spain, shows evidence of rapid shallowing that would have resulted from regional uplift.  The question is, what drove this regional upwarping?  A team from the GEOMAR Research Centre for Marine Geosciences in Kiel, Germany has discovered evidence from the changing geochemistry of Miocene to Pliocene volcanic rocks in the western part of the Mediterranean (Duggen, S. et al, 2003.  Deep roots of the Messinian salinity crisis.  Nature, v. 422, p. 602-606).  Send Duggan and his co-workers found that the lavas underwent a geochemical shift  from affinities with subduction-zone processes to those typical of intra-plate magmatism around 6.3 Ma ago, volcanism largely ending about 4.8 Ma.  The ending in the late Miocene of eastward subduction of Tethyan sea floor beneath the Mediterranean, which had initiated volcanism around 12 Ma, led to foundering of part of the lithosphere and uprise of asthenosphere.  This is marked by a change from high-silica, early magmas to alkaline, more basaltic varieties during the period of the Messinian salinity crisis.  Uplift resulting from this delamination would have pushed the formed connections between the Atlantic and the Mediterranean as much as 800 m above sea level.  Duggan et al. Suggest that the axis of uplift gradually migrated westwards, so that by the end of the Messinian crisis the area now centred on the Straits of Gibraltar would have been bulged up.  Massive gravitational sliding from this edge of the continental lithosphere into the Atlantic may then have opened the narrow passage through which Atlantic water once again flooded.

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