When the Mediterranean evaporated

Much to geologists’ surprise seismic surveys and drilling of the Mediterranean basin revealed that it is floored by an immense thickness of evaporite salts, laid down during the Late Miocene about 6 Ma ago (Messinian Stage).  The event has been dubbed the Messinian salinity crisis, and ascribed to the cutting off from the Atlantic of the Mediterranean Sea causing lowering of sea level by evaporation. The formation of the evaporite sequence has been overshadowed by what happened to restore the Mediterranean: a humongous waterfall at the Straits of Gibraltar. New modelling of the salt-forming event has had a technically surprising outcome (Govers, R. Choking the Mediterranean to dehydration: The Messinian salinity crisis. Geology, v. 37, p. 167-170). It suggests that most of the salt body formed before sea level fell. Sea level lowering reduced the load on the sea floor and allowed isostatic uplift to develop a flow barrier at the Straits of Gibraltar, further cutting off resupply of Atlantic water. The other factor seems to have been the effect of sluggish eastward subduction of a lithospheric slab that eventually resulted in subsidence so that the Atlantic could re-flood the Mediterranean basin.

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