The Zanclean flood, or Zanclean deluge, is theorized to have refilled the Mediterranean Sea 5.33 million years ago. This flooding ended the Messinian salinity crisis and reconnected the Mediterranean Sea to the Atlantic Ocean, although it is possible that even before the flood there were partial connections to the Atlantic Ocean. The re-connection marks the beginning of the Zanclean age, the earliest age on the geologic time scale of the Pliocene. According to this model, water from the Atlantic Ocean refilled the dried-up basin through the modern-day Strait of Gibraltar. Ninety percent of the Mediterranean Basin flooding occurred abruptly during a period estimated to have been between several months and two years, following low water discharges that could have lasted for several thousand years. Sea level rise in the basin may at times have reached rates greater than 10 metres per day (5 fathom/d; 30 ft/d). Based on the erosion features preserved until modern times under the Pliocene sediment, Garcia-Castellanos et al. estimate that water rushed down a drop of more than 1,000 metres (3,300 ft) with a maximum discharge of about 100 million cubic metres per second (3.5 billion cubic feet per second), three orders of magnitude larger than the present-day Amazon River. Studies of the underground structures at the Strait of Gibraltar show that the flooding channel descended gradually toward the bottom of the basin rather than forming a steep waterfall.
Background The geologic history of the Mediterranean is governed by plate tectonics involving the African Plate, the Arabian Plate and the Eurasian Plate which shrank the previously existing Tethys Ocean until its western part became the present-day Mediterranean. For reasons not clearly established, during the late Miocene the Mediterranean was severed from the Atlantic Ocean. It partly dried up when the Guadalhorce and Rifian corridors that had previously connected the Mediterranean to the Atlantic closed. This triggered the Messinian Salinity Crisis with the formation of thick salt deposits on the former seafloor and erosion of the continental slopes. The Nile and Rhône carved deep canyons during this time. Water levels in the Mediterranean during this time dropped by kilometres. The exact magnitude of the drop, and whether it was symmetric between the Western Mediterranean and the Eastern Mediterranean, is unclear; it is possible that interconnected seas remained on the floor of the Mediterranean. The presence of Atlantic fish in Messinian deposits and the volume of salt deposited during the Messinian Salinity Crisis implies that there was some remnant flow from the Atlantic into the Mediterranean even before the Zanclean flood. Already before the Zanclean flood, increased precipitation and runoff had lowered the salinity of the remnant sea, leading to the deposition of the so-called "Lago Mare" sediments. Fluctuations in water levels may have taken place, with water originating in the Paratethys north of the Mediterranean through waterbody capture processes.
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