The Zanclean flood or Zanclean deluge is a flood 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 reconnection marks the beginning of the Zanclean age.
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 have reached rates at times greater than . 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 with a maximum discharge of about , about 1,000 times that of the present-day Amazon River. Studies of the underground structures at the Gibraltar Strait 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 latest Miocene the Mediterranean was severed from the Atlantic Ocean and partly dried up when the Guadalhorce and Rifian corridors that had previously connected the Mediterranean to the Atlantic closed, triggering 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, with some water putatively originating in the Paratethys north of the Mediterranean.
Event
The Zanclean flood occurred when the Strait of Gibraltar opened. Tectonic subsidence of the Gibraltar region may have lowered the sill until it breached. The exact triggering event is not known with certainty; faulting or sea level rise are debatable. The most widely accepted hypothesis is that a stream flowing into the Mediterranean eroded through the Strait of Gibraltar until it captured the Atlantic Ocean and that the Strait did not exist before this erosion event.
During the flood, a channel formed across the Strait of Gibraltar, which starts at the Camarinal Sill in the Strait of Gibraltar, splits around the Vizconde de Eza high of the Alboran Sea and eventually connects with the Alboran Channel before splitting into several branches that end in the Algero-Balear basin. The channel has a U-like shape in its starting region, which is consistent with its formation during a giant flood. The sector of the Zanclean channel that passes through the Camarinal Sill may have a different origin, however.
Whether the Zanclean flood occurred gradually or as a catastrophic event is controversial. The magnitude of a catastrophic flood has been simulated by modelling. One single-dimensional model assumes a catastrophic flood of more than 10–100 sverdrup. Another estimate assumes that after the first breach of the sill, the flowing water eroded the threshold and formed the channel across the Gibraltar strait, increasing the flow of water which in turn increased the erosion until water levels rose enough in the Mediterranean to slow the flood.