Zanclean flood

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 name given to 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.

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.

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. The channel is eroded into the seafloor of the Alboran Sea, 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 formation of the channel mobilized about 1,000 cubic kilometres (240 cu mi) of rock, which was deposited in the Alboran Sea in the form of giant submarine bars. 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, but it was instantaneous by geological standards. 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 Strait of Gibraltar, increasing the flow of water which in turn increased the erosion until water levels rose enough in the Mediterranean to slow the flood.

In such a scenario, a peak discharge of over 100,000,000 cubic metres per second (3.5×10⁹ cu ft/s) occurred with water velocities of over 40 metres per second (131 ft/s; 144 km/h; 89 mph); such flow rates are about 450 times larger than the discharge of the Amazon River and ten times as much as the Missoula floods. This flood would have descended a relatively gentle ramp into the Mediterranean basin, not as a giant waterfall. Later simulations using more explicit geography constrain the flow to about 100 sverdrup or 100,000,000 cubic metres per second (3.5×10⁹ cu ft/s). They further indicate the formation of large gyres in the Alboran Sea during the flooding and that the flood eroded the Camarinal Sill at a rate of 0.4–0.7 metres per day (1.3–2.3 ft/d). The exact size of the flood depends on the pre-flood water levels in the Mediterranean and higher water levels there would result in a much smaller flood.