The Zanclean flood, also called the Zanclean deluge, is believed to have filled the Mediterranean Sea again about 5.33 million years ago. This event ended the Messinian salinity crisis and reconnected the Mediterranean Sea to the Atlantic Ocean. It is possible that the sea had some connection to the Atlantic Ocean even before the flood. This reconnection marks the start of the Zanclean age, which is the earliest period in the geological time scale of the Pliocene.
According to this model, water from the Atlantic Ocean filled the dry Mediterranean basin through the modern Strait of Gibraltar. Ninety percent of the flooding happened quickly over a time period estimated to be between several months and two years. This followed long periods of very slow water flow that may have lasted thousands of years. At times, the sea level in the basin may have risen more than 10 meters (30 feet) each day. Based on erosion features found in Pliocene sediment, Garcia-Castellanos et al. think that water flowed down a drop of more than 1,000 meters (3,300 feet) with a maximum flow rate of about 100 million cubic meters per second (3.5 billion cubic feet per second). This is 1,000 times greater than the flow of the modern Amazon River. Studies of structures below the ground at the Strait of Gibraltar show that the path for the water went down slowly toward the bottom of the basin rather than forming a steep waterfall.
Background
The geologic history of the Mediterranean is influenced by the movement of the African Plate, the Arabian Plate, and the Eurasian Plate. These plates caused the Tethys Ocean to shrink until its western part became the modern Mediterranean Sea. During the late Miocene, the Mediterranean became separated from the Atlantic Ocean. This happened when the Guadalhorce and Rifian corridors, which once connected the Mediterranean to the Atlantic, closed. This event led to the Messinian Salinity Crisis, during which thick salt layers formed on the ocean floor and the slopes of nearby continents were eroded. The Nile and Rhône rivers carved deep canyons during this time. Water levels in the Mediterranean dropped by several kilometers. However, the exact amount of the drop and whether it affected the Western and Eastern Mediterranean equally is unknown. It is possible that some seas remained connected on the Mediterranean floor.
Evidence from Messinian deposits, including Atlantic fish fossils and the large amount of salt deposited, suggests that some water still flowed from the Atlantic into the Mediterranean before the Zanclean flood. Before this flood, increased rainfall and runoff lowered the salinity of the remaining sea, leading to the formation of "Lago Mare" sediments. Changes in water levels may have occurred, with water possibly coming from the Paratethys, a body of water north of the Mediterranean, through processes involving the capture of water from other bodies.
Event
The Zanclean flood happened when the Strait of Gibraltar opened. Movement of Earth’s crust in the Gibraltar region may have lowered a natural barrier until it broke. Scientists are not sure exactly what caused this event, but some believe it was linked to shifting land or rising sea levels. The most accepted idea is that a stream flowing into the Mediterranean Sea eroded through the Strait of Gibraltar until it connected with the Atlantic Ocean. This suggests the Strait did not exist before this erosion happened.
During the flood, a channel formed across the Strait of Gibraltar, beginning at the Camarinal Sill. This channel was carved into the ocean floor of the Alboran Sea, split around a high area in the Alboran Sea, and then connected to the Alboran Channel. It later split into several paths leading to the Algero-Balear basin. The channel’s starting area had a U-shaped form, which matches its creation during a large flood. This process moved about 1,000 cubic kilometers of rock, which was deposited as large underwater ridges in the Alboran Sea. However, the part of the channel passing through the Camarinal Sill may have formed differently.
Scientists debate whether the Zanclean flood happened slowly or suddenly. However, it occurred very quickly by geological standards. Models suggest the flood could have been extremely large, with one estimate showing a flow rate of more than 10 to 100 sverdrup. Another model suggests that after the initial opening of the barrier, water erosion created the channel, increasing the flow until water levels in the Mediterranean rose enough to slow the flood. In this scenario, the flood reached a peak speed of over 100,000,000 cubic meters of water per second, with water moving faster than 40 meters per second. This is about 450 times the flow of the Amazon River and 10 times the flow of the Missoula floods. The water would have flowed into the Mediterranean Sea over a gentle slope, not as a waterfall. Later models suggest the flow was about 100 sverdrup or 100,000,000 cubic meters per second. These models also show large ocean currents formed in the Alboran Sea and that the Camarinal Sill eroded at a rate of 0.4 to 0.7 meters per day. The flood’s size depended on water levels in the Mediterranean before the flood, with higher levels leading to a smaller flood.
At first, the flood only affected the Western Mediterranean because the Malta Escarpment (a rock formation near the present-day Straits of Sicily) acted as a barrier between the Western and Eastern Mediterranean. Water may have flowed into the Eastern Mediterranean through the Noto Canyon across the Malta Escarpment. During this time, swirling water currents and reverse flows carried large amounts of sediment into the Ionian Sea. Early ideas suggested the Eastern Mediterranean would take thousands of years to refill, but later research shows it may have taken much less time, possibly less than a year.
A large flood is not the only explanation for the Mediterranean reconnecting with the Atlantic. Some scientists suggest the Mediterranean may have been refilled more slowly through other water sources or if the Strait of Gibraltar remained open all the time. Evidence from the southern edge of the Alboran Sea suggests a catastrophic flood may not have occurred. However, deposits near the Malta Escarpment indicate a powerful flood may have caused the connection across the Straits of Sicily.
The timing of the Zanclean flood is unclear, but one possibility is that it occurred about 5.33 million years ago. This time is usually linked to the end of the Messinian/Miocene period and the start of the Zanclean/Pliocene period. Some scientists think a smaller flood may have happened before the main event. Deep-sea terraces suggest the Mediterranean may have been refilled in several stages. It is estimated that completely filling the Mediterranean could have taken about a decade.
Consequences
The Zanclean flood formed the Strait of Gibraltar. It is unlikely that earth movements or volcanic activity created the strait, as the main boundaries between tectonic plates do not pass through the strait, and there is little earthquake activity in the area. The current shape of the strait includes two underwater ridges called sills: the Camarinal Sill, which is 284 meters (932 feet) deep at its deepest point, and the deeper Espartel Sill farther west. The narrowest part of the strait is located east of these sills and is much deeper than the sills themselves. These sills may have formed after the flood due to the movement of nearby land caused by gravity.
The Zanclean flood changed the environment of the Mediterranean basin. A type of land deposit called the "Lago Mare" facies was replaced by deep sea deposits from the Zanclean period. The flood might have affected global climate, as a smaller flood caused by the draining of Lake Agassiz led to a cold period. Evidence of this effect was found as far away as the Loyalty Ridge near New Caledonia in the Southern Hemisphere.
Rising sea levels caused the deeply carved Nile River to become a ria, extending inland as far as Aswan, about 900 kilometers (560 miles) from the modern coastline. The Zanclean flood led to the final separation of Mediterranean islands like Crete and disrupted ecosystems, which caused new species of plants and animals to develop there. However, the formation of the Strait of Gibraltar blocked land animals from moving between Africa and Europe. When the strait reconnected, sea animals such as whales, dolphins, seals, and others were able to move from the Atlantic Ocean into the Mediterranean Sea.
Evidence of the flood has been found in sediments from the Zanclean period, both in drilled samples and in sediments that were later lifted above sea level. A clear erosion layer separates the older, pre-Zanclean sediments from the younger, marine sediments.
Water from the flood likely overflowed into the Ionian Sea through Sicily and the Noto submarine canyon near Avola. This overflow was as large as the flood through the Strait of Gibraltar. The speed at which the Mediterranean filled during the flood was enough to cause significant earthquakes. These earthquakes likely triggered large landslides, which could have created tsunamis with waves up to 100 meters (330 feet) high. Evidence of these tsunamis has been found in the Algeciras Basin. The filling of the basin created tectonic stress, which influenced the formation of the Apennine Mountains.
Similar megafloods
Similar floods have happened in other places on Earth throughout history. Examples include the Bonneville flood in North America, when Lake Bonneville flowed through Red Rock Pass into the Snake River Basin, and the Black Sea deluge hypothesis, which suggests a flood from the Mediterranean into the Black Sea through the Bosporus.
Research history
In his book Historia Naturalis, Pliny the Elder wrote about a story that Hercules dug the Strait of Gibraltar, linking the Mediterranean Sea and the Atlantic Ocean. However, the Zanclean flood theory, which explains how the Mediterranean was refilled with water, was not proposed until the 1970s. Scientists discovered that salt deposits and a large area of eroded rock in the Mediterranean were formed during a long period when sea levels were very low. Later, the Mediterranean was quickly refilled with water, likely happening in just a few thousand years or less.