The Zanclean flood, or Zanclean deluge, is believed to have filled the Mediterranean Sea again about 5.33 million years ago. This flooding ended the Messinian salinity crisis and reconnected the Mediterranean Sea to the Atlantic Ocean. It is possible that there were already some connections to the Atlantic Ocean before the flood. This reconnection marks the start of the Zanclean age, the earliest period in the Pliocene era on the geologic time scale.
According to this model, water from the Atlantic Ocean filled the dried-up Mediterranean basin through the modern-day Strait of Gibraltar. Ninety percent of the flooding happened quickly over a period estimated to have lasted several months to two years, following long periods of very little water flow that may have lasted thousands of years. At times, the sea level in the basin rose more than 10 meters per day. Based on erosion features found today under Pliocene sediment, Garcia-Castellanos et al. estimate that water flowed down a drop of more than 1,000 meters, with a maximum water flow of about 100 million cubic meters per second. This is 1,000 times greater than the flow of the present-day Amazon River. Studies of underground structures at the Strait of Gibraltar show that the flooding channel sloped gradually 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 large landmasses called the African, Arabian, and Eurasian plates. These plates caused the ancient Tethys Ocean to become smaller until its western part formed the present-day Mediterranean. During the late Miocene, the Mediterranean became cut off from the Atlantic Ocean. This happened when two underwater paths, the Guadalhorce and Rifian corridors, closed. This event caused the Messinian Salinity Crisis, during which thick salt layers formed on the ocean floor and the edges of continents were worn away. The Nile and Rhône rivers carved deep canyons during this time. Water levels in the Mediterranean dropped by several kilometers. It is not clear how much the water level dropped or if the drop was the same in the Western and Eastern Mediterranean. It is possible that some seas remained connected on the ocean floor.
Evidence of Atlantic fish in Messinian deposits and the large amount of salt formed during the Messinian Salinity Crisis suggest that some water still flowed from the Atlantic into the Mediterranean before the Zanclean flood. Before the Zanclean flood, more rain and water runoff made the remaining sea less salty, leading to the formation of "Lago Mare" sediments. Changes in water levels may have occurred, with water coming from the Paratethys, a sea north of the Mediterranean, through processes where water moved from one body to another.
Event
The Zanclean flood happened when the Strait of Gibraltar opened. Earth movements in the Gibraltar region may have caused the natural barrier there to sink until it broke. Scientists are not sure what exactly caused the flood, but some believe faulting or rising sea levels could have played a role. The most widely accepted idea is that a stream flowing into the Mediterranean Sea eroded through the Strait of Gibraltar until it connected to the Atlantic Ocean. This suggests the Strait did not exist before this erosion event.
During the flood, a channel formed across the Strait of Gibraltar, starting at the Camarinal Sill. This channel was carved into the seafloor of the Alboran Sea, split around a high area in the Alboran Sea, and then connected to the Alboran Channel before branching into several paths that led to the Algero-Balear basin. The channel’s starting area had a U-shaped form, which matches how it might have formed during a large flood. The flood moved about 1,000 cubic kilometers of rock, which was deposited in the Alboran Sea as large underwater rock formations. However, the part of the channel that passed through the Camarinal Sill may have formed differently.
Whether the Zanclean flood happened slowly or suddenly is still debated, but it occurred very quickly by geological standards. Scientists have used models to study the flood’s size. One model suggests the flood could have been more than 10–100 sverdrup, a unit used to measure water flow. Another model suggests that after the barrier first broke, the water eroded the natural barrier, creating the channel and increasing the flood’s speed until water levels in the Mediterranean rose enough to slow the flood.
In this scenario, the flood reached a peak with more than 100,000,000 cubic meters of water flowing every second, moving at speeds over 40 meters per second. These flow rates were about 450 times larger than the Amazon River’s flow and ten times as much as the Missoula floods. The flood would have flowed into the Mediterranean Sea over a gentle slope, not as a waterfall. Later models suggest the flood’s 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 0.4–0.7 meters per day. The flood’s size depended on the water level 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, located 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 process, strong water currents and reverse flows created large amounts of sediment in the Ionian Sea. Early estimates suggested the Eastern Mediterranean would take thousands of years to refill, but later studies of the Strait of Gibraltar’s size suggest the refill could have happened in less than a year.
A large flood is not the only explanation for the Mediterranean reconnecting with the Atlantic and the environmental changes that followed. Some scientists suggest the Mediterranean may have refilled more slowly through other water sources or that the Strait of Gibraltar remained open all the time. Evidence from the southern edge of the Alboran Sea supports the idea that a catastrophic flood did not happen. However, deposits near the Malta Escarpment suggest a single intense flood may have caused the connection across the Straits of Sicily.
The timing of the Zanclean flood is uncertain, but one possibility is that it occurred around 5.33 million years ago. This event is usually linked to the end of the Messinian/Miocene and the start of the Zanclean/Pliocene. Some scientists think the main flood may have been preceded by a smaller flood, and deep sea terraces suggest the Mediterranean may have refilled in several stages. It may have taken about ten years for the Mediterranean to completely refill.
Consequences
The Zanclean flood formed the Strait of Gibraltar. It is unlikely that tectonic or volcanic events created the strait, as the main plate boundaries do not pass through the strait, and there is little seismic 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 movement of nearby land caused by gravity.
The Zanclean flood changed the environment of the Mediterranean basin. Layers of sediment from a time when the region was a large lake, called the "Lago Mare" facies, were replaced by deep sea deposits from the Zanclean period. The flood may have influenced global climate, as a smaller flood that occurred when Lake Agassiz drained caused a cold period. Evidence of this flood’s effects was found as far 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 coast. 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, and seals were able to travel from the Atlantic Ocean into the Mediterranean Sea.
Evidence of the flood has been found in sediments from the Zanclean period, both in drill holes and in sediments that were later lifted above sea level. A clear erosional surface separates older, pre-Zanclean sediments from younger, marine sediments.
Water from the Zanclean flood likely spilled into the Ionian Sea through Sicily and the Noto submarine canyon near Avola. This spillover flood 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 strong earthquakes. These earthquakes likely triggered large landslides, which could have created tsunamis with waves as high as 100 meters (330 feet). Signs of these tsunamis have been found in the Algeciras Basin. The filling of the basin created tectonic stress, which may have contributed to the formation of the Apennine Mountains.
Similar megafloods
Throughout history, similar large floods have happened in other places on Earth. One example is the Bonneville flood in North America, during which Lake Bonneville flowed over Red Rock Pass into the Snake River Basin. Another example is the Black Sea deluge theory, which suggests a flood occurred from the Mediterranean Sea into the Black Sea through the Bosporus.
Research history
In his book Historia Naturalis, Pliny the Elder wrote about a legend that Hercules created the Strait of Gibraltar, connecting the Mediterranean Sea to the Atlantic Ocean. The scientific idea called the Zanclean flood theory was developed in 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 sea rushed back into the region quickly, in just a few thousand years or less.