Satellite image of the Mediterranean Sea. Eric Gaba (Sting), Public domain, via Wikimedia Commons.

A new study provides compelling new evidence that a colossal ‘megaflood’ refilled the Mediterranean Sea millions of years ago.

The study suggests the Zanclean Megaflood ended the Messinian Salinity Crisis, during which the Mediterranean Sea was a vast expanse of salt flats, and which lasted between 5.97 and 5.33 million years ago.

An international team of scientists, including the University of Southampton, have identified a series of geological features around southeast Sicily that point to a massive flood taking place across the region.

Lead author Dr Aaron Micallef, a researcher at the Monterey Bay Aquarium Research Institute in California, said: “The Zanclean megaflood was an awe-inspiring natural phenomenon, with discharge rates and flow velocities dwarfing any other known floods in Earth’s history.

“Our research provides the most compelling evidence yet of this extraordinary event.”

During the Messinian Salinity Crisis, the Mediterranean Sea became isolated from the Atlantic Ocean and evaporated, leading to vast salt deposits which reshaped the region’s landscape.

For years, scientists had thought that this dry period ended gradually, with the Mediterranean refilling over a period of 10,000 years.

But this idea was challenged by the discovery of an erosion channel stretching from the Gulf of Cadiz to the Alboran Sea in 2009.

The finding pointed to a single, massive flooding event, lasting between two and 16 years, which became known as the Zanclean megaflood.

Estimates suggest the megaflood had a discharge from 68 to 100 Sverdrups (Sv), with one Sv equal to one million cubic metres per second.

Comprehensive

The new research combines newly discovered geological features with geophysical data and numerical modelling to provide the most comprehensive picture yet of the megaflood.

The researchers investigated over 300 asymmetric, streamlined ridges in a corridor across the Sicily Sill – a submerged land bridge that once separated the western and eastern Mediterranean basins.

Co-author Professor Paul Carling, an Emeritus professor in the School of Geography and Environmental Science at the University of Southampton, said: “The morphology of these ridges is compatible with erosion by large-scale, turbulent water flow with a predominantly north-easterly direction.

“They reveal the immense power of the Zanclean Megaflood and how it reshaped the landscape, leaving lasting imprints on the geological record.”

By sampling the ridges, the team found they were topped with a layer of rocky debris containing material eroded from the ridge flanks and the surrounding region, indicating it was deposited there quickly and with immense force.

This layer is right at the boundary between the Messinian and Zanclean periods when the megaflood is believed to have occurred.

Using seismic reflection data – a kind of geological ultrasound allowing scientists to see layers of rock and sediment beneath the surface, the researchers discovered a ‘W-shaped channel’ on the continental shelf east of the Sicily Sill.

This channel, carved into the seabed, connects the ridges to the Noto Canyon – a deep underwater valley located in the eastern Mediterranean.

The shape and location of the channel suggest that it acted like a massive funnel.

When the megaflood waters poured over the Sicily Sill, this channel likely carried the water toward the Noto Canyon and into the eastern Mediterranean.

Computer models

The team developed computer models of the megaflood to simulate how the water might have behaved.

The model suggests that the flood would have changed direction and grown in intensity as time went on, reaching speeds of up to 32 metres per second (72 miles per hour), carving deeper channels, eroding more material, and transporting it over longer distances.

Micallef said: “These findings not only shed light on a critical moment in Earth’s geological history but also demonstrate the persistence of landforms over five million years.

“It opens the door to further research along the Mediterranean margins.”

The study is published in Communications Earth & Environment.