A study from the Australian National University examines what unfolded during the mega-flood known as the Zanclean event, which shaped the Mediterranean’s present configuration. At its peak, a colossal waterfall, about 1.5 kilometers high, poured over the region that is now Sicily and helped fill the eastern half of the basin with seawater.
The Zanclean megaflood stands as the largest flood ever documented by science. It marks a turning point that transformed the Mediterranean basin from a largely arid saltwater pool into the vibrant marine ecosystem observed today. The ANU study, published in Nature Geology, sheds light on how this dramatic transition occurred.
Shells can even be found in the Troodos Mountains, one of the island’s broadest ranges. This is a reminder of the Mediterranean’s turbulent geologic past, with shifting tectonics, rising and sinking land, and dramatic flood events.
Udara Amarathunga, the study’s lead author and ANU PhD researcher in paleoenvironments, notes that the Zanclean flood ranks among the most abrupt global environmental changes since the mass extinction that ended the dinosaurs. He describes the event as the “Mediterranean renaissance.”
Gibraltar ‘gate’ closing
“The megaflood ended the Messinian Salinity Crisis. The basin partially dried as the Atlantic–Mediterranean gateway at Gibraltar narrowed and finally closed, leaving large salt deposits and causing widespread die-off,” explains Cosmos magazine.
Straits of Gibraltar and the surrounding waters illustrate what happened, as the sea level and salinity shifted dramatically during this period.
Amarathunga explains that the Mediterranean’s salinity crisis began about six million years ago when the European and African plates moved toward one another, isolating the Mediterranean from the Atlantic where Gibraltar lies today.
He notes that the “gate” did not close abruptly but gradually tightened before becoming effectively sealed around 5.6 million years ago, marking the climax of the salinity crisis.
Although the exact extent of the drying remains debated, the researchers estimate that sea level fell by about 1–2 kilometers, creating eastern and western basins separated by the landmass of what is now Sicily.
A giant waterfall 1.5 kilometers high
But how did the Mediterranean recover from an uninhabitable salt lake?
Amarathunga describes a slow seepage of water from the Atlantic into the Mediterranean driven by erosion at Gibraltar. The megaflood hypothesis, proposed by Spanish scientist Daniel García-Castellanos in 2009, posits that an initial modest flow gradually breached a natural dam, allowing massive volumes of water to surge into the basin.
This flood would have created a towering waterfall at Sicily’s height, flooding the eastern portion of the basin as well.
The energy of this immense water movement would have equaled roughly 500 Niagara Falls over an entire year, and the eastern basin could have risen by more than 10 meters at the flood’s peak.
During the height of the flood, the flow would have reshaped the basin’s dynamics, depositing a layer rich in organic material while fostering anoxic conditions in the eastern part of the sea.
These processes unfolded in stages: first the western basin filled, then, as the barrier at Sicily was breached, a second wave of water surged eastward. The eastern basin would have received large quantities of salt during this phase, altering chemical balances across the basin.
According to Amarathunga, by the flood’s end both basins began to mix, yet the immense storm energy moved most of the salt into the eastern basin. A separate model estimates how long it would take for this excess salt to reach the Atlantic and be diluted, revealing a lag of about 26,000 years before the sea could be considered to have returned to a typical, balanced state.
That extended transition period remained largely unknown to scientists. While the megaflood hypothesis has supporters, it also faces skepticism, and some scientists have argued that the Mediterranean did not dry out on such a large scale. The new ANU study adds weight to the megaflood argument and deepens understanding of the event.
Amarathunga emphasizes that such a dramatic and rapid transformation is rare, making the Zanclean flood a striking example of how quickly an entire ecosystem can change.
Reference: Nature Geology, 2022.
…
Note: Details are based on paleoenvironments research conducted by the ANU and reported in scholarly sources.