Gibraltar’s door to the past and the Mediterranean reset
A study led by the Australian National University examines what happened in the Mediterranean five million years ago during a massive flood that reshaped the region and helped set the sea to its present order. A colossal waterfall, towering 1.5 kilometers, would have surged into the eastern Mediterranean near what is now Sicily, driving a dramatic flood across the basin.
This event, known as the Zanclean Mega-Flood, stands as the largest flood ever documented in scientific records. It transformed the Mediterranean from a drying saltwater pool into the vibrant marine ecosystem we know today. The ANU findings, published in Nature Geology, illuminate the sequence of events that led to this remarkable transition.
Today seashells are found high in the Troodos Mountains on Cyprus, the island’s largest mountain range, evidence of the Mediterranean region’s turbulent history marked by plate movements, island submergence and emergence, and periodic floods.
Udara Amarathunga, the 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 age of dinosaurs. He describes the event as a Mediterranean rebirth that set the stage for modern coastal habitats.
Gibraltar’s doorway closes
The mega-flood ended the Messinian Salinity Crisis by nearly drying the Mediterranean basin after the Atlantic–Mediterranean gateway at Gibraltar narrowed and then closed. In Cosmos Magazine this closing is described as producing large salt deposits and wiping out many life forms.
Amarathunga explains that the crisis began about six million years ago when European and African plates collided, severing the Mediterranean from the Atlantic at Gibraltar. He notes that the gateway is not fully sealed, but access became severely limited, with the peak of salinity occurring about 5.6 million years ago.
Scientists estimate the drying reduced sea level by one to two kilometers, creating two basins, the eastern and western Mediterranean, now separated by Sicily.
A 1.5 kilometer high waterfall
How did the eastern Mediterranean move from a hostile saltwater basin to a life-sustaining sea? Erosion in the Gibraltar region allowed small quantities of Atlantic water to seep into the Mediterranean, setting the stage for the mega-flood hypothesis. This idea was first proposed in 2009 by Daniel García-Castellanos, who suggested that a damlike barrier would eventually fail, unleashing a rush of water into the Mediterranean.
The flood would create an enormous waterfall along Sicily and flood the eastern half of the basin. The energy of that surge would have dwarfed Niagara Falls, with the Mediterranean rising by more than ten meters daily at the flood’s peak.
As the surge built, the flood would carry vast amounts of salt into the eastern basin, altering its chemistry and oxygen levels. In the eastern Mediterranean an organic-rich layer marks low-oxygen conditions, a feature not observed in the western basin. The transition occurred in stages as megacells and organic layers formed across the basin, reshaping the sea’s environmental profile.
Initially the western basin filled, and later a breaching event at Sicily released a massive eastern waterfall. The salt carried by the flood moved toward the Atlantic, and modeling suggests it took roughly twenty-six thousand years to remove the excess salt and restore the Mediterranean to a more typical sea state.
A long transition followed, a period previously unknown to scientists. The mega-flood hypothesis remains debated; some researchers argue the Mediterranean never fully dried out. Yet the ANU study adds compelling evidence that supports the mega-flood scenario and highlights how rare and transformative such a rapid environmental shift can be.
Amarathunga emphasizes that a change of this scale is unusual, making the Zanclean flood a striking example of how quickly entire ecosystems can alter. The study underscores the potential for dramatic climate and hydrological events to recalibrate coastal environments in geologic blink of an eye.
Reference work: Nature Geology on the Zanclean mega-flood and the Mediterranean’s reemergence as a thriving oceanic system.
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