New evidence revives a long-standing debate about the restoration of the link between the Mediterranean and the Atlantic Ocean.
The Great Blue hides a bitter secret: a salt crust about three kilometers thick covers its seabed. These white crystals are the last remnants of an ancient Mediterranean that disappeared millions of years ago. Some scientists are convinced that it dried up completely for a while, much like the southern Sahara.
The mystery surrounding the reasons for this evaporation and the torrents of water that later filled the basin still persists, even after decades of study. Around five million years ago, the Mediterranean was refilled during the greatest flooding ever known. One estimate suggests that the cascade of water that poured into the eastern basin was 500 times the flow of the Amazon River.
« It was a spectacular phenomenon, » says Daniel García-Castellanos of the Jaume Almera Earth Sciences Institute in Spain. In a study published in Earth-Science Reviews, García-Castellanos and his team identified sediments that likely came from this massive flood.
The Mediterranean, as we know it today, wouldn’t exist without this cataclysmic reconnection to the Atlantic Ocean. Ships would not have been able to sail its waters to sustain the rich civilizations that have thrived along its shores since the dawn of human civilization. Today, the Mediterranean serves as a vital pump for global water circulation. Its evaporation adds a dose of extra salt to the Atlantic, fueling the oceanic « conveyor belts » that circle the planet, affecting temperatures, storms, and more.
As current temperatures continue to rise and ice sheets become rarer, understanding why the planet is the way it is today has become « more than crucial, » as explained by Rachel Flecker, a geologist at the University of Bristol.
AN UNPRECEDENTED FLOOD
Today, the millions of cubic kilometers of water in Mare Nostrum continually evaporate, with over a meter of water turning into vapor each year. Rainfall and rivers alone cannot replenish the system. The only source that balances the evaporation is the constant flow of water from the Atlantic Ocean, which passes through a narrow channel between Spain and Morocco: the Strait of Gibraltar.
Millions of years ago, tectonic plate movements deep below broke the vital connection between the Mediterranean and the Atlantic. Water likely continued flowing into the basin, but this rise likely blocked the outlet for thick saline currents that ran along the basin’s floor before reaching the sea. About six million years ago, mineral salts began to accumulate—enough to fill 50 Great Pyramids of Giza for each of the 7.7 billion people on Earth.
Some researchers believe the region almost dried up before the flood, with water levels dropping by more than a kilometer and a half compared to today. A narrow strip of land now forms the present-day Strait of Gibraltar (though the exact width of this ancient land bridge remains unknown).
Around 5.3 million years ago, a cataclysm restored the connection between the ocean and the sea. The scale of the flood remains highly controversial, as does the extent of the drying. With few available clues, García-Castellanos and his team have tried to determine how quickly the empty Mediterranean basin could fill. Based on a 2009 study, they suggest the rupture likely began with runoff along the natural dam linking present-day Europe to Africa. But erosion soon took over. « Very quickly, the process escalated, » says García-Castellanos.
As the water rose, it carved deep openings that allowed even larger amounts of water to rush in. At its peak, this transference would have carried more than 100 million cubic meters of water per second, filling the sea in less than two years. Such an event would have dug out the equivalent of 400 million Olympic swimming pools of sediment, sculpting a channel through the Strait of Gibraltar and a canyon extending down to the seabed.
« It’s like water bursting from a fire hose, » says William Ryan, a marine geologist at Columbia University, who participated in the early work identifying the salt deposits in the Mediterranean.
The cataclysm transformed the entire region, not only by displacing water but also by shearing rocks, sand grains, and other materials in its path. « Such energy doesn’t just bounce sediments into the abyss. Everything is thrown in an extremely turbulent and chaotic manner, » says Victor Baker, a geologist at the University of Arizona and an expert in massive floods.
UNVEILING A PREHISTORIC MYSTERY
Geologists in the 1800s didn’t believe such massive floods were possible. Modern techniques have proven that an ancient geological event like this could have occurred. « The problem is that very large floods are rare, » explains Baker. Like the Chicxulub meteorite catastrophe that forever changed life on Earth, phenomenal floods don’t happen every year—or even every million years.
Scientists began investigating the history of the Mediterranean in the 1950s when they discovered salt deposits along its coasts, revealing the existence of a particularly salty ancient sea. In the 1970s, researchers aboard the drilling ship Glomar Challenger conducted a drilling campaign of the seabed, finally uncovering the salt deposits, remnants of this tumultuous period in the Mediterranean’s history.
Elements resembling the cracked surface of a mudflat exposed to the sun were found in the upper layers of salt—an indication that the waters were not always tumultuous, says Ryan. However, the question of how much water evaporated and how long this lasted remains a subject of intense debate.
Over the years, many researchers have examined these intriguing waters, and with the accumulation of evidence, the situation has become increasingly puzzling. Along the basin, fossils of creatures show that the Mediterranean was full of water before reconnecting to the Atlantic, says Wout Krijgsman, a geologist at Utrecht University in the Netherlands. Before the flood, the region might not have been a desert but a reduced sea.
Where did all the sediments go? This is one of the main questions studied by García-Castellanos and others. More than 1,000 cubic kilometers of sediment are believed to have been scattered throughout the Mediterranean basin, gathering in compartments where water flow was slow. However, sediments formed long before humans set foot in the region are now buried under the seabed.
To locate ancient clues, researchers use a kind of geological probe that sends seismic vibrations from a boat down to the Mediterranean seabed to measure the echoes. A pocket of rocks and sand, likely deposited by the flood, was found east of the border separating the eastern and western basins. By consulting old seismic data, García-Castellanos and his colleagues believe they have found another sediment deposit in the form of a rocky trail extending behind an underwater volcano. Though these sediment pockets are intriguing discoveries, they have not been sampled, so scientists do not know exactly when they formed, says Flecker.
But the mystery could soon be solved. Flecker and others hope to drill in several locations across the Mediterranean to search for additional clues related to these important moments in the region’s geological past.
« Future drilling work could shed light on what truly happened, » concludes Ryan.
Source: nationalgeographic
