The Messinian Salinity Crisis & the Zanclean Flood

The Mediterranean Sea is massive, covering 2.5 million square kilometers (970,000 square miles). It contains 3.75 cubic kilometers of water, enough to fill more than 310 copies of Lake Superior. The body has nourished some of the planet’s greatest civilizations, from the Phoenicians to the Greeks to the Romans.

What would this region of Earth look like if the Mediterranean had no water? Would the societies that ringed it have emerged if this sea, which is open to the Atlantic at just one eight-mile stretch, had instead been a landform? This hypothetical might sound odd today, but the past and possible futures of our planet’s geology actually allow us to envision such a scenario.

If human civilizations had existed during the Messinian age of the Miocene period, a term that transpired between 5.96 million and 5.33 million years ago, they would have reckoned with exactly such a situation.

Because the surface of our planet is constantly in tectonic flux, regions we see today often look slightly different as we move backward through time (and radically different as we move radically backward through time). Six million years ago, the Strait of Gibraltar did not exist. The Mediterranean likely looked a bit similar to today’s sea, but also probably featured a few noticeable differences. It might have appeared something like this:

Suddenly, about 5.96 million years ago, for reasons scientists do not fully understand, the western portion of the Mediterranean closed. The general consensus leans on tectonic activity, which likely uplifted sills near the modern conjunction of the Iberian Peninsula and Africa. Whatever the cause, the Atlantic and the Mediterranean no longer touched.

Geologists refer to the following era as the Messinian salinity crisis.

Without inflow from the Atlantic, the Mediterranean began to desiccate. Though short spurts of higher sea levels might have inched over the new western plug several times, by 5.6 million years ago, the two mighty seas were separated completely. The dry climate in the region went to work and, within 1,000 years, the ancient Mediterranean Sea became a nearly dry basin. Just pockets of brackish lakes likely persisted, similar to today’s Dead Sea. Where a major sea once stood, a 1.9- to 3.1-mile-deep crater lay.

A layer of salt and gypsum formed on the seabed, enabling future geologists to realize what had happened. Today, this layer rests 100 to 200 meters below the current seafloor, evenly distributed across the Mediterranean, indicating the widespread nature of the event.

When the evaporation reached its maximum, the map might have looked something like this:

As you can imagine, a geographical shift of this scale resulted in some seismic changes to the region.

The happening extirpated scores of sea species. The temperature rose to desert-like levels. We now know that oceans are linked to air currents, so this evaporation likely caused massive shifts in weather patterns and the overall climate. Interestingly, the new land did open up the movement of fauna between the two continents, though the combination of salinity and temperature would have dissuaded many animals from staying put.

The lack of water opened the basin up to tremendous downcutting. Though the Atlantic no longer contributed water, the major rivers of the region, such as the Nile and the Rhone, continued to empty into the basin. The results were canyons of wonderful depths. To this day, a buried canyon rests underneath Cairo, carved by the Nile to a depth of at least 7,900 feet.

Of course, the Mediterranean is not today a desiccated basin, which means an event must have transpired to undo the salinity crisis.

That cataclysmic phenomenon is known as the Zanclean flood, which happened approximately 5.33 million years ago. Like the preceding salinity crisis, the cause of the deluge that refilled the Mediterranean is not known with certainty, though several hypotheses exist. Tectonic subsistence might have lowered the land on the western edge of the basin to the point that the Atlantic overtook it or sea levels could have risen globally to the point that they breached the sills. The most widely held belief, however, stems from simple erosion. Over the millennia, water worked on the land, forming streams and channels that one day burst wide open.

Whatever caused the Zanclean flood, the event was Biblical in scale. Some simulations display a complete refilling of the basin within a few months or a couple of years. The water would have poured in at unthinkable speeds, up to 100,000,000 cubic meters per second. That rate is 1,000 times the flow of the Amazon River, the world’s largest river by discharge volume, and 10 times as large as some of the other megafloods of the Earth’s past. Canyons the size of the Grand Canyon were etched by these rapids.

Unfortunately for our imaginations, this influx of water did not occur like a towering waterfall. Mapping of the underwater architecture indicates a gradual slide, instead of a vertical spectacle.

The re-watering of the Mediterranean happened so quickly that many species became endemic to the islands on which they happened to be. The islands we all know today appeared and have remained since. The Strait of Gibraltar became the western gate of the sea. Ocean life returned. To this day, the Mediterranean has a higher concentration of salt than the Atlantic.

If time travel existed, one could certainly do worse than travel to the time of the Zanclean flood to witness the onslaught of water. Few natural spectacles could match its scale. Then again, the show might repeat itself in the distant future. If the Strait of Gibraltar closes, as it’s projected to do, we could see a second crisis and, potentially, another deluge.