When I think of the word ‘crisis’, I imagine many things. Natural disasters, political upheaval, and impeding doom spring immediately to mind. But all of these pale in comparison (and geological significance) in the face of the Messinian Salinity Crisis (MSC) and the Zanclean Flood. Before you panic, let me assure you that you have nothing to fear. The crisis is thankfully over … and has been for the past 5.3 million years.
I sense eyerolls and shaking heads, so let me tell you just how terrifying this event was. Roughly 6 million years ago (hardly a glimmer in the mind of a geologist), the Strait of Gibraltar closed, likely due to movement along the tectonic fault that divides Europe and Africa, This effectively isolated the Mediterranean Sea from the Atlantic Ocean. In just a thousand years, evaporation far exceeding recharge, the Mediterranean was transformed into little more than a puddle. Imagine taking a walk from Spain to northern Africa. Looking to your right, you see the towering cliff that once was the Strait of Gibraltar, rising more than 3 kilometers (2 miles) above the sea floor. To your left, you see pockets of water with as much salt as the Dead Sea occupying what were once the deepest trenches of the Mediterranean.
Artist’s rendition of the extent of the Mediterranean Sea during the peak of the crisis
How can we possibly know this? The two strongest lines of evidence are evaporite deposits and paleocanyons (sorry, I had to throw long words in somewhere). As seawater evaporates, it leaves behind behind salt and other minerals such as gypsum and anhydrite. Rock outcrops at sites across the Mediterranean region show massive deposits of these evaporite minerals with ages between 5.3 and 6 million years (rock deposits from the seafloor have since been pushed upwards by tectonic activity). Seismic surveys have found similar salt deposits below the sea floor. The second clue is the canyons eroded by major rivers as the sea level fell. Sediments near Cairo show that the Nile River cut almost 2.5 kilometers (1.5 miles) down into the rock to come to equilibrium with the new water level. Remember that these processes took place over thousands of years, which gave rivers such as the Nile enough time to transform the landscape from giant waterfalls where they poured off the margins of Europe and Africa to deep canyons, some larger even than the Grand Canyon.
And so we come to the last chapter of our story – the Zanclean Flood. When the Strait of Gibraltar finally reopened 5.3 million years ago, water from the Atlantic Ocean came surging in. So much water, in fact, that the entire basin was refilled in 1 to 2 years. To give you a sense of scale, this would have involved a discharge 1000 times larger than the Amazon River, with sea level in the Mediterranean rising more than 10 meters (30 feet) per day.
Artist’s rendition of the refilling of Zanclean Flood
So, can this event happen again? Not only is it possible but very probable that it will. As Europe and Africa continue to be pushed towards each other by plate tectonics processes, the Strait of Gibraltar will eventually close again. And this time, it may not reopen. In fact, several million years from now, the Mediterranean will likely have disappeared completely as a new supercontinent is formed. But don’t worry, chances are we won’t be around to see it.
This has been your daily dose of mostly harmless science.
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Images from Wikimedia Commons. Cover photo courtesy of Mark, Flickr Creative Commons.