That Time the Mediterranean Sea Disappeared

In 2011, paleontologists working on the Spanish island of Menorca announced the discovery of some very strange fossils. They were bones from a rabbit, but it didn't look like any rabbit you'd ever seen. For one thing, this bunny was a giant, up to six

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s heavier than the average rabbit. It also had short hind limbs, compared to the forelimbs, and feet tipped with claws. And this rabbit almost certainly couldn't jump. It had a rigid spine and splayed toes, very different from the flexible spines and tight toes of living rabbits. Scientists named this enormous bunny Nuralagus rex, "the Rabbit King of Menorca," and determined that he ruled his island kingdom during the Pliocene Epoch, approximately 5 to 3 million years ago.

Now, we've talked before about island gigantism, where small animals left isolated on islands evolve into larger forms due to a lack of predators. And that seems to be what allowed Nuralagus rex to grow so big. But how did the normal-sized ancestor of Nuralagus arrive on a Mediterranean island? Well, it seems that the answer to this biological mystery is actually wrapped up in an even older geological mystery. Since the 19th century, scientists have known that the layers beneath the floor of the Mediterranean Sea were not just made up of the usual sediments, such as mud and sand.

Instead, they are filled with salt crystals (lots and lots of salt) that form mega deposits so large they are some

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s called the Salt Giant of the Mediterranean. And salt deposits like these are often found in places where bodies of water have dried up. So the existence of this Salt Giant suggests that, at some point in history, the Mediterranean Sea must have evaporated. But how could a body of water as large as the Mediterranean simply... disappear? It would take decades and more than 1,000 research studies to even begin to uncover the cause (or causes) of one of the largest disappearing acts in Earth's history.

Today, ocean water flows into the Mediterranean Sea from the Atlantic through a narrow passage between Europe and Africa, called the Strait of Gibraltar. And that is the main source of water for the Sea. Some fresh water also flows into it in the form of rain and rivers, but that is not enough to keep the sea full without water from the Atlantic, because it has very high evaporation rates. . So, to geologists trying to explain the existence of the Salt Giant, it seemed as if the main source of water in the Mediterranean had been turned off somehow, like turning off the faucet in a bathtub.

They called this event the Messina Salinity Crisis, or MSC. But it was not easy to discover how that water source was cut off. And researchers have been arguing about it since the 1970s. How did it happen? How long did it take? In the end, they proposed three main hypotheses to answer these questions and explain how the salt giant got there. First, some scientists thought there was a global cooling event at the beginning of the crisis, in the Late Miocene, about 6 million years ago. If the entire world had cooled into an ice age, then a lot of water would have been taken out of the ocean and frozen in glaciers, reducing the flow of water into the Mediterranean Sea from both the Atlantic and rivers.

An event of this magnitude should have been global, not local. However, this idea was proven wrong quite quickly. Shortly after the Salt Giant was discovered, researchers began studying oxygen isotopes and other geochemical data from sediments and ice cores around the world. Their data showed that the rest of the Earth was not abnormally hot, cold or dry during the crisis. Additionally, they discovered that some of the salt was deposited before any change in sea level. So there didn't appear to be a cooling event large enough to cut off the water flow. That ruled out the first hypothesis.

The second idea was that tectonic events had somehow blocked the Mediterranean from the Atlantic and cut off the flow of water. Some researchers thought that moving ocean crusts were slowly blocking the waterway between the Atlantic and the Mediterranean. As the water remaining in the deep basin evaporated, it became increasingly saltier, depositing layers of salt as it dried. And this explanation was *almost* correct. Other scientists thought it might have been a combination of shifting crusts and climate change that caused the Mediterranean to dry out. This was the third hypothesis. According to this model, the crust under the Strait of Gibraltar rose over time, reducing the flow of water from the Atlantic.

Then, due to changes in the regional climate, such as periods of less rain and higher temperatures, the amount of freshwater reaching the Mediterranean varied. So, according to this way of thinking, the MSC didn't happen all at once. Instead, water levels began to drop after the strait closed and then fluctuated according to changes in the climate. And this hypothesis ended up being… practically correct! Or at least close to it. When geologists began collecting evidence to test these hypotheses, they eventually discovered that the MSC was caused by changes in the Earth's crust, but those changes actually occurred repeatedly, not just all at once.

For example, in sediments near the Nile, geologists found evidence of repeated erosion events, not just one big erosion. This meant that the water level dropped, created a new shoreline for a while, and then dropped again a few more times. Some researchers estimate that there were 16 climate cycles right at the beginning of the MSC. And these changes also correlate pretty closely with what we know about climate cycles. During periods of falling sea level, the position and angle of the Earth changed with respect to the Sun, so there were periods of lower solar energy, and others of higher solar energy, which increased evaporation rates in the sea.

Mediterranean. At the same time, an actively folding and uplifting tectonic belt caused a decline in water supply. The researchers were able to use chemical, and even magnetic, signatures in sediments deposited during this time to estimate how long the MSC lasted. And data suggests it lasted for more than 600,000 years, with the driest period occurring about 5.6 million years ago! At the height of the MSC, external water sources were completely cut off and most of the water remaining in the Mediterranean basin was evaporating. Geologists believe the water level dropped a few hundred meters, the length of several American football fields.

And the water that was left was supersaturated, so the salt continued to precipitate to the bottom of the Sea. The longer there was salt water that could precipitate the salt, the thicker the final deposits became. And the salt giant is possibly up to 3 kilometers thick, meaning the sea was extremely salty for hundreds of thousands of years during the MSC. And this, of course, had enormous effects on living things in and around the Mediterranean. Back then, life at sea was dramatically different. Today, the Mediterranean Sea is home to thousands of marine species and is famous for its crystal clear waters.

But when the sea almost dried up completely, it became uninhabitable. Almost nothing could live there. Most animals and plants that lived in the Mediterranean before the MSC migrated or died because the water was too salty and shallow. Some marine paleontologists think that no true marine organism could have survived, and the evidence for this is quite good. For one thing, the sediments that were deposited during this period were undisturbed, suggesting that no burrowing creatures lived there. And even now there are hardly any deep-sea animals left that are unique to the Mediterranean, because they all died during the MSC.

Some shelled creatures, like gastropods, might have survived, although they weren't exactly thriving. Sediment cores showed that small populations may have eked out a living in isolated areas. But the disappearance of the sea was not entirely bad news for living beings; The lower water levels were also an opportunity for some creatures to thrive. The distribution of fossils of now-extinct megafauna suggests that there was once a land bridge between the mainland and several Mediterranean islands, such as Sardinia and Corsica, which has now

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. This allowed hippos, elephants, and other African megafauna to walk and swim across the Mediterranean.

And we know that it was not only the megafauna that took advantage of the crisis to move. It seems that this was when the ancestors of our giant friend, Nuralagus, left the European continent for Menorca. Paleontologists have found other giant fauna, such as dormice and hamsters, on islands that have not been connected to each other since the MSC, such as Malta and Sicily. This was not the first time that animals made a long journey to lands around the sea; it was just one of several migratory events during the late Miocene. Remains of these migrations are found in the fossilized remains of the hippos and elephants that lived there.

But when the water rose again at the end of the MSC and the land bridges

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, the populations became isolated from each other and from North Africa. That isolation led to small versions of large animals, or insular dwarfism, and large versions of small animals, or insular gigantism, on islands around the Mediterranean. Which solves the mystery of how Nuralagus arrived in Menorca and why he ended up being so big. The Mediterranean Sea, of course, has returned, so obviously the water returned at some point. Ocean crust models suggest that the Strait of Gibraltar opened when tectonic plates shifted again and sediments eroded, reducing the barrier between the Atlantic and the Mediterranean, allowing water to flow through the Strait and into the basin. .

But like the discussions about how the MSC began, there was also much debate about how it ended. At first, some scientists thought it was filled by a giant waterfall falling into the Mediterranean, with water falling so fast that it was filled in just a few months. They even found sediment deposits that suggested rapid flooding had occurred. And while that sounds incredible, it seems like it's not true, at least not the waterfall part. More recently, other geologists, using seismic data, discovered that the slope between the Atlantic and the Mediterranean was not steep enough for a waterfall to have occurred.

Instead, what refilled the sea probably looked more like a river. However, the basin filled quickly; Recent estimates say it only took about 2 years to end the MSC, during an event sometimes called the Zanclean Flood. But that doesn't mean this story is over. The Earth's plates are always moving, and if the perfect storm of plate movement and climate change were to repeat itself, it could happen again. Scientists are still finding more evidence of the MSC, from those fossilized mini elephants to Nuralagus and salt crystals found in the soil and under the Mediterranean Sea. And while some events leave obvious marks on the planet's surface, such as mountains and craters, there are others where you have to look deeper to find traces.

Although the MSC shaped life in and around the Mediterranean for hundreds of thousands of years, its fingerprints lie largely out of sight, buried beneath the waves. But the fossils of those dwarf elephants and giant rabbits let us see their effects and help us remember that geological moment in which the Mediterranean Sea disappeared. Hey! Do you love PBS shows? Well, the PBS Video app is your home for amazing educational programs ranging from science films from NOVA to concerts at Austin City Limits to documentaries from Ken Burns and Frontline, all without ads or algorithms. Oh, and eons. We are there too!

If you live in the United States, check out the link in our description to start watching anytime, on almost any device. Now, giant kicks to this month's eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, Sean Dennis, Hollis, and Steve. Pledge your support and become an Eonite at patreon.com/eons! And as always, thank you for joining me at the Konstantin Haase Studio. Be sure to subscribe at youtube.com/eons.